CABI SILWOOD LIBRARY
24 0066169 2
INSECT HERBIVORE LOAD
AND
PLANT SUCCESSION
GARETH EDWARDS-JONES
B.Sc. (Hons) Manchester
A thesis submitted for the degree of
Doctor of Philosophy of the
University of London and for the
Diploma of Imperial College
Department of Pure and Applied Biology
Imperial College
Silwood Park
A sc o t
Berkshire September 1988 TABLE OF CONTENTS
Sum m ary 1 List of figures 7.
List of tables 3
CHAPTER 1 : Introduction 17
1.1 O v e rv ie w 17
1 . 2 .i Host plant spatial distribution 18
1, 2 .ii Host plant density 19 Patch composition 1. 2 . ii 19 1.3 Nutrient availability in the host plant 20 1.4 Chemical defences 21 1 .4 ,i C y a n id e 22 1 .4 . ii A lk a lo id s 23
1 .4 . i i i Glucosinolates 26
1 ,4 .iv F la v e n o id s 28
1 . 4 . v S a p o n in s 28
1 ,4 .iv Terpenes & Resins 29
1 .4 . vii T a n n in s 31
1,4 .v iii Miscellaneous defences 33 1.5 Review of plant/herbivore community and 34
evolutionary theory
C H A P T E R 2: M ethods 44 2.1 Experimental sites 44 2.1 .i Site preparation 44 2 . 1. i i Site age and nomenclature 44 2. 1. iii Marking sites 45 2.2 Sam pling 46 2. 2 . i Insect sampling 47 2 . 2 .ii Plant sampling 48 2.3 Data handling and analysis 50 2 . 3 . i Rationale behind database 50 2 . 3 . ii Distribution of the data 52 2 .3 . iii Gamma distribution 52 2.3. iv Generalised linear interactive models (GLIM) 53 2.3.v Other statistics 56 CHAPTER 3: Community Patterns Through Succession 57
3.1 Introduction 57
3.2 Methods 59 3.3 R e su lts 60 3.3. a Plant community 60
3.3. a . i Plant species composition 65 3.3. a.ii Leaf area and the plant community 65
Total leaf area
Annual fluctuations in leaf area in a serai stage
Trends in the leaf area of major plant families
within and between serai stages
Proportion of serai stage leaf area, provided by
each major plant family
Contribution of individual plant species to the
total leaf area of a serai stage
3 . 3 . b. Insect Community 84 3.3. b.i Total insect abundance during succession 84
3.3. b.ii Abundance of major herbivore taxa associated with 84
different serai stages
3 . 3 . b.iii Annual variation in insect abundance 86
Total numbers
Adult numbers
3 . 3 . b.iv Annual variation in abundance of insect groups 92 3.3. b.v Herbivore species composition 92
Variation in species richness with serai stage
Annual variation in species richness
Annual variation in species richness of insect
g ro u p s
Species richness of insect groups feeding on
major plant families
3 . 3 .b.vi Variation in insect species composition with 100
successional age
Annual variation in insect herbivore species
composition 3 . 3 . b . v i i The birch community 1 0 2
Variation in species richness between canopies
and y e a rs
Proportion of adult individuals in each insect
g ro u p
Variation in abundance of Oncopsis between
canopies and dates
3 .4 D iscu ssio n 1 0 5
C H A P T E R 4: Absolute Abundance over a Successional Gradient 1 1 0 1 1 0 4.1 Introduction
4.2 M ethods i n
4.3 R esults 1 1 2
4 .3 . i Distribution of absolute abundance 1 1 2
4 . 3 . ii Absolute abundance in relation to serai stage 1 1 2
Cicadellidae
Delphacidae
Curcuiionoidea
Heteroptera
Minor insect groups - Psyllidae, Cercopidae,
Chrysomelidae
4 .3 . ii i Absolute abundance of woody and non-woody 1 2 6
plants
4 .3 . iv Absolute abundance on major plant families 1 2 6
4 .3 .V Absolute abundance and species composition of
Cicadellidae feeding on Holcus and Agrostis spp 1 4 1
4 . 3 . vi Comparison of absolute abundance of phloem and 1 4 1 mesophyll-feeding Cicadellidae
4 .4 D iscu ssio n 1 4 4
C H A P T E R 5: Vegetation Structure and the Insect Community 1 4 9
5.1 Introduction 1 4 9
5.2 M ethods 1 5 3 5.3 R esults 1 5 3 Vegetation structure and small scale variation in 5 .3 . i 1 5 3 herbivore abundance and species richness
5 .3 . i i Variation in absolute abundance of species 1 5 8 between different serai stages 5 .3 . i i i Variation in overwintering strategies and species 168 ric h n e s s
5 .3 . iv Variation in absolute abundance of birch 168
herbivores associated with the upper and lower
ca n o p y 178 5.4 D iscu ssio n
C H A P T E R 6 : Variation in Food Quality and Performance of 182
Erannis defoliaria (Lepidoptera: Geometridae) 6.1 Introduction
6.2 Experiment 1 : The effect of leaf age on the 184
performance of Erannis defoliaria
6.2.i Aim s 184
6 . 2 . i i Materials and methods 184 6.2.11. a The organisms 184 6 . 2 .11. b Sampling and experimental procedure 185 M oths
B irc h 6 . 2 . i i . c Measurement of food quality 185 Water content
T o u g h n e s s
Nitrogen and tannin content
Nitrogen content
Tannin content
6 . 2 . iii R esu lts 187 6 . 2 . i i i . a M oths 187 6 . 2 . i i i . b Food quality
6.3 Experiment 2 : The effect of the freezing of
B . pendula leaves on larval growth 197
6 . 3 . 1 Aim s 197 6 .3 .ii Materials and methods 197 6 .3 . i i . a M oths 197 6 . 3 . i i . b Food 197 6 .3 . iii R esults 197 6.4 Experiment 3: Investigation of the early spring 198
phenology of E^ defoliaria 6 . 4 . 1 Aim s 198 6 .4 . ii M ethods 198 6 . 4 . iii R esults 198 6.5 Experiment 4: The effect of leaf toughness on 200 survival of early instar larvae
6 . 5 . i Aim s 200
6 .5 . i i Materials and methods 202
6 .5 . i i i R e su lts 202 6.6 D is c u ssio n 208 6.7 C o n c lu sio n 209
CHAPTER 7: General Discussion 210
Acknowledgements 214
Bibliography 215
A p p e n d ic e s 232 SUMMARY
1. The thesis aims to test Lawton S M cN eill's (1979) h y p o th e sis
that the absolute abundance of herbivorous insects
(expressed as number/leaf area of host plant), associated
with plants of early succession, is greater than that of
herbivores associated with late successional plants.
2. The predicted differences in absolute abundance is related
to the postulated differences in the chemical defences of
early and late successional plants, being defended by
qualitative and quantitative defences respectively.
3. The hypothesis was tested for herbivorous Hemiptera and
C oleo ptera d u r in g 1985 a n d 1986 o v e r an e x perim en tal
successional gradient at Silwood Park, Berkshire U.K.,
spanning from very early colonisation of bare ground,
through mature pasture to a birch woodland.
4. The hypothesis was found to be true. Differences in
absolute abundance between serai stages dominated by
herbs and grasses is most likely due to variation in factors
other than that in chemical defences, eg predation or
nutrient availability.
5. Small scale variation in several community attributes., was
correlated with the abundance and species richness of
herbivores. Curculionoidea and Auchenorrhyncha differed
in their response to small scale variation. Host plant leaf
area and structure were important for some species.
6 . Examination of patterns of abundance and diversity of
herbivorous insects and plants revealed that diversity of
plants is greatest in early midsuccession and insects in
early succession. Species richness and abundance of
insects were comparable between years. 2
7. Experiments under controlled conditions examined the effect
of food quality on the performance of Erannis defoliaria
(Lepidoptera: Geometridae). The development time and
pupal weight of larvae fed young and old birch leaves were
compared. Larvae fed on older leaves generally performed
better. These results are contrary to current theory, and
are discussed in the light of current plant/herbivore
th e o ry . 3
LIST OF TABLES
2.1 Age and nomenclature of serai stages
2.2 Model analysis of deviance table for two-way analysis
of deviance (ANODEV) (taken from McCullagh &
Nelder, 1985).
3.1 Sorensons Index of Similarity for plant species in each
major plant family between in different serai stages.
3.2 Proportion of leaf area provided by the dominant
species to the community and major plant families on
each date, (a) ruderal 1985, (b) ruderal 1986,
(c) early 1985, (d) early 1986, (e) early mid 1985 , (f)
early mid 1986, (g) late mid 1985, (h) late mid 1986,
and species richness of each plant family and
co m m u nity.
3.3 Total, adult and nymphal abundance of each major
insect group on sites of different successional age over
two years (* = significant at P < 0.05).
3.4 Sorensons Index of Similarity for species in each major
insect group between years on serai stages of the same
a g e .
3.5 Sorensons Index of Similarity for species in each major
insect group between serai stages of different
successional age. Data summed over two years.
3.6 Species richness of each insect group on lower and
u p p e r ca n o p y 1985 a n d lower ca n o p y 1986 in m on th ly
sam ples.
3.7 Abundance and proportion of total abundance on
Oncopsis (Cicadellida: Hemiptera) in each species on
three dates in upper and lower canopy 1985 and lower
canopy 1986. 4
4.1 Mean absolute abundance and standard errors of
Curculionoidea by host plant family and species feeding
on woody and non-woody plants. abhf, ^9^) ~
150.26, P < 0 . 001); a b h s , F (1 2g2) = 430.97, P <
0 . 001.
4.2 Mean absolute abundance and standard errors of
Curculionoidea by host plant family and species feeding
on woody and non-woody plants on each date. Data
pooled over two years, (abhf, date F,„f 4 3941 = 44.04, P < 0.001, data.plant F = 1.5006, P > 0 .0 5 ; (4,394) abhs, date F = 53.23, P < 0.001, date.plant (4,282) F = 1.506, P > 0.05. (4,282)
4.3 Mean absolute abundance and standard errors of
Curculionoidea by (a) host plant family, (b) host plant
species on major plant families in different serai stages.
See text for significance levels.
4.4 Mean absolute abundance of Heteroptera by (a) host
plant family, (b) host plant species on major plant
families in different serai stages. Differences between
plant families and serai stages tested by
Kruskall-Wallis one way ANOVA.
4.5 Adult abundance of Cicadellidae specialising on
(a) Agrostis spp, (b) Holcus spp in different serai
sta g e s .
5.1 Spearman's Rank Correlation Coefficients for adult
abundance of common herbivore species in subplots
with several measured vegetation attributes during
A u g u s t 1985 in (a) r u d e r a l, (b) e a rly s u c c e s s io n ,
(c) early midsuccession, (d) late succession.
* P < 0.05, ** P < 0.01, *** P < 0.001.
5.2 Pearson's Correlation Coefficient for species richness
of Heteroptera, Curculionoidea, Cicadellidae and
Delphacidae in subplots with several measured
vegetation attributes during August 1985 in ruderal,
early succession, early midsuccession and late
succession. * P < 0.05, ** P < 0.01, *** P < 0.001. 5
5.3 Correlation coefficient for autocorrelation between
vegetation attributes for host plants in subplots during
August 1985. Separate tables for the host plant of
hervibores in each serai stage; (a) Trifolium pratense
on ruderal site; (b) Trifolium pratense and Trifolium
spp (in parenthesis) in early succession; (c) Trifolium
pratense and Trifolium spp (in parenthesis) in early
midsuccession; (d) Agrostis capillaris and Holcus
lanatus (in parenthesis) on ruderal site; (e) grasses
in early succession; (f) Holcus lanatus in early
midsuccession; (g) Holcus lanatus and Dactyl i s
glomerata (in parenthesis) in late midsuccession;
(h) Spergula arvensis on ruderal site;
(i) Tripleurospermum inodorum in early succession; (j)
Cirsium arvense in late midsuccession. * P < 0.05, **
P < 0 .0 1 , *** P < 0 .0 0 1 .
5.4 Pearson's Correlation Coefficient for species richness
of Heteroptera, Curculionoidea, Cicadellidae and
Delphacidae with that of the other insect groups
during August 1985, in ruderal, early, early mid and
late midsuccession. * P < 0.05, ** P < 0.01, *** P < 0.001.
6.1 Survival of E. defoliaria larvae on diets of young
j3. pendula leaves (treatment) and normally aging
leaves (control). Asterisks represent level of
s ig n ific a n c e , ** P < 0 .0 1 , *** P < 0 .0 0 1 ).
6.2 Surival of E. defoliaria larvae on diets of fresh
_B. pendula leaves (control) and on leaves frozen for
24 hours prior to larval feeding (treatment). (N.S. =
not significantly different, P = 0.29).
6.3 Pupal weight (mg) and larval development rate
(inverse of days from hatching to pupation) of
E. defoliaria fed fresh ]3. pendula leaves
(control) and leaves which had been frozen for 24
hours (treatment). (Mean ± 1 S.E., N = 30,
N.S. = not significantly different.) 6
6.4 Comparison of toughness of young B_. pendula leaves
with that of mature leaves (mean ± 1.S.E.) 7
LIST OF FIGURES
3.1 Number of plant species recorded in each of two years
on ruderal (R), early (E), early mid (EMS) and late
mid (LMS) successional sites.
3.2 Number of plant species recorded on four serai stages
by summing two sites of the same age (ruderal (R),
early (E), early mid (EMS), late mid (LMS)).
3.3 Number of plant species recorded in each of the major
p la n t fam ilies d u r in g 1985 and 1986 on sites o f
different successional age, ((a) ruderal, (b) early,
(c) early mid, (d) late midsuccession; mf = minor
families; Leg = Leguminosae, Co = Compositae, Cr =
Cruciferae, Po = Polygonaceae, Gr = Gramineae).
3.4 Number of plant species recorded on each date during
1985 and 1986 on sites of different successional age
((a) ruderal, (b) early, (c) early mid, (d) late
midsuccession).
3.5 Total leaf area on serai stages of different successional
age, data summed over two years (R = ruderal, E =
early, EMS = early mid, LMS = late mid, L = late
succession).
3.6 Leaf area recorded on sites of different successional
age at monthly intervals over two years.
3.7 Mean leaf area per bag at monthly intervals on upper
and lower canopy birch 1985 and lower canopy birch
1986.
3.8 Leaf area of (a) Leguminosae, (b) Compositae, (c)
Cruciferae, (d) Polygonaceae, (e) Graminae, (f) minor
families on serai stages of different successional ages.
Data pooled over two years. (R = ruderal, E = early,
EMS = early midsuccession, LMS = late midsuccession). 8
3.9 Proportion of total leaf area provided by major plant
families on serai stages of different successional age.
(a) ruderal 1 985, (b) ruderal 1986, (c) early 1985 ,
(d) early 1986, (e) early mid 1985, (f) early mid 1986,
(g) late mid 1985, (h) late midsuccession 1986.
3.10 Total number of (a) insect herbivore individuals (b)
adult insect herbivores (excluding Aphididae,
Coccoidee , AleyrocW»Aea (Hemiptera\ Thysarp ptera and
Lepidoptera) on serai stages of different successional
age. Data summed over two years (R = ruderal, E =
early, EMS = early mid, LMS = late midsuccession, L =
la te ) .
3.11 Total number of individuals in each major insect group
on serai stages of different successional age. Data
pooled over two years (R = ruderal, E = early, EMS =
early mid, LMS = late mid, L = late succession).
3.12 Total number of adults in each major insect group on
serai stages of different successional age. Data pooled
over two years (R = ruderal, E = early, EMS = early
mid, LMS = late mid, L = late succession).
3.13 Total number of insect herbivore individuals (excluding
Aphididae, Coccoidae,, Aleyro&diAea (Hemiptera),
Thysanoptera and Lepidoptera) recorded on monthly
samples over two years on serai stages of different
successional age.
3.14 Total number of adult herbivores (excluding
Aphididae, Coccoid^ai, AleyroAo\dea (Hemiptera'),
Thysanoptera and Lepidoptera) recorded on monthly
samples over two years on serai stages of different
successional age. 9
3.15 Mean number of insect herbivore species (excluding
Aphididae, Coccoidea. , Aleyro^oidea (Hemiptera\
Thysanaptera and Lepidoptera) on serai stages of
different successional age (a) data summed over two
years, (b) for each year (R = ruderal, E = early, EMS
= early mid, LMS = late mid, L = late succession).
Bars are standard errors. Clear = 1985, hatched =
1986.
3.16 Number of insect herbivore species (excluding
Aphididae, Coccoidea, Aleyro
Thysanoptera and Lepidoptera) in monthly samples
over two years on serai stages of different
successionai age.
3.17 Number of species in each major insect group during
two years on serai stages of different successional age
(Het = Heteroptera, Psy = Psyllidae, Cere =
Cercopidae, Del = Delphacidae, Cic = Cicadellidae,
Cure = Curculionoidea, Chry = Chrysomelidae).
3.18 Host plant family of species in each major insect group
and summed over all groups (Gen = generalist, Leg =
Leguminosae, Co = Compositae, Cr = Cruciferae, P =
Polygonaceae, Gr = Gramineae, PP = partial predator,
B = birch, mf = minor families). Shaded columns on
Cicadellidae indicate Typhlocybinae. Hatched columns
on (h) indicate chewing insects.
3.19 Proportion of adults in each major insect group on
u p p e r a n d low er c a n o p y b ir c h 1985 and lower ca n o p y
1986.
4.1 Mean absolute abundance of Cicadellidae by (a) host
plant family, (b) host plant species in different serai
stages (R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession, L = late
succession). Bars are standard errors. 10
4.2 Mean absolute abundance of Cicadellidae by (a) host
plant family, (b) host plant species from May to
September in different serai stages (R = ruderal, E =
early succession, EMS = early midsuccession, LMS =
late midsuccession, L = late succession). Bars are
standard errors.
4.3 Mean absolute abundance of Delphacidae by (a) host
plant family, (b) host plant species in different serai
stages (R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession). Bars are
standard errors.
4.4 Mean absolute abundance of Delphacidae by (a) host
plant family (b) host plant species from May to
September in different serai stages (R = ruderal, E =
early succession, EMS = early midsuccession, LMS =
late midsuccession). Bars are standard errors.
4.5 Mean absolute abundance of Curculionoidea by (a) host
plant family, (b) host plant species in different serai
stages (R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession, L = late
succession). Bars are standard errors.
4.6 Mean absolute abundance of Curculionoidea by (a) host
plant family, (b) host plant species from May to
September in different serai stages (R = ruderal, E =
early succession, EMS = early midsuccession, LMS =
late midsuccession, L = late succession). Bars are
standard errors.
4.7 Mean absolute abundance of Heteroptera by (a) host
plant family, (b) host plant species in different serai
stages (abhf, Kruskall- Wallis = 45.602, n = 342, P <
0.0001 ; abhs, Kruskall-Wallis = 79.522, n = 167, P <
0.001) (R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession, L = late
succession). 11
Mean absolute a b u n d a n c e of Heteroptera by (a) host
plant family, (b) host plant species from May to
September in d iffe r e n t serai stages (abhf, May,
Kruskall-Wallis = 3 .6 2 1 , n = 25, P = 0.46; June,
Kruskall-Wallis = 6.918, n = 37, P = 0.14; July,
Kruskall-Wallis = 1 6 .3 0 4, n = 74, P < 0 . 01; A u g u s t ,
Kruskall-Wallis = 30.703 , n = 97, P < 0.0001;
September, Kruskall-Wallis = 1.9218, n = 90, P =
0.7501 ; abhs, May, Kruskall-Wallis = 2.2319, n = 12, P
= 0.5257; June, Kruskall-Wallis = 13.047, n = 22, P <
0.01; July, Kruskall- Wallis = 23.651 , n = 39, P <
0.0001 ; August, Kruskall-Wallis = 36.439, n = 52, P <
0 . 001); September, Kruskall-Wallis = 11.314, n = 41, P
< 0.05) (R = ruderal, E = early succession, EMS =
early midsuccession, LMS = late midsuccession, L = late
succession).
4.9 Mean absolute abundance of Chrysomelidae by (a) host
plant family, (b) host plant species in different serai
stages (R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession). (abhf,
Kruskall-Wallis = 9.6894, n = 58, P < 0.05; abhs,
Kruskall-Wallis = 12.46, n = 23, P < 0 . 001).
4.10 Mean absolute abundance of Psyllidae by (a) host plant
family, (b) host plant species in different serai stages
(R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession, L = late
succession), (abhf, Kruskall-Wallis = 24.9296, n = 38,
P < 0.0001 ; abhs, Kruskall-Wallis = 25.794, n = 48, P < 0.0001).
4.11 Mean absolute abundance of Cercopidae by (a) host
plant family, (b) host plant species in different serai
stages (R = ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession, L = late
succession), (abhf, Kruskall-Wallis = 3.6276, n = 20,
P = 0.4587; abhs, Kruskall-Wallis = 11 .2561 , n = 20, P
= 0 .0 2 3 8). 12
4.12 Mean absolute abundance of Cicadellidae, Heteroptera
and Psyllidae by host plant family and host plant
species feeding on woody and non-woody plants. Bars
on (a) and (b) are standard errors. Heteroptera,
abhf, Kruskall- Wallis = 26.678, n = 308, P < 0.0001 ;
abhs, Kruskall-Wallis = 71 .558, n = 166, P < 0 . 0001.
Psyllidae, abhf, Kruskall-Wallis = 31.096, n = 57, P <
0 . 001; abhs, Kruskall-Wallis = 24.783, n = 48, P <
0 . 0001.
4.13 Mean absolute abundance of Cicadellidae by (a) host
plant family, (b) host plant species feeding on woody
and non-vvoody plants from May to September. Bars
are standard errors. (abhf, ^94) = 19 .3 3 , P <
0.001; abhs, Fflt = 12.24, P <'0.001). (.4,1 /b J
4.14 Mean absolute abundance of Heteroptera by (a) host
plant family, (b) host plant species feeding on woody
and non-woody plants (hatched bars = woody plants,
clear bars = non-woody plants) ( a b h f, M a y ,
Kruskall-Wallis = 0.4875, n = 24, P = 0.4851; June,
Kruskall-Wallis = 2.4217, n = 35, P = 0.1197; July,
Kruskall-Wallis = 7 .4 4 3 , n = 74, P < 0.01; August,
Kruskall-Wallis = 2 4 .8 3 9 , n = 100, P < 0 . 001;
September, Kruskall-Wallis = 0.2026, n = 88, P =
0.6592; abhs, May, Kruskall-Wallis = 0.7200, n = 14,
P = 0.3961 ; June, Kruskall-Wallis = 7.031, n = 25, P <
0 . 01; July, Kruskall- Wallis = 14.986, n = 42, P <
0.0001 ; August, Kruskall-Wallis = 28.309, n = 55, P <
0.001); September, Kruskall-Wallis = 6.079, n = 43, P
< 0 .0 5 ).
4.15 Mean absolute abundance of Psyllidae by (a) host plant
family and (b) host plant species feeding on woody
and non-woody plants. Insufficient data to allow
statistical comparisons. 13
4.16 Mean absolute abundance of Curculionoidea by (a) host
plant family, (b) host plant species on major plant
families (data pooled over all dates and all serai
stages) (L = Leguminosae, Co = Compositae, Cr =
Cruciferae, Po = Polygonaceae, B = birch). Bars are
standard errors.
4.17 Mean absolute abundance of Heteroptera by (a) host
plant family, (b) host plant species on major plant
families (data pooled over all dates and all serai
stages) (L = Leguminosae, Co = Compositae, Cr =
Cruciferae, Po = Polygonaceae, B = birch, Gr =
Gramineae, MF = minor families, G = generalist (species
associated with more than one plant family)).
4.18 Mean absolute abundance of Psyllidae by (a) host plant
family, (b) host plant species on major plant families
(L = Leguminosae, Co = Compositae, Po =
Polygonaceae, MF = minor families, B = birch).
4.19 Mean absolute abundance of Heteroptera by (a) host
plant family, (b) host plant species on herbs, grass
and birch (data pooled over all dates and serai
s t a g e s ) .
4.20 Mean absolute abundance of Cicadellidae by host plant
species feeding on (a) Agrostis and (b) Holcus
(Gramineae) in different serai stages (R = ruderal, E =
early succession, EMS = early midsuccession, LMS =
late midsuccession). Bars are standard errors.
4.21 Mean absolute abundance by (a) host plant family of
Cicadellidae excluding Typhlocybinae (b) host plant
species of Cicadellidae excluding Typhlocybinae (c)
host plant family of Typhlocybinae and (d) host plant
species of Typhlocybinae in different serai stages (R =
ruderal, E = early succession, EMS = early
midsuccession, LMS = late midsuccession, L = late
succession). Bars are standard errors. 14
4.22 Mean absolute abundance by (a) host plant family of
Cicadellidae excluding Typhlocybinae, (b) host plant
species of Cicadellidae excluding Typhlocybinae, (c)
host plant family of Typhlocybinae and (d) host plant
species of Typhlocybinae on woody and non-woody
plants (hatched columns = woody, clear columns =
non-woody plants). Bars are standard errors.
5.1 Host plant family and adult abundance of abundant
herbivore species recorded on the four younger serai
stages. Data pooled over two years. (a) Apion
apricans Fp = 8.93, P < 0.01; (b) Apion assimile,
Fp = 9.571, P < 0.01; (c) Apion aethiops, Fp
= 21.52, P < 0 . 001; (d) Sitona lineatus, Fp =
4.459, P < 0.05; (e) Sitona hispidulus, F^ ^ = 2.315,
P > 0.05; (f) Sitona sulcifrons, F 0.236, P > (3,3) 0.05; (g) Macrosteles laevis, F 62 .3 5 , P < (3 ,8) 0 . 001; (h) Recilia coronifera, F = 4.43, P > 0.05; (3 ,7) (i) Adarrus ocellaris, F p 13^ = 11 .829, P < 0.01; (j)
Psammotettix confinis, Frr) = 12.19, P < 0.01; (k) ------13,DJ Eusceiis incisus, Fp = 4 1 .1 1 , p < 0 . 001; (I)
Mocydiopsis parvicauda, F 11.07, P < 0 . 01; (3,11) 6 .0 6 , P < 0 .0 5 ;
P < 0.00 1 ;(n) Z y g in id ia s c u t e lla r is , F p 12^ 2 7 .8 6 , P < 0.001;(n)
(o) Javesella pellucida, F p ^ = 8.713, P < 0 . 01; (p)
Dicranotropis hamata,hama Ffo 0. = 2.904, P > 0.05; (q) ------13,»J Paralabqrnia aalei, F = 6.33, P < 0.05. P < (3,8) 0.05, ** P < 0.01, *** P < 0.001.
5.2 Mean absolute abundance by host plant species of
abundant herbivore species recorded on the four
younger serai stages. Data pooled over two years,
(a) Apion assimile, F, = 11 .91 , P < 0 . 01; (b) (3,8) Apion apricans, F^ .4 8 , P < 0 . 001; (<:) Apion (3 ,1 7 ) = 1 aethiops, Fp ^ = 8.97, i > 0 . 05; (d) Psammotettix
confinis, Fp ^ = 5 .0 4 , P > 0 . 05; (e) Elym ana
sulphurella, F P > 0.05; (f) Macrosteles (3 ,5) “ la e v is , F = 18.36, P < 0 ,.05; Recilia (3 ,3 ) (g) coronifera, F = 7 .9 5 , P > 0 . 05; (h) A d a r r u s (3,4) ocellaris, Fp ~ 10-05, P < 0 . 001; (i) Zyginidia scutellaris, F 29.14, P < 0.001; (j) (1,9) Dicranotropis hamata, Fp = 2.84, P > 0.05; (k) Paralaburnia dalei F,~l *3 / O J = 14.523, P < 0.01. * P < 0 .0 5 , ** P < 0 .0 1 , *** P < 0 . 001.
Mean absolute abundance by host plant family of
Cicadellidae overwintering as eggs and adults. Data
pooled over all sites and two years.
Leaf holding apparatus for measuring leaf toughness.
Arrangement of leaf holding apparatus during
measurement of leaf toughness.
Inset: position of toughness measurement of leaf,
o = position of measurement.
Mean pupal weight (mg) of E. defoliaria on control and
treatment diets. Bars are standard errors. Numbers
are sample size.
Mean pupal weight (mg) of male and female
E. defoliaria on control and treatment diets.
Treatment diet represented by hatching. Bars are
standard errors.
Mean development time (days) for EE. defoliaria larvae
on control and treatment diets. Treatment diet
represented by shading.
Mean wet and dry weight (g) of IB. pendula leaves
during spring and early summer.
Mean leaf toughness (g/cm) of E3. pendula during
spring and early summer as measured by a
penetrometer. Bars are standard errors.
Total leaf nitrogen (mg/g dry wt) of B_. pendula
during spring and early summer. 16
6.9 Percentage tannic acid equivalents as measured by
Wint's method of IB. pendula leaves during spring and
early summer. Bars are standard errors.
6.10 Abundance of _E. defoliaria and other Lepidoptera
larvae on E3. pendula during the period immediately
following bud burst 1987. 17
CHAPTER ONE
INTRODUCTION
1.1 "We have long been accustomed to comprehend many manifestations of the morphology (of plants) of vegetative as well as reproductive organs, as being due to the relations between plants and animals, and nobody, in our special case here, will doubt that the external mechanical means of protection of plants were acquired in their struggle (for existence) with the animal world. In the same sense, the great differences in the nature of chemical products, and consequently of metabolic processes, are brought nearer to our understanding, if we regard these compounds as means of protection, acquired in the struggle with the animal world. Thus, the animal world, which surrounds the plants, deeply influenced not only their morphology but also their chemistry." (Stahl, 1888, cited in Fraenkel, 1959).
Despite this unequivocal statement a century ago and much recent work, controversy over the importance of herbivores in the evolution of plants and vice versa still rages in the ecological literature. Several theories seeking to explain the plant/animal interaction and coevolution have been formulated, but none universally accepted. This thesis seeks to test one recent hypothesis on the effects of the chemical defences of plants on the population dynamics of insect herbivores: that is that the chemical defences of long-lived perennial plants e.g. trees significantly reduce the intrinsic rate of increase of herbivores, but the chemical defences of shorter lived perennials and annuals have little effect on the intrinsic rate of increase of herbivores. Such a dichotomy in effect on population processes would lead to higher herbivore densities on shorter lived plants than on longer lived ones.
Before considering this hypothesis and other theories on plant/animal interactions, three major attributes of plants known to affect the distribution, abundance and diversity of insect herbivores will be discussed; these are host plant spatial distribution, nutrient availability in the host plant and the chemical defences of plants. The latter subject is presented as a series of resumes on the distribution, structure and biological activity of the main groups of chemical defence.
1.2• i Host Plant Spatial Distribution
The effect of island size on species diversity was described in
The Theory of Island Diogeoqraphy (MacArthur & Wilson, 1967).
According to this theory, the number of species on an island will be set by a dynamic balance between rates of immigration and extinction. The rate of immigration may depend on the distance of the island from a pool of potential colonists. Colonisation rates may also be related to island size, as the larger the island, the larger the "target" for colonists to land on by chance. The extinction rates of species on islands are inversely related to total population size and are therefore usually inversely related to island size. Patches of plants may be treated as islands.
Insect herbivore species richness and population density may increase with patch size (Cromartie, 1975; MacGarvin, 1982;
Rey, 1981), but may not do so in the same way for all species within a patch (MacGarvin, 1982). Rey (1981) showed that extinction rates decreased and immigration rates increased with increasing patch size as predicted by theory. The number of species and the total number of individuals per plant may also increase with increasing patch size (MacGarvin, 1982). Strong,
Lawton & Southwood (1984) suggest that these observed patterns may be due to patch size per se, or to increased habitat heterogeneity with increasing patch size.
Experimental isolation of host plant patches from other patches of the same species has been shown to have an effect on insect herbivore species richness (Davis, 1975). There are, however, few such reports from observations of isolated host plant patches
in natural communities (Rey, 1981; Rigby & Lawton, 1981).
Cromartie (1975) showed that different species responded differently to host plant distribution. Each herbivore species had its maximum colonisation potential under slightly different situations, none was able to occupy plants in all situations with equal success.
Mathematical models indicate that a randomly searching herbivore should find evenly distributed host plants more easily than clumped host plants (Cain, 1985). Field experiments have verified this model (Cain, Eccleston & Kareiva, 1985). (See
Kareiva (1986) for a review of patchiness and insect herbivore community dynamics.)
1. 2 .ii Host Plant Density
The response of insect herbivore to host plant density is extremely variable. Some species occur in greater abundance on high density plots (van der Meijden, 1979), whereas other species have been observed to oviposit differentially on low density patches (Thompson & Price, 1977; Solomon, 1981; Root &
Kareiva, 1984). Strong et a[ (1984) conclude that it is impossible to generalise about the effects of host density on herbivore abundance and species density.
1 . 2 .iii Patch Composition
Monocultures are colonised more rapidly and may support greater densities of insect herbivores than polycultures (Root, 1973;
Bach, 1980; Tahvanainen & Root, 1972). This may be because the presence of a second plant renders the host plant less apparent (sensu Feeny), either by physically hiding the host plant or by masking its chemical odours. Risch (1980) showed that if one plant species in a two species mixture was not a host of a chrysomelid beetle then the number of beetles per host plant was less than in a monoculture. When both plant species in a two species mixture were host plants, beetle numbers per plant were greater than in a monoculture. Tahvanainen (1983) suggested that the effects of non-host plants on a herbivore's abundance may be greater on herbivores of unapparent than apparent plants. Three explanations of increased herbivore load in monocultures have been proposed (Crawley, 1983); the resource concentration hypothesis, the enemies hypothesis and the microclimate hypothesis. The resource concentration hypothesis states that
"herbivores are more likely to find and remain on hosts that are growing in dense or nearly pure stands, and that the most
specialised species frequently attain higher relative densities in
simple environments" (Root, 1973). The enemies hypothesis is based on the observation that natural enemies are more numerous
in weedy crops (polyculture) than in weed free systems
(monoculture), and the correlation between higher predator densities and lower pest numbers per plant. However, detailed studies of mono and polycultures have failed to demonstrate that
rates of predation and parasitism are significantly higher in
polycultures (Root, 1973; Bach, 1980b; Tahvanainen S Root,
1972). The microclimate hypothesis suggests that the microclimate in monoculture may be conducive to higher fecundity or a lower death rate. Reproductive rates may be lower in
polycultures (Tahvanainen S Root, 1972; Bach, 1980b), however,
the causal mechanism is unclear and the effect is not universal
(Bach, 1980a). The microclimate in a polyculture may be more
favourable to enemies and unfavourable to herbivores (Luginbill
& McNeal, 1958).
1.3 NUTRIENT AVAILABILITY IN THE HOST PLANT
As with all organisms, if the nutrients essential for metabolism in
herbivorous insects are limiting in their environment or food,
then their performance will be reduced. It has been suggested
that plants may decrease nutrient availability to herbivores as
part of a defence mechanism (Feeny, 1976; Moran & Hamilton,
1980). Nitrogen is an essential element in the structure of
proteins. Much literature now suggests that nitrogen availability
is of vital importance to herbivores (McNeill 6 Southwood, 1978;
Mattson, 1980; White, 1984).
Work on aphids has shown how nitrogen availability can affect
individual fecundity, survival and population dynamics (Dixon,
1963, 1966, 1969). The Sycamore Aphid (Drepanosiphum
platanoides) feeds on phloem, thus it is the level of nitrogen in the phloem sap which affects this aphid's performance. The performance of D. platanoides in the field is correlated with soluble nitrogen levels in host plant tissues. As the concentration of nitrogen in the phloem sap drops, aphids feeding on the phloem became smaller and less fecund (Mittler,
1958; Dixon, 1963, 1975). The variable nitrogen content of leaves may cause aphids to aggregate on leaves of the best quality, especially in the autumn (Dixon, 1966; Wratten, 1974).
Such aggregation may affect the population dynamics of a species, either directly through mutual interference (Dixon,
1966) and/or via their interaction with predators and parasitoids
(Hassell, 1981).
Other studies confirm that the nitrogen content of food has major consequences for herbivore performance. Fox & Macauiey (1977) report that the performance of Paropsis atomaria (Coleoptera:
Chrysomelidae) on Eucalyptus can be directly related to nitrogen concentration. Similar effects have been reported for
Tyria jacobaeae (Lepidoptera: Arctiidae) where size, fecundity and larval survival increased with increasing nitrogen concentration in the host plant (Myers & Post, 1981). The nitrogen content of grasses and its availability has been shown to have a major effect on the population dynamics of leafhoppers
(Hemiptera: Cicadellidae, Delphacidae) (Prestidge, 1982a, 1982b;
Prestidge & McNeill, 1983b). However, negative effects of nitrogen on an insect herbivore's performance have, been
reported at high nitrogen concentrations (Wayne-Brewer,
Capinera, Desham & Walmsley, 1985; Prestidge, 1982a).
Similarly, Faeth, Mopper & Simberloff (1981) report that
densities of leaf miners on oak were significantly and negatively
correlated with total nitrogen content of leaves.
1.4 CHEMICAL DEFENCES
Many compounds have been isolated from plant tissue which have
been postulated to have a defensive function against herbivores
or disease-causing organisms. In the following section the major
groups of chemical defence found in plants are reviewed. 1.4.i C y a n id e
The presence of cyanide in plants is widespread, occurring in species among 500 genera, comprising 100 families. The families which are particularly noted for cyanogenesis are Rosaceae,
Leguminosae, Gramineae, Araceae, Compositae and Passifloraceae
(Gibbs, 1974).
In order for a plant to be cyanogenic it must contain both cyanogenic glucosides and the appropriate /$ -glucosidase enzymes capable of hydrolysing the glucosides and releasing hydrogen cyanide (HCN) when the tissues are damaged. The genetic basis of cyanogenesis in Trifolium repens and Lotus corniculatus has been discussed by Nass (1972) and Jones
(1977). The system is controlled by genes at two unlinked loci.
One pair of alleles controls the production of the cyanogenic glucosides and the other pair controls production of the enzyme.
This system leads to polymorphism in the ability to produce cyanide, with three genotypes being acyanogenic and one genotype being cyanogenic. Such a system is ideal for the study of plant/herbivore interactions.
The defensive capacity of cyanogenesis has been shown in
several studies. It is known that HCN forms complexes
reversibly with heme proteins, notably cytochrome oxidase, the enzyme which catalyzes the terminal step in aerobic respiration
(Conn, 1979).
Jones (1962) showed that the acyanogenic morph of Lotus
corniculatus was chosen in preference to the cyanogenic morph
by the vole Microtus agrestis, the snails Arianta arbustorum and
Helix aspersa, and the slug Agriolimax reticulatus. Corkhill
(1952) showed that rabbits avoided cyanogenic T. repens. Dirzo
& Harper (1982) demonstrated that cyanogenesis markedly
reduced, but did not wholly prevent, damage to T. repens by
four species of mollusc. Subsequent experiments showed that
slugs which fed on cyanogenic leaves of T. repens attained
smaller live weight gains or lost weight faster than slugs fed
acyanogenic leaves. Both Dirzo & Harper (1982) and Jones, 23
K eym er 8 Ellis (1978) found a negative relationship between the
density of cyanogenic morphs of T. repens and L.. corniculatus
respectively and the density of molluscs in the field.
Bracken (Pteridium aquilinum) also demonstrates cyanogenesis.
Cooper-Driver 8 Swain (1976) found that cyanogenic bracken was
less preferred by deer and sheep than was acyanogenic bracken.
In laboratory experiments they fed bracken to the locust,
Schistocera gregaria: only 4% of cyanogenic fronds were eaten although around 50% of acyanogenic fronds were eaten.
In 1976 Lawton suggested that high levels of cyanide and other chemicals in bracken in the spring were responsible for the low diversity of insect herbivores which fed on bracken at this time of year. The concentration of cyanide in bracken decreases
throughout the year, whereas insect herbivore diversity peaks in
July (Lawton, 1976). Although later he considered that architecture was the major force structuring the insect herbivore
community (Lawton, 1978).
Although there is good evidence for cyanide acting as a chemical
defence against herbivores, there could be other functions for cyanide in a plant e.g. nitrate reductase regulation (Solomonson
8 Spehar, 1977; Eck 8 Hageman, 1974).
1.4. i i Alkaloids
The alkaloids are grouped together because they contain
nitrogen, frequently in a heterocyclic ring, and not because of
any common metabolic origin (Whittaker 8 Feeny, 1971). The
alkaloids are split into several groups of more closely related
compounds; the pyrrolidines e.g. nicotine, the tropanes e.g.
cocaine, the purines e.g. caffeine and the steroids e.g.
solanidine.
Alkaloids are widespread in the plant kingdom. Cromwell (1955)
claims that one seventh of all angiosperm families contain alkaloid
bearing species, while Williamson 8 Schubert (1955) report the
presence of alkaloids in one third of all angiosperm species. The majority of alkaloid bearing plants are dicotyledons, but some are reported from monocotyledons and gymnosperms. Levin
(1976) suggests that alkaloids occur more frequently in annual/herbaceous plants than in perennials.
Alkaloids generally occur in metabolically very active tissues, typically epidermal and hypodermal tissues, vascular sheaths and latex vessels (Robinson, 1979) and in all young tissues
(Mothes, 1955). The percentage dry weight of alkaloids in leaves tends to decrease as the leaves age (Mothes, 1955). As well as occurring in leaves, alkaloids have also been reported to occur in flowers (Dollinger, Ehrlich, Fitch & Breedlove (1973) and in seeds (Bell, 1978). McKey (1974) suggested that the alkaloids may be distributed within a plant in accordance with the value of the tissue to be defended i.e. valuable tissues should contain higher alkaloid concentrations.
Once ingested, alkaloids tend to either disrupt developmental processes or affect the nervous system of an animal. Alkaloids are known to disrupt DNA replication, RNA transcription, protein synthesis and membrane transport processes, inhibit enzymes and block receptor sites for endogenous chemical transmitters (Robinson, 1979). The adverse effects of alkaloids on vertebrate grazers is well documented; cattle, sheep and meadow voles (Microtus californicus) show a preference for reed canary grass (Phalaris arundinacea) which has low total alkaloid content (Simons & Marten, 1971; Marten, Barnes, Simons &
Wooding, 1973; Kendall & Sherwood, 1975). If fed alkaloidal plants, these animals may suffer reduced growth (cited in
Robinson, 1979)/ become blind (Hermann, 1966), produce abnormal offspring (Keeler, 1969) or die (Marsh & Clauson,
1916).
In the ecology of insects, alkaloids typically play one of three roles; as an attractant, as in the case of the alkaloid sparteine and the broom aphid Acyythosiphon spagtii (Hemiptera:
Aphididae) (Smith, 1966), as a sequestered toxin which confers protection on the sequestering insect against predators, as in the case of the alkaloids in Senecio jacobaeae and the cinnabar 2 5
moth Tyria jacobaeae (A plinr Benn & Rothschild, 1968), and as a feeding deterrent/toxin acting as a defence against herbivores. This latter role will now be discussed in greater detail.
In a detailed study, Harley & Thorsteinson (1967) examined the effect of twenty alkaloids on the performance of a polyphagous grasshopper Melanoplus bivittatus (Say) (Acrididae: Orthoptera). Of the twenty alkaloids in an artificial diet, ten had no effect on survival to adulthood, one increased survival, three decreased survival and five were lethal to nymphs in any dose. Of the ten alkaloids that had no effect on survival, five had no effect on the rate of weight gain or adult weight, two decreased adult weight and three decreased the rates of weight gain. In feeding preference experiments, diets containing innocuous chemicals were sometimes discriminated against, while diets containing chemicals producing lethal effects were accepted. Three of the alkaloids found to be lethal were present in known host plants of M. bivittatus. The authors speculated that these alkaloids may be restricted to certain stages of the plant's life cycle e.g. seedlings, and that the grasshopper may discriminate against such stages.
The work of Harley & Thorsteinson is important as it shows that the effects of alkaloids need not be an all or nothing response, as supposed by some theories of plant/herbivore interactions (Feeny, 1976; Rhoades S Cates, 1976) (see section 1 .5 ), but may affect aspects of herbivore performance, such as adult weight which may be correlated with fecundity.
Unfortunately, few studies are as detailed as the above, although many report the adverse effects of alkaloids on herbivorous insects. The Colorado beetle (Leptinotarsa decemlineata) (Coleoptera; Chrysomelidae) feeds on cultivated potatoes, Solanum tuberosum, containing the alkaloid solanine with apparently no adverse effects on performance. However demissine, which is similar to solanine in structure and occurs in Solanum demissum, acts as a feeding deterrent to L. decemlineata, as does tomatine from tomatoes. A 2mM/kg solution of tomatine painted on potato leaves reduces larval 26
feeding by 50%, whereas a solution of 3mM/kg causes 100% larval mortality (cited in J.B. Harborne, 1 978).
Dollinger et al (1973) examined a system involving a lycaenid butterfly, Glaucopsyche lygdamus ( Lepidoptera: Lycaenidae) and three species of lupin, Lupinum bakeri, L. caudata and L. floribundus ( Leguminosae). They discovered that populations of lupin which suffered little predation from (j. lygdamus contained fewer alkaloids than lupin populations suffering greater predation, which had increased alkaloid concentrations. However, it was not the absolute concentration of alkaloids which conferred a defence on the lupin, but the variability of the alkaloid mixtures present in the plant populations. The "high alkaloid" lupin populations which suffered the least damage had very variable alkaloid profiles. The populations suffering most damage contained nine alkaloids but they were constant between individual plants. The authors hypothesize that the individual variability in alkaloid content is an anti-specialist chemical defence mechanism, "Such individual variability may be advantageous to plant populations by reducing the possibility of selection for strains of specialist herbivores capable of detoxifying plant defensive compounds", Dollinger et a [ (1973).
1 • 4. iii Glucosinolates
Glucosinolates are found in families belonging to the order Capparales, namely the Capparaceae, the Resedaceae, the Moringaceae and the Cruciferae (Kjaer cited in van Etten & Tookey, 1979). The majority of work on glucosinolates has concentrated on the latter family. Indeed, all species of Cruciferae so far investigated have been found to contain one or more glucosinolate. These chemicals are typically present in plants in small quantities, eg isothiocyanate is present in Brussel sprouts at 0.0568% dry weight and in cauliflower at 0.0083% (Lichenstein, Morgan & Mueller, 1964).
As a chemical group, all glucosinolates contain >3 -D-thioglucose and sulphate moieties (Ettliger & Kjaer, 1968) which are well known toxins. Sinigrin is toxic because it releases the mustard oil, allylisothiocyanate, into the gut (Erickson s Feeny, 1974). The mustard oils are powerful antibiotics (Virtonen, 1965) and damage mammalian tissues typically the thyroid, liver and kidneys (Tookey, van Etten & Daxenbichler, 1979).
The glucosinolates may act as feeding attractants to adapted insect herbivores (Feeny, Poauwe £ Demong, 1970; Schoonhoven, 1969; Thorsteinson, 1953), and as attractants to egg laying females of some Pierid butterflies (Chew, 1975). Glucosinolates have also been reported as sequestered toxins (Rothschild, 1972). The toxic properties of glucosinolates against non-adapted insect herbivores are well documented. For example, Erickson 6 Feeny (1974) reared larvae of the Umbelliferae specialist, the black swallowtail butterfly, Papilio polyxenes) (Lepidoptera: Papilionidae), on celery leaves cultured in sinigrin solution of different concentrations. They found that feeding rates were not significantly affected, but growth and development were substantially reduced as was pupal weight and the number of eggs produced per female. At sinigrin concentrations of 0.1%, 100% larval mortality occurred (concentrations in the plant varied between 0.03-0.1% dry weight). In a further study on the same system, Blau, Feeny & Contardo (1978) showed that the toxicity of allylglucosinolate was greatest to P. polyxenes, intermediate on the generalist Spodoptera eridania (Lepidoptera; Noctuidae), where larval growth was inhibited at high allylglucosinolate concentrations and least on the Cruciferae specialist Pier is rapae (Lepidoptera: Pieridae). Even at artificially high concentrations of the glucosinolate the performance of Pieris rapae was not affected.
Scriber (1981) observed reduced larval growth of S_. eridania on a diet of cabbage, relative to growth on other food plants. He ascribed this observation to the glucosinolate content of the cabbage. Other studies tend to corroborate the effect of glucosinolates on non-adapted herbivores, e.g. studies quoted in Erickson 5 Feeny (1974) all report reduced larval growth and survival of Lepidoptera larvae when fed cruciferous food plants; Torri & Morri (1948) ( Bombyx mori), Wallbauer (1960) (Manduca sexta), Soo Hoo (1963) (Proderria eridania) and Brower (1969) ( Danaus plexippus) . 28
Glucosinolates have also been reported to have an effect on sap-feeding insects. Klingauf, Senganco S Bennewitz (1972) showed that a 0.1% solution of sinigrin reduced the uptake of sucrose in the aphids, R.hopalosiphum padi and Aphis fabae (Hemiptera: Aphididae), but increased the sucrose uptake of Brevicoryne brassicae and Myzus persicae (Hemiptera: Aphididae).
I.U .iv Flavonoids
Flavonoids occur throughout the angiosperms and gymnosperms and regularly in ferns, mosses and liverworts. There are about two thousand known naturally occurring flavonoids. The most widespread classes of flavonoids are anthocyanins, flavones and flavanols. Chemically flavonoids are aromatic, heterocyclic compounds derived from fiavone. Flavone has a -pyrone ring with ether-linked oxygen as a part of its structure (Harborne, 1979).
Flavonoids are extremely toxic. The isoflavin vestital, has been shown to be toxic to the larvae of Heteronychus aratoo (Coleoptera: Curculionidae) which feed on the roots of Lotus spp (Russell, Sutherland, Hutchinson & Christmas, 1978), and as a feeding deterrent to Pieris brassicae (Lepidoptera: Pieridae) (Schoonhoven, 1972). Flavonal glycosides in Ulmus europea have been shown to act as feeding attractants to Scolytus multistriatus (Coleoptera; Scolytidae) (Doskotch, Mikhail & Chatterji, 1973). Furth & Young (1988) claim that the flavonoid content of Rhus spp. (Aracardiaceae) correlated with the feeding preference of two chrysomelid beetles.
1.4 .v Saponins
Saponins are glycosides in which the aglycone portion of the molecule is either a sterol or a triterpene. The number and nature of sugar units combined with the aglycone is very variable (Birk, 1969). Saponins have been reported from five hundred species of plants from eighty different families (Basu & Rostagi, 1967), including leg jmes (Walter, 1961) and some woody 29
plants (Takahashi, Miyazaki, Yasue, Imamura & Honda (1963) cited in Applebaum & B irk, 1979). They have been described from all parts of the plant, including roots and seeds. The toxic effect of saponins is due to a hydrophobic/hydrophilic asymmetry and consequently they lower surface tension, thereby affecting cell membranes. Applebaum & Birk (1972) suggest that saponins may alter the permeability of the insect gut with toxic results and that they may also cause a hormone Imbalance, resulting in incomplete development. !shaaya& Birk (1965) show that saponins can function as inhibitors of proteolytic enzymes and thus may act as both a qualitative and quantitative defence, (cited in Appeibaum & Birk, 1979). Saponins have been shown to confer resistance against the root-feeding larvae of the grass grub Costelytra zeaiandica (Kain & Atkinson, 1970) and the white grub Melalontha vulgaris (Coleoptera; Scarabeiidae) (Horber, 1965), and to inhibit the development of Callosobruchus chinensis (Coleoptera: Bruchidae) and Tribolium castaneum (Coieoptera: Terebrionidae) (Ishaaya, B irk, Bondi & Tencer (1969) (all cited in Appeibaum & B irk, 1979).
The leafhopper, Empoasca fabae (Hemiptera: Cicadellidae) suffered increased mortality when saponin was introduced into its artificial diet (Roof, Horber & Sorenson (1972), cited in Applebaum & Birk (1979)). Acyrthosiphon pisum (Hemiptera: Aphididae) responded in a similar manner. Applebaum & Birk (1972) report that saponin acted as a feeding deterrent when included in artificial diets fed to Myzus persicae (Hemiptera: Aphididae), a polyphagous aphid. This suggestion that saponin may act as a feeding deterrent as well as a toxin has not been fully pursued.
1.4.iv Terpenes & Resins
Terpenoids may function in many metabolic processes in the plant, as well as a pollinator attractant and herbivore deterrent. They are organic molecules based on repeating 5-carbon subunits. The best known are the pyrethroids, obtained from Chrysanthemum spp (Compositae) which have excellent 30
insecticidal properties. However, less than 1% of known terpenoids have been investigated for their feeding and deterrent properties (Mabry & Gill, 1979).
Sequiterpene lactones are common in Compositae but occur infrequently in other families. The sequiterpene lactone glaucolide A of Vernonia (Compositae) was shownto be a deterrent to several Lepidoptera species, and also to increase their larval development times and decrease their growth. Rabbits and Whitetail deer also avoided Vernonia plants containing glaucolide-A (Mabry & Gill, 1979; Burnett, Jones & Mabry, 1978 and references therein).
Resins of conifer trees, a major defence against insects, commonly contain monoterpenes. After being attacked some conifer species increase monoterpene levels in their tissues which then become more active against bark boring beetles (Coleoptera: Scolytidae), (Smith, 1965). Smith (1975) suggested that tree resistance is related to both monoterpene content and to the level of resin flow. The results of Larsson, Bjorkman & Gref (1986) show that resin acid, containing diterpenoids, of Scots Pine ( Pinus sylvestris L .) has a complex effect on Neodiprion sertifer (Hymenoptera; Diprionidae). High resin acid concentrations increased development times and mortality but had no effect on pupal weight. Final instar larvae showed no decrease in mean relative growth rate when fed diets containing three times the resin acid content of the control diet. Indeed, there is some suggestion that final instar larvae actually search out high concentration resin acid needles and bark, perhaps in order to utilise the resin acids in their own defence.
The effect of resins produced by chapparral shrubs on their herbivores has been particularly well studied. Rhoades & Cates (1976) studied two Larrea species which produced a resin containing up to 90% phenolic compounds and comprising up to 44% of the dry weight of young leaves and 15% of mature leaves. Most herbivores studied preferred mature leaves of Larrea. However, a specialist feeder, Insara couilleae (Orthoptera: Tettigonidae), preferred young leaves. Removal of resin from 31
the food plant increased its palatability to generalist feeders and decreased it to a specialist feeder, which used resin as a feeding stimulant. Similar results were obtained with herbivores of the bush Eriodictyon californicum (Hydrophyllaceae) by Williams, Lincoln & Ehrlich (1983). The flavonoid aglycone resin decreased feeding by Euphydryas chalcedony (Lepidoptera: Nymphalidae), possibly due to the mechanical difficulty it caused larvae. The resin also decreased larval survivorship, growth rate and pupal weight in a simple dose-dependent method (Lincoln, Newton, Ehrlich & Williams, 1982). Johnson, Brain S Ehrlich (1984) showed that, despite low resin concentration foliage being preferred for feeding by larvae and for ovipositing by adults of Trirhabda diducta (Coleoptera: Chrysomelidae), an increase in resin concentration of five times had no effect on adult or larval growth rate. No effect of the resin acting to precipitate proteins was found, as resin concentration increased in the diet the beetles ate more.
1.4.vii Tannins
Tannins have been defined as any naturally occurring phenolic compounds with a molecular weight between 500 and 3000 which are able to form effective cross linkages with proteins and other macromolecules (Swain, 1965). Tannins have been found in all classes of vascular plants, but do not occur in prokaryotes, fungi or animals (Swain, 1979). Swain (1979) recognises four groups of tannin, proanthocyandin or condensed tannins, hydrolysable tannins, oxytannins and B-tannins. Condensed tannins are the most widespread of the four groups, hydrolysable tannins occur in dicotyledons, oxytannins are only formed on injury to the plant and B-tannins comprise a wide variety of lower molecular weight compounds.
During the 1970’s, it was believed that tannins acted in a dose-dependent manner on all herbivores and that they were very difficult to overcome in the evolutionary 'arms race1. The deleterious effects of the chemicals being produced by forming complexes with proteins in the food and with enzymes in the herbivore's gut, thereby decreasing digestive efficiency. These 32
ideas arose mainly from the work of Feeny (1968, 1970, 1976) who showed in 1968 that the addition of tannins to the diet of Operophtera brumata L. (Lepidoptera: Geometridae) reduced larval growth. From this he postulated that the presence of tannins in the mature leaves of trees was one reason why the spring-feeding habit predominated amongst deciduous tree-feeding temperate Lepidoptera (Feeny, 1970). Prior to Feeny's work, Bennett (1965) had found tannic acid to be a repellent and to reduce survivorship in Hypera postica (Coleoptera: Curculionidae). More recent work has also shown evidence of reduced herbivore performance in herbivores feeding on diets containing tannin (Manuwoto & Scriber, 1986; Berenbaum, 1983; Roehrig & Capinera, 1983; Chan, Waiss, Binder & Ellinger, 1978; Schoonhoven & Derksen-Koppers, 1976). No-one has found conclusive evidence of tannins, however, at concentrations found naturally in the host plant affecting (all aspects of) herbivore performance in the classic dosage dependent manner postulated by Feeny (1976). During the 1980's, several workers have questioned the classical view of the action of tannins (Bernays, 1978, 1981; Berenbaum, 1983; Zucker, 1983; Lawson, M erritt, Martin, Martin & Kukar, 1984). Indeed Bernays & Woodhead (1982) describe a mechanism by which phenols are used to an insect's advantage; the Tree Locust, Anacridium melanorhodon (Orthoptera: Acrididae), utilises the breakdown products of phenols to stabilise proteins in the cuticle, thereby conserving amino-nitrogen. It has also become increasingly evident that hydrolysable and condensed tannins may have different effects on insect herbivores (Bernays, 1981). Bernays (1981) states that hydrolysable tannin cannot be counted as a quantitative defence as perceived by Feeny (1976). Condensed tannins may cause quantitative effects on several species of insect, but the response between species is different. However the pH of the midgut of Lepidoptera larvae may be sufficiently alkaline to eliminate the digestibility-reducing properties of this group of tannins (Berenbaum, 1980) (see also Krieger, Feeny & Wilkinson, 1971 & Brattsten, Wilkinson & Eisner, 1977, on the role of mixed function oxidases in the gut of Lepidoptera). The separate effects of condensed and hydrolysable tannins are often difficult to determine, as both types may co-occur in the same plant. 33
The changes of opinion of ecologists on the importance of tannins to individuals and populations of herbivores is of extreme importance to this thesis as at the time of formulating the hypothesis tested here, Lawton & McNeill (1979) believed the classic Feeny (1976) view on tannins.
1.4.viii Miscellaneous Defences
Other groups of chemicals not previously discussed but known to have an effect on herbivores include insect hormone analogues, furanocoumarins and non-protein amino acids.
Substances which interfere with the hormonal control of growth and development in insects have been isolated from Pteridophytes, Cymnosperms and several Angiosperm families. Plants possessing these substances tend to be woody perennials rather than herbaceous plants (Slama, 1979). The chemical ageratochromere from Ageratum plants induce premature metamorphosis in nymphs of Oncopeltus (Hemiptera: Lygeidae) (Bowers, 1975). Vertebrate hormones and analogues have also been discovered in plants (Heftmann, 1975).
Furanocoumarins are known from eight angiosperm families, but occur regularly only in the Umbelliferae and Rutaceae (Berenbaum, 1981a). These chemicals have been shown to be toxic towards insects and may play an important role in structuring the insect herbivore community on umbellifers (Berenbaum 1978, 1981a, 1981b, 1981c, 1983; Berenbaum & Feeny, 1981).
It is postulated that if non-protein amino acids are incorporated in proteins, they render the protein inactive. Approximately 260 non-protein amino acids have been isolated from higher plants, the majority of which are family, genus or species specific, and have been isolated from all parts of the plant. Plants containing non-protein amino acids are extremely toxic to mammals and insects. One of the best known non-protein amino acids is canavanine, which occurs in certain legume species. It is an analogue of the amino acid arginine, 34
and is known to be toxic to a wide range of organisms including insects. The inclusion of 1.0mM of canavanine in the diet of Manduca sexta causes abnormal development and decreased survival. The seed feeding beetle Caryedes brasiliensis (Coleoptera: Bruchidae) can, however, detoxify canavanine (Rosenthal & Bell, 1979).
Plants have also produced an array of physical defences against herbivores, e.g . thorns, spines and pubescence. For a recent review of the importance of physical defences see Myers (1987) and references therein.
Despite the vast amount of evidence documenting the effects of chemical defences on herbivores, it is impossible to prove that these chemical actually evolved under the selection pressure provided by herbivores. Jermy (1984) emphasises this point stating that "as opposed to many entomologists, botanists and phytochemists are reluctant to consider phytophagous insects (or herbivores in general) as significant selection factors which have caused the evolution of new metabolic pathw ays ...... neither do they regard such substances as defence mechanisms".
1.5. REVIEW OF PLANT/HERBIVORE COMMUNITY AND EVOLUTIONARY THEORY
In 1976 Paul Feeny and David Rhoades & Rex Cates independently published papers reviewing work on plant chemical defences and insect herbivores. Both papers then attempted to place their own and other people's work into one cohesive theory, which could explain the abundance and structure of chemical defences against insects across all communities. As both papers seek to explain the same observed patterns of chemical defences in plants and are fairly similar in their argument they will be reviewed together.
Both papers start with the same basic observation; that ephemeral plants tend to possess low concentrations of chemical defences which vary structurally between and within plant families, whereas long-lived perennial plants typically contain higher concentrations of defensive compounds with little diversity in structure between species.
Ephemeral plants are generally defended by compounds such as cyanogenic glucosides, alkaloids, glucosinolates, non-protein amino acids, insect hormone analogues etc (see section 1 .4 .i-v for a review of their structure and action). These compounds are postulated to act as feeding deterrents to non-adapted herbivores, but to have little or no effect on the performance of adapted herbivores. Feeny (1976) terms these compounds qualitative defences, whereas Rhoades S Cates (1976) call them toxins. Long-lived plants are typically defended by digestibility-reducing defences (Rhoades & Cates, 1976) or quantitative defences (Feeny, 1976). These defences include the tannins and resins (see section 1 .4 .v i-v ii for a review of their structure and function). Both papers then proceed to consider the predictability of the plant as a resource for the herbivore and its commitment to defence. Feeny (1976) coins the term "apparency" which he defines as describing the "susceptibility to discovery" of a plant by a herbivore. This term may be applied either to individual plants or to tissues within a plant. For example, annual herbs are small in size, are only available to herbivores for a short time each year and their occurrence is unpredictable both in time and in space. Feeny (1976) considers that annuals have low apparency when compared with trees which are large, available all year round and are predictable in space for many years. Trees are therefore referred to as apparent. The young leaves of a tree are, however, considered unapparent as they are only available for a short time each year and their occurrence in time is relatively unpredictable. Rhoades & Cates (1976) note the same dichotomy between predictable plants and tissues and unpredictable ones, but do not assign terms.
The papers then consider the metabolic cost of chemical defence production. It is assumed that energy put into the production of chemical defences is energy which could otherwise have gone into growth or reproduction, and that evolution would select for the optimal allocation of resource, depending on the relative selective pressures of herbivores and other aspects of the individual's biology.
Feeny (1976) states "For early successional herbs selection is unlikely to favour a large metabolic allocation for defensive compounds, especially since the selective pressures of predation are reduced as a result of escape from many enemies in time and space. The adaptive emphasis in such species is thus likely to favour defensive compounds which are especially effective in small quantities."
In long-lived plants such as trees, a greater commitment to defence would seem a better strategy, as according to apparency theory such plants are more likely to be discovered by herbivores. In addition, according to r-k theory (Pielou, 1967) such long-lived organisms are not under such intense pressure to maximise growth and reproduction each season.
Having noted the differences in defence cost, structure and function between ephemeral and long-lived plants the papers then consider the effects of qualitative (toxins) and quantitative (digestibility-reducing) defences on insect herbivores.
They postulate that qualitative defences have little or no effect on adapted herbivores, but they serve as barriers to non-adapted insects. That is, they deter non-adapted herbivores from feeding on them, and if such a herbivore does feed on a plant containing a toxin it cannot detoxify, it may die. In contrast, quantitative defences act on all herbivores. Tannins complex with proteins, either enzymes in the gut of the herbivore or proteins in the food. They therefore serve to make the food less digestible. It is an important premise of the theory that quantitative defences should act in a dosage-dependent manner, that is the greater the herbivore's intake of the chemical, the greater will be its effect. Quantitative defences typically reduce the growth rate of herbivores, which serves to increase development time, thereby exposing the herbivores to natural enemies for a longer period 37
and also possibly resulting in smaller adult insects which are less fecund than larger adults. Both papers suggest that it is very difficult for herbivores to evolve a detoxification mechanism against quantitative defences. The authors also consider that low nutritional value of foliage, e.g. low nitrogen concentration, low water concentration and high toughness serve to reinforce the presence of specific chemicals as a quantitative defence.
Feeny (1976) then provides a further discussion of apparency and the rationale behind correlating an increasing commitment to defence with increasing apparency. Rhoades & Cates (1976) however, take their ideas in a different direction, although they do recognise that the amount of digestiblity-reducing defences is related to the predictability of the plant or tissue. They conclude their paper with a discussion of the importance of specialist and generalist herbivores to a plant and coevolution.
Rhoades & Cates (1976) consider that the ability of an annual plant to escape in time and space would be more effective against specialist herbivores than generalists, as specialist herbivores have no alternative food source. They predict that the more ephemeral the host plant, the greater will be the mortality of the specialist in searching for its host plant. "The predictability and availability of any individual resource is of less consequence for a generalist, however, since it can opportunistically utilize whatever resource happens to be available. Therefore, predictability of resource should select for specialism and ephemerality for generalism. It follows that ephemeral plants and tissues should escape specialists more effectively than generalists. Thus the defences evolved in ephemeral plants and tissues should be directed particularly against generalism. This in turn, should and has, given rise to a divergent system of chemical defences in such plants and tissues" (Rhoades & Cates, 1976). The generalist herbivore should than have selective forces acting upon it which would make it capable of coping with the "average defensive chemistry" of its host plant range. Thus, plant species or individuals with defences which deviate most widely from this average should be at a selective advantage when compared with plants of the "average chemical" type. 38
Rhoades S Cates (1976) believe that such a process is responsible for the diverse array of defensive chemicals in ephemeral plants and tissues.
By similar reasoning, it can be seen that the defences of predictable plants and tissues should be directed particularly against specialist herbivores. Rhoades & Cates (1976) predict a convergence of chemical defences for predictable plants, as in contrast to the evolution of defences in ephemeral tissue, the defence evolved by any one plant species will not be affected by the defences evolved in other plant species in the community. They propose "that the defensive system, that has been converged upon, is the digestibility reduction mode of defence" which shows very limited diversity. Rhoades & Cates (1976) then consider the next step in the "evolutionary arms race". They predict that convergent defences should select for generalism, "since if the resource defences are all very similar, it should not be adaptive for the herbivore to expend time and energy seeking a particular subgroup of the resources" (Rhoades & Cates, 1976). Conversely, divergent defences should select for specialism in the herbivore, "since all the resource defences are very different, the ability of a generalist to accommodate any one of the defences should be significantly less than of a specialist herbivore . . . . and the selective advantage should accrue to specialists".
Such coevolutionary theories are extremely difficult to test as it is difficult, if not impossible, to place any individual herbivore-host plant interaction on the correct "step" of the arms race. Such ideas are further complicated by the fact that a plant will be evolving in response to its whole guild of herbivores, each of which could be exerting different selective pressures on the plant. Further to this point, both Feeny (1976) and Rhoades & Cates (1976) are in agreement that although they have discussed chemical defences in the context of insect herbivores, the plant's chemistry will also have evolved under selective pressures from fungal, viral and bacterial pathogens, other invertebrates and vertebrate herbivores and maybe in response to other plants e.g . allelopathy. 39
Although the main thrust of the papers is that ephemeral plants are defended by qualitative/toxic defences and long-lived perennials by quantitative/digestibility-reducing defences, both accept that toxins may be present in perennial plants and digestibility-reducing compounds in ephemeral plants.
Two important questions remain to be answered satisfactorily before these theories can be fully accepted: firstly is there any effect of toxins on the performance of adapted herbivores and secondly, do quantitative defences really act in a dosage-dependent manner against herbivores? Very little work has been done in an attempt to answer the first of these two questons, and hence they is little evidence for or against this assumption. Feeny (1976) recognised this when he said; "I must emphasize though that the experimental evidence for this suggestion is very limited". He quotes work by van Emden & Bashford (1969) and van Emden (1972) as an example of a qualitative defence affecting fecundity of an insect herbivore (Brevicoryne brassicae (Hemiptera: Aphididae) on Brussel sprouts). There is some evidence that qualitative defences are not an all or nothing defence against non-adapted herbivores from the work of Dirzo & Harper (1982) (see page2 ! ) / Harley & Thorsteinson (1967) (see page25 ) and references on page27 .
The effects of tannins on herbivore performance, once believed unequivocally to be a universal dosage-dependent phenomenon reducing insect performance is now under question (see section 1.4.vii and Chapter 5). In a brief review of chemical defences and insects Strong, Lawton & Southwood (1984) conclude; "In short, although the distinction between qualitative and quantitative defences was a reasonable theoretical beginning for this field current discoveries are making it increasingly difficult to maintain this distinction for specialised insects".
Three years after the publication of Feeny's and Rhoades & Cates' 1976 papers, Lawton & McNeill (1979) published "Between the devil and the deep blue sea: on the problems of being a herbivore", in which the hypothesis tested in this thesis was formulated. In their paper they review the mechanisms which 40
determine the characteristic levels of abundance of herbivorous insects. They investigated the independent effects of parasitoids and predators and food plant quality on the population dynamics of herbivores and then considered the combined effect of these two factors on herbivorous insect populations. After a brief review on herbivores, they conclude that there is "very little room for doubt about the importance of natural enemies, particularly parasitoids in reducing population sizes for phytophagous insects". A consideration of several models of host-parasitoid interactions predict that the equilibrium population size of the host increases as r (intrinsic rate of increase) increases, and that very high r values may lead to oscillations in population size. If the host-parasitoid model incorporates an element of spatial heterogeneity, as in that of
Free,Beddington '• & Lawton (1977), it is seen that small changes in the host's r have large effects on the host's equilibrium population size. They conclude that if the host equilibrium density is low, say due to the unpalatability of the available food, the the effect of parasitoids and predators will be to make the host even rarer.
Lawton & McNeill then discuss the effect of plant chemistry on the rates of increase of adapted herbivores. They argue that if more insects mean more damage to a plant, then it must be selectively advantageous for plants to reduce the rates of increase of their adapted herbivores. This may be achieved by reducing growth rates of the herbivores and thereby increasing generation times and reducing the number of generations a year; any reduction in survival and fecundity would also reduce r. The authors then review the effects of quantitative and qualitative defences, and the availability of nutrients on the rates of increase of herbivores (see section 1.3 and 1.4). They conclude that "plant chemistry has been shown to have a marked effect on one or more components of r , even in adapted herbivores. The clearest examples are provided by plant nutrients (particularly nitrogen) and quantitative defences, but qualitative (toxic) defences may also play a part. In consequence, control by parasitoids and predators is facilitated and equilibrium population sizes are reduced", Lawton & McNeill (1979). 41
Finally, Lawton & McNeill attempt to predict what the characteristic levels of abundance of herbivorous insects would be on apparent and non-apparent plants. They say that there are no easy answers, but some simple predictions suggest themselves:
"Obviously we should compare like with like (eg Lepidoptera with Lepidoptera) on a common scale - the insects per unit weight or area of a plant for example. Within these constraints, and other things being equal, we would predict that insects attacking ephemeral, early successional herbs and weeds (non-apparent plants with largely qualitative defences (Feeny, 1976) will have higher r values than those exploiting perennial long-lived plants (apparent species with considerable investment in quantitative defences)... Again other things being equal, high r values imply large equilibrium population sizes and a greater propensity for the population to become unstable and to outbreak in populations controlled by predators and parasitoids. Hence we might expect the average population sizes of insects which attack non-apparent plants to be higher than those attacking apparent plants and to fluctuate more", Lawton & McNeill, (1979). They go on to say that the predictibility of herbivore control by predation and parasitism on ephemeral plants will "serve to reinforce the differences that already exist" between apparent and non-apparent plants.
This thesis sets out to test these predictions over a successional gradient within a comparatively small spatial scale, as suggested by the authors. The main aim of the project was to measure the absolute abundance of insect herbivores, that is the number of individuals per unit of plant, on plants typical of different stages of succession, i.e. apparent and non-apparent plants. Within the time-scale of a Ph.D. it is not possible to attempt to measure the stability of a population. Thus the prediction that insect populations on ephemeral plants are more likely to outbreak than those on long-lived plants is not tested, neither is the importance of predators and parasitoids as controlling agents on herbivores on ephemeral and long-lived plants. It is recognised that predators and parasitoids alone could produce 42
patterns of insect herbivore abundance in accordance or disagreement with Lawton & McNeill's (1979) prediction. Consequently, an obvious sequel to this thesis is a study of the importance of predator and parasitoids to herbivore populations on ephemeral and long-lived plants. It is only then that Lawton & McNeill's (1979) hypothesis can be accepted or rejected. There has already been one attempt to test Lawton & McNeill's (1979) hypothesis by Godfray (1985). In this the absolute abundance of leafmining insects on the same successional sites at Silwood Park as will be examined in this thesis were examined and found to vary in the manner predicted.
Also relevant is the work of Coley (1980, 1983a, 1983b, 1988) and Coley, Bryant & Chapin (1985) who aimed to explain the observed patterns of defence allocation in the field. Coley et £[ (1985) sought to use resource allocation as an explanation for differing defence investments in plants. They divided plants into slow and fast growers and predicted that slow-growing plants are favoured over fast-growing plants in an environment where resources are limited and vice versa, and that the optimal level of defence investment increases as the potential growth rate of the plant decreases. The reasons given for this are three-fold: firstly, as growth rates become limited by resource availability, then replacement of resources lost to herbivores becomes more expensive; secondly, a given rate of herbivory represents a larger fraction of the net production of a slow grower than that of a fast grower; thirdly, because the relative cost of defence increases as growth rates increase, lower levels of defence in resource rich environments might be expected. The authors also predict that growth rates of plants may influence the type of defence as well as the amount. It is suggested that as quantitative defences (sensu Feeny) are present in higher concentrations than qualitative defences, they represent a higher initial construction cost than qualitative defences, and they also have lower continued maintenance costs than qualitative defences which are continually being produced and broken down. They suggest that qualitative defences would not be expected to be common in long-lived leaves as the continued metabolic costs summed over leaf life time would be 43
larger than a fixed investment in quantitative defences. However, long-lived leaves can afford the metabolic cost of producing quantitative defences as they have "more time over which to spread these costs" Coley et a[ (1985). They conclude that "resource availability in the environment is the major factor influencing the evolution of both the amount and type of a plant defence", Coley et a[ (1985).
Coley (1988) presents evidence from tree species in lowland rain forest which wholly support the ideas propounded in Coley et a| (1985). Southwood, Brown & Reader (1986) working on a temperate system, agree with Coley (1983b) that foliage palatibility and rate of herbivore damage are inversely correlated with leaf life expectancy. However, they are unable to agree with all the ideas presented in Coley et a[ (1985), as they found vast differences in leaf life expectancies between their sites but no difference in net primary production. Southwood et a[ (1986) found that palatability level decreased and herbivore damage level increased with increasing age of the plant community and they suggest these results conform with Feeny's (1976) apparency theory. Southwood et a[ (1986) conclude their paper with an attempt to reconcile the ideas of Feeny (1976) and Coley et a[ (1985) with their own findings in the context of Southwood's (1977) habitat templet. 44
CHAPTER TWO
METHODS
2.1 EXPERIMENTAL SITES
The fieldwork in this project was carried out at Imperial College, Silwood Park, Berkshire, UK, during 1985 and 1986. Silwood Park is situated in 93Ha of arable land, acidic grassland and woodland areas, mainly birch and oak, at 51°21'N and 0°39*VV at an altitude of 91m. The study sites lie on Bagshot sands.
The study utilised sites of known successional age which are part of a series of sites initiated by Sir Richard Southwood and Dr. V .K . Brown in 1977. The sites are described further in Southwood, Brown S Reader (1979), Brown (1982a; 1982fc>; 1984; 1985), Brown & Southwood (1987), Stinson (1983), Godfray (1983) and Hyman (1983).
2.1. i. Site Preparation
The sites were originally prepared by ploughing and applying herbicide during the autumn before site establishment. The sites were then reploughed, harrowed and lightly rolled during the following spring before being left to recolonise naturally. Rabbits had been excluded from the the sites by fencing since 1976.
A woodland, Hell Wood, was also sampled. This is approximately 200m from the other sites. It consisted mainly of Betula pubescens, Betula pendula with the occasional Quercus spp, Fagus sylvaticus, I lex aquilifolium and Castanea sativa. The understory consisted mainly of Pteridium aquilinum.
2.1. ii Site Age and Nomenclature
A range of sites representing different successional stages was chosen for study. The sites will be referred to in the following manner - ruderal sites sampled in 1985 and 1986, at 0-1 years 45
old, early successional sites sampled in 1985 and 1986 at 1-2 years old, early midsuccessional sites sampled in 1985 at 6 years old and 1986 at 7 years old, late midsuccessional sites sampled in 1985 at 9 years old and in 1986 at 10 years old and a late successional site at 64+ years old. The ruderal site in 1985 became the early successional site in 1986. The same site was sampled in both years for early and late midsuccession and late succession. It can be seen that some sites were sampled in both years and some only in one year giving a mixture of site and year comparisons. In order to avoid confusion over nomenclature a site is taken to mean the plot of land of one successional age and serai stage is employed when data from both years samples are pooled, i.e . the plant species richness on the ruderal serai stage is the total number of plant species recorded on a ruderal site in 1985 and 1986. Therefore five serai stages were sampled over two years from eight different sites (see Table 2 .1 ).
2 .1 .Hi Marking Sites
Apart from the birch site, each site was divided into 3m x 3m subplots which were marked by stakes. There were 45 subplots on all sites except the 1985 site where only 40 subplots were defined. Quadrats of 1m x 1m, each divided into 4 smaller quadrats of 0.5m x 0.5m were marked in the centre of each subplot. This procedure was followed on all subplots, except where a tree or woody shrub occurred in the central area. In this case the quadrat was placed in a predetermined corner of the subplot. On the 1985 site where only 40 subplots occurred, five subplots were chosen at random and quadrats marked in a predetermined corner. Ten subplots were unavailable for sampling on 1984 site as they were being used by other workers and here a second quadrat was placed in 10 subplots to be as far from the worker’s subplots as possible. Each of the 0.5m x 0.5m quadrats was designated a, b, c or d on a systematic basis.
During early April 1985 sixty branches in the lower canopy (1-3m high) of Betula spp and twenty branches in the upper 46
Successional 0 - 1 1 - 2 6 - 7 7-10 6M+ Age (yrs)
Name ruderal early early mid late mid late successional succession succession
Symbol RE EMS LMS L
Site = patch of land of one successional age Serai stage = data from sites of same age summed over two years
Table 2.1 Age and nomenclature of serai stages 47
canopy (5-8m high), each approximately the size of the sampling bag described in Southwood, Brown & Reader (1979), were marked with plastic tags. The upper canopy was reached by a fixed walkway 5m high. It was not possible to sample the upper canopy during 1986 so an additional 40 branches in the lower canopy were marked in April 1986.
2.2 SAMPLING
2 .2 .i. Insect Sampling
Insects were sampled from all sites once every month between May and September inclusive. The first samples were taken from the birch immediately after 50% of the trees had burst their buds.
Samples were taken from birch with a beating bag, and all arthropods were collected from the bag in the field with an aspirator (pooter). Samples from the other sites were taken with a D-Vac suction sampler (D ietrik, 1961; Thornhill, 1978). This method has been shown to be the most efficient sampling technique for most groups of insects on non woody plants (Heikinheimo & R&atikaiven, 1962; Henderson & Whittaker, 1977; Tormala, 1982).
One D-Vac sample lasting 30 seconds was taken from each subplot on all sites at each sampling date. The subquadrat from which the sample was taken varied at each sampling date, revolving systematically, the first sample of 1985 being taken from subquadrat a, the second from b, and continuing on a rotation basis. This prevented depletion of natural insect populations, whilst providing the most comparable sample sites.
All insect samples were sorted in the laboratory as soon as possible after collection. Insects were separated from plant matter and other arthropods using an aspirator. The sorting took place in a wooden sorting hood, with a light source from behind. After sorting, insects were stored in plastic tubes in 70% alchohol, to which a drop of glycerine was added. All adult 48
herbivorous insects were subsequently identified to species using a Kyowa stereoscopic microscope. Most herbivorous taxa were identified although it proved impossible to identify all larval Lepidoptera, sawfly (Hymenoptera: Symphyta) and Coleoptera to species. Certain other groups were not identified to species due to a combination of taxonomic and sampling difficulties: Aphididae, Aieyrodoidea / Coccoidea (Hemiptera; Homoptera), Thysanoptera and Lonqitarsus spp (Coleoptera: Chrysomelidae). The following keys were used to identify the herbivorous insects to species; Heteroptera (Southwood & Leston, 1959) Coleoptera (Joy, 1932), Delphacidae (Le Quesne, 1960), Cicadellidae (Le Quesne, 1956, 1969; Le Quesne & Payne, 1981) and Psyllidae (Hodkinson & White, 1979). The Silwood insect collection and certain colleagues confirmed identification as necessary.
2 .2 .ii Plant Sampling
During the 30 seconds of insect sampling, plastic markers were placed around the edge of the D-vac hood, thus marking the area sampled. A circular wire quadrat exactly the same size as the D-vac sampling area (0.1m2) , was placed on the area of ground sampled by the D-vac. All plants within the quadrat were identified to species using Clapham, Tutin & Warburg (1981), Fitter, Fitter & Blarney (1980), Hubbard (1984) and Fitter, Fitter & Farrer (1984).
All the leaves of dicotyledonous species were counted and recorded; the tiller of monocotyledon were also counted and recorded. Estimates of the leaf area for each plant species at each sampling date were obtained by collecting leaves of plant species from each site. All plant species recorded during leaf counting were designated as either common or rare. A common plant was defined as one which was recorded in at least 20% of quadrats on the previous sampling date. A rare plant occurred in less than nine quadrats on the previous sampling date. On the first sampling date of the season, the designation of common and rare was made on that month's leaf count. On each site a leaf of a common dicotyledon plant was collected from each subplot in which it occurred. The leaves were chosen by 49
selecting the leaf/plant closest to a randomly positioned point quadrat in each subplot. When plants with a strong vertical component in their habit eg Cirsium arvense, Chamaenervon angustifolium and Pulicaria dysenterica were being sampled, the plant closest to the point quadrat was sampled and a single leaf chosen at random. This method ensured that all leaves on such a plant had a chance of being sampled, otherwise the largest leaf would always be the one closest to the point quadrat. All rare plants were sampled every other sampling date so as to avoid depletion of the natural population. Estimates for the area of a grass tiller were obtained by multiplying the mean area of a grass leaf by the mean number of leaves on a tiller. These data were obtained for a grass species on a site at a given date by selecting the tiller closest to a randomly positioned point quadrat in every subplot and recording the number of leaves on each tiller. (A leaf was defined as the area of tissue between the leaf apex and the junction of the leaf blade and leaf sheath).
All leaves were frozen within 1hr of collection at -11 °C and kept in a freezer. In 1985 leaf area was measured using an Optomax and in 1986 with an IBM compatible Digithurst Microsight I. If less than 30 leaves of a species from a site on a date had been collected then all leaves were measured for area. If more than 30 leaves had been collected from a site on one date, then a subsample of 30 leaves was randomly chosen for area measurement.
When no data were available for the leaf area of a species on a site on a given date, then data obtained at the next sampling date were used as an estimate. Indeed, such estimates of leaf area were frequently used for "rare" plant species. When data were completely lacking for a species on a particular site, data from the next oldest site were used as an estimate. This occurred very rarely when a species was represented by only one individual which could not be found for leaf area analysis.
Immediately after sampling a branch of birch for insects the number of leaves sampled was counted. Leaves for area analysis were collected randomly from upper canopy and lower canopy in 1985 at each date, and only from the lower canopy in 1986. 50
2.3 DATA HANDLING AND ANALYSIS
2.3.i Rationale behind database
Absolute abundance is defined as the number of individuals of a species per unit resource of host plant. The resource used here is host plant leaf area. The information on a species' host plant is available in the literature. However, the reliability of these data varies both within and between insect orders. In order to make best use of the available data, but not be solely dependent on potentially unreliable data, two measures of absolute abundance were calculated, one using the host plant species, if specified in the literature, and a second using the host plant's family. The latter, absolute abundance by host plant family (abhf) is calculated as:
_ number adult insects in sp y total leaf area of host plant's family(m2)
This measure of absolute abundance allows for literature records of species found feeding on only one plant species to be unreliable. It is particularly useful for species which may feed on more than one species in a genus or family, e.g among the weevils Apion aethiops feeds on Vicia spp, Apion assimile on Trifolium spp, Apion hydrolopathi on Rumex spp, Sitona hispidulus on Leguminosae (Hyman, 1983), in the Cicadellidae and Delphacidae which may be associated with one grass species but may feed on others, e.g . Arthaldeus pascuellus feeds on grasses (Ossiannilsson, 1983) especially Agrostis capillaris (Waloff & Solomon, 1973), Elymana sulphurella feeds on grasses (Ossiannilsson, 1983) especially Holcus (Waloff & Solomon, 1973) and also for those species for which no specific host plant is known, e .g . Agallia ribauti grasses (Le Quesne, 1965) and Aphrodes bifasciatus on grasses (Le Quesne, 1965).
The other measure of absolute abundance, absolute abundance by host plant species (abhs) is calculated as:
number adult insects in sp y abhs leaf area of host plant of sp y(m2 ) 51
This measure of absolute abundance is only as reliable as host plant records. If an insect species is recorded as having more than one host plant and both were present in a plot, then the sum of their leaf area was taken. These measures assume no difference in the insect's preference for host plants. Little data was available on host plant preferences of oligophagous species, and if they were, such data would be difficult to assimilate into the measure of absolute abundance. If no known host plant is recorded for a species is was assumed to be an absolute generalist, e.g. Stygnocoris pedestris (Heteroptera); in this case the denominator in the equation for absolute abundance was the total leaf area on that date/site. If a species had a host plant record but the host plant was not recorded on the sites, then absolute abundance was assigned a missing value and excluded from analyses.
Only adult insects were used in estimating absolute abundance for two reasons. Firstly, it is generally very difficult to identify immature stages of insects to species level and such identification to species is imperative in order to determine the host plant. Secondly, absolute abundances of species could vary within their lifecycle and, in order to render absolute abundance comparable, only one lifecycle stage should be considered.
A database was constructed on an Amstrad PC1512 HD20 using the "STATISTIX" statistical package (NH Analytical Software USA). In this, the number of nymphs and adults of each species on a subplot on each sampling date was recorded for each site. The leaf area of the insects host plant family and its host plant species (if present) on that subplot was also recorded. Simply,
absolute - num^er °f adult individuals in sp y on subplot x abundance host plant leaf area on subplot x(m2)
this would be the absolute abundance for each plot. However, unexploited host plants were recorded - that is, a given species of insect did not occur on all patches of its potential host plant(s). These patches were assumed to be available for 52
exploitation, however, and should therefore be included in the analysis of absolute abundance. The resulting equation if number of individuals = 0 is clearly nonsensical. Thus, in order to get around the problem, absolute abundance was calculated at the site level.
akak _ number of adult individuals in sp y on site x host plant leaf area on site xim 2)
2.3.ii Distribution of the data
The absolute abundance of insect species is not distributed normally, being highly skewed towards low values. Common statistical transformations failed to normalise the data. The distribution of absolute abundance accurately reflects the natural situation as it implies that most herbivorous insects are uncommon relative to their host plants, although some are extremely common. This pattern has been recorded in several communities (Lawton & McNeill, 1979; Strong, Lawton & Southwood, 1984).
Before statistical analysis may be undertaken it is usual to specify a mathematical function that fits the distribution of the data, e.g . normal, Poisson, binomial. Such a function may be reached by one or two methods. Firstly, it may be assumed for good biological reasons that the data will approximate to a certain distribution. Secondly the distribution of the data may be tested against various functions in order to determine the closeness of fit to the data. Due to the non-normality of the data, the second approach to distribution fitting was adopted.
2 .3 .iii Gamma distribution
The form of the data suggested that it might follow a gamma distribution. There is a constant coefficient of variation in gamma distribution (a normal distribution has constant variance). If coefficient variation (SD) k x
SD = standard deviation x = mean k = a constant
then x = SD.k
It is therefore possible to test a data set in order to determine whether it follows a gamma distribution by plotting the coefficient of variation against the mean, or by plotting the mean against standard deviation and fitting a line by linear regression. In the former case, the slope of the fitted line must not be significantly different from zero, and in the second the fitted line must be significantly greater than zero and pass through the origin. The database was tested by insect group in order to determine whether it followed a gamma distribution. It is possible to analyse data with gamma errors by using Generalised Linear Interactive Modelling (GLIM) (Royal Statistical Society 1986).
2.3.iv Generalised linear interactive models (GLIM)
Simply, GLIM is a method for fitting statistical models by maximum likelihood. According to McCullagh & Nelder (1985) "Fitting a model to data may be regarded as a way of replacing a set of data values y by a set of fitted values p derived from a model involving (usually) a relatively small number of parameters. In general, the ps will not equal the ys exactly and the question then arises of how discrepant they are, because while a small discrepancy may be tolerable a large discrepancy is not. Measures of discrepancy (or goodness of fit) may be formed in various ways, but we shall be primarily concerned with that formed from the logarithm of a ratio of likelihoods to be called the deviance".
The analysis of deviance is the GLIM equivalent to analysis of variance. "The terms in the analysis of variance can usefully be 54
thought of as the first differences of the goodness of fit statistic for a sequence of models, each including one term more than the previous one. Thus the factorial model for two factors A and B gives rise to an analysis of variance with three terms, A, B and A.B. The sums of squares for these are the first differences of the residual sums of squares obtained from fitting successively the models 1, A, A + B and A + B + A .B . As an example consider the following analysis of an unreplicated 4 x 3 table indexed by A and B (see Table 2 .2 ).
On the left is the sequence of models with their discrepancies, as measured by the residual sums of squares: note that the last model is the full model, i.e . has as many parameters as observations, so the degrees of freedom (df) and the discrepancy are both zero. On the right is the analysis- of-variance (anova) table, with the sums of squares (s.s.) obtained from the first differences of the discrepancies. Note that the discrepancy for model 1 is just the total sum of squares about the mean in the anova table. The form of the generalisation is now clear. Given a sequence of nested models we can use the deviance as our generalised measure of discrepancy and form our analysis of deviance (anodev) table by taking the first differences" McCullagh & Nelder (1985). When gamma distribution is specified, the change in deviance between successive models is checked for significance in F tables as in standard ANOVA.
One important point about GLIM is that although standard distributions are referred to such as normal, binomial, Poisson or gamma, the principal conclusions depend only on second- moment assumptions, rather than on the correctness of the assumed distributional form, that is on the variance to mean relationship and uncorrelatedness. "This is fortunate because one can rarely be confident that the assumed distributional form is necessarily correct" (McCullagh S Nelder, 1985).
Standard error bars with gamma distribution are asymmetric. 55
Analysis of Variance Model df Discrepancy ss df Term
1 11 1000 400 3 A ingnoring B A 3 600 200 2 B eliminating A A + B 6 400 400 6 A.B eliminating A + B + A. B 0 0 A and B
Table 2.2
Model analysis of deviance table for two-way analysis of deviance (ANODEV) (taken from McCullagh & Nelder, 1985). 56
2.3.v Other statistics
Where appropriate, non-parametric statistical tests are employed. Although non-parametric tests are considered less powerful than their equivalent parametric tests, if the assumption underlying parametric statistical analysis are in some way violated, then they are more powerful. One problem with two of the most frequently used non-parametric tests, Mann Whitney-U test and the Kruskall-Wallis test, is that they actually compare the distribution of the data and not the means. They are, however, frequently treated as tests for differences between means (Zar, 1984). The power efficiency of the Mann Whitney-U test and Kruskall-Wallis test, when applied to data which may be properly analysed by the most powerful parametric tests, t-test and F-test respectively, is approximately 95% (depending on sample size) (Siegel, 1956). 51
CHAPTER THREE
COMMUNITY PATTERNS THROUGH SUCCESSION
3.1 INTRODUCTION
Changes in plant diversity, species richness and stability during succession have been much studied and are well-documented (Clements 1916; Gleason 1917, Margale? 1968; Odum 1969; Drury & Nisbet 1973, Denslow 1980). Fewer studies have considered patterns in insect communities (eg VVitkowski, 1973; Itamies, 1983). Trends along a secondary successional sere at Silwood Park have been described in detail by Southwood, Brown & Reader (1979), Brown (1982a, 1982b, 1984-, 1985) and Brown & Hyman (1986). Major conclusions from these studies have shown that community stability, plant structure and architecture increase with successional age, although plant diversity peaked in early succession (Southwood, Brown & Reader, 1979). These community attributes are associated with the change from communities of annual herbs to those dominated by grasses and perennial herbs, and finally to woodland. Studies on insects have showed that the abundance of chewing and sap-feeding herbivores, and parasitoids and predators increase during succession (Brown S Southwood, 1987). Diversity of phytophagous and predatory insects peaked in early succession, mirroring that of plant diversity (Southwood, Brown & Reader, 1979). Community analysis suggested that guild structure was not constant between serai stages: the proportion of sap feeding insects decreased with successional age, while that of tourists, predators and parasitoids increased (Brown & Southwood, 1983). It has also been observed that the life history strategies of insects varied during succession with the herbivorous species colonising ruderal sites having shorter generation times, wider feeding niches and higher reproductive potential than insects of later stages (Brown & Southwood, 1987).
This chapter aims to describe the plant and insect herbivore communities along the Silwood successional gradient during the 58
two years of the study. It is not intended as an in-depth community analysis, but as background information on the patterns and abundance of plants and herbivores through succession. Such information will allow greater understanding of hypotheses tested later in this thesis. The chapter has two major sections; one describing the plant and one the insect community. The former compares the species composition of communities between years on sites of the same age and between serai stages of different age. Similarly, species composition between years and stages is considered for major plant families. As leaf area is the resource used to obtain a measure of absolute abundance, it is relevant to consider changes in leaf area within and between serai stages of the whole community and of major plant families. Annual variation within and between serai stages of leaf area is examined. May (1975) suggested that all meaningful ecological information about diversity could be gained by examining species richness and the extent to which the community is dominated by certain species. This method was employed to give some insight into plant diversity. Dominance of a community by a species was calculated by considering contribution to the community's leaf area.
Similar analyses were carried out for the insect community; variation between years and serai stages in abundance and species richness of the whole herbivorous insect community and each major insect group were considered. The similarity in the species composition of the major insect groups between years and serai stages gave some information on the turnover rates of herbivore species through succession. Due to the different sampling methods employed on birch, some aspects of its insect community were analysed separately. Species richness and the structure of the insect community was compared between upper and lower canopy and between years. The abundance of a single very common genus of sap-feeder on birch, Oncopsis, was also considered. The genus Oncopsis (Cicadellidae) is taxonomically very difficult; six species are known to feed on trees in Britain (Claridge, Reynolds S Wilson, 1977). Discrimination between species on purely morphological grounds is difficult, the genitalia provide the most useful characters 5 9
(Claridge & Nixon, 1981). Analysis of male courtship songs suggests the existence of further sibling species similar to 0 . flavicollis (Claridge & Reynolds, 1973), which are as yet indiscriminate from flavicollis on purely morphological characters. Due to the confusion over the taxonomy of this genus, and the difficulty in their identification only one species was recognised throughout the analysis. However, in order to provide some information on the abundance of different species in the genus between canopies and dates, subsamples were taken at random, dissected and identified to species using the RESL Handbook.
3.2 METHODS
General field methods are described in Chapter 2. Extensive use of regression analysis was employed in order to determine the slope of the fitted line describing various relationships. P values quoted after regression equations refer to the significance of the difference of the slope of the fitted line from zero. Non parametric tests were employed where the data did not comply with the assumptions of a normal distribution tested by Bartlett's test of inequality of variances and/or the Wilts-Shapiro test. The Kolmogorov-Smirnoff test was used to test for differences between distributions. Although it is a powerful test, the small number of data points decrease its sensitivity, and it will act conservatively.
The similarity of species composition between communities was described by Sorensons Index of Similarity (see Southwood, 1978).
Cs = 2] i a T b )
where j = number of species common to the two samples and a and b are the total number of species in each sample. Cs (the similarity coefficient) may vary between zero and one, zero if there are no species common to both samples, and one if all species are common. This index does not take abundance into 60
consideration and therefore gives equal weight to rare and common species. It is, however, appropriate for the comparison of species richness.
The dominance of a plant community by a species (P ^ was calculated by
P.j = leaf area of dominant species total leaf area
P.j was calculated for the dominant species in the community and for each of the five major families on each date, and over all dates for each serai stage in both years.
The insect community on birch was analysed separately; comparisons were made between upper and lower canopy 1985 and between the lower canopy 1985 and 1986. Due to the different sample sizes in the years and canopies the communities were not directly comparable. There were also temporal differences between the 1985 and 1986 data; sampling commenced on birch when approximately half the trees had burst their buds, this occurred on April 18, 1985 and on May 5, 1986. The regime of one sample per month thus led to six samples being taken in 1985 and five in 1986. Comparisons between years were made by pairing the first sample of each year and so on through the year. This leads to the sample taken in September 1985 having no equivalent sample in 1986. This method of comparison was deemed preferable to one pairing samples by calendar month, as it should be more biologically meaningful.
3.3 RESULTS
3 .3 . a Plant Community
3 .3 . a.i Plant species composition
Plant species richness was greatest on the early midsuccessional site in 1985 and least on the ruderal site in 1985 (Fig. 3 .1 ). There was no effect of year of sampling on plant species Number of plant species recorded in each of two years on ruderal (R ), early (E ), early mid (EMS) and late mid (LMS) successional sites.
Number of plant species recorded on four serai stages by summing two sites of the same age (ruderal (R ), early (E), early mid (EMS), late mid (LMS)). FIG 3 .1 PLANT SPECIES RICHNESS PLANT SPECIES RICHNESS
SERAL STAGE Fig 3.3 Number of plant species recorded in each of the major plant families during 1985 and 1986 on sites of different successionai age, ((a) ruderal, (b) early, (c) early mid, (d) late midsuccession; mf = minor families; Leg = Leguminosae, Co = Compositae, Cr = Cruciferae, Po = Polygonaceae, Cr = Gramineae). NUMBER OF SPECIES FIG FIG 3.3 c)EARLY MED SUCCESSION c)EARLY PLANT FAMILY PLANT d)LATE MIDSUCCESSION d)LATE I 1985 I I 1986 n ~ n 61 62
richness (t = 0.25, df = 6, P = 0.81). Plant species richness increased with successional age until early midsuccession, when 67 species were recorded, and then declined to 52 in late midsuccession (Fig 3 .2 ). Species richness in each major plant family was similar between years within each serai stage (Fig. 3.3a - d ). Species richness on the ruderal serai stage was not significantly different between years (X2 = 1.62, df = 5, P = 0.89). The species richness of Gramineae and of minor plant families in early succession was greater in 1985 than 1986, with 15 more species belonging to minor plant families being recorded in 1985 than 1986. However, there was no significant difference in total species richness between years CX2 = 3.859, df = 5, P = 0.569). Neither were there any significant differences in total species richness between years in either of the midsuccessional serai stages (EMS, X 2 = 1.94, df = 5, P = 0.8574; LMS, X? = 1.122, df = 4, P = 0.89), with the largest difference between years occurring in the minor plant families.
Seasonal trends in plant species richness can be seen in Fig. 3.4. Species richness in the ruderal serai stage increased throughout the season in 1985 and 1986 but the trend was not significant (1985, species richness = 17.4 + 1.6 date, P = 0.09; 1986, species richness = 23.1 + 1.9 date, P = 0 .3 ). In contrast in early succession, the number of plant species recorded decreased slightly throughout the year in both years (1985, species richness = 21.0 - 0.2 date, P = 0.55; 1986, species richness = 34.0 - 0.6 date, P = 0.25) (Fig. 3.4b). In the early midsuccessional site plant species richness increased throughout the year, and was significant in 1985 (1985, species richness = 22.4 + 1.6 date, P < 0.05; 1986, species richness = 28.3 + 1.7 date, P = 0.77). In the late midsuccessional site species richness decreased in both years (1985, species richness = 30.6 - 1.6 date, P = 0.125; 1986, species richness = 31 - 0.4 date, P = 0.4075). With the exception of early succession, more plant species were recorded in 1986 than 1985.
There was greater constancy in plant species composition between years with increasing age of the community, with that of the two midsuccessional serai stages being most similar between Fig 3.4 Number of plant species recorded on each date during 1985 and 1986 on sites of different successional age ((a) ruderal, (b) early, (c) early mid, (d) late midsuccession). SPECIES RICHNESS 33 0 - 20 20 20 13 30 « 3 - 23 13 0 - 40 O - JO 3 - 13 3 - 33 10 0 - 10 0 - 3 3
c)EARLY MIDSUCCESSION c)EARLY a)RUDERAL FIG FIG 3.4 A JN JL A< SEPT AU Serai Stage Plant Family Ruderai Early Early mid Late mid Minor families 0.25 0.41 0.65 0.82 Leguminosae 0.50 0.77 0.93 0.71 Compositae 0.70 0.63 0.75 0.67 Cruciferae 0.67 - -- Polygonaceae 0.83 0.33 1.00 0.50 Gramineae 0.50 0.80 0.92 0.90 Overall 0.56 0.56 0.79 0.78 Table 3.1 Sorensons Index of Similarity for plant species in each major plant family between years in different serai stages. 65 years (Table 3 .1 ). Plant species in the Polygonaceae showed least variation between years in the ruderal serai stage and Leguminosae and Gramineae the most. In contrast, these latter two families showed least variation between years in the early serai stage and Polygonaceae the most. A similar pattern occurred in early midsuccession where species of Leguminosae and Gramineae remained fairly constant between years, however, the same three species of Polygonaceae, Rumex acetosella, R. acetosa and R^ crispus occurred in both years. In the late midsuccessional serai stage Polygonaceae were the most variable, and again Leguminosae and Gramineae the most stable of the major families. The species composition of the minor families was also similar between years in this serai stage. 3 .3 .a. ii Leaf area and the plant community Total leaf area Total leaf area displayed a distinct successional pattern, with increases occurring from ruderal to early midsuccession and from midsuccession to late succession. There were, however, no significant differences in leaf area between serai stages (Kruskall-Wallis = 0.8957, P = 0.83) or between years on any serai stages (Kruskall Wallis = 4.1, P = 0.7584) (Fig. 3.5). The greatest leaf area sampled was in late succession, these data were not included in the analysis as the sampling method was not comparable with that on the other serai stages. Annual fluctuations in leaf area in a serai stage Total leaf area of the ruderal serai stage was generally greater in 1985 than 1986, but the difference was not significant (t (unequal variances) = 0.38, df = 4,4, P = 0.72). In 1985 leaf area increased significantly with date (Fig. 3.6) (leaf area = 9659 3 date — 6543.8 , P < 0 .0 1 ), whereas in 1986 the slope of Zl the fitted line was negative (leaf area = 2.236 x 10 - 888.75 date, P = 0.5155 (Fig. 3.6). The high value for leaf area in May 1986 was due to Chenopodium album which did not occur in 1985. The increase in leaf area during July and August 1985 Fig 3.5 Total leaf area on serai stages of different successional age, data summed over two years (R = ruderal, E = early, EMS = early mid, LMS = late mid, L = late succession). 6 6 FIG 3.5 400 -r C\J E ^0) 350 - "o i' 300 - CO 'oi-H 250 - XI | 200 - 150 - 1 100 - 50 - 0 -■ EMSLMS SERAL STAGE 67 was mainly as a result of the grasses which were not present in such abundance in 1986. In the early successional sites there was a significant difference in total leaf area between years (t = 2.61, df = 8, P = 0.03) with the higher values occurring in 1985. Leaf area decreased significantly through the year in both years (1985, leaf area = 3.158 x 104 - 3214.4 date, P = 0.01; 1986 leaf area = 5r623 x 104 - 6646.1 d ate , P < 0 .0 5 ). L e af area showed a similar seasonal trend in both years in the mid- successional sites. Leaf area peaked in August in both sites and years, although the peak was less pronounced in 1986. There were significant differences in total leaf area between years on both sites (EMS, t = 3.44, df = 8, P < 0.01; LMS, t = 4.36, df = 8 , P < 0.01). However, in early midsuccession in 1985 the slope of the fitted line was negative, whereas in 1986 and in both years in late midsuccession leaf area tended to increase through the year (EMS 1985, leaf area = 3.959 x 104 - 985.92 date, P = 0.699; 1986, leaf area = 2.461 x 104 + 381.4 date, P = 0.716; LMS 1985, leaf area = 3.201 x 104 + 1140.1 date, P = 0.626; 1986 leaf area = 2.21 x 104 -+- 534.13 date, P = 0.652). The leaf area of birch, expressed as the mean leaf area per bag, displayed clear seasonal trends (Fig. 3.7). Mean leaf area per bag of birch on lower canopy 1985 increased until June and then gradually declined until the end of the year. In 1986 mean leaf area per bag increased from May to August and fell by September. The greatest leaf area per bag on the upper canopy was observed in July, leaf area then declined until September. There were significant differences in mean leaf area per bag between dates for all canopies (LC 1985, Kruskali-Wallis = 104.37, P < 0.001; UC 1985, Kruskall-Wallis = 37.477, P < 0.001, LC 1986, Kruskall-Wallis = 153.33, P < 0.001). The relationship between mean leaf area per bag and date did not differ significantly between upper and lower canopy in 1985 (Kolmogrov-Smirnoff = 0.17, P = 0.99) or between lower canopies in 1985 and 1986 (Kolmogrov-Smirnoff = 0.30, P = 0.19). Fig 3. Leaf area recorded on sites of different successional age at monthly intervals over two years. LEAF AREA xlO ~3 (cm2) /4-5m2 - a)RUDERAL FIG 3.6FIG A JN JL AO SEPT AUO JULY JUNE MAY ■ ■ + + ■ ♦ + ■ +• DATE 0 - 20 0 - 40 0 - 30 0 - 10 bYEARLYSUCCESSION ■ • + * ■ ♦ 1985 + 1986 ■ ■ - " 4- + 8 6 Fig 3. Mean leaf area per bag at monthly intervals on upper a n d lower ca n o p y b ir c h 1985 a nd lower c a n o p y b ir c h 1986. 69 FIG 3.7 LOWER CANOPY 1985 UPPER CANOPY 1985 LOWER CANOPY 1986 DATE APRIL GHHD MAY JUNE I I JULY EZ3 aug liiil sept 70 Trends in the leaf area of major plant families within and between serai stages Different successional trends in total leaf area were seen in the major plant taxa (Fig. 3.8). In the Leguminosae leaf area was greatest in the early serai stage and least on late midsuccession (Fig. 3.8a). There were significant differences in leaf area between serai stages (Kruskall-Wallis = 16.539, P < 0.001) but not between years within a serai stage. There were no significant differences in leaf area of Compositae between serai stages (Kruskall-Wallis = 4.44, P = 0.22) or between years within a serai stage. The greatest leaf area of Compositae was recorded during early midsuccession (Fig. 3.8b). The leaf area of Cruciferae declined sharply with increasing successional age, with the family not being represented during late midsuccession ( F ig . 3.8c). Differences in leaf area between serai stages on which Cruciferae occurred were significantly different (Kruskall-Wallis = 18.18, P < 0 .0 0 1 ), although differences between years within a serai stage were non significant. Leaf area of Polygonaceae declined from ruderal to early midsuccession, but then increased slightly to late midsuccession ( F ig . 3.8d). In this family, the leaf area in early succession in 1986 was s ig n ific a n tly g re a te r th an in 1985 ( K ru s k a ll-W a llis statistic = 23.95, P < 0 .0 1 ), no other significant differences between years occurred. The leaf area of grasses was least during the ruderal stage of succession and greatest during early midsuccession (Fig. 3.8e) and differed significantly between serai stages (Kruskall-Waliis = 15.9644, P < 0.01). There were no significant differences between years within a serai stage. No consistent trends in leaf area of species in minor families appeared through succession (Fig. 3.8f). The area provided by minor families was greatest in late midsuccession, probably due to Epilobium spp, Chamaerverion angustifolium and Rubus fruticosus agg, and was also high on the ruderal sites, due mainly to the abundance of Spergula arvensis. There were no significant differences in leaf area of minor families between serai stages (Kruskall-Wallis = 0.8957, P = 0.826) or between years within a serai stage (Kruskall-Wallis = 4.184, P = 0.760). Fig 3.8 Leaf area of (a) Leguminosae, (b) Compositae, (c) Cruciferae, (d) Poiygonaceae, (e) Graminae, (f) minor families on serai stages of different successional ages. Data pooled over two years. (R = ruderal, E = early, EMS = early midsuccession, LMS = iate midsuccession). 71 FIG 3.8 c\Ej 0) ao a SERAL STAGE FIG 3.8 o> SERAL STAGE 72 FIG 3.8 CME CD CM 6 O 9 CME CD CM ao a SERAL STAGE 73 FIG 3.8 c\] E 05 OJ 8 o s SERAL STAGE f)MINOR FAMILIES C\J E 05 8 O 9 SERAL STAGE 74 Contribution of the plant families to the total leaf area of a serai stage In the ruderal site 1985, Leguminosae and grasses were the dominant contributors to leaf area and increased in importance during the first two years of succession (Figs. 3.9a). Compositae showed a similar increase although their overall contribution to leaf area was lower. Cruciferae and Polygonaceae made relatively minor contributions and were more important during the first year of succession. In 1986 the ruderal site showed the same general trends, although the Compositae, Polygonaceae and Cruciferae made greater contributions to community leaf area than in 1985 (Fig. 3.9b). The leaf area of grasses and minor families in early succession was greater in 1985 than in 1986 but the contribution of Leguminosae to the total leaf area was lower (Fig. 3.9c&d). The proportion of total leaf area provided by the major plant families was similar on the midsuccessional sites where both commuryties were dominated by grasses, with Compositae and minor families making important contributions to the sites leaf area (Figs. 3.9e-h). Leguminosae, Cruciferae and Polygonaceae made very little contribution to leaf area on either site. The proportion of total leaf area provided by the major plant families was comparable between years on both sites. Contribution of individual plant species to the total leaf area of a serai stage The dominant plant species on the ruderal site in 1985 in terms of leaf area for the whole season was Holcus lanatus. However, during May and June Spergula arvensis was the dominant species while Trifolium pratense dominated in August (Table 3.2a). In 1986 another grass species, Agrostis stolonifera was the season dominant on the ruderal site, although the annual herbs Chenopodium album and Spergula arvensis dominated the community early in the year, and a late germinating annual, Fig 3.9 Proportion of total leaf area provided by major plant families on serai stages of different successional age. (a) ruderal 1985, (b) ruderal 1986, (c) early 1985, (d) early 1986, (e) early mid 1985, (f) early mid 1986, (g) late mid 1985, (h) late midsuccession 1986. 75 FIG 3.9 a)RUDERAL 1985 MAY JUNE JULY AUG SEPT « LEGUMINOSAE CRUCIFERAE COMPOSITAE POLYGONACEAE GRAMINAE MINOR FAMILIES PROPORTION OFTOTAL LEAF AREA 0.1 0.2 0.3 0.4 0.3 0.6 0.7 ).9 J.8 0 i EUIOA CRUCIFERAE LEGUMINOSAE POLYGONACEAE GRAMINAE MINOR FAMILIES MINOR GRAMINAE POLYGONACEAE FIG FIG 3.9 d)EARLY SUCCESSION 1986 SUCCESSION d)EARLY DATE i e a sit o p m o c 6 7 PROPORTION OFTOTAL LEAF AREA EUIOA CUIEA COMPOSITAE CRUCIFERAE LEGUMINOSAE POLYGONACEAE GRAMINAE MINOR FAMILIES MINOR GRAMINAE POLYGONACEAE FIG FIG 3.9 A JN JL AG SEPT AUG JULY JUNE MAY A JN . UY U SEPT AUG JULY . JUNE MAY f)E A R L Y MIDSUCCESSION 1986 MIDSUCCESSION Y L R A f)E )ARYMDUCSIN 1985 MIDSUCCESSION RLY e)EA DATE 77 78 FIG 3.9 g)LATE MIDSUCCESSION 1985 f f l hJ § MAY JUNE JULY AUO SEPT h)LATE MIDSUCCESSION 1986 1 -1 12 2cu MAYJUNE JUEY SEPT DATE ■ □ LEGUMINOSAE CRUCIFERAE COMPOSUAE POLYGONACEAE GRAMINAE MINOR FAMILIES 79 Galinsoga parviflora became dominant in September (Table 3.2b). Within the major plant families pratense was the dominant legume in 1985, whereas in 1986 T_^ repens dominated. In 1986, G. parviflora was the dominant Compositae, this species did not occur in 1985 and Hypochaeris radicata contributed most to the the family's area. Raphanus raphanistrum was the only crucifer in 1985 and Polygonum persicaria was the dominant representative of the Polygonaceae. In the early successional site in 1985, Agrostis stolonifera was the dominant species over the whole season, and apart from early in the year it was the dominant species in each monthly sample (Table 3.2c). However, in 1986 a herb, T^ pratense was the overall dominant species, and dominated monthly samples in June and July (Table 3.2 d ), with Holcus lanatus dominant in May, August and September. Within individual families, T . repens was the dominant legume in 1985 and T^ pratense in 1986. The dominant species in the Compositae also differed between years, with radicata dominant throughout the season in 1985, and a series of species in 1986. Within the Polygonaceae, Rumex obtusifolius and Rumex crispus dominated in 1985, and either Rumex acetosa or F\_ persicaria in 1986. In midsuccession the grasses were the dominant species. In early midsuccession in 1985 lanatus was dominant, although this species never became dominant in 1986 and the thistle Cirsium arvense assumed dominance (Table 3.2e & f ). As in ruderal and early succession the dominant legume varied between years, vAV jq. dominated the family in 1985 and Lotus uliginosus in 1986. Compositae were more similar between years, with either arvense or Senecio jacobaeae dominating on all dates. However, C^ arvense was the overall dominant for the family in both years. No Cruciferae were recorded in 1985, and only FU raphanistrum in 1986. In the Polygonaceae, R j_ crispus was only recorded in 1986 when it dominated the family, while in 1985 R^ acetosa was dominant. The dominant grass species differed between years, H^ lanatus dominated in 1985 and Dactylis glomerata in 1986. The monthly samples in 1985 from June to September were dominated by Holcus lanatus, whereas in 1986 P,*ve species dominated in different months. Table 3.2 Proportion of leaf area provided by the dominant species to the community and major plant families on each date (P^). (a) ruderal 1985, (b) ruderal 1986, (c) early 1985, (d) early 1986, (e) early mid 1985, (f) early mid 1986, (g) late mid 1985, (h) late mid 1986, and species richness (sp. no.) of each plant family and community. (Leg = Leguminosae, Co = Compositae, Cr = Crucifer, Pol = Polygonaceae, Gr = Craminae). 80 Tahiti 3.2.1 DATE May June Ju ly August September O verall f-'amily Pj sp. no. Pj sp. no. Pj sp. no. Pj sp. no. Pj sp. no. Pj sp. no. 0.699 3 0.919 9 0.55 5 0.929 9 0.895 9 0.613 6 U tj T. pratense T. pratense T. hybridum T. pratensc T. pratense T. pratensc 0.698 2 0.927 5 0.806 3 0.53 9 0.659 6 0.559 6 Co T. inodoruin C. capillaris C. capillaris T. inodorum H. radicata II. radicata - 0 1.0 3 0 1.0 1 - O 0 C r R. raphinostrum R. raphinostrum 0.638 2 0.509 3 0.818 3 0.53 9 0.626 3 0.799 3 Pol P. lipathifolium P. persicaria P. persicaria P. persicaria P. persicaria P. persicaria 0.735 U 0.673 5 0.709 5 0.63 5 0.600 5 0.623 5 C r A. stolonifera II. lanatus H. lanatus H. lanatus 11. lanatus II. lanatus 0.6099 0.296 0.239 0.355 0.301 0.159 Total S. arvensis S. arvensis II. lanatus T . pratense II. lanatus II. lanatus Table 3. 2b DATE May June August September Overall Family Pj sp. no. P1 sp. no. Pj sp. no. Pj sp. no. P.j sp. no. Pj sp. no. 0.65 2 0.95 3 0.79 5 0.53 5 0.59 3 0.38 6 Leg V . cracca V. tetraspermum V. tetraspermum T. repens T. repens T . rep ens 0.90 9 0.922 10 0.612 8 0.97 9 0.67 9 0.96 12 Co C. arvense C. parviflora G. parviflora S. asper C. parviflora G. parviflora 0.77 2 0.7817 2 0.80 2 1.00 1 0.99 2 0.83 2 C r R. raphinastrum R. raphinastrum R. raphinastrum C. b-pastoris R. raphinastrum R. raphinastrum 0.58 9 0.526 6 0.685 3 0.91 9 0.91 9 0.59 7 Pol R. crisp u s R. obtusifolius R. aviculare P. persicaria P. persicaria P. persicaria 0.99 2 0.987 3 0.99 2 0.97 3 0.99 2 0.98 3 Or A. stolonifera A. stolonifera A. stolonifera A. stolonifera A. stolonifera A. stolonifera 0.776 20 0.35 32 0.25 30 0.19 33 0.23 29 0.211 Total C. album S. arvense S. arvense A. stolonifera G. parviflora A. stolonifcra 81 Table 3.2c DATE May June Ju ly A u gu st September O verall Family Pj sp. no. Pj sp. no. P.j sp. no. Pj sp. no. Pj sp. no. Pj sp. no. 0.329 7 0.953 6 0.537 7 0.958 5 0.629 6 0.303 Ley M. lupilina V. hirsu ta T . repens T . repens T . repens T . rcpen s 0.597 8 0.39 7 0.959 8 0.297 7 0.257 6 0.159 Co T. officinale T . inodorum C. canadensis C. canadensis T. officinale T. officinale 0 0 0 0 — O - C C r 0.10 1 0.752 2 0.593 3 1.0 1 1.0 1 0.556 2 Po' R. obtusfolius R. crisp u s R. obtusfolius R. obtusfolius R. obtusfolius R. cris p u s 0.297 7 0.578 6 0.795 6 0.722 5 0.558 5 0.589 C r Poa annua A. stolonifera A. stolonifera A. stolonifera A. stolonifera A. stolonifera 0.12 0.193 0.269 0.919 0.305 0.25 Total T. officinale V . hirsu ta A. stolonifera A. stolonifera A. stolonifera A. stolonifera T able 3.2d DATE May June Ju ly A u gu st September O verall Family Pj sp. no. Pj sp. no. P 1 sp . no. P 1 sp. no. ?! sp. no. ?! sp . no. 0.782 5 0.85 6 0.91 5 0.83 5 0.89 5 0.85 6 Leg T. pratense T. pratense T. pratense T. pratense T. pratense T. pratense 0.86 C 0.87 9 0.61 3 0.91 5 0.93 5 0.86 8 Co H. radicata II. radicata II. radicata II. radicata II. radicata II. radicata 0 0 1.0 1 0 0 1.0 1 Cr R. raphinastrum R. raphinastrum 0 1.00 1 1.0 1 0 1.0 1 O. 89 2 Pol R. acetosella P. persicaria R. acetoseila P. persicaria 0.79 6 0.75 9 0.63 9 0.76 3 0.83 9 0.79 6 C r 11. lanatus 11. lanatus II. lanatus 11. lanatus 11. lanatus II. lanatus 0.32 20 0.38 22 0.90 20 0.28 20 0.38 20 0.31 Total hi. lanatus T. pratense T. pratense H. lanatus 11. lanatus T. pratense Table 3.2e DATE May June Ju ly Au gu st September O verall Family Pj sp. no. Pj sp. no. Pj sp . no. Pj sp. no. Pj sp. no. Pj sp. no. 0.6C S 0.45 6 0.34 7 0.25 7 0.62 5 0.43 7 Ley V . sativa V . sativa V . sativa M. lupilina T . repens V . sativa 0.51 5 0.48 6 0.60 7 0.59 6 0.87 5 0.65 7 Co C . arvense C . arvense C . arvense C. arvensfc S. jacobcae C . arvense 0 0 0 0 0 0 Cr 0 1.0 1 0.72 2 1.00 1 0.56 2 0.83 3 Pol R . acetosa R. acetosa R. acetosa R. acetosella R. acetosa 0.34 6 0.58 5 0.69 6 0.77 5 0.84 7 0.68 7 O r A. capillaris H . lanatus II. lanatus II. lanatus II. lanatus II. lanatus 0.18 23 0.28 26 0.38 29 0.48 28 0.73 30 0.19 Total A. capillaris H. lanatus II. lanatus H . lanatus II. lanatus 11. lanatus Table 3. 2f DATE May June July A u gu st September O verall Family P 1 sp. no. P, sp. no. Pj sp . no. P 1 sp. no. P 1 sp. no. Pj sp. no. 0.32 5 0.32 4 0.43 6 0.74 7 0.37 5 0.33 7 Ley T . repens L. ulignosus V. sativa L. ulignosus L. ulignosus L. ulignosus 0.42 5 0.53 4 0.68 6 0.77 7 0.71 8 0.50 8 Co S. jacobeae 5. jacobeae C. arvense C. arvense C. arvense C. arvense 1.0 1 0 1.0 1 1.0 1 1.0 1 Cr — O R. raphinastrum R. raphinastrum R. raphinastrum Ft. raphinastrum 1.0 1 0.79 2 0.85 3 0.52 2 0.54 2 0.83 3 Pol R. crisp u s R . crisp u s R. crisp u s R. acetoseila R. acetosa R. cris p u s 0.47 9 0.23 8 0.38 10 0.34 7 0.39 7 0.33 11 C r D. glomerata H. lanatus D. glomerata A. stolonifera H. lanatus D. glomerata 0.20 31 0.14 29 0.23 35 0.22 36 0.18 36 0.08 Total D. glomerata R. repens T. officinale A. stolonifera C. arvense D. glomerata 83 Table 3.2q DATE May June Ju ly A u gu st Septemoer O vcral l Family Pj sp. no. Pj sp. no. Pj sp. no. Pj sp. no. Pj sp. no. Pj sp. no. 0.72 5 0.62 4 0.77 4 0.44 4 1.0 1 0.49 5 Leg V . sativa V . h irsu ta V. h irsu ta L. corniculatus L. corniculatus V . h irsu ta 0.42 5 0.37 5 0.43 5 0.61 6 0.33 5 0.57 ] Co T. officinale C. arvense C. arvense C . arvense C . arvense C . arvense 0 0 0 0 0 0 C r 1.0 1 1.0 1 0.97 2 1 .0 1 1.0 1 0.99 ! Pol R. acetosa R. acetosa R. acetosa R. acetosa R. acetosa R. acetosa 0.40 ii 0.54 7 0.50 6 0.69 7 0.56 C 0.65 a C r H. lanatus II. lanatus II. lanatus II. lanatus II. lanatus II. lanatus 0.24 28 0.31 27 0.29 27 0.50 27 0.34 20 0.34 Total II. lanatus H. lanatus H. lanatus II. lanatus H . lanatus II. lanatus T able 3.2h DATE May June Ju ly August September O verall Family Pj sp . no. Pj sp. no. Pj sp . no. Pj sp. no. Pj sp. no. Pj sp. no. 0.88 4 0.88 5 0.88 3 0.54 4 0.88 3 0.83 5 Leg L. corniculatus L. corniculatus L. corniculatus L. corniculatus L. corniculatus L. corniculatus 0.61 7 0.52 7 0.63 7 0.51. 0.51 7 0.6.- 8 Co C . arvense C. arvensc C . arvense C. arvense C . arvense C. arvense 0 0 0 0 0 0 Cr 1 .0 1 1.0 1 1.0 1 1.0 1 1.0 1 1 .0 1 Pol R. acetosa R. acetosa R . acetosa R. acetosa R. acetosa R. acetosa 0.43 7 0.58 9 0.56 G 0.62 9 0.57 9 0.56 11 C r D. glomeratn D. glomerata D. glomerata D. glomerata D. glomcrata D. glomerata 0.17 30 0.26 31 0.22 29 0.39 31 0.29 30 0. 27 Total D. glomerata D. glomerata D. glomerata D. glomerata D. glomernta D. ylomerat;. 84 In both years in late midsuccession grasses v/ere dominant in every monthly sample, Holcus lanatus in 1985 and glomerata in 1986 (Table 3.2g). The Leguminosae were dominated by Vicia spp early in 1985, but by Lotus corniculatus later in the season and throughout 1986. C. arvense was generally the dominant Compositae in both years. No Cruciferae were recorded in either year and only one member of the Polygonaceae, acetosa. 3 .3 .b . Insect Community 3.3. b.i. Total insect abundance during succession Peak total and adult insect abundance occurred in early succession and was lowest on the ruderal and the late successional stage (Fig. 3.10). There were significant differences in abundance between serai stages (total, Kruskall-Wallis = 12.798, P < 0.001; adult, Kruskall-Wallis 11.737, P < 0.001). 3 .3 . b.ii Abundance of major herbivore taxa associated with different serai stages The total number and number of adult Heteroptera was greatest on late midsuccession (Fig 3.11 & 3.12). This was mainly due to the abundance of the Lygaeid Ishnodemus sabuleti. Numbers of Heteroptera were low on both the ruderal and late serai stages. There was a significant difference in total Heteroptera number and number of adults between serai stages (total, Kruskall- Wallis = 26.308, P < 0.001; adult, Kruskall-Wallis = 18.97, P < 0.001). The number of Psyllidae was generally low in the early successional stages but increased in late midsuccession and on birch (Fig. 3.11), but was higher in the former due to the broom feeding species Arytainiila spartiophila and Arytaina genistae. There was a significant difference in abundance between serai stages (Kruskall-Wallis = 14.885, P < 0.001). No nymphal Psyllidae were recorded. Fig 3.10 Total number of (a) adult insect herbivores (b) insect herbivore individuals (excluding Aphididae, Coccoidea,, Aleyro^oidea (Hemiptera), Thysanoptera and Lepidoptera) on serai stages of different successional age. Data summed over two years (R = ruderal, E = early, EMS = early mid, LMS = late midsuccession, L = late). 85 FIG 3.10 a)ADULT ABUNDANCE 10000 -T 9000 - w 8000 - u 2 7000 - < 6000 - Q 2 5000 - 4000 - {3 3000 - 2000 - 1000 - o J - 14000 SERAL STAGE b)TOTAL ABUNDANCE 13000 - w 12000 - U 11000 - 2 10000 - < 9000 - Q 8000 - 2 7000 - D 6000 - m 5000 - < 4000 - 3000 - 2000 - 1000 - SERAL STAGE 86 Among the Auchenorrhyncha the abundance of Cercopidae was greatest in early succession, and gradually decreased through succession (Fig. 3.11). However, no significant differences in abundance between serai stages occurred ( Kruskall-Wallis = 5.209, P = 0.267). No nymphal Cercopidae were recorded. The abundance of Delphacidae increased from ruderal to late midsuccession, however none were recorded on birch (Figs. 3.11 & 3.12). There were significant differences in total and adult abundance of Delphacidae between serai stages (total, Kruskall- Wallis = 21 .673, P < 0.001; adult, Kruskall-Wallis = 19.0472, P < 0.001). The total number of Cicadellidae peaked in early midsuccession, with the lowest number being recorded in the ruderal stage (Figs. 3.11 & 3.12). The differences in abundance between serai stages were significant (total, Kruskall-Wallis = 19.77, P < 0.001; adult, Kruskall-Wallis = 11.269, P < 0.001). Due to their habit, no larval Curculionoidea were recorded; the number of adults was greatest in early succession. The differences in abundance between serai stages were significant (Kruskall-Wallis 26.102, P < 0.001). Total and adult abundance of Chrysomelidae were highest in the ruderal stage but were low on the other serai stages. No Chrysomelidae were recorded on birch. There were significant differences in total and adult abundance between serai stages (total, Kruskall-Wallis = 20.694, P < 0.001; adult, Kruskall-Wallis = 13.615, P < 0.001) (Figs. 3.11 & 3.12). The abundance of Chrysomelidae in the ruderal serai stage was due to the abundance of Cassida spp adults and nymphs which feed on Stellaria spp, Cirsium spp, Urtica spp and Sperguia arvensis. 3.3.b.iii Annual variation in insect abundance Total numbers The abundance of herbivorous insects in the ruderal stage was similar between years, peaking in August in both years (Fig. 3.13) (1985, number = 191.2 + 206 date, P = 0.149; 1986, number = -414.4 + 308 date, P < 0.05). In early succession, the pattern of abundance was different between years: in 1985 Fig 3.11 Total number of individuals in each major insect group on serai stages of different successionai age. Data pooled over two years (R = ruderal, E = early, EMS = early mid, LMS = late mid, L = late succession). TOTAL ABUNDANCE WOO• 2000 - 4000 • 5000 7000 O ' 1 ' MOO - l coo- FIG3.11 " 1 I I 1 » I I I » 1 * " o • ■ ■ a)HETEROPTERA * 8 M LS L LMS BMO 8 R SERALSTAGE ------8 M LS l LMS BMi 8 R 87 Fig 3.12 Total number of adults in each major insect group on serai stages of different successional age. Data pooled over two years (R = ruderal, E = early, EMS = early mid, LMS = late mid, L = late succession). ADULT ABUNDANCE KB 0 0 0 2 2200 - 2400 - 2600 1000 1200 - 1400 - 1600 2SG0 - 2SG0 - I wo- 600 • • 600 - too 400 - _ _ - 400 200 0 0 ------• a)HETEROPTERA FIG 3.12 FIG SERAL STAGE SERAL d)CHRY d)CHRY SOMELID AE b)DELPHACIDAE 88 89 numbers had increased to 2600 by July, then fell progressively (number = 602.7 + 135.3 date, P = 0.659), whereas in 1986 numbers increased until August then fell in September (number = 365.3 + 290.7 date, P = 0.701). The seasonal pattern of abundance was also different between years in early midsuccession. In 1985 abundance increased progressively through the season (Fig. 3.13c) (number = 377.9 + 183.3 date, P < 0.05). In 1986, however, numbers peaked in July and then declined during August and September (number = 865.9 + 6.7 date, P = 0.940). The patterns of abundance in late midsuccession were very different to those in early midsuccession (Fig. 3.13d). Abundance peaked in August in 1985 and in June in 1986 (1985, number = 656.6 + 152.4 date, P = 0.14: 1986, number = 1370.7 - 43.7 date, P = 0.876). In late succession the slope of the fitted regression line was negative for the upper and lower canopy in 1985 and the lower canopy in 1986 (LC 1985, number = 538.8 - 89.6 date, P = 0.075; UC 1985, number = 221 .2 - 24 date, P = 0.266; LC 1986, number = 601 .7 - 46.7 date, P = 0.644). Numbers peaked in June for the lower canopy 1986 and for the upper canopy 1985, and in May for the lower canopy 1985. The increased numbers on the lower canopy 1986 were probably due to the increased sampling effort. Adult numbers The abundance of adult insects in the monthly samples from the ruderal sites was similar in both years (number = -135.3 + 135 date, P = 0.069; number = -206.3 + 161.5 date, P < 0.05) (Fig. 3.14). Numbers were also similar in May, June and September of both years in early succession. Peak abundance switched between the two years being in July in 1985 and in August in 1986 (1985, number = 602.7 + 135.3 date; 1986, number = 365.3 + 290.7 date, P = 0.701). Adult abundance increased almost linearly through 1985 in early midsuccession (number = -55.8 + 182.8 date, P < 0.001). in 1986 the number of adults recorded in June, July and August were similar to those in the same months during 1985, however in contrast, numbers decreased dramatically in September 1986 (number = 220.5 + 66.5 date, P = 0.228) (Fig. 3.14c). Adult abundance in late midsuccession also Fig 3.13 Total number of insect herbivore individuals (excluding Aphididae, Coccoidea , Aleyro&aidea (Hemiptera\ Thysanoptera and Lepidoptera) recorded on monthly samples over two years on serai stages of different successional age. TOTAL ABUNDANCE 000 10 1000 11001200 1300 MOO 600 200 300 400 300 900 200 400 600 700 900 700 300 300 too 100 100 100 0 0 NEARLY MIDSUCCESSION (PLATE MIDSUCCESSION (PLATE MIDSUCCESSION NEARLY elLATE SUCCESSION elLATE a)RUDERAL FIG 3.13 FIG A JN' JULr JUNE' MAY DATE 0 0 2 2 1000 1200 1400 2000 2600 2100 1600 1100 2400 3000 400 600 200 too + LOWER CANOPY 1985 1985 CANOPY LOWER + 0 o UPPER CANOPY 1985 1985 CANOPY UPPER o ■ LOWER CANOPY 1986 CANOPY LOWER ■ b)EARLY SUCCESSION b)EARLY 18 ■ 1986 ■ 1985 + A JN JL AO SEPT AUO JULY JUNE MAY A JN JL AO SETT AUO JULY JUNE MAY I ■—» i i » — "■ I l 90 Fig 3.14 Total number of adult herbivores (excluding Aphididae, Coccoidea , A ley ro^ bides. (Hemiptera\ Thysanoptera and Lepidoptera) recorded on monthly samples over two years on serai stages of different successional age. ADULT ABUNDANCE FIG 3.14 FIG a)RUDERAL e)LATE SUCCESSION e)LATE A nB UY U SETT AUQ JULY nmB MAY may jm n iuly Alia Alia sept DATE 1000 1200 1400 two taoo 2000 TXT} <00 800 400 0 0 2 0 b)EARLY SUCCESSION b)EARLY + LOWER CANOPY 1985 CANOPY LOWER + ■ LOWER CANOPY 1986 CANOPY LOWER ■ O UPPER CANOPY 1985 CANOPY UPPER O A JN JL AO SEPT AUO JULY JUNE MAY 1985 + 1986 9] 92 increased almost linearly from May to August 1985 (number = 75.7 + 105.5 date, P = 0.881). In 1986 adult abundance was generally higher and peak adult abundance was recorded in June, due mainly to the abundance of lshnodemus sabuleti (Hemiptera: Heteroptera) and Arytainilla spartiophila (Hemiptera: Psyllidae) (number = 734 - 29.6 date, P = 0.184). The number of adults on lower canopy birch 1985 followed the same pattern as that already described for total abundance (number = 430.6 - 79.4 date, P = 0.097). However, on both lower canopy 1986 and upper canopy 1985, adult abundance was greatest in midsummer (1985 UC, number = 221 .2 - 24 date, P = 0.443; 1986 LC, number = 601 .7 - 46.7, P = 0.721). 3.3.b.iv Annual variation in abundance of insect groups There were no significant differences in numbers of adults or nymphs between years on any serai stage as tested by a Mann Whitney U Test (Table 3.3 ). However, eleven Psyllidae were recorded in the ruderal stage in 1986 and none in 1985, and 46 in early succession in 1985 and none in 1986. Similarly Chrysomelidae were recorded in early midsuccession in 1985 and not in 1986. Cicadellidae were the most abundant group in ruderal, early midsuccession and late succession but were also important on the other serai stages. Curculionoidea were numerically dominant in early succession and Heteroptera in late midsuccession. Cercopidae, Psyllidae and Chrysomelidae were less abundant in all stages, with Cercopidae being particularly rare. 3 .3 .b .v Herbivore species composition Variation in species richness with serai stage Insect herbivore species richness was greatest in midsuccession, and least in late' succession (Fig 3.15a). There were significant differences in species richness between serai stages (one way ANOVA, ^ = 9.32, P < 0.001). However, species richness did not differ significantly between years in any serai stage, although significant differences between years of different serai stages were detected, but not tested for (one way ANOVA Fg ^ = 3.98, P < 0.001) (Fig. 3.15b). Table 3.3 Total, adult and nymphal abundance of each major insect group on sites of different successional age over two years (na = number of adults, nn = number of nymphs, NT = total number). Table 3.3 Serai Stage Ruderal Early Early mid Late mid BLC BUC BLC Insect Group 1985 1986 1985 1986 1985 1986 1985 1986 1985 1985 1986 na 97 256 779 766 694 787 616 1769 634 88 158 Heteroptera nn 88 920 846 684 630 696 2176 1816 128 73 94 NT 185 1176 1625 1450 1324 1483 2792 3585 762 161 252- na 0 11 46 0 10 44 122 625 86 305 139 Psyllidae nn ------NT 0 11 46 0 10 44 122 625 86 305 139 na 1 2 11 7 2 11 6 2 2 0 Cercopidae nn ------NT 1 2 11 7 2 1 1 3 6 2 2 0 na 37 14 85 183 240 in 361 209 0 0 0 Delphacidae nn 47 4 67 230 470 417 626 531 0 0 0 NT 84 18 152 418 710 689 987 740 0 0 0 na 618 517 889 1120 1017 770 644 478 161 81 684 Cicadellidae nn 530 125 681 791 1073 1217 803 618 260 166 728 NT 1148 642 1570 1911 2090 1987 1447 1096 421 247 1412 na 434 444 3205 2348 476 216 199 119 79 31 505 Curculionoidea nn ------NT 484 440 3205 2398 476 216 199 119 79 31 505 na 119 147 28 8 24 0 16 20 0 0 0 Chrysomelidae nn 109 120 0 0 3 0 3 7 0 0 0 NT 208 267 28 8 27 0 19 27 0 0 0 Total NT 2130 2556 6637 6187 4639 4430 5569 6198 1350 746 2308 GT = 42,750 CD CO Fig 3.15 Mean number of insect herbivore species (excluding Aphididae, Coccoidea, Aleyro&oides (Hemiptera\ Thysanoptera and Lepidoptera) on serai stages of different successional age (a) data summed over two years, (b) for each year (R = ruderal, E = early, EMS = early mid, LMS = late mid, L = late succession). Bars are standard errors. Clear = 1985, shaded = 1986. 94 FIG 3.15 co U 2 CO W8 cocw SERAL STAGE b)SPECIES RICHNESS IN 1985 AN D 1986 50 -i------ I I 1985 1986 95 Annual variation in species richness Herbivore species richness increased almost linearly in the ruderal stage in 1985 (Fig. 3.16a) (species richness = 1.2 + 7 date, P < 0.05) with 36 species being recorded in September. Species richness in 1986 was similar to 1985 in all months except August, when 55 species were recorded in 1986 compared with only 30 in 1985 (1986, species richness = 1.9 + 8.5 date, P = 0.136). The relationship between species richness and date did not differ significantly between years in the ruderal stage (Kolmogrov-Smirnoff = 0.08, P = 0.98) or in the early serai stage (Kolmogrov-Smirnoff = 0.04, P = 0.98). The number of species in early succession increased from May until August during both years, and then fell by September (Fig. 3.16b). However, the slope of the fitted line was not significantly different from zero in either year (1985, species richness = 14.8 + 7 date, P = 0.055; 1986, species richness = 21.7 + 5.5 date, P = 0.195). Species richness also peaked in August in early midsuccession in 1985, (species richness = 15.5 + 7.9 date, P < 0.05) but not until September in 1986 (species richness = 13.7 + 9.5 date, P < 0.005), although the relationship between species richness and date was not significantly different between years (Kolmogrov-Smirnoff = 0.04, P = 0.97). Species richness in late midsuccession peaked in August in both years, and again the relationship did not differ between years (Kolmogrov-Smirnoff = 0.03, P = 0.84) (1985, species richness = 30.1 + 2.9 date, P = 0.270; 1986 species richness = 37.8 + 1.8 date, P = 0.627). During both years on late succession species richness increased from May until August and then decreased in September (Kolmogrov-Smirnoff = 0.11, P = 0.805). The greater species richness in 1986 could be due to increased sampling effort (1985, species richness = 8.6 + 1.11 date, P = 0.465; 1986, species richness = 5.3 + 4.3 date, P = 0.117). Annual variation in species richness of insect groups Curculionoidea and Heteroptera were the two most species-rich groups in the ruderal and early successional serai stage, with Cercopidae, Psyllidae and Delphacidae being the least Fig 3.16 Number of insect herbivore species (excluding Aphididae, Coccoide^, Aleyrodoidea (Hemiptera), Thysanoptera and Lepidoptera) in monthly samples over two years on serai stages of different successions! age. SPECIES RICHNESS 50 55 40 20 10 15 25 50 50 40 60 10 30 50 70 0 5 0 FIG 3.16 FIG nL A JN JL AX SEPT A1X3 JULY JUNE MAY AnUL A JUNE MAY )AL )AEMIDSUCCESSION O I S S E C C U S D I M d)LATE N O I S S E C C U S D I M c)EARLY e)LATE UCCESSION S e)LATE a)RUDERAL U sm s AUQ DATE 96 1985 + 1986 A JN JULY JUNE MAY b)EARLY UCCESSION S b)EARLY 96 97 species-rich (Figs. 3.17a&b). There were no significant differences in the species richness of any group between years on either serai stage (r u d e r a l,^ 2 = 2.092, df = 6, P = 0.911; early, X 2 = 4.927, df = 6, P = 0.553). A similar pattern occurred in the two midsuccessional stages; the Heteroptera was the most species-rich group in 1986 on both sites, whilst the Curculionoidea was the most species-rich in 1 985. Cercopidae and Psyllidae again showed low species richness, but the Delphacidae increased from the level found in the two earlier serai stages. There were no significant differences in species richness between years in either of the midsuccessional stages (EMS, \ 2 = 0.654, df = 6, P = 0.995; LMS,^.2 = 5.019, df = 6, P = 0.542). Species richness of insect groups feeding on major plant families A total of 76 species of Heteroptera were recorded, 20 of these were grass-feeders and 15 generalists. Nine Heteroptera species were recorded feeding on trees (Fig. 3.18a). Only one species, Eurydema oloracea specialised on Cruciferae. Ten partially predatory species were recorded (Nabidae and Anthocoridae were considered wholly predatory and consequently not included in this analysis). The majority of the 16 Psyllidae species recorded fed on trees and 4 on minor plant families. All 12 Delphacidae species were grass feeders as were the majority of Cicadellidae. Fourteen of the 17 species of Cicadellidae recorded from trees were members of the subfamily Typhlocybinae, but only one species of this subfamily, Zyginidia scutellaris fed on grass. The only generalist recorded in this group, Empoasca decipiens was also a member of the Typhlocybinae. Two of the three Cercopidae species were grass feeders, and Cercopis vulnerata was deemed a generalist. Both of the Coleoptera groups showed wide host ranges. Curculionoidea were recorded feeding on all major plant families except grass and Chrysomelidae from all except the Compositae and birch. Leguminosae had the most species of herbivorous Coleoptera feeding on them and grasses the least. Fig 3.17 Number of species in each major insect group during two years on serai stages of different successional age (Het = Heteroptera, Psy = Psyilidae, Cere = Cercopidae, Del = Delphacidae, Cic = Cicadellidae, Cure = Curculionoidea, Chry = Chrysomelidae). SPECIES RICHNESS 30 - - 30 - 35 20 - 29 t c)EARLY MIDSUCCESSION c)EARLY e)LATE SUCCESSION e)LATE • FIG 3.17 FIG INSECT GROUPINSECT d)LATE MIDSUCCESSION d)LATE I18 ■■ 1986 ■ ■ 1985 I I 98 Fig 3.18 Host plant family of species in each major insect group and summed over all groups (Gen = generalist, Leg = Leguminosae, Co = Compositae, Cr = Cruciferae, P = Polygonaceae, Gr = Gramineae, PP = partial predator, B = birch, mf = minor families). Shaded columns on Cicadellidae indicate Typhlocybinae. Hatched columns on (h) indicate sap-sucking species. 99 FIG 3.18 . c)DELPHACIDAE (DCICADELLIDAE Leg Co Cr P» Or PP ■ MF HOST PLANT FAMILY TYPHLOCYBINAE I ICICADELLIDAE FIG 3.18 e)CURCULIONOIDEA f)CHRYSOMELIDAE I I CHEWING INSECTS l - ^vl SAP-SUCKING INSECTS HOST PLANT FAMILY 100 Overall, 70 of the 255 herbivore species fed on grasses and of these only one, Chaetocnema hortensis (Chrysomelidae) was a chewing insect (Fig. 3.18h). Fifty species fed on birch, 15 of these were chewing insects. The plant family with fewest herbivore species was the Cruciferae, which had 7 chewing and one sap-sucking herbivores. Twenty one species were generalists and 23 specialised on plant families not included in any of the major families. Ten species, all Heteroptera, were partially predacious. 3 .3 .b .v i Variation in insect species composition with successional age Species composition of Heteroptera was most similar between the two midsuccessional stages. There was little difference in the Index of Similarity between the early successional stage and two midsuccessiona! stages and none in that between ruderal and the early and midsuccessional stages (Table 3 .5 ). There was little overlap in species composition of Psyllidae between stages, late mid succession and birch had the most similar fauna, probably due to insects from young trees present on the late midsuccession falling to the ground. The relatively high Index of Similarity of Cercopidae reflects little preference between the serai stages of the few species recorded. Not surprisingly, species composition of Delphacidae on the midsuccessional sites was similar, as was that of the ruderal and early serai stages. More surprising was that the highest Index of Similarity recorded for Cicadellidae occurred between the ruderal and early serai stage. However, comparisons between most stages showed relatively high Indices of Similarity for this group. Indices of Similarity of Curculionoidea were generally lower, the greatest similarity occurred between early and early midsuccession. The same Chrysomelidae species were recorded on the ruderal as on the early successional stage, other comparisons showed little similarity in species composition between stages. The group mean Index of Similarity showed that insect herbivore species composition in the ruderal stage was most similar to that in the early successional stage, and equally similar to both Table 3.4 SERAL STAGE R E EMS LMS L (LC ) X INSECT CROUP 85 - 86 85 - 86 85 - 86 85 - 86 85 - 86 85 - 8 Heteroptera 0.68 0.52 0.76 0.65 0.69 0.66 Psyllidae - - 0.4 0.66 0.5 0.52 Cercopidae 1.0 0.66 0.66 0 - 0.58 Curculionoidea 0.66 0.57 0.64 0.66 0.73 0.65 Delphacidae 0.33 1 .0 0.88 0.53 - 0.69 Chyrsomelidae 0.8 0.5 1.0 0.82 - 0.78 Cicadellidae 0.69 0.63 0.90 0.66 0.64 0.70 X 0.69 0.65 0.75 0.56 0.64 Table 3.4 Sorensons Index of Similarity for species in each major insect group between years on serai stages of the same age. (R = ruderal, E = early, EMS = early midsuccession, LMS = late midsuccession, L = late succession). 102 midsuccessional stages. Species composition in the early serai stage was also equally similar to the two midsuccessional stages, but less similar than to the ruderal stage. The two midsuccessional stages had a species composition most similar to each others than to that on any other serai stage. Not surprisingly the species composition on birch was most similar to that in late midsuccession. These results indicate that herbivore species composition changes with community age, and overall is most comparable between serai stages of the closest age. However, this may not be true for individual insect groups. Annual variation in insect herbivore species composition Cercopidae and Chrysomelidae showed the least variation in species composition between years in the ruderal serai stage (Table 3 .4 ). Species of Delphacidae varied considerably between years in the ruderal serai stage, but not at all on the early successional stage. The three major insect groups, Heteroptera, Cicadellidae and Curculionoidea, varied less between years on the ruderal serai stage than in the early stage. Species of Delphacidae, Cicadellidae and Chrysomelidae varied little between years in the early midsuccession serai stage, but to a greater extent in late midsuccession. Curculionoidea were the most comparable group between years on birch, and Psyllidae the least. The mean Index of Similarity for each year suggested that herbivore species in early midsuccession changed least between years, and those in late midsuccession the most. However, the Index of zero for Cercopidae in this stage lowered this mean. Exclusion of this point gave a mean of 0.66. A similar crude analysis suggested that Chrysomelidae species changed least between years in a serai stage. 3 .3 .b .v ii The Birch Community Variation in species richness between canopies and years Forty two herbivore species were recorded on lower canopy 1986, 33 on lower canopy 1985 and 30 on upper canopy 1985 (Table 3 .6 ). The combined species list for 1985 (upper and lower 103 Table 3.5 INSECT CROUP R-E R-EMS R-LMS R-L E-EMS E-LMS E-L EMS-LMS EMS-L LMS-L o cr Sf ■tr Heteroptera O 0.44 0.263 0.57 0.54 0.27 0.64 0.17 0.13 Psyllidau 0 0 0.22 0 0.3 0.5 0 0.4 0 0.8 Cercopidae 0.67 0.67 0.5 0 1.0 0.8 0 0.8 0 0 Delphacidae 0.8 0.57 0.63 - 0.71 0.63 - 0.8 - - Curculionoidea 0.59 0.57 0.42 0 0.70 0.64 0.07 0.71 0.04 0.08 Clccadellidae 0 0.8 0.66 0.68 0.73 0.77 0 0.77 0 0.08 Chrysomelidae 1 .0 0.31 0.31 - 0.31 0.31 - 0.43 - - X 0.61 0.46 0.46 0.053 0.59 0.598 0.068 0.65 0.042 0.218 Table 3.5 Sorensons Index of Similarity for species in each major insect group between serai stages of different successionai age. Data summed over two years. Table 3.6 Species richness of each insect group on lower and upper canopy 1985 and lower canopy 1986 in (a) monthly samples and (b) annual totals. Table 3.6a DATE 1 2 3 4 5 6 year 85 86 85 86 85 86 85 86 85 86 85 86 canopy LU L LU L L U LL U L LU LL UL Heteroptera 3 2 2 3 4 5 8 4 9 8 5 8 9 4 5 3 2 - Psyllidae 0 1 1 1 1 2 Q 2 3 1 1 3 1 1 2 1 0 - Ciccadellidae 1 1 2 1 2 3 5 5 8 5 3 10 7 2 10 3 5 - Curculionoidea 3 2 3 8 6 8 6 4 7 3 3 5 5 1 3 1 1 - Total 7 6 8 13 13 18 19 15 27 17 12 26 22 8 20 8 8 - Table 3.6b year 85 85 canopy LC UC LC + UC LC 86 Heteroptera 13 8 15 13 Psyllidae 2 4 4 6 Typhocybinae 8 7 9 10 Cicadellidae 2 2 3 4 Sum 10 9 12 14 8 Curculionoidea 11 8 11 104 Total 33 30 39 42 105 canopy) showed 39 species, 3 less than recorded in 1986 (50 samples in each year). Species richness of each group except Curculionoidea peaked in midsummer on both canopies, Curculionoidea species richness peaked on date two in upper and lower canopy in both years. Proportion of adult individuals in each insect group Heteroptera and Cicadellidae were the dominant groups on the lower canopy in 1985 and 1986 respectively and the Psyllidae on the upper canopy 1985 (Fig. 3.19). The proportion of individuals in insect groups differed significantly between upper and lower canopy 1985 (Kolmogrov-Smirnoff = 0.49, P < 0.001) and between the lower canopy in 1985 and 1986 (Kolmogrov- Smirnoff = 0.57, P < 0.001). Variation in abundance of Oncopsis between canopies and dates Abundance of tristis and (X subangulata peaked in July and O, flavicornis in May on both upper and lower canopy in 1985 and 1986. The dominant species of the genus, expressed as a proportion of total, varied between years and canopies, with Ch flavicornis dominating throughout 1986 and in May 1985, and tristis dominating on the other two dates in 1985. The abundance of each species differed between canopies and years, and no clear patterns emerged from this limited data set (Table 3 .7 ). 3.4 DISCUSSION Plant species rapidly accumulated onto a ruderal site, and there was some turnover of species during the first year of succession. During the second year of succession, species- richness and leaf area increased over that of ruderal serai stage, although the number of species and leaf area declined through the second year, the surviving ruderal species dying out and typical midsuccessional species becoming established. Plant species-richness increased through the year in early Fig 3. 9 Proportion of adults in each major insect group on upper and lower canopy birch 1985 and lower canopy 1986. % INDIVIDUALS/GROUP 100 40 50 60 70 90 20 80 30 10 0 LOWER CANOPY 1985 UPPER CANOPY 1985 LOWER CANOPY 1986 CANOPY LOWER 1985 CANOPY UPPER 1985 CANOPY LOWER FIG FIG 3.19 HETEROPTERA CICADELLLDAE CANOPY PSYLLLDAE CURCULIONOIDEA 106 1 0 7 Table 3.7 CANOPY LOWER UPPER SPECIES 0. tristis 0. flavicornis 0. subanquiata 0. tristis CL flavicornis 0. subanaulata DATE no P no P no P no P no P no P 10/5/85 0 C 11 1.0 0 0 0 0 19 0.9 2 0. 1 4/7/85 16 0.55 8 0.28 5 0.17 6 0.46 1 0.46 6 0.08 24/7/85 6 0.75 2 0.25 0 0 1 1 .0 0 0 0 0 10/6/86 6 0.032 128 0.69 51 0.28 7/7/86 37 0.26 87 0.61 18 0.13 11/8/86 3 0.25 8 0.75 0 0 Abundance (no) and proportion (P) of total abundance on Qncopsis (Hemiptera: Cicadellidae) in upper and lower canopy 1985 and lower canopy 1986. 108 midsuccession. During this serai stage Leguminosae became less important in terms of leaf area, grasses and Compositae becoming dominant. The early midsuccessional serai stage was the most species-rich, as well as producing most leaf area. Little change occurred from early to late midsuccession, both species richness and leaf area decreased slightly, but the community changed little, being dominated by grasses, though several new species did occur on this site eg Betula, Quercus, Cytisus, Rubus and Chamaerverion. Despite differences between serai stages, significant differences in community attributes between years in stages of the same age were rare. Species richness, the proportion of the community's leaf area provided by each family and dominant species/family were all comparable between years, although leaf area did differ between years on all stages except the ruderal stage. The insect community also changed with succession. Total species richness increased with the age of the serai stage up to late midsuccession, whereas insect abundance was greatest on early succession and least on late succession. The different insect groups showed different patterns of numerical abundance with succession, reflecting primarily the abundance of their respective host plants. Species richness increased through the season on all serai stages, however the pattern of insect abundance varied with serai stage, it increased with date on the ruderal stage, decreased on birch and peaked in midsummer during midsuccession. There was remarkable similarity in species richness and abundance of herbivorous insects between years on sites of the same age. This suggests the existence of some structuring force(s) in these communities. The results of this study agree well with those of other studies on this system (see section 3.1 for references). Southwood, Brown & Reader (1979) report 38 plant species at the end of the first year of succession and the "extinction of primary colonisers" within 18 months, these are similar to results gained here. They report the oC-diversity of plants and insects peaking in early succession. No such analysis was undertaken in this study. Plant species richness was highest in early 109 midsuccession and slightly less in early succession, a consideration of the total leaf area provided by the dominant species (P^) suggests that such a measure of diversity would be greatest in early midsuccession. Although the species richness of insect herbivores was slightly greater in late midsuccession than early and early midsuccession, the abundance of insects in early succession suggests a calculated diversity index would be greatest for this serai stage. It is interesting to note that a consideration of herbivore abundance between serai stages based on equal sample sizes alone, would yield no results either for the total insect community or for any individual group, in agreement with the prediction of Lawton & McNeill (1979). 110 CHAPTER FOUR ABSOLUTE ABUNDANCE OVER A SUCCESS1CNAL GRADIENT 4.1 INTRODUCTION This chapter presents data which aim to test Lawton S McNeill's (1979) hypothesis that the absolute abundance of insect species on early successional plants is greater than that of species on late successional plants. Godfray (1985) tested this hypothesis for leaf-mining insects and found it to be true. However, only one year's data were presented and leaf miners may well differ from external-feeding insects in their relationship with the host plant. The hypothesis was examined in several different ways. Initially, variation in the pattern of absolute abundance with the successional age of the habitat was examined; the null hypothesis being that absolute abundance did not vary between different successional stages. These data were then split by date and the hypothesis tested for individual dates. Then data for non-woody plant-feeding species were compared with those for woody plant feeding species. Here, the null hypothesis was that the absolute abundance of species feeding on non-woody and woody plants was not different. In reality this provided a comparison of the absolute abundance of species feeding on plants defended by qualitative defences with that of species feeding on plants defended by quantitative defences. Again the null hypothesis was tested for the whole sampling period and for separate dates. For major plant families, the absolute abundance of associated species was compared over the whole successional gradient and within each serai stage to test the null hypotheses that absolute abundance of species feeding on these families was consistent within serai stages, and that absolute abundance of species feeding on a particular plant family did not vary between different serai stages. This analysis aimed to provide some insight into the effects of different qualitative defences on the absolute abundance of the associated herbivores, and on the I ll variation in the abundance of herbivores feeding on any one plant family in different successional stages. In the latter case, it is assumed that species feeding on a plant family would be subjected to similar chemical defences although other attributes of the habitat would change according to successional age. Taking a single insect family, the Cicadellidae, a similar analysis compared the absolute abundance of species regarded as specialists on Holcus spp and Agrostis spp (Gramineae) in habitats of different successional age. Here the null hypothesis being tested was that there was no difference in the abundance of species feeding on the same host plant when it occurred on sites of different successional age. Holcus and Agrostis spp were chosen as they were common on the four younger serai stages. A difference in feeding habits of one subfamily of the Cicadellidae, the Typhlocybinae, allowed a unique analysis to be undertaken. The majority of Cicadellidae species feed on phloem, whereas Typhlocybinae feed on mesophyll cells (Claridge & Wilson 1981). Thus, if the concentration and/or nature of defensive compounds varied between phloem and mesophyll, the patterns of abundance of species feeding on these tissues may differ. In this case, the null hypothesis was that the absolute abundance of species of Typhlocybinae varied in a similar manner to that of phloem-feeding Cicadellidae. 4.2 METHODS All field methodology and rationale are discussed in Chapter 2. The null hypotheses outlined in the introduction were tested for dominant herbivore groups: Cicadellidae, Delphacidae, Cercopidae, Psyllidae (Hemiptera: Homoptera); Heteroptera (Hemiptera) and Chrysomelidae, Curculionoidea (Coleoptera). Initially, each group was tested to determine whether it followed a gamma distribution, as described in section 2.3. Since no a posteriori significance tests were employed, overall probability values are presented and patterns of absolute abundance discussed. 112 4.3 RESULTS 4.3.i Distribution of absolute abundance In the Cicadellidae, Delphacidae and Curculionoidea the fitted line of standard deviation passed through the origin and its slope was significantly different from zero, indicating that the data were gamma distributed. However, this was not the case in the Heteroptera, the data for which were analysed by standard non- parametric tests. The same tests were applied to the minor insect groups (Chrysomelidae, Psyllidae and Cercopidae) where the sample sizes were small. The regression analyses are given in Appendix 3. 4 .3 .ii Absolute abundance in relation to serai stage Cicadellidae There was a significant difference in the absolute abundance of Cicadellidae species by host plant family and host plant species between serai stages (abhf, = 104.03, P < 0.001; abhs, F,.. = 139.96, P < 0.001). Generally absolute abundance by l ^ J host plant family and species decreased with increasing successional age (Fig. 4 .1 ). There were significant effects of date and date, sera I stage interaction for absolute abundance by host plant family and species. There were some clear trends in monthly abundance by host plant family and species, with a tendency for the highest abundance to be associated with the ruderal stage (Fig. 4.2) and generally declining with increasing successional age. Although there was no overall seasonal change, abundance by host plant family in July and August was greatest in early succession, and in June and July the value for late succession was marginally higher than for late midsuccession. No value for absolute abundance by host plant species could be calculated for species on the ruderal serai stage during May. By June, however, large numbers of Macrosteles gave values of absolute abundance approaching 1000/m2 of leaf area. Fig 4.1 Mean absolute abundance of Cicadellidae by (a) host plant family, (b) host plant species in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession, L = late succession). Bars are standard errors. 113 FIG 4.1 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY 8 1 SERAL STAGE b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES i ill R E EMS LMS L SERAL STAGE Fig 4.2 Mean absolute abundance of Cicadellidae by (a) host plant family, (b) host plant species from May to September in different serai stages. Bars are standard errors. 114 FIG 4.2 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY MAY JUNE JULY AUG SEPT DATE | [ RUDERAL B | EARLY MIDSUCCESSION FT] EARLY SUCCESSION □ LATE MIDSUCCESSION LATE SUCCESSION 115 Delphacidae In this group there was a significant difference between serai stages in absolute abundance by host plant species (F ^ = 32.82, P < 0.001), but not in absolute abundance by host plant family (F^ = 1.59, P > 0.05), with the greatest abundance occurring in early midsuccession (Fig. 4 .3 ). This was particularly clear when abundance by host plant species was considered and was due mainly to high numbers of Stenocranus minutus in September 1985 which feeds on Dactyl is glomerata. There was a significant effect of date in the fitted model for absolute abundance by host plant family and species, although there was only a significant serai stage.date interaction for absolute abundance by host plant family. Monthly data for absolute abundance by host plant family and host plant species showed few clear trends. As might be expected from the above, the highest values were generally associated with midsuccession, being particularly apparent in September (Fig. 4.4). Curculionoidea This group also showed significant differences in absolute abundance by host plant family and species between serai stages (abhf, F(4 j = 164.59, P < 0.00 ; abhs F(4 287) = 133*04' P < 0.001). In both cases, the lowest abundance was recorded in late succession, with abundance by host plant family and species being greatest in ruderal and midsuccession respectively (Fig. 4.5). There was a significant effect of date and date.serai stage in the fitted model of absolute abundance by host plant family. It was not possible to fit the effect of date for absolute abundance by host plant species as the fitted mean was out of range of the model. A Kruskall-Wallis test showed that there was a significant difference in the distribution of absolute abundance by host plant species between dates (Kruskall-VVallis = 15.227, P < 0.01, n = 294) with abundance being generally higher later in the year. The pattern of absolute abundance by host plant family changed through the year, being greatest in the ruderal stage early in the year, and in midsuccession in September. In contrast, absolute abundance Fig 4.3 Mean absolute abundance of Delphaciaae by (a) host plant family, (b) host plant species in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession). Bars are standard errors. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE FIG 4.3 FIG SERAL STAGE SERAL 116 Fig 4.4 Mean absolute abundance of Delphacidae by (b) host plant family (a) host plant species from May to September in different serai stages. Bars are standard errors. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE ^ LATE SUCCESSION LATE ^ RUDERAL □ FIG4.4 EARLY SUCCESSION EARLY DATE S LATE MIDSUCCESSION LATE S MIDSUCCESSION EARLY P H 117 Fig 4.5 Mean absolute abundance of Curculionoidea by (a) host plant family, (b) host plant species in different serai stages. Bars are standard errors. 118 FIG 4.5 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY e 008 SERAL STAGE b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES 8 5 coI 5 SERAL STAGE Fig 4.6 Mean absolute abundance of Curcuiionoidea by (a) host plant family, (b) host plant species from May to September in different serai stages. Bars are standard errors. 119 FIG 4.6 800 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY 8 700 - 600 - 500 - 5 400 - 300 - COO 200 H 100 - s 0 MAY JUNE JULY AUG SEPT DATE □ RUDERAL EARLY MIDSUCCESSION fvTl EARLY SUCCESSION LATE MIDSUCCESSION LATE SUCCESSION 120 by host plant species was greatest in midsuccession throughout the year (Fig. 4.6). Heteroptera Absolute abundance by host plant family and species was distributed in a significantly different manner between serai stages, showing basically similar patterns for both measures of abundance, with the highest levels being seen in the ruderal and early midsuccessional stages (Fig. 4 .7 ). The pattern of abundance between serai stages was different on each month (Fig 4.8) . No discernible patterns appear from these data. Minor insect groups - Psyllidae, Cercopidae, Chrysomelidae There were significant differences in the distribution of absolute abundance by host plant family of Chrysomelidae and Psyllidae between serai stages, but not of Cercopidae. Absolute abundance by host plant species of Psyllidae and 'Chrysomelidae was distributed differently between serai stages, while there was no difference in the distribution of Cercopidae Absolute abundance of Chrysomelidae was greatest in early succession and declined progressively with increasing successional age (Fig 4 .9 ) . No Chrysomelidae were recorded on birch. The patterns of absolute abundance by host plant family and species of Psyllidae differed markedly: abundance by host plant family was greatest in late midsuccession, whereas abundance by host plant species decreased with increasing successional age (Fig. 4.10). The high abundance by host plant family in late midsuccession was due to the broom-feeding species Arytainilla spartiophila. This species was recorded in D-vac samples and therefore had to be considered as a legume feeder, although it is probably species specific. Exclusion of broom-feeding species from the analysis did not affect the results (Kruskall-Wallis = 18.436, n = 47, P < 0.01). Absolute abundance by host plant family of Cercopidae was greatest in late midsuccession, and abundance by host plant species on the ruderal stage. One individual of Aphrophora alni was recorded from birch in each year (Fig. 4.11). Fig 4.7 Mean absolute abundance of Heteroptera by (a) host plant family, (b) host plant species in different serai stages (abhf, Kruskall- Wallis = 45.602, n = 342, P < 0.0001; abhs, Kruskall-VVallis = 79.522, n = 167, P < 0.001) (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession, L = late succession). 121 FIG 4.7 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY 8 3 3 SERAL STAGE 1200 b)ABSOLUTE ABUNDANCE BY HOST PLANT SP iCEES 1100 - 8 1000 - 900 - 800 - 700 - 600 - 500 - o 400 - co 300 - 200 - 100 - 0 EMS LMS SERAL STAGE Fig 4.8 Mean absolute abundance of Heteroptera by (a) host plant family, (b) host plant species from May to September in different serai stages (abhf, May, Kruskall-Wallis = 3.621, n = 25, P = 0.46; June, Kruskall-Wallis = 6.918, n = 37, P = 0.14; July, Kruskall-Wallis = 16.304, n = 74, P < 0.01; August, Kruskall-Wallis = 30.703, n = 97, P < 0.0001; September, Kruskall-Wallis = 1.9218, n = 90, P = 0.7501 ; abhs, May, Kruskall-Wallis = 2.2319, n = 12, P = 0.5257; June, Kruskall-Wallis = 13.047, n = 22, P < 0.01; July, Kruskall- Wallis = 23.651 , n = 39, P < 0.0001 ; August, Kruskall- Wallis = 36.439, n = 52, P < 0.001); September, Kruskall-Waliis = 11.314, n = 41 , P < 0.05). 1 2 2 FIG 4.8 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY EMS DATE 3500 b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES B 3000 - 2500 - 2000 - 1500 - OCO $ 1000 - 500 - EMS LMS L DATE □ RUDERAL EARLY MIDSUCCESSION [W] EARLY SUCCESSION LATE MIDSUCCESSION 5^1 l a t e s u c c e s s io n Fig 4.9 Mean absolute abundance of Chrysomelidae by (a) host plant family, (b) host plant species in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession). (abhf, Kruskall-Wallis = 9.6894, n = 58, P < 0.05; abhs, Kruskall-Wallis = 12.46, n = 23, P < 0 . 0 0 1 ). ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE 3500 2000 - 2500 00 - 3000 1000 - 1500 200 250 FAMILY PLANT HOST BY ABUNDANCE a)ABSOLUTE 400 300 300 350 0 - 500 100 150 150 50 50 0 - - b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES PLANT HOST BY ABUNDANCE b)ABSOLUTE FIG 4.9 FIG E M LS L LMS EMS E R SERAL STAGE SERAL SERAL STAGE SERAL EMS LMS 3 2 1 Fig 4.10 Mean absolute abundance of Psyllidae by (a) host plant family, (b) host plant species in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession, L = late succession), (abhf, Kruskall-Wallis = 24.9296, n = 38, P < 0.0001; abhs, Kruskall-Wallis = 25.794, n = 48, P < 0.0001). 1 2 4 FIG 4.10 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY R EMS LMS SERAL STAGE Fig 4.11 Mean absolute abundance of Cercopiaae by (a) host plant family, (b) host plant species in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession, L = late succession). (abhf, Kruskall-Wallis = 3.6276, n = 2 0 , P = 0.4587; abhs, Kruskall-Wallis = 11 .2561 , n = 2 0 , P = 0.0238). ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE 0 - - 400 0 - - 300 0 - - 600 - 700 200 0 - - 800 900 0 - - 500 100 -- 0 - - b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES PLANT HOST BY ABUNDANCE b)ABSOLUTE FIG 4.11FIG a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY PLANT HOST BY ABUNDANCE a)ABSOLUTE SERAL STAGE SERAL SERAL STAGE SERAL EMS LMS 1 2 6 4.3.iii Absolute abundance of woody and non-woody plants Absolute abundance by host plant family and species of Cicadellidae, Psyllidae, Heteroptera and Curculionoidea was significantly greater on non-woody plants than on woody plants over the whole season, and generally on each individual date (Figs. 4.12 - 4.15 & Tables 4.1 & 4.2). However, absolute abundance by host plant family of Heteroptera on woody plants in May was greater than that on non-woody plants. This anomaly was due to the abundance of Kleidocerys resedae (Lygaeidae) on birch in that month. 4.3.iv Absolute abundance on major plant families There were significant differences in the absolute abundance of insects associated with major plant families in all groups tested (Curculionotied'. abhf, ^ 7 9 ) = 198.4, P < 0.001, abhs, F^s 273) = 157.74, P < 0.001; Heteroptera: abhf, Kruskall-Wallis = 74.02, n = 321, P < 0.001, abhs, Kruskall-Wallis = 73.767, n = 179, P < 0 . 0 0 1 ; Psyllidae: abhf, Kruskall-Wallis = 31.922, n = 57, P < 0 . 0 0 1 , abhs, Kruskall-Wallis = 25.029, n = 48, P < 0.001). Absolute abundance of Curculionoidea by host plant family was greatest on Cruciferae, while by host species it was greatest on the Leguminosae with Compositae and Cruciferae also having high abundances of insects (Fig. 4.1 A). Abundance of Heteroptera by host plant family was also greatest on Cruciferae, although that by host species was greatest on Compositae (Fig. 4.17). The high value for host species on Compositae reflects the abundance of Tingis ampliata (Tingidae) which feeds on Cirsium arvense. A very different pattern of abundance occurred in the Psyllidae, where absolute abundance by host plant family was greatest on Leguminosae and that by host plant species on Polygonaceae. The latter analysis excluded broom-feeding species, whereas analysis of abundance by host plant family included as legume feeders (Fig. 4.10). Exclusion of broom-feeding species did not alter the results of the analysis by host plant family (Kruskall-Wallis = 16.619, n = 47, P < 0.001). In all cases absolute abundance on birch was lower than on any major plant family. There was a significant family.serai Fig 4.12 Mean absolute abundance of Cicadellidae, Heteroptera and Psyllidae by host plant family and host plant species feeding on woody and non-woody plants. Bars on (a) and (b) are standard errors. Heteroptera, abhf, Kruskall- Wallis = 26.678, n = 308, P < 0.0001 ; abhs, Kruskall-Wailis = 71 .558, n = 166, P < 0.0001 . Psyllidae, abhf, Kruskall-Wailis = 31.096, n = 57, P < 0.001; abhs, Kruskall-Wallis = 24.783, n = 48, P < 0 . 0 0 0 1 . ABSOLUTE ABUNDANCE CD CD 0 - 30 - 40 so 20 10 a) ABSOLUTE ABUNDANCE b) ABSOLUTE ABUNDANCE ABSOLUTE b) ABUNDANCE ABSOLUTE a) c) ABSOLUTE ABUNDANCE ABUNDANCE ABSOLUTE c) - - all FIG 4.12FIG Y OTPATFMILY FAM PLANT HOST BY BY HOST PLANT FAMILY BY HOST PLANT SPECIES PLANT HOST BY FAMILY PLANT HOST BY srecms Y D O O W Y D O O W N O N .BROOM FEEDERS PLANT TYPE PLANT CICADELLIDAE HETEROPTERA PSYLLIDAE 0 « 300 0 ■[ 1 ■ 1 ■ - 400 [ ■ 500 100 200 d) ABSOLUTE ABUNDANCE ABSOLUTE d) f) ABSOLUTE ABUNDANCE ABSOLUTE f) I ■ T '■ I * 0 • - BY HOST PLANT SPECIES PLANT HOST BY BY HOST PLANT SPECIES PLANT HOST BY NONWOODY Y D O O W Y D O O W N O N 7 2 1 Fig 4.13 Mean absolute abundance of Cicadellidae by (a) host plant family, (b) host plant species feeding on woody and non-woody plants from May to September. Bars are standard errors. (abhf, F^ ^ 9 4 ) = 19.33, P < 0.001; abhs, F^ 1?6) = 12.24, P <'0.001). 1 2 8 FIG 4.13 DATE I 1 NONWOOD Y PLANTS CHJ WOODY PLANTS Fig 4.14 Mean absolute abundance of Heteroptera by (a) host plant family, (b) host plant species feeding on woody and non-woody plants from May to September (hatched bars = woody plants, clear bars = non-woody plants) (abhf, May, Kruskall- Wallis = 0.4875, n = 24, P = 0.4851 ; June, Kruskall-Wallis = 2.4217, n = 35, P = 0.1197; July, Kruskall-Wallis = 7.443, n = 74, P < 0 . 0 1 ; August, Kruskall-Wallis = 24.839, n = 100, P < 0.001; September, Kruskall-Wallis = 0.2026, n = 8 8 , P = 0.6592; abhs, May, Kruskall-Wallis = 0.7200, n = 14, P = 0.3961 ; June, Kruskall-Wallis = 7.031, n = 25, P < 0 . 0 1 ; July, Kruskall- Wallis = 14.986, n = 42, P < 0.0001 ; August, Kruskall- Wallis = 28.309, r, = 55, P < 0.001); September, Kruskall-Wallis = 6.079, n = 43, P < 0.05). 1 2 9 FIG 4.14 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES DATE I I NONWOODY PLANTS F T l WOODY PLANTS Fig 4.15 Mean absolute abundance of Psyllidae by (a) host plant family and (b) host plant species feeding on woody and non-woody plants from May to September. Insufficient data to allow statistical comparisons. 1 3 0 FIG 4.15 a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES I INONWOODY PLANTS E D WOODY PLANTS Absolute Abundance Plant Type Lowest SE Mean Highest SE host plant family non-woody 96.99 105.22 114.97 host plant species non-woody 1933.7 2228.6 2629.8 host plant family woody 0.758 0.933 1.214 Table 4.1 Mean absolute abundance and standard errors of Curculionoldea by host plant family and species feeding on woody and non-woody plants, abhf, ^9 4 ) = 150.26, P < 0.001); abhs, F(1 282) = 430.97, P < 0.001. Table 4.2 Mean absolute abundance and standard errors of Curcuiionoidea by host plant family and species feeding on woody and non-woody plants on each date. Data pooled over two years, (abhf, date F(4 , 3 9 4 ) = 't'K0It- P < 0 -0 0 1 - data.plant F{a = 1 .5006, P > 0.05; abhs, date 282) = 53.23, P < 0.001, date.plant = ^*^06, P > 0.05. Table U.2 (a) Absolute abundance by host plant family Non-woody Plants Woody Plants Date Lowest SE Mean Highest SE Lowest SE Mean Highest SE May 101.0 121.9 153.7 0.079 0.136 0.449 June 39.02 45.97 55.96 0.057 0.093 0.244 July 38.64 45.72 55.97 0.047 0.082 0.306 August 162.6 188.2 223.3 0.G35 0.060 0.226 September 165.3 193.3 232.5 0.040 0.089 0.042 (b) Absolute abundance by host plant species Non-woody Plants Woody Plants Date Lowest SE Mean Highest SE Lowest SE Mean Highest SL \lay 2466.99 3430.53 5630.63 0.079 0.136 0.449 June 363.57 492.61 763.65 0.057 0.093 0.244 July 2744.55 3732.74 5832.90 0.047 0.082 0.306 August 4044.3 5243.8 7454.89 0.035 0.060 0.226 September 1557.87 2076.4 3112.35 0.040 0.089 0.042 132 Fig 4.16 Mean absolute abundance of Curculionoidea by (a) host plant family, (b) host plant species on major plant families [data pooled over all dates and all serai stages) (L = Leguminosae, Co = Compositae, Cr = Cruciferae, Po = Poiygonaceae, B = birch). Bars are standard errors. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE 00 - 3000 2200 - 2400 - 2600 - 2800 2000 1000 1200 - 1400 60 - 1600 - 1800 5200 5200 0 - 400 200 0 - 600 - 800 b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES PLANT HOST BY ABUNDANCE b)ABSOLUTE -- 0 - - - - - t a)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY PLANT HOST BY ABUNDANCE a)ABSOLUTE FIG 4.16 FIG ------PLANT FAMILY PLANT T N FAMILY A L P ” T Cr T" Po Birch 3 3 1 Fig 4.17 Mean absolute abundance of Heteroptera by (a) host plant family, (b) host plant species on major plant families (data pooled over all dates and all serai stages) (L = Leguminosae, Co = Compositae, Cr = Cruciferae, Po = Polygonaceae, B = birch, Cr = Gramineae, MF = minor families, G = generalist (species associated with more than one plant family)). ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE b)ABSOLUTE ABUNDANCE B Y HOST PLANT SPECIES PLANT HOST Y B ABUNDANCE b)ABSOLUTE FIG 4.17 FIG YBOUEAUDNEBY OTPATFMILY FAM PLANT HOST Y B ABUNDANCE aYABSOLUTE L o r o r F B MF Gr Po Cr Co L G PLANT FAMILY PLANT LN A ILY FAM PLANT 4 3 1 Fig 4.18 Mean absolute abundance of Psyllidae by (a) host plant family, (b) host plant species on major plant families. Data pooled over all dates and serai stages. (L = Leguminosae, Co = Compositae, Po = Polygonaceae, MF = minor families, B = birch). ABSOLUTE ABUNDNACE ABSOLUTE ABUNDNACE 1000 0 - 700 0 - 400 - 600 - 900 0 - 800 200 - 300 - 500 1 00 - 100 - 0 - b)ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES PLANT HOST BY ABUNDANCE b)ABSOLUTE FIG 4.18 FIG Co T PLANT FAMILY PLANT PLANT FAMILY PLANT oMF Po T 5 3 1 Fig 4.19 Mean absolute abundance of Heteroptera by (a) host plant family, (b) host plant species on herbs, grass and birch (data pooled over all dates and serai stages). ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE 20 0 - 30 - 40 50 10 - - a)ABSOLUTE ABUNDANCE BY HOSTFAMILY PLANT BY ABUNDANCE a)ABSOLUTE FIG 4.19 FIG b) ABSOLUTE ABUNDANCE BY HOST PLANT SPECIES HOSTPLANT BY b) ABUNDANCE ABSOLUTE S S A R G S B R E H PLANT TYPE PLANT PLANT TYPE PLANT BIRCH 136 1 3 7 stage interaction for Curculionoidea for abundance by host plant family and species (abhf, = 14.49, P < 0.001, abhs F(9 273) = 42,65' P < 0 . 0 0 1 ). In the younger serai stages absolute abundance by host plant family of Cruciferae-feeding species was the greatest, with that of Compositae-feeding species being the least (Table 4.3). In addition, there were different patterns of absolute abundance in the two midsuccessional serai stages, with the Polygonaceae-feeding species being dominant in early midsuccession and the Leguminosae-feeding species in late midsuccession. Apart from the ruderal stage where Cruciferae- feeding species were again dominant, absolute abundance by host plant species showed different patterns; with that of Leguminosae-feeding species being the greatest throughout the other serai stages, and Polygonaceae-feeding species being the least. Absolute abundance by host plant family of Heteroptera- feeding on major plant families differed significantly in distribution in all serai stages except for early succession where insects associated with • Leguminosae were dominant (Table 4.4). In contrast the distribution of absolute abundance by host plant species between plant families was only significantly different on the ruderal serai stage. Cruciferae-feeding species were only found on the ruderal stage, whereas Leguminosae- feeding species reached their highest abundance in early succession. In both midsuccessional stages, however, Polygonaceae-feeding species showed the greatest absolute abundance and were particularly marked in EMS. Analysis by host plant species revealed different patterns, with Leguminosae-feeding species dominant in late midsuccession, and Compositae-feeding species in early midsuccession. Only Leguminosae-feeding species showed significant differences in the distribution of absolute abundance by host plant family between serai stages, and only grass-feeding species by host plant species. The Heteroptera occurred at all successional stages and demonstrated that species feeding on herbaceous dicotyledons had greater absolute abundances than those feeding on grass and these in turn had greater absolute abundances than birch-feeding species (Fig. 4.IS) (abhf, Kruskall-VVallis = 30.0142, P < 0.0001 , abhs, Kruskall-Wallis = 61.33, P < 0.0001 ). 138 Table 9.3a plant family Crucitcrae Polygonaceae B irch SERAI. STAGE Lcguminosae Compost tae mean '10.5 13.2 3 31.9 37.1 Kuderal SB 32.6 53.5 5.26 27.3298.2 998.8 25.15 70.62 mean 7'1.8 u.7 915.2 198.8 E arly succession SE 6 <1.8 88.6 1.9 0.8 298.9 126.9135.2 375.9 mean 86.5 2.11 11 .92 192.8 E a rly mid midsuccession SE 7«.2 103.7 1.2 8.9 9.76 25.3 85.95 939.8 mean 137.9 16.72 - 22.1 Late mid midsuccession SE 117.0 167.8 8.96 125.5 13.67 56.9 mean _ -- 0.93 Late succession SE 0.76 1.197 Table 9.3b p lan t family SERAL STAGE Leguminosae Compositae Cruciferae Polygonaceae Minor families mean 93.9 569.9 6218.9 38.31 76.7 Ruderal SE 27.9 96.9 307.7 3831.9 3978.6 19232.9 2133 187.9 96.7 215.9 mean 9616.8 953.3 399 52.99 392.8 E a rly succession SE 3693.7 6298.8 960.8 1800.1 196.3 9950 27.61 657.9 183.0 269S.9 mean 2932.5 180.5 155.8 19.16 29.5 E a rly mid midsuccession SE 1859.1 3517.«»ISO.9 299.9 99.0 307.9 9.355 33.21 9.8 97.9 mean 6006 6.095 17.27 Late mid midsucccssion SE 9319.1 9881.9 1.87 13.52 5.50 39.5 Table 4 .3 Mean absolute abundance and standard errors of Curculionoidea by (a) host plant family, (b) host plant species on major plant families in different serai stages. See text for significance levels. 1 3 9 Significance Serai Sta KW = 10.79 n = IS K u ileral 0.95; 13.38 612.9 30.35 2.972 5.936 - P < 0.05 KW = 1.3969 n = 23 E arly 130.80 - | - 8.53 3.860 11.250 - P = 0.51 succession ! KW = 29.791 9 n = 82 Early miii 45.46 j1 - j i 137.70 0.880 59.10 - j P < 0.0001 succession KW = 11 .0209 n = 71 Late mid 19.20 3.51 - 22.76 7.30 5.12 - P < O.OS succession 9.796 ' Late -- 2.08 succession i KW = 19.086KW =1.33 KW = 5.081KW = 9.88 KW = 6.97 Siyitiiicauce n = 29 - n = 15 n = 90 it = 123 - P < 0.01 P = 0.298 P = 0.160 P = 0.299 P = 0.166 Table 9.9a Scr.il Stage Legummosaei Compost tae C ruciferae Polygonaceae Craminae Generalist Uirclt Significance j KW = 29.01 Kudcral j 7,079 - 182.9 209.2 0.6099 299.5 - n = 10 1 P < 0.00 1 KW = 3.19 E a rly j 63.39 - 97.19 160.6 . - n = 59 succession ! ! P = 0.53 KW = 1.703 E a rly mid j 197.8 9708.0 j - 209.6 259.7 - n s 38 succession 1 P = 0.636 KW = 3.908 Late mid J 57.98 95.29 - - 98.89 96.69 - n = 92 succession ij P S 0.99 Late “ * " - - 2.00 9.796 - succession KW = 9.69 KW = 2.37 KW = 12.7 KW = 16.22 Significance n = ie n = 17 - - n = 98 n = 37 -- P = 0.20 P = 0.12 P < 0.01 P < 0.05 Table 9.9b Table 4.4 Mean absolute abundance of Heteroptera by (a) host plant family, (b) host plant species on major plant families in different serai stages. Differences between plant families and serai stages tested by Kruskall-YVallis one way ANOVA. Fig 4.20 Mean absolute abundance of Cicadellidae by host plant species feeding on (a) Agrostis and (b) Holcus (Gramineae) in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession). Bars are standard errors. 140 FIG 4.20 alAGROSTIS spp w 1000 y 900 < 800 § 700 9 600 500 H-JR 400 o 300 200 100 0 E EMS LMS SERAL STAGE blHOLCUS spp SERAL STAGE 1 4 1 4.3.v Absolute abundance and species composition of Cicadellidae feeding on Holcus and Agrostis spp There was a significant difference in the absolute abundance of species feeding on both Agrostis and Holcus between serai stages (Agrostis: 29 ) = 34.55, P < 0 . 001; Holcus: 57) = 10.14, P < 0.001). In both grass species the greatest insect absolute abundance occurred on the ruderal stage, with the lowest value of Agrostis-feeding species occurring in early midsuccession and in early succession for Holcus-feeding species (Fig. 4.20). Three of the four species of Cicadellidae which fed specifically on Agrostis spp and three of the six Hoicus spp specialists were recorded on the four younger serai stages (Table 4.4). The abundance of these species varied between serai stages. There was no discernible pattern in the abundance of Agrostis feeding species, although the Holcus-feeding species tended to increase in abundance with increasing successional age (Table 4.5). 4.3.vi Comparison of absolute abundance of phloem and mesophyll-feeding Cicadellidae Different patterns of absolute abundance emerged when phloem and mesophyll-feeding Cicadellidae were considered separately (Fig. 4.21). Absolute abundance by host plant family of phloem-feeding Cicadellidae was greatest on the ruderal stage and least in late midsuccession, while that of Typhlocybinae combined to decrease with increasing successional age. There were significant differences in absolute abundance by host plant family and species for Typhlocybinae and phloem-feeding Cicadellidae (phloem-feeding Cicadellidae: abhf, F, (4,315) 57.57, P < 0.001; abhs, FffI(4,116) 11C, = 35.34, P < 0.001; Typhlocybinae: abhf, F( (4,64) = 582.8, P < 0.001; abhs F (4,74) 162.9, P < 0.001). Absolute abundance by host plant species of Typhlocybinae and phloem-feeding Cicadellidae decreased with increasing successional age, although the abundance of the latter in early midsuccession was lower than that in late midsuccession. 1 4 2 Serai Stage Species Ruderal Early Early mid Late mi a Arthaldeus pascuellus 0 12 1 0 Psammotettix confinis 18 69 3 0 Doratura sylata 1 1 12 Macrosteles laevis 224 84 2 1 TOTAL 243 166 8 13 Table 4.5a Serai Stage Species Ruderai Early Early mid Late mid Adarrus ocellaris 45 139 548 243 Cicadula persimilis 0 1 0 1 Dipiocoienus abdominalis 0 0 1 0 Recilia coronifera 6 6 59 219 Aphrodes albifrons 0 0 1 7 Aphrodes bicinctus 2 6 23 21 TOTAL 53 152 631 491 Table 4.5b Table 4.5 Adult abundance of Cicadellidae specialising on (a) Agrostis spp,. (b) Holcus spp in different serai stages. Fig 4.21 Mean absolute abundance by (a) host plant family of Cicadellidae excluding Typhlocybinae (b) host plant species of Cicadellidae excluding Typhlocybinae (c) host plant family of Typhlocybinae and (d) host plant species of Typhlocybinae in different serai stages (R = ruderal, E = early succession, EMS = early midsuccession, LMS = late midsuccession, L = late succession). Bars are standard errors. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE 700 300 400 300 600 0 0 2 100 ------a)ABSOLUTE ABUNDANCE ABUNDANCE a)ABSOLUTE BY HOST PLANT FAMILY PLANT HOST BY BY HOST PLANT FAMILY PLANT HOST BY c)ABSOLUTE ABUNDANCE ABUNDANCE c)ABSOLUTE FIG 4.21 FIG L 3 M L S M R B R L 3 M L S M C B R L S M L S M C B R i * o • * • i i CICADELLIDAE(phloem feeding species) feeding CICADELLIDAE(phloem TYPHLOCYBINAE SERAL STAGE SERAL SERAL STAGE SERAL 41200 41200 41000 41000 41100 40900 40900 1000 0 0 2 300 600 300 300 400 300 400 300 600 600 700 900 900 200 200 100 100 100 0 BY HOST PLANT SPECIES PLANT HOST BY b)ABSOLUTE ABUNDANCE ABUNDANCE b)ABSOLUTE d)ABSOLUTE ABUNDANCE ABUNDANCE d)ABSOLUTE BY HOST PLANT SPECIES PLANT HOST BY i 1 3 4 1 1 4 4 Absolute abundance by host plant family and species of non-woody plant feeders was greater than that of woody plant feeders regardless of their mode of feeding (Fig. 4.22). However, there was no significant difference between the absolute abundance by host plant family of woody and non-woody plant phloem-feeding species (F^ = 0.5009, P > 0.05), whereas all other comparisons were significantly different (Typhlocybinae: abhf, F^ = 110.3, P < 0.001; phloem feeding Cicadellidae F^ = 8.097, P < 0.05). 4.4 DISCUSSION The absolute abundance of insect groups varied between serai stages. However, the two measures of abundance (by host plant family and host plant species) did not always show consistent trends. Absolute abundance by host plant family of Cicadellidae, Heteroptera and Curculionoidea was greatest on the ruderal stage and least on birch, but only the former decreased progressively with increasing successional age, while for abundance by host plant species the Psyllidae also showed this trend. The lack of consistent trends in absolute abundance throughout the season was not unexpected since gross changes in host plant chemistry are known to occur (Feeny, 1970; Lawton, 1976; Thompson & Price, 1977). However, despite seasonal fluctuations, absolute abundance was generally lowest in late succession and highest during the early years of succession. A progressive decline in absolute abundance with increasing successional age of the habitat is unlikely for two reasons; firstly, there were several plant species and genera common to two or more serai stages (section 3.3). Such species would be likely to be consistent in their chemical defences and their relationship with herbivores between stages. Secondly, certain insect species may be particularly abundant on a single site during a single year. This could occur as a result of a decrease in the defensive chemistry of the plant, an increase in nutritional value of a plant/patch, favourable microenvironment for reproduction and survival, decreased predation, or alternatively the species could be in a boom part of a cycle. Fig 4.22 Mean absolute abundance by (a) host plant family of Cicadellidae excluding Typhlocybinae, (b) host plant species of Cicadellidae excluding Typhlocybinae, (c) host plant family of Typhlocybinae and (d) host plant species of Typhlocybinae on woody and non-woody plants (hatched columns = woody, clear columns = non-woody plants). Bars are standard errors. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE )BOUEAUDNE b)ABSOLUTE ABUNDANCE BY HOST PLANT FAMILY a)ABSOLUTE ABUNDANCE FIG 4.22 FIG BY BY HOST PLANT FAMILY c)ABSOLUTE ABUNDANCE CICADELLIDAE(phloem feeding species) feeding CICADELLIDAE(phloem TYPHLOCYBINAE PLANT TYPE PLANT PLANT TYPE PLANT y b d)ABSOLUTE ABUNDANCE BY BY HOST PLANT SPECIES HOST PLANT SPECIES 5 4 1 146 Clearly a larger data set would clarify such anomalies. Among the herbivorous groups examined, it was perhaps the Heteroptera which were least likely to show any pattern, since within this group species show a range of trophic relations, including folivorous species, seed and flower-feeders and partial predators (Southwood S Leston, 1959; V/etton & Gibson, 1987). Such potential for polyphagy or omnivory would tend to make patterns of abundance based on plant chemistry less consistent that in totally folivorous groups. Lawton £ McNeill's (1979) hypothesis predicts a difference between the absolute abundance of herbivores of typical early successional plants and that of herbivores of late successional plants, ie trees. Effectively, this is a difference in the absolute abundance of herbivores feeding on plants defended by qualitative and those defended by quantitative defences. V/hen tested it was seen that with one exception, there were significantly more herbivores/m2 leaf area on non-woody plants than on woody plants. The group which diverged from this trend was the phloem-feeding Cicadellidae, which when analysed by host plant family were not significantly more abundant on non-woody plants than woody plants. This result is of interest, since it may reflect a greater effect of host plant on mesophyll- feeding Cicadellidae than on phloem-feeding Cicadellidae. Consideration of the biochemistry of tannins lends support to this hypothesis. Condensed tannin is probably stored as a complex in structural protein and is usually associated with organelles or cells walls, while hydrolysable tannin is usually stored in vacuoles. Membranes tend to be non-permeable to tannins due to their size and relative polarity (Zucker, 1983) so, unless active transport occurs (which is unlikely), the concentration of tannin in phloem would probably be very low or even non-existent. It is difficult to explain the difference in the pattern of absolute abundance with serai stage observed in the Cicadellidae and Delphacidae, since the two groups are close taxonomically and ecologically. However, some ideas are explored in Chapter 5. Grasses, the food of both groups, are defended by a mixture of qualitative and quantitative defences: alkaloids are present in 1 4 7 some species when young (Harley & Thorsteinson, 1967) and silica may be present in older individuals (Crawley, 1983). From the standpoint of a herbivorous insect, grasses may be as apparent (sensu Feeny) as trees, since clones of stoloniferous grasses eg. Holcus mollis may cover many acres and be of considerable age (Harper, 1977). Grasses therefore seem to lie in the middle of the qualitative-quantitative defence spectrum but are not considered in detail by Feeny (1976), Rhoades & Cates (1976) or Lawton & McNeill (1979). In the present study, the Heteroptera enabled this to be considered. The abundance of Heteroptera decreased in the order herbaceous dicotyledons > grass > trees with the absolute abundance of grass-feeding species being closer to that of tree-feeding species than to that of species feeding on herbaceous dicotyledon plants. These results support the position of grasses as intermediate between qualitatively-defended early successional plants and quantitatively-defended late successional plants. There were differences in absolute abundance of grass-feeding species between serai stages, with the absolute abundance of both specialist Holcus and Acjrostis-feeding species being greatest in the ruderal stage. This difference could be due to reduced predation, to the reduced action of chemical defences, or to differences in the nutritional status of the soil/plant. After disturbance and the commencement of succession, soil nutrients may change in several ways. In nutrient-rich soils, nutrients may not accumulate during succession and may actually be lost (Aarrssen & Turkington, 1985) while in nutrient-poor soils, such as those at Silwood Park, there may be a period of nutrient accumulation (Inovxye, Huntley, Tilman, Tester, Stillwell 5 Zinnel, 1987). Also, in tropical forests where soils are generally extremely nutrient-poor, there is a pulse of nutrients immediately after disturbance which rapidly falls away (Vitousek 6 Walker, 1987). Nitrogen is generally the limiting nutrient for plant growth (Tilman, 1988) and both total and available nitrogen have been reported to increase during succession (Til man, 1988). Although no measures of nutrient status of the soil have been made in the successional sere at Silwood Park, the small spatial scale and relatively small age differences 1 4 8 between stages make it unlikely that there would be substantial differences in the concentration of nutrients. However, the lower levels of plant competition in the ruderal stage may increase the amount of available nutrients. Such a suggestion would fit with the pattern of absolute abundance of specialist feeders on Holcus and Agrostis, since these were very abundant on the ruderal sites with little difference in abundance on the older sites. Although this thesis generally assumes ecological and evolutionary pressures to be equal on all species within a group, it must be realised that the absolute abundance of species may be subject to many different selective pressures. The major conclusion from the analysis of absolute abundance of herbivore species feeding on plant families at different serai stages is simply that qualitative defences cannot be lumped together and assumed to have the same effect on herbivore populations. The differences in absolute abundance of species feeding on dicotyledonous plant families, both within and between serai stages, stresses the different relationship between herbivore and host plant, and how these can be affected by a multiplicity of factors. In conclusion, it seems that Lawton & McNeill (1979) were correct, and that the absolute abundance of herbivore species feeding on plants defended by qualitative defences is greater than that of species feeding on plants defended by quantitative defences. However, it is more difficult to establish why absolute abundance varies between different serai stages of predominantly qualitatively defended plants. Variation in predation pressures and soil nutrient availability is certainly worthy of further investigation. 1 4 9 CHAPTER FIVE VEGETATION STRUCTURE AND THE INSECT COMMUNITY 5.1 INTRODUCTION The main aim of this chapter is to investigate the effect and importance of small-scale differences in community attributes on insect herbivore abundance and species richness. Particular attention has been focussed on the importance of plant species and community architecture or structure. Plant architecture has two major attributes, the size and/or spread of structures and their variety. Since only the size and spread of plant structures is considered here, it is preferable to refer to plant structure. These attributes of the community are best measured by a foliage height index (Lawton, 1978), such as that utilised by Murdoch, Evans S Peterson (1972) and Stinson & Brown (1983). Plant architecture is important as it permits greater niche diversification for, inter alia competitor and predator avoidance, and hence architecturally more complex plants should support more herbivore species (Lawton, 1983). This has been shown to be the case by Lawton & Schroder (1977), as herbivore species richness on different plant growth forms declines in the order, trees > weeds and other annuals > monocotyledons (excluding grasses). Many studies have considered changes in architecture on herbivore species richness, and found good positive correlations between plant architecture and herbivore species richness (Cameron, 1972; Denno, 1977, Lawton, 1978; Morris, 1971, 1979, 1981; Morris & Lakhani, 1979; Murdoch et al, 1972; Niemela & Haukioja, 1982; Niemela, Tahvanainen, Sorjo\oen, Hokkaren & Neuvonnen, 1982; Stinson & Brown, 1983). These studies tend to fall into one of two categories; either they are based on literature reviews of plant attributes and of herbivore host plant specificity and occurrence (both of which may be subject to error), or they utilise seasonal changes in plant architecture as "natural experiments". Such methodology does not take into account the many other seasonally fluctuating 1 5 0 factors which may affect herbivore abundance and diversity, e.g. plant nutritional quality, plant chemical defences, leaf toughness, temperature, predator and competitor abundance. Few studies have considered the effect of small scale variation in plant architecture on herbivore abundance and diversity. Despite demonstrating that the species diversity of Homoptera correlated with foliage height diversity and mean vegetation height between fields, Murdoch et a[ (1972) found only poor correlation between plant diversity and structure and insect diversity in small scale quadrats within a field. Two other studies (Cornell 8 Washburn, 1979 and Fowler, 1985) report little difference in herbivore species diversity between architecturally simple and complex systems. The aim of this chapter is two-fold: firstly to examine the importance of various community attributes, including host plant structure and the structure of the surrounding vegetation, on the abundance and species richness of insect herbivores on a single date in each of four serai stages, and to compare patterns between them, and secondly to test the null hypothesis that the absolute abundance of individual species does not differ between serai stages. If absolute abundance is determined solely by the relationship between the herbivore and the chemical defences of its host plant, then there should be no differences in absolute abundance between serai stages. However, if this relationship is modified in some way by the surrounding community then differences in absolute abundance could occur. Work at the species level allowed some comparisons between Cicadellidae and Delphacidae to be made in an attempt to understand their different relationships with their host plants. Finally, this chapter tests the null hypothesis thab absolute abundance of birch-feeding insects does not vary between upper and lower canopy. Studies by Claridge, Edington & Murphy (1968) and Fowler (1985) conclude there is no overall vertical stratification of species in a birch canopy, although individual species may show preferences for different parts of the canopy. Fowler (1985) demonstrated that Operophtera (Lepidoptera: Geometridae), Epinotia (Lepidoptera: Tortricidae) and Euceraphis spp (Hemiptera: Aphididae) showed a significant preference for upper canopy, whereas six other species, Oncopsis spp (Homoptera: Cicadellidae), Apoch eima 1 5 1 pilosaria, Erannis defoliaria, Agriopis spp (Lepidoptera: Geometridae), Coleopihera serratella (Lepidoptera: Coleophoridae) and Eriocrania spp (Lepidoptera: Eriocraniidae) showed no preference. In a comparable study on beech, Phillipson & Thompson (1983) recorded greatest herbivore damage in the lower canopy, where 75-85% of leaves were attacked and accounted for 35% of the total herbivore damage. 5.2 METHODS General field methods are described in Chapter 2. The abundance of adults of the most commonly occurring species in each subplot was correlated with several community attributes. Only species for which more than 15 individuals were recorded from a single site during the August 1985 sample were included in the analysis. August was selected since peak insect abundance and species richness generally occurred during that month (see section 3.4). The late successional site was omitted from this analysis. The community attributes considered were host plant species leaf area, host plant family leaf area, total leaf area of subplot, mean height of host plant species, mean height of host plant family, overall mean height of subplot, number of leaves in host plant family, number of leaves in subplot and oC-diversity of plants in subplot. The measurement of these parameters is described in Chapter 2. Data for the mean height index were gathered by using a point quadrat frame 50cm in length with 10 equally spaced pins. Each pin (3mm diameter) was marked at intervals of 2, 4 , 6, 8 and 10cm and then at successive 5cm intervals from soil level to the maximum height of the vegetation. A single frame was randomly placed in each subplot in August 1985. A measure of vegetation height, based on a weighted mean height of touches, was derived from: N i = 1 where h = midpoint of class 'i1, n = number of touches of height class 'i1, N = number of height classes represented in the sample. 1 5 2 The non-normality of the insect abundance data necessitated the use of Spearman's Rank Correlation. This non-normality of the data violated several assumptions of multiple regression techniques, which would otherwise have been utilised. Since it was likely that many of the community attributes were themselves related, Spearman's Rank Correlation was used to test for any autocorrelation between the variables which could affect interpretation of the results. The species richness of Cicadellidae, Delphacidae, Heteroptera and Curculionoidea in each subplot during August 1985 was also correlated with the community attributes. A final analysis considered the relationship of species richness of each insect group with that of each other insect group. The analyses of insect species richness utilised Pearson's Correlation Coefficient. In order to test for differences in absolute abundance of a single species between serai stages the most abundant species occurring on all of the four younger serai stages were analysed using GLIM, as described in Chapter 2. Patterns of adult abundance between serai stages were compared with those of absolute abundance. Analyses comparing these parameters statistically would not be valid as adult abundance is inherent in the calculation of absolute abundance. Data was pooled over two years and dates. Differences in absolute abundance of birch herbivores between the upper and lower birch canopy were assessed by GLIM where appropriate, however, Kruskall-Wallis non-parametric one-way ANOVA was utilised for the analysis of canopy differences of Heteroptera, and for single species responses. The absolute abundance of species of Cicadellidae overwintering as eggs and those overwintering as adults were compared using GLIM (data on overwintering strategies was taken from VValoff, 1981). 153 5.3 RESULTS 5.3.i Vegetation structure and small scale variation in herbivore abundance and species richness Weevil abundance on the ruderal site was positively correlated with most community attributes, with host plant leaf area and mean height being particularly important. Abundance of Cassida vittata was also positively related to these two attributes, although abundance of Auchenorrhyncha displayed more varied responses to community attributes (Table 5.1a). Host plant leaf area was also strongly correlated with weevil abundance in early succession (Table 5.1b). Mean height was important to Apion dichroum, but not to A^ apricans or A^ hookeri. However, total leaf number, total leaf area and plant oC-diversity were generally poorly correlated with weevil abundance, and no single community attribute was significantly related to the abundance of the Auchenorrhyncha species considered. The abundance of Apion species in early midsuccession was also positively correlated with host plant species area, mean height and leaf number. However, abundance of the generalist weevil, Sitona lineatus was not significantly correlated with any community attribute. As on the younger sites, there appeared to be little consistency in factors influencing Auchenorrhyncha abundance (Table 5.1c). The abundance of Dicranotropis hamata and to a lesser extent Adarrus ocellaris was significantly correlated with host plant leaf area, and Zyginidia scutellaris with host plant mean height. On the late midsuccessional site no weevil species was sufficiently abundant to permit analysis, although several Heteroptera species were. Factors affecting Heteroptera abundance were generally similar to those for the weevils on the younger sites, in that abundance was significantly correlated with host plant area and mean height, with the exception of the grass-feeding species Stenodema laevigatum (Table 5.1d). The abundance of Auchenorrhyncha showed similar patterns to those for other sites. The abundance of A^ ocel laris and Stenocranus minutus was significantly correlated with host plant leaf area, while host plant mean height was also important for S^_ minutus. Euscelis incisus showed significant correlation with leaf number Table 5.1 Spearman's Rank Correlation Coefficients for adult abundance of common herbivore species in subplots with several measured vegetation attributes during August 1985 in (a) ruderal, (b) early succession, (c) early midsuccession, (d) late succession. * P < 0.05, ** P < 0.01, *** P < 0.001. SPECIES COMMUNITY ATTRIBUTES Host plant Host plant Total Host plant Host plant Subplot Number of Number of Plant species family subplot species mean family mean mean leaves in leaves in leaf area leaf area leaf area height height height host family subplot ©(diversity Apion 0.05095 0.5183 0.5032 0.4872 0.4645 0.4781 0.4347 0.3979 0.2742 assimile *** *** *** ** ** + * ** * Apion 0.3486 0.3716 0.2747 0.4974 0.45 0.3322 0.3241 0.3819 0.3473 apricans ♦ * *+* ** * * * * Cassida 0.3548 - 0.0051 0.4427 - 0.0179 - 0.1111 -0.0803 vittata * ** Macrosteles -0.2777 0.0678 0.3670 0.0852 0.2754 0.3172 -0.2461 0.3497 0.0212 laevis * * * Zyginidia 0.1126 0.2024 -0.1423 0.1098 0.0612 0.0114 0.4341 -0.2381 0.0754 scutellaris ** Dicranotropis 0.1607 -0.0056 0.1780 0.0302 0.2763 0.3507 0.0359 0.2888 -0.1898 hamata * 4 5 1 Table 5.1a (Ruderal) COMMUNITY ATTRIBUTES Host plant Host plant Total Host plant Host plant Subplot Number of Number of Plant species family subplot species mean family mean mean leaves in leaves in leaf area leaf area leaf area height height height host family subplot oUiiversity Apion 0.5306 0.5014 0.3802 0.3673 0.4123 0.3656 0.4121 0.2785 0.1695 assimile *** * * ** * ** Apion 0.7232 0.5609 0.2811 0.5363 0.3984 0.2107 0.4955 0.2437 0.2947 #** apricans *** *** *+ * Apion 0.4487 0.4020 0.1390 0.3403 0.3723 0.5284 0.4087 0.2214 0.1914 dichroum ** ** * ' * *** Apion 0.7382 - 0.3157 0.6629 - 0.2642 0.3587 0.3223 0.4254 hookeri *** * *#* * * ** Jav.esella - 0.0170 0.0581 - 0.0219 0.0275 -0.0975 -0.0686 0.1939 pellucida Euscel \s 0.0245 0.1358 “ 0.813 0.1788 -0.0538 -0.0133 -0.0542 incisus 155 Table 5.1b (Early succession) SPECIES COMMUNITY ATTRIBUTES Host plant Host plant Total Host plant Host plant Subplot Number of Number of Plant species family subplot species mean family mean mean leaves in leaves in leaf area leaf area leaf area height height height host family subplot cCdiversity Apion 0.6091 0.3737 0.1677 0.6386 - 0.0309 0.4178 0.3569 0.3750 assimile *** ** *♦ « *♦ Apion 0.7453 0.4163 0.1016 0.7133 - 0.1291 0.3862 0.2362 0.2268 apricans ** **+ Sitona - 0.2307 -0.0900 -- 0.1577 0.2119 0.0741 0.2343 lineatus Adarrus 0.3617 0.2301 0.1638 -0.0945 -0.022 0.0122 0.0445 -0.1040 0.0873 ocellarus * Mocydiopsis - -0.0671 -0.3413 - -0.0853 0.0661 -0.0795 -0.0789 0.1352 parvicauda Recilia -0.0402 -0.0203 -0.1803 0.1271 0.1683 0.0539 0.3557 0.1481 -0.1624 coronifera Zyginidia 0.1399 0.2206 0.2053 0.5031 0.2711 0.1588 0.0588 0.063 0.2974 scutellaris *+* Dicranotropis 0.4496 0.4715 0.3341 0.0149 0.0183 0.1983 0.2938 -0.0295 -0.0753 hamata ** +* * Javesella - 0.1226 0.1124 - 0.2619 0.3434 0.2648 0.3159 0.3599 pellucida * * + 156 Table 5.1c (Early midsuccession) SPECIES COMMUNITY ATTRIBUTES Host plant Host plant Total Host plant Host plant Subplot Number of Number of Plant species family subplot species mean family mean mean leaves in leaves in leaf area leaf area leaf area height height height host family subplot ^diversity Stenodema - 0.1982 0.2415 - 0.0661 0.0804 0.1721 0.1787 0.1376 laevigatum Phytocoris - 0.3372 0.3792 - 0.4561 0.4989 0.3051 0.3092 0.1458 varipies * * ** **+ * ♦ Ishnodemus 0.7329 0.2385 0.2983 0.5022 0.3947 0.3864 0.097 0.0961 0.2022 +** # sabuleti *+* ** ** Tingis 0.5424 - 0.2732 0.5357 - 0.2489 0.4456 0.2269 0.1688 ampliata ** Zyginidla 0.045 0.043 0.2146 0.2490 0.0061 0.0293 0.1695 0.2991 -0.0473 scutellaris * Stenocranus 0.428 0.1976 0.1159 0.4666 0.2093 0.2539 0.1899 0.2254 0.1719 minutus ** ** Dicranotropis 0.1237 0.032 0.0525 0.0955 -0.0238 0.1275 0.2481 0.1848 0.0722 hamata Adarrus 0.333 0.248 0.2880 0.1714 0.0247 0.0638 0.1894 0.2724 0.1423 ocellarus * Eusceli‘s - 0.2498 0.3005 - 0.2171 0.2221 0.3151 0.3611 0.2364 incisus * * Mocydiopsis - 0.2181 0.2211 - 0.0212 0.0277 0.2537 0.1230 0.0128 parvicauda 157 Table 5.1d (Late midsuccession) 1 5 8 on this site (Table 5.1d). Insect species richness was only correlated with two community attributes (Table 5.2) both of these occurring on the ruderal site. As might be expected there was a high degree of autocorrelation between plant community attributes. These have been assessed for host plants of the individual dominant herbivores, and were especially evident amongst the Leguminosae. On all sites, host species area of Legume-feeding species was generally significantly positively correlated with all other variables (Table 5.3a-c). Attributes of the grasses also demonstrated autocorrelation. On the ruderal site, the three measures of leaf area were highly correlated, as were the measures of mean height (Table 5.3d). In early succession, family leaf area and mean height were highly correlated with total leaf area and mean height respectively (Table 5.3e). The three measures of leaf area were also autocorrelated in early midsuccession, but were not related to any measure of structure (Table 5.3f). Total structure was highly correlated with grass structure on this site and in late midsuccession. Both lanatus and glomerata leaf area in late midsuccession were correlated with most other community attributes (Table 5.3g). In contrast to the majority of results, no measured attributes of Spergula arvensis were autocorrelated (Table 5.3h). However, leaf area of Tripleurospermum inodorum in early succession was significantly correlated with all measured community attributes, and that of Cirsium arvense in late midsuccession with all community attributes except mean height (Tables 5.3i—j). 5.3.ii Variation in absolute abundance of species between different serai stages Absolute abundance by host plant family of three species of Apion and Sitona lineatus differed significantly between serai stages, whereas the absolute abundance of Sitona sulcifrons and Sitona hispidus was similar between serai stages (Figs 5.1a-f). Based on host plant species the absolute abundance of A. assimile and A^ apricans also differed significantly between serai stages, although that of A^ aethiops showed no significant 159 Coeiunity attributes Serai Insect Leaf area Grass Plot Grass Ho, of Ho, of Plant stage Group of plot leaf area aean lean leaves grass ocdiversity height height in plot leaves Heteroptera 0,0007 - -0,165 0,1287 0 , 4 7 0 6 * * * Ruderal Curculionoidea 0,1498 - 0,0802 - 0,2611 - 0,1606 Cicadeilidae 0,1809 -0,1685 -0,1756 -0,0435 0.2705 -0,2853 0,2784 Delphacidae 0,3719* 0,2015 -0,2202 -0,1592 0,2106 0,0275 0,1347 Heteroptera -0,18S7 _ 0,1150 -0.0377 _ 0,1398 Early Curculionoiaea 0,0555 - 0,1107 - -0,0516 - -0,1959 Cicadeilidae 0,1310 0,1122 -0,1204 -0.0506 -0,2104 -0,0572 0,0796 Delphacidae 0,1198 0,0333 -0,2807 -0,1908 -0,1740 -0,1742 -0,0058 Heteroptera 0,0240 -0,0821 _ -0,0755 . 0,0154 Early Curculionoidea -0.0368 - -0,1340 - 0,0208 - -0,0299 Hid Cicadellidae -0,1295 -0,0702 0,0869 0,0962 -0,0547 -0,0638 0,0064 Delphacidae -0,1618 0,0640 -0,0484 -0,0884 -0,1535 0,2989 -0,0447 Heteroptera 0.1794 0.2504 0,0900 . -0,0446 Late Curculionoidea 0,3146 - 0,1673 - 0,0358 - 0.0670 aid Cicadellidae 0,1023 -0,0601 -0,2704 -0,2658 0.2719 -0,0281 -0,0976 Delphacidae -0,1333 -0,1435 0,1234 0,0145 -0,1069 -0,1415 -0.0780 Table 5.2 Pearson's Correlation Coefficient for species richness of Heteroptera, Curculionoidea, Cicadeilidae and Delphacidae in subplots with several measured vegetation attributes during August 1985 in ruderal, early succession, early midsuccession and late midsuccession. * P < 0.05, ** P < 0.01, *** P < 0.001. Table 5.3 b Correlation coefficient for autocorrelation between vegetation attributes for host plants in subplots during August 1935. Separate tables for the host plant of hervibores in each serai stage; (a) Trifolium pratense on ruderal site; (b) Trifolium pratense and T rifolium spp (in parenthesis) in early succession; (c) T rifolium pratense and T rifolium spp (in parenthesis) in early midsuccession; (d) Agrostis capillaris and Holcus lanatus (in parenthesis) on ruderal site; (e) grasses in early succession; (f) Holcus lanatus in early midsuccession; (g) Holcus lanatus and Dactyli s glomerata (in parenthesis) in late midsuccession; (h) Spergula arvensis on ruderal site; (i) Tripleurospermum inodorum in early succession; (j) Cirsium arvense in late midsuccession. * P < 0.05, ** P < 0.01, *** P < 0.001. H a s t H o s t T o ta l H a s t Fa m i 1 y T o ta l Na. family T o t a l n o . P I a n t H o s t H o s t T o t a l H o s t F a a i 1 y T o ta l Ho. family I c t a l no. F I a n t s p e c i e s F a m i1 y s u b p l o t s p e c i e s m ean m ean l e a v e s o f l e a v e s d i v e r s i t y s p e c i e s F a m i l y s u b p l o t s p e c i e s m ean m ean 1 e a v e s c r l e a v e s a i v e r s i t y l e a f a r e a l e a f a r e a l e a f a r e a mean height h e i g h t h e i g h t l e a f a r e a l e a f a r e a l e a f a r e a mean height h e i g h t h e i g h t Host species Host species l e a f a r e a l e a f a r e a Host Farai1y 0 . 9 6 1 4 Host Family t i l l e a f a r e a l e a f a r e a Total subplot 0 . 9 1 9 8 0 . 8 7 2 6 Total subplot - 0 . 0 1 4 0 III III l e a f a r e a l e a f a r e a Host species 0 . 6 6 8 5 0 . 6 6 9 3 0 . 5 7 9 7 Host species 0 . 2 1 6 3 Q. 0 4 1 9 mean height i l l HI i l l mean height Host Family 0 . 6 3 4 0 0 . 6 4 0 3 0 . 5 4 0 0 0 . 9 1 2 6 Host Family mean height III III III III mean height Total mean 0 . 3 0 9 2 0 . 2 5 1 2 Q . 1 7 5 3 0. 4 4 6 6 0 . 3 8 1 3 Total mean 0 . 0 6 1 1 0 . 1 7 3 5 0 . 2 9 2 8 h e i g h t l 1 II l h e i g h t No. fam ily 0 . 9 3 3 8 0 . 9 2 1 8 0 . 8 4 0 8 0 . 5 8 2 8 0 . 6 4 4 2 0 . 2 1 0 5 No. fam ily l e a v e s i l l i n III h i III l e a v e s T o ta l no. 0 . 7 0 6 7 0 . 6 3 2 8 0 . 6 1 0 1 0. 4 4 3 0 0 . 5 0 7 2 0 . 0 5 9 0 0 . 7 9 5 4 T o t a l n o . -0.1592 0.6101 0 . 1 8 7 6 0 . 0 5 9 0 o f l e a v e s III i l l III II i l l III o f l e a v e s P l a n t 0.0088 0.0102 - 0 . 0 1 2 9 0 . 2 3 1 9 0.2502 0.0614 0.0367 - 0 . 2 9 9 0 P l a n t 0.0927 -0.0129 0.1621 0.0614 - 0 . 2 9 9 0 d i v e r s i t y d i v e r s i t y 160 Table 5.3b H o s t H o s t T o ta l H o s t F a m i1 y T o ta l No. family T o t a l n o . P I a n t H o s t H o s t T o t a l H o s t Fam i 1 y T o ta l No. family T o t a l na. P I a n t Fam i 1 y s p e c i e s s u b p l o t s p e c i e s m ean m ean 1 e a v e s o f l e a v e s d i v e r s i t y s p e c i e s F a r a i1 y s u b p l o t s p e c i e s m ean m ean 1 e a v e s o f l e a v e s d i v e r s i t y l e a f a r e a l e a f a r e a l e a f a r e a nean height h e i g h t h e i g h t l e a f a r e a l e a f a r e a l e a f a r e a nean height h e i g h t h e i g h t Host species Host species l e a f a r e a l e a f a r e a Host Fanily - 0 . 2 1 9 9 Host Family l 0 . 6 1 1 9 l e a f a r e a l e a f a r e a l l ( 0 . 6 7 9 7 ) Total subplot - 0 . 6 3 1 3 0 . 5 2 7 0 Total subplot 0 . 5 1 1 0 l e a f a r e a III III l e a f a r e a III ( 0 . 6 0 3 4 ) 10. 4 0 4 2 ) II Host species 0 . 3 4 3 9 0 . 3 1 2 1 - 0 . 1 3 1 2 Host species 0 . 5 9 7 2 0 . 2 4 1 8 tie an height mean height III (0. 4 9 3 2 ) ( 0 . 2 3 1 5 ) ( 0 . 1 6 7 2 ) 1 0 . 1 1 4 4 ) ( 0 . 0 6 9 2 ) 0 . 1 1 5 3 Host Fanily - 0 . 2 9 5 7 0 . 2 6 7 6 - 0 . 1 0 2 1 Host Fanily 0 . 5 1 3 2 0. 4319 0.8423 nean height nean height III (0. 4 1 9 9 ) ( 0 . 3 9 1 4 ) ( 0 . 4 1 2 1 ) III Total mean 0 . 3 2 8 3 0 . 0 1 1 3 0 . 1 7 5 3 - 0 . 1 7 8 1 0 . 7 8 3 4 Total mean 0. 4 5 8 1 -0. 0273 0.1917 0 . 4 3 2 1 h e i g h t III h e i g h t III ( 0 . 3 5 0 8 ) ( 0 . 3 4 1 6 ) ( 0 . 1 6 0 4 ) ( 0 . 0 7 7 7 ) No. fami 1 y 0 . 5 4 5 2 0 . 5 7 0 7 - 0 . 1 4 7 4 0. 2 4 9 5 - 0 . 1 2 8 4 - 0 . 1 9 2 9 No. fam ily I I 0 . 4 8 3 9 0 . 0 8 8 9 0 . 1 1 5 0 0.5927 0.0170 HI III l l e a v e s 1 e a v e s ( 0 . 0 7 1 5 ) ( 0 . 1 3 4 2 ) ( 0 . 6 5 7 1 ) ( 0 . 1 7 1 8 ) III T o t a l no. - 0 . 2 7 2 0 0 . 0 1 1 6 0 . 6 1 0 1 0 . 1 0 1 3 0 . 0 5 4 6 0 . 0 5 9 0 - 0 . 2 1 7 9 T o t a l no. I I 0 . 4 9 2 3 0. 4 2 3 8 0 . 1 8 0 3 0 . 1 2 0 0 0. 1401 0.3581 o f l e a v e s HI o f l e a v e s ( 0 . 1 6 0 5 ) ( 0 . 1 1 0 4 ) ( 0 . 3 4 8 8 ) ( 0 . 0 5 2 0 ) l P l a n t - 0 . 2 2 1 7 - 0 . 1 3 3 9 - 0 . 0 1 2 9 0 . 0 1 3 4 0.1228 0.0614 -0. 2897 - 0 . 2 9 9 0 P l a n t 0 . 3 3 7 6 -0.2435 0.2777 0 . 0 2 4 1 -0.0966 -0.0553 -0.1405 d i v e r s i t y d i v e r s i t y i ( 0 . 0 5 7 3 ) ( 0 . 1 0 0 3 ) 161 ( - 0 . 2 1 7 8 ) ( 0 . 0 5 0 3 ) Table 53c Jflble 5.3d H o s t H o s t T o t a l H o s t F a m i l y T o t a l No. fami 1 y T o ta l no. P I a n t H o s t H o s t T o t a l H o s t F a m i l y T o t a l No. f 3T.i y T o ta l no. P I a n t s p e c i e s Fa m i 1 y s u b p l o t s p e c i e s m ean m ean 1 e a v e s o f l e a v e s d i v e r s i t y s p e c i e s Fa m i 1 y s u b p l o t s p e c i e s m ean m ean 1 e a v e s o f l e a v e s d i v e r s i t y l e a f a r e a l e a f a r e a mean height h e i g h t h e i g h t l e a f a r e a l e a f a r e a l e a f a r e a l e a f a r e a mean height h e i g h t h e i g h t Host species Host species l e a f a r e a l e a f a r e a Host Family Host Fam ily l e a f a r e a l e a f a r e a Total subplot 0 . 3 3 9 0 Total subplot 0 . 8 7 0 1 l e a f a r e a l e a f a r e a III Host species 0 . 7 1 7 3 0 . 0 7 2 1 Host species mean height H i mean height Host Family Host Family 0 . 0 1 4 1 0 . 0 4 6 6 mean height mean height 0. 4 8 5 3 - 0 . 0 2 7 3 0 . 3 9 0 4 Total mean Total mean -0.1078 -0. 0907 0 . 7 5 0 7 III II h e i g h t h e i g h t HI 0 . 6 1 6 4 0.3091 0. 2405 No. fam ily - 0 . 0 2 3 3 No. fam ily 0 . 8 6 5 1 0 . 7 5 6 9 0.0466 -0. 0907 III l e a v e s l e a v e s III III T o t a l n o . 0 . 4 5 9 2 0 . 4 2 3 8 0 . 1 6 7 4 0 . 1 4 0 1 T o t a l no. 0 . 3 1 7 8 0. 4 2 3 8 -0.0814 0. 1401 0.4432 o f l e a v e s III l l o f l e a v e s II II P l a n t 0 . 5 0 1 1 - 0 . 2 4 3 5 0 . 3 5 3 9 - 0 . 0 9 5 6 - 0 . 1 4 0 5 P l a n t - 0 . 1 1 0 3 0 . 0 2 6 3 -0. 0956 -0.2955 -0.1405 d i v e r s i t y III 1 d i v e r s i t y 162 Table 5.3e H o s t T o t a l F a s i 1 y !io. fam ily H o s t H o s t T o t a l H o s t F a m i l y T o t a l No. family T o t a l n o . P I a n t H o s t H o s t T o ta l T o t a l no. F I a n t s p e c i e s F a m i l y s u b p l o t s p e c i e s F a m i l y s u b p l o t s p e c i e s m ean m ean l e a v e s o f l e a v e s d i v e r s i t y s p e c i e s mean m ean i e a v e s o f l e a v e s d i v e r s i t y l e a f a r e a l e a f a r e a l e a f a r e a l e a f a r e a l e a f a r e a mean height h e i g h t h e i g h t l e a f a r e a mean height h e i g h t h e i g h t Host species Hast species l e a f a r e a l e a f a r e a Host Fami1y 0 . 5 8 8 3 Host Family 0 . 7 8 5 3 l e a f a r e a i l l l e a f a r e a i l l ( 0 . 5 8 7 0 ) III Total subplot 0 . 3 4 5 8 - 0 . 1 4 7 6 Total subplot 0 . 5 9 4 9 0 . 6 3 8 8 l e a f a r e a 1 l e a f a r e a III III ( 0 . 1 0 3 8 ) Host species 0 . 3 9 8 8 0 . 8 4 8 3 0 . 0 0 8 9 Host species 0.1181 0.1734 0. 0 7 8 3 ~°an height 1 mean height ( 0 . 8 4 9 1 ) ( 0 . 8 5 1 3 ) ( - 0 . 0 1 0 3 ) Host Family 0 . 3 0 5 8 0 . 4 8 7 8 0.0361 0.0899 Host Family 0 . 0 0 6 8 0.8160 0.1958 0.4885 mean height 1 mean height 1 ( 0 . 8 5 6 1 ) ( 0 . 1 0 3 1 ) 1 Total mean 0 . 3 8 4 3 0 . 1 8 6 8 -0. 8016 0.0031 0 . 1 1 8 1 Total mean - 0 . 1 4 1 8 0.0088 -0. 8016 0.1957 0.6000 h e i g h t 1 i l l h e i g h t III ( 0 . 0 8 0 5 ) ( 0 . 0 1 9 7 ) No. family 0 . 8 7 8 8 0 . 8 7 8 8 -0 . 853 1 0 . 1 0 0 9 0. 8 9 8 9 0 . 0 9 1 0 No. fam ily 0. 8 6 8 3 0 . 4 1 3 9 0. 08 7 1 - 0 . 0 0 4 0 0.1105 -0.1080 l e a v e s HI III l e a v e s l ( 0 . 5 9 7 7 ) ( 0 . 1 3 1 0 ) HI T o t a l no. 0. 4 3 5 0 0 . 6 4 0 7 - 0 . 0 5 1 7 0 . 1 8 1 3 0.134? -0.1508 0 . 8 8 5 0 T o t a l no. - 0 . 8 0 0 1 -0.8884 -0. 0517 - 0 . 1 7 4 7 - 0 . 0 7 8 3 -0.1508 9.1801 o f l e a v e s II III o f l e a v e s II (0.4896) ( 0 . 0 1 3 9 ) P I a n t 0 . 3 7 8 6 - 0 . 0 6 7 4 -0.0989 0.0847 0. 04 9 1 0. 3 3 8 4 -0.0854 -0.0739 P l a n t - 0 . 3 6 9 6 - 0 . 1 6 7 0 - 0 . 0 9 8 9 - 0 . 0 5 5 3 0.1341 0.3384 - 0 . 8 8 5 6 - 0 . 0 7 3 9 d i v e r s i t y l 1 d i v e r s i t y 1 ( 0 . 1 0 9 1 ) ( 0 . 0 9 3 1 ) 163 Table 53*1 Table 5,3h T o ta l H o s t Fam i 1 y T o ta l Ho. family T o t a l no. P l a n t H o s t H o s t H o s t H o s t T o t a l H o s t Fam i 1 y T o ta l No. family T o ta l no. P I a n t F a m i1 y s u b p l o t s p e c i e s m ean m ean l e a v e s o r l e a v e s d i v e r s i t y s p e c i e s F a m i l y s p e c i e s s u b p l o t s p e c i e s m ean m ean 1 e a v e s o f l e a v e s d i v e r s i t y m ean h e i g h h e i g h t l e a f a r e a l e a f a r e a l e a f a r e a h e i g h t l e a f a r e a l e a f a r e a l e a f a r e a mean height h e i g h t h e i g h t Host species Host species l e a f a r e a l e a f a r e a Host Family Q. 5 7 7 9 Host Family l e a f a r e a III l e a f a r e a ( 0 . 2 6 2 3 ) Total subplot 0 . 5 7 5 3 0 . 8 6 0 0 Total subplot 0. 4 7 2 0 l e a f a r e a III III l e a f a r e a III ( 0 . 2 1 2 8 ) Host species 0 . 3 3 4 1 0 . 0 3 7 3 0 . 0 2 5 8 Host species 0 . 3 9 7 3 0. 25 8 1 mean height l mean height II ( 0 . 6 5 4 3 ) ( 0 . 0 7 7 3 ) ( - 0 . 0 5 1 4 ) i l l Host Family - 0 . 3 1 9 6 -0.1165 -0.2499 0 . 2 2 2 6 Host Family mean height 1 mean height ( 0 . 5 3 7 0 ) ( - 0 . 2 4 9 9 ) ( - 0 . 2 4 9 9 ) III - 0 . 0 9 4 4 0 . 2 7 0 3 H 0 . 8 5 2 8 Total mean - 0 . 1 9 5 3 - 0 . 0 0 3 4 Total mean - 0 . 0 3 8 2 - 0 . 0 9 4 4 0. 2 9 2 6 h e i g h t 1 h e i g h t ( 0 . 5 2 8 6 ) ( - 0 . 0 9 4 4 ) ( 0 . 1 7 0 7 ) III I I 0 . 6 8 6 8 • 0 . 5 4 9 5 0. 4991 0 . 3 3 1 7 - 0 . 0 9 9 1 - 0 . 0 0 3 5 No. fam ily No. fami 1 y 0 . 8 7 0 1 0. 2 7 2 6 0. 3 0 5 9 - 0 . 1481 l e a v e s II III l e a v e s III i ( - 0 . 0 9 3 9 ) (0 . 4 9 9 1 ) ( - 0 . 2 0 2 5 ) I I 0 . 4 8 6 1 0 . 3 2 8 1 0. 4791 0 . 2 9 9 6 - 0 . 2 4 8 6 -0.1056 0.5528 T o t a l no. T o t a l no. 0 . 3 6 3 8 0 . 4 7 9 1 0 . 4 2 2 7 - 0 . 1 0 5 6 III o f l e a v e s l i l l II o f l e a v e s l III II ( - 0 . 1 9 9 5 ) ( - 0 . 2 8 9 8 ) 0 . 1 1 5 0 -0 1770 -0. 2081 0.3138 -0.0492 -0.1361 0 . 0 6 3 7 - 0 . 0 7 7 2 P I a n t P l a n t 0 . 1 1 1 7 - 0 . 20 8 1 0 . 3 1 8 8 -0. 1361 -0.0772 d i v e r s i t y 1 d i v e r s i t y ( - 0 . 0 8 3 7 ) ( - 0 . 1 8 1 2 ) 4 6 1 Table 5.3L Fig 5. Host plant family and adult abundance of abundant herbivore species recorded on the four younger serai stages. Data pooled over two years. (a) Apion apricans = 8.93, P < 0.01; (b) Apion assimile, F^s ^ = 9.571, P < 0.01; (c) Apion aethiops, F^ = 21.52, P < 0.001; (d) Sitona lineatus. (3,14) 4.459, P < 0.05; (e) Sitona hispiduius, F (4,5) = 2.315, P > 0.05; (f) Sitona sulcifrons, F (3,3) = 0.236, P > 0.05; (g) Macrosteles laevis, F^ = 62.35, P < 0.001; (h) Recilia coronifera, F (3,7) ' = 4.43, P > 0.05; (i) Adarrus ocellaris, F (3,13) = 11 .829, P < 0.01; (j) ------Psammotettix confinis,------F,0 = 12.19, P < 0.01; (k) Eusceiis incisus, F,(3,13) = 41.11, P < 0 .001; (!) Mocydiopsis parvicauda, F^ = 11.07, P < 0.01 (m) Elymana sulphurella-*, F^ = 6.06, P < 0.05 (n) Zyqinidia scute!iaris, F^ 12) = 27-86' P < 0.001 (o) Javeseila pellucida, F^ ^ = 8.713, P < 0.01; (p) Dicranotropis hamata, F^ = 2.904, P > 0.05; (q) Paralabwrnia dalei, F (3,8) = 6.33, P < 0.05. P < 0.05, ** P < 0.01, *** P < 0.001. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE ■ FIG5.1 e)SirONA HISPIDULUS e)SirONA |o 1 c)AP10N AETHIOPS c)AP10N a ) A T l U I N A J r K J SERAL STAGE SERAL SERALSTAGE o BS LMS BMS 8 CATO o ADULTABUNDANCEO ■ ABSOLUTE ABUNDANCE ABSOLUTE ■ o ”• ” 9 J 0 0 _ 1000 w 9 § , 100' ,J S | l”‘ | — £ 1 § 5 soo. y 03 350 «i • J ------1 r B K nsrroNA nsrroNA B R jo------dlSlTONA LINEATUS dlSlTONA SERALSTAGE SERALSTAGE sa s s ~ i r~" o ulof s n o fr o l su M LMSEM* P o • • > ADULT ABUNDANCE ADULT ABUNDANCE ADULT ABUNDANCE ADULT 165 ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE FIG5.1 r i)ADARRUS OCELLARIS i)ADARRUS )MACROSTELE5 LAEVIS )MACROSTELE5 SERAL STAGE SERAL SERALSTAGE SERAL STAGE SERAL Bi LMS BMi B ADULT ABUNDANCEO ■ ABSOLUTE ABUNDANCE ABSOLUTE ■ Q I £ & MCDOSSPRIAD ** PARVICAUDA DMOCYDIOPSIS ftPSAMMOTETnX CONFINE ftPSAMMOTETnX SERALSTAGE ADULT ABUNDANCE e ADULT ABUNDANCE ADULT ABUNDANCE ADULT ABUNDANCE ADULT e ABUNDANCE ADULT 166 167 e f HH o to 8 H D N v a N n e v a u n o s a v HDNvaNnav amiosav ADULT ABUNDANCE ADULT ADULT ABUNDANCE ABUNDANCE ADULT 8 °)JAVESELLA PELLUCID A *♦ __ p)DICRANOTROPIS HAMATA g H D N v a N n a v a u n o s s v H D N v a N f i Q v a m i o s a v ADULT ADULT ABUNDANCE ADULT ADULT ABUNDANCE I 9 H D N v a N n a v a u n o s H V ADULT ABUNDANCE 8 § | jjj 9 4 ► § ■ 8 8 § § § o 1 6 8 difference (Figs 5.2a-c). The patterns of weevil abundance with succession were similar to those of absolute abundance by host plant family, except for A^_ aethiops, where adult abundance varied as described for absolute abundance by host plant species. Absolute abundance by host plant family of all Cicadellidae species investigated differed significantly between serai stages, each species showing a different pattern of absolute abundance (Figs. 5.1g-m). Analysis of absolute abundance by host plant species, however, revealed that only three of the six Cicadellidae species considered differed significantly between serai stages (Figs 5.2d-i). Adult Auchenorrhyncha abundance tended to vary between serai stages in a similar manner as absolute abundance. Of the three species of Delphacidae, only hamata showed no difference between serai stages for either measure of absolute abundance (Figs. 5.1o-q, 5.2j—k). 5.3.iii Variation in overwintering strategies and species ric h n e s s The analysis of absolute abundance of Cicadellidae overwintering as eggs and adults revealed no significant difference between these strategies (Fig. 5.3) (F^ = 0.0123, P > 0.05). The species richness of Heteroptera was significantly positively correlated with that of Curcuiionoidea and Cicadellidae on the ruderal site and that of Cicadellidae with Curcuiionoidea. No significant relations were observed in early or early midsuccession, however in late succession the species richness of Cicadellidae was significantly positively correlated with that of Heteroptera and Delphacidae (Table 5.4) 5.3.iv Variation in absolute abundance of birch herbivores associated with the upper and lower canopy There were no significant differences in the absolute abundance of insect groups occurring on the upper and lower canopy (Fig 5.4a-e). However, with the exception of the Psyllidae, there was a significant date effect for all groups (Figs. 5.5a-e). Generally, absolute abundance was high early in the year, fell in Fig 5. Mean absolute abundance by host plant species of abundant herbivore species recorded on the four younger serai stages. Data pooled over two years, (a) Apion assimile, gj = 11.91, P < 0 . 0 1 ; (b) Apion apricans, F^ ^ = 11.48, P < 0.001; (c) Apion aethiops, F^ = 8.97, P > 0.05; (d) Psammotettix confinis, F = 5 .0 4 , P > 0.05; (e) Elymana ( 2 , 2 ) sulphurella, F^ ^ = 0.79, P > 0.05; (fl Macrosteles laevis, F^ = 18.36, P < G.05; ( g } R e d l ia coronifera, F^ ^ = 7 .9 5 , P > 0.05; (h) Adarrus oceilaris, frr> = 10.05, P < 0.001; (i) Zyginidia (3,27) scutellaris, F^ ^ 2 9 .1 4 , P < 0.001; (j) Dicranotropis~~~ ——— hamata, F,_ l O * O D,J = 2.84, P > 0.05; (k) ------Paraiaburnia --- dalei F,n li,oJ = 14.523, P < 0 . 0 1 . * P < 0 .0 5 , ** P < 0 .0 1 , *** P < 0 .0 0 1 . 1 6 9 FIG 5.2 a)APION ASSIMUJE *** 18000 17000 - 16000 - 15000 - 8 14000 - 13000 - 12000 - 11000 - 3 10000 - 9000 - *000 - 7000 - 6000 - 5000 - 4000 - a 3000 - 2000 - 1000 - E EMS SERAL STAGE b)APION AP RICANS 8 § 3 8 SERALSTAGE c)APION AETHIOPS & I S 8 ■ ABSOLUTE ABUNDANCE O ADULT ABUNDANCE 1 7 0 FIG 5.2 d)PSAMMonrrnxconfinis e)ELYMANA SULPHURELLA g)RECILIA CORONIFERA £ 1 £ a 0 hYADARRUS OCELLARIS no a 0 SERAL STAGE m ABSOLUTE ABUNDANCE O ADULTABUNDANCE 171 FIG 5.2 j)DICRANOTROPIS HAMATA 150 B ADULT ABUNDANCE ADULT ABUNDANCE 200 -1 O CO 3 B EMS SERAL STAGE k)PARALABHRNIA DALEI ** B 3 co(3 9 EMS LMS SERAL STAGE ABSOLUTE ABUNDANCE O ADULT ABUNDANCE Fig 5.3 Mean absolute abundance by host plant family of Cicadellidae overwintering as eggs and adults. Data pooled over all sites and two years. 1 7 2 FIG 5.3 OVERWINTERING STAGE Table 5.4> Pearson's Correlation Coefficient for species richness of Heteroptera, Curculionoidea, Cicadeilidae and Delphacidae with that of the other insect groups during August 1985, in ruaeral, early, early mid and late midsuccession. * P < 0.05, ** P < 0.01, *** P < 0.001. 173 INSECT GROUP Serai Stage Insect Group Heteroptera Curculi onoi dea Ci cadel1 i dae Del phaci dae Ruderal Heteroptera - 0.3750 0. 4125 0. 0962 * f t Curcul i onoi dea - 0. 3470 -0. 0037 f t Cicadel1 i dae - 0. 1206 Del phaci dae - Earl y Heteroptera - -0. 1000 -0.0225 -0. 2102 Curcul ionoidea - 0.1651 0. 0038 Cicadel1 i dae - 0. 1692 Oel phaci dae - Early Mid Heteroptera - 0. 1531 -0. 0465 0. 1479 Curcul ionoidea - 0. 1691 0. 0475 Cicadel1 i dae - 0.2403 Del phaci dae - Late Mi d Heteroptera - 0. 1503 0. 3420 0. 2695 * Curcul i onoi dea - -0. 0422 0. 0053 Cicadel 1 i dae - 0. 3223 * Delphaci dae - TABLE 54 Fig 5.4 Mean absolute abundance of herbivores on the upper and lower canopy of birch in 1985. (a) Curcuiionoidea, ^ ^ = 0.53, P > 0.05; (b) Cicadellidae (excluding Typhlocybinae), F^ ^ = 0.141, P > 0.05; (c) Typhlocybinae, F^ ^ = 0.68, P > 0.05; (d) Psyilidae, F (1 ^ = 0.035, P > 0.05; (e) Heteroptera, Kruskall-Wallis = 0.018, n = 1202, P > 0.05. Data pooled over all dates. Hatched columns = upper canopy, open columns = lower canopy. Bars represent standard errors. ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE o U\ AOY CANOPY CANOPY ABSOLUTE ABUNDANCE ABSOLUTE ABUNDANCE 174 Fig 5.5 Mean absolute abundance on birch during each month in 1985. (a) Curcuiionoidea, ^ ) = 23.56, P < 0 . 0 01; (b) Cicadellidae (excluding Typhlocybinae), ^ = 35.02, P < 0.01; (c) Typhiocybinae, F ^3 = 26.43, P < 0.001; (d) Psyllidae, F ^5 ^ = 1.95, P > 0.05; (e) Heteroptera, lower canopy, Kruskall-Waliis = 65.3951 , n = 857, P > 0.001; upper canopy, Kruskall-VVallis = 77.8627, n = 345, P < 0 . 001. Hatched columns = upper canopy, open columns = lower canopy. Bars (a)-(d) represent standard errors. Data pooled over both canopies for all groups except Heteroptera, as no significant canopy.date interaction. 03 FIG 5.5 ooooooS ABSOLUTEABUNDANCE ABSOLUTEABUNDANCE ou.oSo&i©£§ ou.oSo&i©£§ AUGUST 176 FIG 5.5 0 S DATE ABSOLUTE ABUNDANCE 100 20 30 40 50 60 70 90 80 10 0 FIG 5.5 FIG APRIL I I LOWER CANOPY ILOWER I UE JULY JUNE Y A M t- — — n e)HETEROPTERA DATE UPPER CANOPY UPPER AUG SEPT 7 7 1 1 7 8 midsummer and increased during autumn. Adult Typhlocybinae were not recorded until June, after which their absolute abundance increased until the end of the season. Consideration of single species showed that only the heteropteran, Kleidocerys resedae showed a difference in absolute abundance between upper and lower canopies, being significantly greater in the lower canopy. 5.4 DISCUSSION In view of the high degree of autocorrelation between measured host plant and community attributes, conclusions about the importance of any single attribute on the abundance of a species must be made with caution. However, certain over-riding and consistent patterns were apparent. Weevil abundance was strongly correlated with most community attributes on the younger successional sites, whereas the abundance of Auchenorrhyncha displayed far lower degress of correlation. This dichotomy may occur for two reasons: firstly, differences in mode of feeding or secondly, differences in host plant specificity. The weevil species considered are highly host specific and on the younger sites their host plants, species of Leguminosae, are important determinants of community structure (see section 3.3). If weevil abundance is determined by any one host plant attribute, then by virtue of the observed autocorrelation it would be related to other attributes. On the ruderal and early succession sites, weevil abundance was correlated with both host plant and community attributes, whereas in early midsuccession where Leguminosae are not important components of community structure (see section 3.3), abundance was only correlated with host plant attributes. This suggests that general community attributes may not be important determinants of weevil abundance. It appears that small scale variation in weevil abundance is affected more by certain features of the host plant. Leaf area and the structure of the host plant are likely to be important, but it is impossible to separate the relative importance of these attributes with these d a ta . 179 On the other hand, the Auchenorrhyncha showed little correlation with any measured host plant or community attribute. This could be due to incorrect host plant identification from the literature records, or that as grass-feeders their abundance is determined more by food quality than quantity. Some element of community structure has been indicated as important to certain species on most sites, and leaf area is positively correlated with abundance of Adarrus ocellaris, Dicranotropis hamata and Stenocranus minutus on older sites. Generally the abundance of the Heteroptera was strongly correlated with host plant leaf area and structure. However, the abundance of the generalist grass-feeder, Stenodema laevigatum, was unrelated to any measured community attribute. Only for the Chrysomelid Cassida vittata is it possible to state unequivocally that host plant structure is of primary importance and host plant area of less importance. In contrast to other studies (Brown & Southwood, 1983; Lawton, 1978; Morris, 1971, 1979, 1981.; Murdoch et al, 1972; Southwood, Brown S Reader, 1979), no relationship between species richness and any measured community attribute was apparent. Such a result is therefore surprising (however, see Sedlacek, Barrett & Shaw, 1988). It could be that the scale of the sampling was too small to reflect variation in species richness, a suggestion which could only be upheld by similar studies employing different sampling scales (e.g. Murdoch et a[, 1972). In conclusion, small-scale variation in community attributes may be important to herbivorous insects, although different aspects of the community are probably important to different insect groups. Plant structure, as measured here, appeared to be of some importance, especially to Auchenorrhyncha and vittata. However, the relative important of specific aspects of vegetation structure and the ecological reasons for their importance will probably only be identified by manipulative experiments. These results also underline the importance of the appropriate scale of study in ecological research. 180 The majority of species showed significant differences in absolute abundance between serai stages when related to host plant family. However, analysis of absolute abundance by host plant species revealed significant differences for only half the Auchenorrhyncha species and two of the three weevil species. The latter is probably the most relevant measurement of abundance in this analysis, as generalist insect species may well switch hosts in different serai stages, and thereby affect the results. No single factor appears to determine which species differ in absolute abundance between serai stages. It is interesting that species sharing a host plant often differ in their pattern of absolute abundance between serai stages. This suggests that a host plant species is not necessarily a uniform resource for all herbivore species. Certain plant attributes may vary between serai stages to a greater extent than others, and the differential importance of certain attributes between serai stages may explain, at least in part, the difference in abundance of species sharing the same host plant. It was not expected that absolute abundance of birch herbivores would differ between the upper and lower canopy, since any differences in the defensive chemistry of birch related to foliage height are unlikely. Of the individual species studies, only Kleidocerys resedae showed a significant difference in absolute abundance between canopies, and this probably reflects its preference for catkins (Southwood S Leston, 1959) which were more abundant in the lower canopy. Seasonal changes in absolute abundance agree with known changes in birch chemistry (see Chapter 6 ). The results for the Psyllidae are difficult to explain, and may be complicated by their low abundance. The most interest^ point to emerge from the correlation analysis of insect group species richness was the general lack of a significant relationship between Cicadellidae and Delphacidae species richness'. As both groups are ecologically similar and share host plants, a significant relationship may be expected, its form depending on the importance of different structuring forces, i.e. competition, or combined use of good quality resource. The analysis of Cicadellidae overwintering as eggs 1 8 1 and adults revealed no difference in absolute abundance, and therefore sheds no light on the different patterns of Cicadellidae and Delphacidae absolute abundance, which remains unexplained. These results suggest that despite many similarities these two groups are ecologically very different. This chapter aimed to explain the variation in the relative and absolute abundance of single herbivore species to small-scale community differences. Such analyses of single species are necessary but the interpretation of results require an in-depth knowledge of the natural history of the species. Perhaps what is needed to provide a fuller understanding of the variation in individual species response and relations to vegetation attributes is "to do more elegant and sophisticated natural history. It is in nature that both the questions and answers lie" (Dingle, 1983). 1 8 2 CHAPTER SIX VARIATION IN FOOD QUALITY AND PERFORMANCE OF ERANNIS DEFOL1AR1A (LEP1DOPTERA: CEOMETR1DAE) 6.1 INTRODUCTION It is a vital component of Lawton S McNeill's theory (1979) that the intrinsic rate of increase (r) of a herbivore population is reduced when feeding on a diet containing quantitative defences (sensu Feeny). In order for the intrinsic rate of increase to be reduced, individual herbivores must have increased generation times, greater mortality, reduced fecundity, or any combination of these three parameters. This chapter aims to investigate whether any of these parameters are affected by quantitative defences in a common herbivore of birch. Quantitative defences are generally associated with apparent plants e.g. trees (Feeny, 1976). The leaves of trees undergo considerable chemical changes during their lifetime. Typically, young leaves are less tough than older leaves and have higher water and nitrogen contents. The phenolic content of leaves tends to increase with leaf age and the nitrogen and water content decreases (Feeny, 1970; Haukioja, Niemela, Iso-livari, Ojala & Aro, 1978; Ayres & MasClean, 1987; Scriber, 1977; Faeth, Mopper & Simberloff, 1981). Phenolic compounds, e.g. tannins and resins alone do not constitute a quantitative defence. Feeny (1976) states "the properties of decreasing nitrogen and water content, combined with tough leaves and the presence of tannins all confer upon oak leaves a 'quantitative' defence against herbivores and pathogens". Many workers have investigated the effects of quantitative defences on herbivore performance. It has been suggested that leaf toughness is important to herbivores. Feeny (1970) and Kraft & Denno (1982) suggest that the seasonal increase in leaf toughness is the main reason why many larvae feed on young foliage. Scriber (1977) showed that Lepidoptera larvae fed leaves low in water grew more slowly than larvae fed 183 leaves fully supplemented with water. Miles, Aspinall & Cornell (1982), however, report the performance of Paropsis atomaria (Coleoptera: Chrysomelidae) to be similar on host plants subject to different water regimes. The majority of work has, however, been conducted on the effects of tannin and nitrogen on herbivore performance. The interactions of both tannins and plant nutrients with herbivores has been reviewed in the general introduction (sections 1.3 & 1.4). Several workers have taken an holistic approach and have investigated the effect of all the constituents of quantitative defences on herbivore performance. These studies tend to utilise the seasonal changes which occur in leaves as natural manipulations of quantitative defences. Feeny (1970) found that 0 . brumata fed on young oak leaves had increased survival and greater pupal weights than larvae fed older oak leaves. In a study of fifteen species in the tribe Lithophanini (Lepidoptera: Noctuidae) Schweitzer (1979) found that twelve species had greater larval weights on a diet of young foliage than those on a diet of older foliage. Similar results were obtained by Hough & Pimentel (1978) with Gypsy Moth (Lymantria dispar L. Lepidoptera: Geometridae) feeding on oak; Kraft & Denno (1982) with Trirhabada bacharidis (Weber) (Coleoptera: Chrysomelidae) feeding on a woody perennial shrub, Baccharis halimifolia (Compositae); Ayres & Maciean (1987) with Epirrita autumnata (Bonkhausen) (Lepidoptera; Larentiinae) on Betula pubescens ssp. tortuosa and by Damman (1987) with Omphalocera munroei Martin (Lepidoptera: Pyralidae) feeding on leaves of the genus Asimina (Annonacc.eae). Thomas (1987) reports that leaf age is more important than host plant species in host plant selection by Monomacra sp and Disonycha quinquelineata (Coleoptera: Chrysomelidae) on Passiflora vines. A field experiment by Fowler & Lawton (1985) showed that most species of birch herbivore colonised young birch foliage in preference to older foliage. The preference for young foliage is not, however, universal. Cates (1980) reports that larvae of monophagous and oligophagous insect herbivores preferred young leaves, while polyphagous species preferred mature leaves of their various host plants. 1 8 4 It can be seen that, despite much work, the underlying trends in the relationship between insect herbivores and their host plants are not clear. There is ample evidence of herbivores performing better on young leaves than on older ones, although experiments seeking to investigate the effects of single parameters of food quality have been less revealing and often contradictory. There seems to be little evidence supporting the idea of quantitative defences acting in a dosage dependent manner as postulated by Feeny (1976). The role of phenolic compounds is particularly unclear. The only studies which demonstrate conclusively an effect of quantitative defences on the population dynamics of herbivores are the work on aphids (Dixon, 1963, 1966, 1969, 1975). This system is peculiar, however, as the herbivore is linked so closely to the nitrogen levels of the host plant. 6 .2 Experiment 1 THE EFFECT OF LEAF AGE ON THE PERFORMANCE OF ERANNIS DEFOLIARIA 6 . 2 .i Aim s The aims of this experiment are to investigate whether quantitative defences reduce individual performance and can affect herbivore population dynamics. 6.2.ii Materials and Methods 6 . 2 .ii.a The organisms Erannis defoliaria Clerck (Lepidotera: Geometridae) (Mottled Umber) is a common moth in Great Britain. It is polyphagous, reported from Betula, Corylus, Crataegus, Quercus, Lonicera, Rosa, (South, 1908) and Acer (S. Warrington, pers. comm.). The adult moth flies from October to December and occasionally through to March. The wingless females lay eggs on the twigs of the host plant. The larvae occur between the end of March and June and pupate during June on or below ground at the base of the host tree (Scorer, 1913). 1 8 4 Betula pendula Roth (Betulaceae) (Silver Birch) is a common tree in woods throughout the British Isles, especially on light dry soils and heaths, but it is rare in Scotland and on calcareous soils in England and Wales (Clapham, Tutin & Warburg, 1981). 6 .2.ii.b Sampling and experimental procedure M oths Female E. defoliaria were caught at Silwood Park, Berkshire and Wytham Wood, Oxford during the winter of 1985-86. They were placed in transparent plastic butter dishes ( 10cm diameter, 4cm deep) covered with muslin. The dishes were placed in an outside insectory. If the moths had mated prior to being caught, they usually laid their eggs on the muslin covering of the butter dishes. The eggs were collected from the dishes and kept well ventilated and moist, again in plastic butter dishes in an outside insectgry until 28th April 1986. The eggs were then moved into a constant temperature room at 15°C, 70% relative humidity and 16 hours day length in order to induce hatching. Newly hatched larvae were placed in a large plastic container and fed fresh young leaves. Within 48 hours of being placed in the constant temperature room the majority of eggs had hatched. The larvae were randomly allotted to controls or treatments. Each treatment consisted of one hundred larvae. The larvae in each treatment were distributed equally between ten butter dishes. The twenty butter dishes were then positioned randomly in an outside insectory. The larvae were fed daily, and the dishes monitored regularly to ensure that at no time larvae were without food. This never occurred. The survival of the larvae in each treatment was monitored and the date of pupation recorded for each individual. The pupae were weighed using an Unimatic CL41 balance as soon as the cuticle had hardened sufficiently to ensure that no damage occurred during handling. Weighing always took place within 12 hours of the beginning of pupation. 1 8 5 B irc h Newly burst buds of Betula pendula were collected from Hell Wood, Silwood Park between 1st - 3rd May 1986. The buds varied in phenology, from leaves which were just unfurled to fully opened complete young leaves. All the leaves were placed in polythene bags and frozen at - 11°C. These leaves constituted the treatment diet. The control diet constituted JB. pendula leaves collected from Hell Wood, Silwood Park on Mondays from 5th May - 30th June 1986. These leaves were frozen at -11°C for one week and then fed to the control larvae during the following week. The control leaves were therefore frozen for six to twelve days before larval feeding. All leaves were removed from the freezer the afternoon preceeding larval feeding to allow for them to thaw completely. 6 . 2 .ii.c Measurement of food quality Water content Leaves used to estimate water content were collected from Hell Wood on 12th & 20th May and 2nd & 16th June 1986. On each occasion five leaves were individually weighed within 30 minutes of collection. They were then placed in paper bags in an oven at 80°C. The leaves were weighed daily to constant weight. T o u g h n e s s Four leaves were collected on 12th & 20th May and 2nd & 16th June. Each leaf was placed between two perspex sheets (a & b on F ig .6 .1). The leaf holding apparatus was then balanced on two pieces of wood 50cm above the bench (Fig. 6.2.). A push-pull fruit tester (John Chatillon & Sons Inc., New York, Cat. no. 516-1000) was used to measure leaf toughness. Four toughness readings were taken from each leaf: two from each side of the midrib (see Fig. 6 . 2 ). Care was taken not to include any major veins in the area of leaf tested. Fig 6. Leaf holding apparatus for measuring leaf toughness. Fig 6. Arrangement of leaf holding apparatus during measurement of leaf toughness. Inset: position of toughness measurement of leaf, o = position of measurement. 186 FIG 6.1 FIG 6.2 Penetrometer Leaf holding apparatus T Wooden block Bench 1 8 7 Nitrogen and tannin content Twenty leaves collected on 3rd May, 2nd June and 18th June were freeze dried in an Edwards Freeze drier using phosphorus pentoxide as a dessicant and then placed in a freezer at -11°C. During November 1986 the leaves were ground to a powder using a ball grinder. The powdered leaves were stored in polythene topped glass tubes in a refrigerator at 5°C until analysis. Nitrogen content The total nitrogn content of the leaves was calculated by a Kjeldhar method (see Appendix 2). Tannin content The tannin content of the leaves was estimated by VVint’s method for the estimation of the nutritive inhibitor content of the host plant leaves (S. McNeill & V.C. Brown, pers. comm.) (see Appendix 2). 6 .2 . iii R esults 6 .2 . Mi. a M oths The survival of control larvae to pupation was significantly greater than that of treatment larvae (Table 6.1, binomial t-test, t = 3.95, P < 0.001). The survival of pupae to emergence showed a similar pattern. Significantly more treatment pupae died than control pupae (2 x 2 contingency table, Yates corrected \ 2 = 9.64, P < 0.01) (E v e re tt 1977). The mean pupal weight of the control larvae was significantly greater than that of treatment larvae (Fig. 6.3) (one way A N O V A , F ^ 7 2) = 8*0 9 6 ' p < 0.01). When the sex of the pupae was taken into account (Fig. 6.4) it can be seen that females in both treatments were significantly heavier than the males (one way ANOVA, treatment F^ ^ ) = 4*26, P < 0.05, control F^ ^ = 5.23, P < 0.05). Female control pupae were heavier than 1 8 8 D iet Control Treatment Number of larvae 100 100 Number pupating 50 *** 24 Number of pupae 47 15 producing adults Number dying as pupae 3 * * 9 T a b le 6.1 Survival of E. defoliaria la rv a e on diets of young B. pendula leaves (treatment) and n o rm a lly aging leaves (control). Asterisks represent level of significance, ** P < 0.01, *** P < 0 .001). D iet Control T reatment Number of larvae 30 30 Number pupating 20 N.S. 15 T a b le 6.2 Surival of E. defoliaria la rv a e on diets of fresh B. pendula leaves (control) and on leaves frozen for 24 hours prior to larval feeding (treatment). (N.S. = not significantly different, P = 0.29). F ig 6.3 Mean pupai weight (mg) of E. defoiiaria on control and treatment diets. Bars are standard errors. Numbers are sample size. 1 8 9 FIG 6.3 Fig 6.4 Mean pupal weight (mg) of male and female EE. defoliaria on control and treatment diets. Treatment diet represented by hatching. Bars are standard errors. 190 FIG 6.4 E\\1 TREATMENT I I CONTROL 191 treatment females, but the difference was not significant (one way ANOVA, = 2.16, P > 0.05). Male control pupae were significantly heavier than treatment males (one way ANOVA, F^ ^ j = 5.68, P < 0.05). The development rate (inverse of time in days from hatching to pupation) of control insects was significantly higher than that of treatment insects (one way ANOVA, F (1 y2) = 67.08, P < 0.01) (Fig. 6.5). 6. 2.iii.b Food quality It can be seen from Fig. 6.6 that the wet weight of Betula pendula leaves increased between the first and final sampling -4 -3 date (wet weight = 8.98 x 10 date + 1.35 x 10 , r = 0.9/7, P < 0.05). Little increase in wet weight occurred between sampling dates three and four. The dry weight of the leaves increased throughout the sampling period (regression slope is positive and significant, P < 0.03 (one-tailed)). The water content of the leaves decreased throughout the sampling period (dry wt. = 2.24 x 10“3 date + 2.984, r = 0.98, P < 0.05). Date explained 82% of the variation about the regression line. Leaf toughness increased with time and varied significantly between dates (two way nested ANOVA, ^(3 37) = 188.77, P < 0.001) (Fig. 6.7). However, there was no significant difference in toughness between individual leaves on a single date (two way nested ANOVA, F ^ 37) = 2.20, P > 0.05). Leaf total nitrogen (mg/g leaf dry weight) decreased during the sample period (nitrogen = -0.641 date + 42.74, r = 0.999, P < 0.05) (Fig. 6. 8 ). Tannic acid equivalents increased throughout -5 the sampling period (Fig. 6.9) (% tannic acid = 5.989 x 10 date + 2.337 x 10“2, r = 0.95, P < 0.05). Fig 6.5 Mean development time (days) for E^. defoliaria larvae on control and treatment diets. Treatment diet represented by shading. 192 FIG 6.5 26 24 22 20 18 16 14 a 12 10 8 6 4 2 0 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 LARVAL DEVELOPMENT TIME(days) I [CONTROL TREATMENT Fig 6. Mean wet and dry weight (g) of iB. penduia leaves during spring and early summer. 193 FIG 6.6 0.15 -r 0.14 - —100 — 50 0.01 - WATER CONTENTS fresh weight) 0 - i I " ' i i MAY 12 MAY 20 JUNE 2 JUNE 16 SAMPLING DATE (1986) WET WEIGHT + % WATER o DRY WEIGHT Fig 6.7 Mean leaf toughness (g/cm) of 13* pendula during spring and early summer as measured by a penetrometer. Bars are standard errors. 194 FIG 6.7 260 240 220 200 180 160 140 120 100 80 60 40 20 0 Fig 6. Total leaf nitrogen (mg/g dry wt) of B^. pendula during spring and early summer. 195 FIG 6.8 Fig 6.9 Percentage tannic acid equivalents as measured by Wint's method of E3. pendula leaves during spring and early summer. Bars are standard errors. 196 FIG 6.9 0.03 - r CO 0.029 - W 0.028 - 0.027 - a w 0.026 - ue 0.025 - c u 0.024 - 1 0.023 - 0.022 - 0.021 - 0.02 -■ MAY 3 JUNE 2 JUNE 18 SAMPLING DATE (1986) 6.3 Experiment 2 THE EFFECT OF THE FREEZING OF 3. PENDULA LEAVES ON LARVAL GROWTH 6.3.i Aims The aims of this experiment were to determine whether freezing birch leaves prior to larval feeding had any effect on the performance of the larvae. 6.3.ii Materials and Methods 6.3.11. a Moths Sixty newly hatched jE. defoliaria larvae were placed randomly in six plastic butter dishes on 23rd April 1987. Each dish contained ten larvae. Three dishes were allocated at random to a treatment diet and the remaining three dishes were controls. The larvae were kept in an outdoor insectory until they pupated and were fed treatment and control diets daily. The dishes were monitored throughout the day to ensure that food was always available (this was always the case). The date of pupation and the pupal weight for each larvae was recorded as before. The survival of larvae in each dish was also monitored. 6.3.11. b Food Each day from 23rd April 1987 E$. pendula leaves were collected from trees in Silwood Park. Half of the leaves were allocated to the control diet and half to the treatment diet. The control leaves were fed to larvae within 30 minutes of being collected. The treatment leaves were frozen at - 11°C for 36 hours before being thawed for 12 hours and then fed to larvae. 198 6.3. iii Results It can be seen from Table 6.2 that survival to pupation did not differ significantly between the two diets OL 2 2 by 2 contingency table, Yates' corrected X 2 = 1.1, P = 0.2949). The development rate did not vary significantly between larvae on the two diets (two way ANOVA, = 1.48, P = 0.234), and there was no difference in development rate between the sexes (two way ANOVA, F^ 2Q) = 1*36, P > 0.05) (Table 6.3). Pupal weight did not vary significantly between treatments (two way ANOVA, ^(1 27) = P = 0*277). In both treatments the pupal weight of females was significantly higher than males (two way ANOVA, F(1 = 10.73, P < 0.05). 6.4 Experiment 3 INVESTIGATION OF THE EARLY SPRING PHENOLOGY OF E. DEFOLIARIA 6.4 .i Aims The aims of this investigation were to examine the relationship between the hatching of E. defoliaria and the phenology of the tree relative to other spring feeding Lepidoptera larvae. E. defoliaria larvae are quoted in the literature as being spring feeders (South, 1908; Scorer, 1913). This investigation was carried out to determine whether E. defoliaria larvae in the field normally feed on newly burst buds and very young leaves; that is leaves of a similar age to those constituting the treatment diet in the main experiment. 6 .4 .ii Methods One hundred birch branches were marked in Hell Wood, Silwood Park as part of the major field experiment (see section 2 .1 ). These branches were utilised in this investigtion. Every few days between 24th April 1987 and 27th May 1987, ten of the marked branches were randomly selected and sampled with a beating bag. No branch was sampled twice. All Lepidoptera 199 DIET T reatment Control Mean development rate £.13 x 1C “ P 3.54 x 10 N. S. 2.04 x 10~ 2 ± 3.54 Mean pupal weight 87.72 0 2.33 N.S. 84.06 1 2.33 Mean pupal weight of females 91.25 P 4.19 N.S. 91.35 = 3.19 Mean pupai weight of males C4.19 ± 2.57 N.S. 76.77 - 4.48 Table 6.3 Pupal weight (mg) and larval development rate (inverse of days from hatching to pupation) of E. defoliaria fed fresh j3. penaula leaves (control) and leaves which had been frozen for 24 ho (treatment). (Mean i 1 S.E., In = 30, N.S. = not significantly different.) 200 larvae collected were identified as accurately as possible. The date of the first burst on birch in Hell Wood was also recorded. 6.4. i i i Results Bud burst of birch was recorded in Hell Wood on 20th April 1987 and as Figure 6.10 shows, E. defoliaria larvae were first recorded on 27th April 1987. The temporal distribution of E. defoliaria larvae was not significantly different from that of other Lepidoptera larvae recorded in the sample (Kolmogrov-Smirnoff statistic =0.28, P > 0.05). 6.5 Experiment 4 THE EFFECT OF LEAF TOUGHNESS ON SURVIVAL OF EARLY INSTAR LARVAE 6 .5 .i Aims The aim of this experiment was to determine whether newly hatched larvae of E. defoliaria could initiate feeding and survive on mature leaves as well as on their normal diet of young leaves. These results should give some indication as to whether leaf toughness is an important factor in determining the larval feeding period of E. defoliaria as suggested by Feeny (1970) for O. brumata. 6 .5 .ii Materials and Methods defoliaria eggs, which had been laid during the winter of 1987, were placed in a refrigerator at 5°C on 25th March 1987. This served to retard their development and prevent hatching. On 28th May 1987 the eggs were removed from the refrigerator and placed in a controlled environment room at 15°C, 70% relative humidity and 16 hours day length. All larvae hatched within 48 hours. Ten larvae were placed in each of two plastic butter dishes. The dishes were placed in an outside insectory and the Fig 6.10 Abundance of E. defoliaria and other Lepidoptera larvae on E3. pendula during the period immediately following bud burst 1987. NUMBER OF LARVAE FIG 6.10 FIG ■ E.defoliaria E.defoliaria ■ + OTHER LEPIDOPTERA OTHER + BUD BURST BUD 201 202 larvae fed daily with freshly collected j3. pendula leaves. The survival of the larvae was monitored for 7 days beginning on 30th May 1987. Four leaves of EK pendula were collected from the field on 30th May 1987 and 5th June 1987. The toughness of these leaves was measured as described in section 6. 2.ii.c . 6.5. iii Results After seven days only eight larvae were alive compared with the high survival of larvae fed fresh leaves in Experiment 2 (twenty two out of thirty). The survival of larvae in this experiment was significantly reduced compared with Experiment 2 (X2 2 by 2 contingency table, Yates' corrected "^2 = 4.25, P < 0.05). Some larvae had initiated feeding as damage was observed on some leaves and frass was present in the dishes. However, it was impossible to ascertain which larvae had fed. The toughness of the birch leaves increased significantly between sampling dates (two way ANOVA, log data as variances were unequal, 35^ = 280.82, P < 0. 001). Table 6.4 shows that the mean toughness of leaves on 30th May 1987 was three times greater than that of the youngest leaves sampled during 1986 (no measurements of the toughness of young leaves in 1987 were taken, so comparisons were made with samples from 1986). 6.6 DISCUSSION Contrary to theory (Feeny, 1976; Lawton & McNeill, 1979), E. defoliaria performed better for a range of measures on older leaves, generally regarded as poor quality food, than on young leaves, which are generally regarded as high quality diet (Feeny, 1970; Rausher, 1981; Lawson et a[, 1982; Raupp & Denno, 1983; Damman, 1987). These results present a paradox with respect to current ecological theory and the life history of E. defoliaria. It seems that accepted theory of what constitutes a diet of high quality e.g. high water content, high nitrogen 203 Date Toughness (q/cm) 12th May 1986 58.75 ± 2.61 30th May 1987 195.33 ± 11.49 5th June 1987 247.92 ± 6.62 Table 6.4 Comparison of toughness of young B_. pendula leaves with that of mature leaves (mean ± 1 .S .E .) 204 levels, low toughness and low defensive chemical levels (McNeill & Southwood, 1978; Feeny, 1976; Rhoades & Cates, 1976) does not hold when applied to E. defoliaria. No explanation can be provided for this, although it is relevant to note current dispute on the role of quantitative defences in plant/herbivore interactions (Bernays, 1978, 1981 ; Moran & Hamilton, 1980; Zucker, 1983). In view of these unexpected results, it is important to consider the experimental procedure. The experimental design was similar to that used by Feeny (1970) who fed previously frozen oak leaves to O. brumata, but obtained different results from those obtained here. The experiment was carried out under artificial conditions in the laboratory using excised leaves, both of these factors may influence the plant/herbovore interaction. The use of previously frozen leaves may also have affected the interaction. Several of these potential problems were tested in individual experiments. Firstly, the effects of fresh and previously frozen leaves on defoliaria were tested (Experiment 2) and no differences were found. Secondly, the relative phenology of the host plant and larvae in the field (Experiment 3) revealed that the leaves offered in the experiments were a normal source of food for E. defoliaria. Given that the experimental procedure was appropriate, two biological explanations for the observed results can be presented. Firstly, there could be a chemical (or chemicals) present in the buds and/or very young leaves, which providing the larvae were subjected to this for a short period had little effect on larval performance. However, if larvae are forced to feed on such food for" longer periods the effects are detrimental. The chemical could perhaps be acting in a dosage-dependent manner. Such a hypothesis would be compatible with the idea that plants defend their tissues in relation to their apparency (Feeny, 1976). For example, unapparent tissues (buds and young leaves) are optimally defended by qualitative defences, whereas more apparent plant parts (mature leaves) are best 205 defended by quantitative defences (Feeny 1976). Fowler (1984) found that "unapparent" seedlings of B_. pendula were less well defended by quantitative defences than more "apparent" saplings. No attempt was made, however, to examine the qualitative defences of seedlings and saplings. The idea that any such chemical would act in a dose-dependent manner, as suggested by these results, does not agree with theory on the action of qualitative defences (Feeny, 1976). The second explanation of the results is that later instars of E. defoliaria larvae require some nutritional factor in quantities not present in young leaves, but which are present in mature leaves. Neither of these hypotheses can be tested with the results available. It is interesting to note, however, that other workers have found that E. defoliaria behaves in a similar manner. S. Hartley (cited in Lawton, 1986) found using choice chambers that E. defoliaria preferred damaged foliage to undamaged foliage. Theory concerning rapidly induced chemical defence (Edwards & VV ratten, 1985; Fowler & Lawton, 1985; Haukioja £ Hanhimaki, 1985) predicts that damaged tissue contains a greater concentration of phenolic compounds than undamaged tissue. However, in view of the current debate on the importance of induced defences, it may be best not to read too much into that result; it is interesting to note, however, that other moths tested (0 . brumata, Apocheima pilosaria and Euproctis similis, showed no preference between damaged and undamaged foliage, or preferred undamaged foliage. Haukioja ( pers. comm.) reports that E. defoliaria preferred mature foliage to immature foliage. It is concluded that no obvious explanation for the above results can be presented. Clearly E. defoliaria shows interesting behaviour and there are several hypotheses which require testing before the situation will be clarified. The results also point to other questions about the life histoy strategy of E. defoliaria. Given that it does perform better on older leaves than on young ones, why then do the larvae emerge so early in 206 the season when young leaves constitute the only food source? There could be several pressures selecting for early spring larval emergence apart from the presence of higher quality food in the spring. By having a life cycle in which larvae emerge in spring, the moth could be reducing potential parasitism or predation on one of the life states. Ekanoyake (1967) showed that the key factor in the population dynamics of E. defoliaria was "winter disappearance", defined as the difference between the log number of eggs/m 2 and the log number of fully developed larvae falling to the ground. Winter disappearance accounted for 76% of total mortality. The major mortality factor contributing to winter disappearance was the asynchrony between caterpillar hatching and bud burst. This could be lessened if the eggs hatched later in the season. This suggests that presence of a large selective pressure for larvae to hatch in early spring. EkanoyaVe (1967) found that larval mortality due to parasitoids, predators and disease accounted for 5.8% of total mortality. Pupal mortality accounted for 18% of the total mortality; this means that only 10% of larvae pupating emerge as adults. (Adult mortality was assumed to be negligible). This key factor analysis suggests that it is the pupal stage which is most susceptible to predation. Perhaps if the pupae were in the soil at a different time of year they would suffer even greater mortality. Such a hypothesis is easily testable. Another pressure which could perhaps select for spring larval feeding would be escaping interspecific competition. This seems unlikely as herbivore density is at its peak during spring. Few authors have found interspecific competition to be an important structuring force in herbivorous insect communities (Lawton & Strong, 1981), but Karban (1986) presents evidence suggesting interspecific competition could be important to insect herbivores. It has been shown here (Experiment 4) that first instar larvae survive less well on mature foliage than on young leaves. Unfortunately, the sample size in this experiment was small, but nevertheless it provides an important insight into understanding the life-cycle strategy of E. defoliaria. Clearly, if early instars 207 are unable to feed on mature foliage then it is imperative that they hatch in early spring and feed on young foliage. It seems that, as suggested by Feeny (1970) and Kraft & Denno (1982), the seasonal increase in leaf toughness is the reason for early spring feeding by JE. defoliaria. As stated in the introduction, if quantitative defences are to affect the population dynamics of a herbivore, they must either reduce survival, reduce fecundity or increase generation time. Experiments which claim to show an effect of quantitative defences on insect herbivores can usually demonstrate an effect on the first two parameters. Reduced fecundity is usually demonstrated by showing reduced pupal weights on specific diets and assuming insect body weight is positively correlated with fecundity as shown by Miller (1957), Cook (1961) and Haukioja & Neuvonnen (1985). An increase in larval development time does not necessarily result in that individual having an increased generation time. Such a statement can only be made if larval development time is positively correlated with a longer pupal development time. This has never been demonstrated. it is also important to consider the relevance of an increase in development time of a matter of days as shown here and by Hough & Pimentel (1978). In univoltine insects, such as most spring feeding Lepidoptera it is difficult to see how an increase in development time of a few days can have an effect on the population dynamics, as the number of generations per year will remain unaffected. There could, however, be effects on the mortality of the larvae, and it has been suggested that increased development times could lead to increased predation or parasitism (Feeny, 1976; Moran & Hamilton, 1980; Price et aj_, 1980). However, Clancy & Price (1987) and Dammon (1987) conclude that longer development time does not necessarily lead to greater predation. I would like to suggest that the situation may be complex and that a series of trade-offs could be occurring. For example, if death per unit time of larvae was lower than death per unit time of pupae then, assuming all other things to be equal, it could be 208 advantageous for an individual to prolong its larval development time. The data on the population dynamics of E. defoliaria (Ekan^yrke, 1967) suggests that such a mechanism would be feasible in EE. defoliaria. If such trade-offs do occur then we need to rethink the analysis of experimental data and theories of plant/animal interactions. 6.7 CONCLUSION E. defoliaria larvae consistently perform better on what theory predicts should be a poorer diet than on a good diet. No explanation can be given to explain these results. It is suggested that leaf toughness is important in selecting for spring feeding in E. defoliaria. No evidence is provided in support of the theory that quantitative defences can reduce the intrinsic rate of increase of an insect herbivore. 209 CHAPTER SEVEN GENERAL DISCUSSION This thesis aimed to test a specific hypothesis in which the absolute abundance of insect herbivores was predicted to change in relation to the successional age of the habitat (Lawton & McNeill, 1979). This hypothesis and several related topics were examined in Chapter 4, but prior to this, in Chapter 3, patterns in the communities used to test the hypothesis were described. In many respects this analysis confirmed the now well- documented changes in plant and insect communities which occur during succession (Cray, Crawley & Edwards, 1987; Til man, 1988 and references therein), together with a somewhat surprising constancy in insect species richness and abundance between the two years of sampling. In Chapter 5 it was seen that certain attributes of the successional communities may affect the relative and absolute abundance of some insect species. It was, however, difficult to analyse the effects of community attributes individually as they were often interrelated. Even so, an awareness of the interrelatedness of the various community attributes, and a realisation of the difficulty inherent in measuring such attributes is considered preferable to a consideration of any one attribute in isolation which may lead to a false interpretation of the observed patterns of insect abundance. Chapter 6 aimed to examine one aspect of the original hypothesis under controlled laboratory conditions, where confounding influences of the surrounding community were absent. The results of this section were unexpected, and despite following the methodology of Feeny (1970), gave completely different results. Although controlled laboratory experiments are of great value in ecology, when the results are contradictory to accepted theory some difficulty remains in their interpretation, since the artificial environment of the laboratory may be considered a confounding factor. The unexpected results described in Chapter 6 emphasise that even in a well-studied system such as birch (Fowler, 1984, 1985; Fowler & MacGarvin, 1986; Hartley, 1988; Hartley & Lawton, 1987; Haukioja, Niemeia, Isolivar'v, Ojala & Aro, 1978; Haukioja, 210 Niemela & Isolivari, 1978; Haukioja & Niemela, 197&; Niemela, Aro & Haukioja, 1979; Haukioja & Niemela, 1976, Ayres & Maclean 1987), there remain questions which will only be answered by a combination of experimentation in the field and laboratory. The results of this work show unequivocally that the absolute abundance of herbivores feeding on plants defended by qualitative chemical defences is greater than that of herbivores on plants defended by quantitative chemical defences. However, some accounts in the literature contain information contrary to these results and these must be considered before the current work is taken as vindication of plant/herbivore theory as proposed by Feeny (1976) and Rhoades & Cates (1976), and as understood by Lawton & McNeill (1979). To date little work has been undertaken on the relationship between qualitative defences and adapted insect herbivores, with some results suggesting that there may be effects on herbivore performance eg weight (Dirzo & Harper, 1982), growth rate (Erickson & Feeny, 1974; Scriber 1981), survival (Harley & Thorsteinson, 1967) and fecundity (I.M . Evans pers. comm.). Such reductions in individual performance may well be translated into effects on population densities by similar mechanisms as those postulated for quantitative defences (Lawton & McNeill, 1979). Also, it is now far from certain that tannins act on all herbivores in the manner suggested by Feeny (1976) (eg Berenbaum, 1983; Bernays, 1978, 1981; Chan, Waiss, Binder & Ellinger, 1978; Lawson et al, 1982, 1984; Manuwoto & Scriber, 1986; Schweitzer, 1979). On the other hand there is some evidence that herbivores are affected in some way by tannins (Bennett, 1965, Bernays, 1978; Berenbaum, 1983; Chan et a[, 1978; Feeny, 1968, 1970; Manuwoto & Scriber, 1986, Roehrig & Capinera, 1983; Schweitzer, 1979). Consequently, there is considerable difficulty in interpreting this contraditory evidence, and there is a considerable need for good biochemical experimentation in ecology (see Martin & Martin, 1982). Methods, such as those of Hartley (1988), where phenolic production was prevented by blocking essential enzymes in the Shikimic pathway, could provide a novel experimental tool. The present controversy over the effects of chemical defences on We«\>ivores poses questions 211 about the rationale underlying the hypothesis tested here. Lawton & McNeill's (1979) prediction of different aged successional communities having characteristic levels of insect herbivore abundance are confirmed by the results of the present study and, in the absence of good contradictory evidence, it must be assumed that their initial assumptions on the effects of chemical defences on insect herbivores were correct. It is possible hov/ever, that observed patterns of abundance were due to predation and not solely to plant chemistry. The importance of predators in herbivore population dynamics is evident from the literature on biological control (De Bach, 1974; Huffaker, 1980). The data from biological control is often obtained from unusual circumstances, although data from natural popuations confirms the importance of predation and disease in herbivore dynamics (Anderson & May, 1980, 1981 ; Buckner & Turnock, 1965; Ekanoyake, 1967; Holmes, Schultz & Nothnagle, 1979; Ohgushi & Sawada, 1985; Varley, Gradwell & Hassell, 1973 and references therein). Little work on variation in predation pressures and efficiency between community types has been undertaken, although available data demonstrates considerable variation between habitats (Ohgushi & Sawada, 1985; Eickwort, 1977). Both of these studies equate higher levels of predation with a greater diversity of background vegetation maintaining predator numbers. These studies did not, however, consider differences in predation between woodland and herbacious communities, and in the absence of such data one cannot speculate on the possible impact of such a phenomenon on herbivore abundance: it remains an important and potentially productive avenue of research. The lower absolute abundance of herbivores on late successional plants does, however, suggest that some aspect of their chemistry reduces the intrinsic rate of increase, r, of their hervivores to a greater degree that that of early successional plants. It is not possible to say whether early successional plants have any effect on the intrinsic rate of increase of their herbivores. It is apparent from Chapter 5 that other aspects of the environment may affect the absolute abundance of certain species. Such results suggest that local variation in community 212 attributes, such as predation pressure, vegetation structure and soil nutrients may be important. The importance of vegetation structure was difficult to ascertain from the results of the analysis in Chapter 5, although it is clearly important to some species, and could be interacting with variation in predation pressure. Soil nutrients have been shown to have an effect on herbivore abundance (Prestidge & McNeill, 1983; Onuf, Teal & Valiela, 1977). and are "the principal effect of habitat on average food quality" (Crawley, 1983). However, increased nutrient levels need not necessarily lead to greater herbivore densities (Auerbach & Strong, 1981; Wilcox & Crawley, 1988). Any combination of these three factors could affect patterns of herbivore abundance between serai stages and are worthy of further study. There are several weaknesses in this study. Firstly, the host plant records, which are crucial to the calculation of absolute abundance, were of necessity extracted from the literature and are almost certainly subject to error. Consideration of both absolute abundance by host plant family and species helped nullify such errors, but which of these two measures is most likely correct, may vary from species to species. Records of the larger, more consipicuous species are more likely to be complete and reliable, so for Chrysomelidae, most Heteroptera and some weevil species absolute abundance by host plant species is probably the best measure, although within certain weevil genera where there are known taxonomic difficulties, eg Apion and C^uV^orhynchus, it is probable that host plant specificity is less accurate. In the case of grass-feeding species, absolute abundance by host plant family is undoutedly the best measure, since relatively few records distinguish between grass species. For each group, however, absolute abundance utilising both measures was greater on early successional plants than on late successional species. The second major problem with the system studied here is the lack of replication in space. Due to constraints on land availability only one site of each age is initiated every year. Statistically, this results in differences between serai stages which may be purely site differences, as site and serai stage are confounded variables (Hurlbert, 1984). 213 Intuitively, however, it seems that observed differences are the result of the different community types on each site, and the constancy of the results over time reinforces this assumption. Thirdly, this study only considered a limited range of plant and insect species over a relatively short time span. In this context it would be interesting to consider the absolute abundance of herbivores on a range of late successional plant species to confirm the consistency of these results, as it remains possible that birch may be peculiar in its relationship with its herbivores. Further study in several geographic regions over longer timespans would provide a better test of Lawton & McNeill's (1979) hypothesis. Finally, all hervivores were assumed to be folivores. This is not strictly true as some species may feed at least in part on flowers and/or seeds. Literature records are generally unclear as to the specific plant tissue utilised by a species, and no indication of preference for the different plant parts is available. This study has tested a hypothesis under field conditions using a novel technique. The measurement of absolute abundance, at the scale used here, is to my knowledge unique and demonstrates a powerful tool for comparisons between communities. If the interpretation of the results and assumptions of Feeny (1968, 1970, 1976) on the action of tannins had been accepted without controversy, this thesis could be seen as good evidence for his theories, and those of Rhoades & Cates (1976). In view of the recent controversy however, its contribution to ecological theory is less clear. The results of this thesis do, however, add considerable weight to the view that qualitative and quantitative defences have very different effects on herbivore performance and population dynamics. Only after a better understanding of the mechanisms and effects of chemical defences on herbivores, and the relative importance of their predators, will it become clear whether chemical defences are the prime determinants of characteristic levels of abundance of insect herbivores. "The common view that low nutritive quality of plant tissue is an antiherbivore adaptation must be allowed as plausible, but it remains uncertain. It will be hard to refute". (Moran & Hamilton, 1980). 214 ACKNOWLEDGEMENTS I would like to thank my supervisor, Dr. V.K. Brown, for her continued help and guidance, and Professor M.P. Hassell for allowing me to use the facilities available at Silwood Park. Statistical advice was freely provided by M. Rees, T. Ludlow, M. Crawley, A. Cange and C. Godfray. Taxonomic difficulties were overcome with help from P. Kirby, P. Hyman, M. Cox and J. Hollier. I would like to thank the many people who provided field assistance, especially to intrepid D-Vac assistants, Bernie Briscoe, Simon Pilchard, Charlotte Ford and Debbie Pro<^rer, and to Mr. H .A . "L evitt and his technical staff for all their help. I greatly appreciate the interesting and valuable conversations held with Drs. A. Gange, S. McNeill, M. Crawley, C. Godfray, S. Hartley, S. Fowler and A. Le Masurier. Special thanks for the many stimulating discussions with Ian Evans, MarkRees and Andrew Wilcox and extra special thanks to Helen for her continued support throughout the duration of this project. This work was financially supported by N.E.R.C. and Charles Square discretionary awards. 215 REFERENCES AARSSEN, L.W. & TURKINGTCN, R (1985). 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Biostatistical Analysis. Prentice-Hall International, London. ZUCKER, W.V. (1983). Tannins: Does structure determine function? An ecological perspective. American Naturalist 121: 335-365. 233 Appendix 1 Insect species Host plant R e f . Hemiptera: Heteroptera Acalypta parvula Short moss. (1) Acetropis gimmerthali Grasses esp. oat grass. (1) Adelphocoris lineolatus Always on Papilionaceae esp. restharrow, meadow vetchling, milk vetch & clovers. (1) Aelia acuminata Tall & rank grasses. (1) Agramma leata Sedges & rushes. (1) Amblytylus nasutus Grasses esp. meadow-grass. (1) Berytinus minor Trifolium repens, Ononis repens. (1) Berytinus montivagus Medicago lupilina. (1) Berytinus signoreti Medicago lupilina, Hippocrepis canos a. (1) Blepharidopterus angulatus Apple, alder, elm, birch, lime & other trees. (1) Calocoris norvegicus On growing points, buds, flowers, & unripe fruits of Urtica, Canpositae (incl. T. inodorum, Senecio, Cirsium) and clovers. (1) Campyloneura virgula Many trees esp. Crataegus, oak & hazel. (1) Capsus ater Grasses esp. Lolium perenne, Agropyron repens, also Phleum, Holcus lanatus & others. (1) Ceraleptus lividus Trifolium pratense, T . campestre, T. arvense. (1) Chlamydatus pullus Lucerne, sorrel & knotgrass - found on probably black medick & white clover are food plants. (1) Coreus marginatus Rumex acetosa, R. acetosella, R. crispus, Polygonum persicaria, P. aviculare. (1) Corianeris denticulatus Medicago lupilina, Melilotus spp., T. arvense. (1) Cymus claviculus Polygonum aviculare, Juncus bufonius. (1) Cymus melanocephalus Lotus ulignosus, Lysimachia vulgaris, rushes. (1) Dolycoris baccarum Dicyphus epilobi Epilobium, Chamaenerion spp. (1) Dicyphus errans (1) Drymus brunneus Mosses, fungi & ? (1) Drymus sylvaticus Mnium & other mosses. (1) Elasmucha grisea Birch. (1) Elasmostethus interstinctus Birch. (1) Eurydema oleracea Larvae may be carnivorous seeds of Alliaria petiolata, Raphanus raphanistrum, Armoracia rusticana & many other crucifers. (1) Harpocera thoracica Oak. (1) 234 Insect species Host plant Ref. Heterotana merioptera Much vegetation esp. Urtica many trees & shrubs all stages predatory. (1) Ischnodemus sabuleti Deschampsia cespitosa, reeds Arrhenatherum elatius, Glyceria maxima, G. fluitans, D. glomerata (pers obs). (1) Kleidocerys resedae Birch, alder. (1) Leptopterna dolabrata Grasses esp. timothy, Agropyron repens, meadow foxtail, Holcus lanatus & Dactylis glcmerata. (1) Leptopterna ferrugata Grasses esp. red fescue, common bent, wavy hair-grass, oat- grass & meadow grass. (1) Lopus decolor Agrostis spp. & other grasses. (1) Lygocoris contaminatus Birch, alder occasionally nettles. (1) Lygocoris pabulinus Hawthorn, apple, currant, plum, cherry & lime, nettles, creeping thistle, groundsel, dandelion, black nightshade, potato, bittersweet, white deadnettle, sunflower, dock, fat hen, rose-bay willowherb & common cow wheat. (1) Lygus rugulipennis Many herbaceous plants & shrub esp. Chenopodium album & other Chenopodiaceae, T. inodorum, Rumex spp., Urtica spp., Trifolium spp. (1) Miris striatus Oak, alder, willow, elm, hazel, hawthorn & other trees. (1) Megaloceraea recticomis Grasses. (1) Monalocoris filicis Bracken & ferns. (1) Myrmus miriformis Many species of grass, leaves & unripe seeds. (1) Neottiglossa pusilla Grasses esp. meadow grass. (1) Notostira elongata Grasses esp. Agropyron repens. (1) Nysius thymi Numerous esp. Composites e.g. Inula conyza, Erigeron spp. (Conyza) party insectivorous. (1) Orius niger Heathers, wide range of low plants esp. Artemsia vulgaris predatory. (1) Pentatcma rufipes Oak, alder & most native deciduous trees. (1) Pantilius tunicatus Hazel, alder & birch. (1) Peritrechus geniculatus Moss & low vegetation. (1) Peritrechus lundi Potatoes. (1) Phytocoris longipennis Deciduous trees esp. hazel, oak & hawthorn. (1) Phytocoris tiliae All deciduous trees esp. oak, ash, lime & apple. (1) 235 Insect species Host plant Ref. Phytocoris varipes Grasses esp. Bromus, Phleum, & Trifolium y Rumex, A ch illea, T. inodorum. (I) Piesma maculatum Chenopodiaceae (L) Pithanus maerkeli Grasses & rushes. (1) Plagiognathus arbustorum Many esp. Urtica spp. (1) Plagiognathus chrysanthemi Senecio jacobeae , T. inodorum, Achillea millefolium, Medicago lupilina, Urtica. (1) Podops inuncta Rung (saprophage or carnivore). (L) Psallus betuleti Partly phytophagous on birch partly predatory on aphids etc. (L) Psallus falleni ? U) Rhopalus parumpunctatus Cranesbill, St. John's worts, spurreys, storksbill & cocks foot grass esp. common mouse ear chickweed. (1) Rhopalus subrufus Wild basil, herb Robert esp. St. John's wort. (I) Scolopostethus thonsoni ? nettles ? (L) Stenodema calcaratum Grasses on buds & unripe grains esp. on Agrostis & meadow fox t a il. (1) Stenodema laevigatum Grasses esp. meadow foxtail, timothy, red fescue, cannon bent and wavy hair grass esp. on flowering heads, on buds & grains. (1) Stenotus binotatus Grasses esp. Phleum & Dactyl is & occassionally on Canpositae flowers. (1) Stygnocoris fuligineus ? (1 ) Stygnocoris pedestris ? (1) Tingis ampliata Cirsium arvense. (1) Tingis cardui Cirsium vulgare, Carduus nutans, Cirsium palustre. (1) Trigonotylus ruficomis Grasses esp. wavy hair grass, common bent, red fescue, timothy & others. (1) Troilus luridus Trees, predator. (1) Tytthus pygmaeus Predatory on eggs & nymphs (1) Hemiptera: Hanoptera - Psyllidae Aphalara polygoni Polygonum aviculare, P. amphibium. (2) Arytaina genistae Cytisus scoparius, C^. austriacus, Genista tinctoria. (2) Arytainilla spartiophila Cytisus scoparius. (2) Craspedolepta nervosa Achillea millefolium, A. ptarmica, A. gerberi, Artemesia vu lgaris. (2) Craspedolepta subpunctata Chamaenerion angustifolium. (2) 236 Insect species Host plant Ref. Psylla alni Alnus glutinosa, A. incana, A. viridis, A. japonica, A. hirsuta. (2) Psylla betulae Betula pendula. (2) Psylla brunneipennis Salix spp. (2) Psylla hartigi Betula pendula, B. pubescens, B. platyphylla. (2) Psylla melanoneura Crataegus. (2) Psylla pulchra Salix spp. (2) Psylla sorbi Sorbus. (2) Trioza abdominalis Chrysanthemum sp p ., Alchemilla vulgaris agg. (2) Trioza chenopodii Atriplex patula, A. hortensis. (2) Trioza remota Quercus robur, Q. petraea. (2) Trioza urticae Urtica dioica, U. dubia, U. urens. (2) liptera: Homoptera - Cicadellidae Adarrus ocellaris Grasses. (3) Arrhenatherum elatius. (6) Holcus spp. (5) Agallia ribauti Grasses. (4) Agallia venosa Grass. (4) Allygus carmutatus Trees, shrubs, sometimes grass- land. (8) Aphrodes albifrons Grasses. (4) Holcus spp., Dactylis glcmerata. (5) Aphrodes bicinctus Grasses. (4) Holcus spp., Dactylis glcmerata. (3) Aphrodes bifasciatus Grasses. (4) Aphrodes histrionicus Grasses. (4) Aphrodes trifasciatus Grasses. (4) Arthaldeus pascuellus Grasses. (8) Grasses & Juncus. (6) Agrostis capillaris. (5) Athysanus argentarius Saltmarshes. (8) Damp meadows & clover fie ld s. (6) Balclutha punctata Grasses. (8) Deschampsia flexuosa. (5) Cicadula persimilis Juncus, Carex. (8) In meadows. (6) Dactylis glcmerata, Holcus spp. (5) Cicadella viridis Grasses in marshy places. (4) Juncus effusus. (5) Conosanus obsoletus Grasses. (4) Diplocolenus abdominal is Grasses. (8) Grasses. (6) Holcus spp. (5) Doratura stylata Fields & meadows, Nardus str ic ta . (6) Agrostis capillaris, Festuca rubra. (5) 237 Insect species Host plant Ref. Elymana sulphurella Grasses. (8) Holcus m ollis, Phleum pratense, grasses. (6) Holcus spp. (5) Euscelis incisus Grasses. (8) Euscelis lineolatus Grasses. (8) Graphocephala fennahi Rhododendrons. (4) Graphocraerus ventralis Grasses. (8) Poa pratensis & Anthoxanthum odoraturn. (6) Issus coleoptratus ivy, holly & various trees or in moss. (7) Lampottetix octopunctatus Trees. (8) Macropsis scutellata Nettles. (4) Macrosteles cristatus Clover fields, in potato fields, Polygonum, Linum. (8) Macrosteles laevis Grasses, oats, barley, Lolium perenne. (8) Agrostis capillaris. (5) Macrosteles sexnotatus Grasses & clover? (8) Megophthalmus scanicus Grass roots & bushes. (4) Mocydiopsis parvicauda Grasses. (8) Oncopsis flavicollis Betula. (4) Oncopsis subangulata Betula pendula. (4) Oncopsis tristis Betula. (4) Psammotettix confinis Grasses. (8) Arrhenatherum elatius. (6) Agrostis capillaris. (5) Recilia coronifera Short grasses. (8) Holcus spp. (5) Tachycixius pilosus Thamnotettix confinis Trees & lower vegetation. (8) Betula, willows & herbs. (6) Thamnotettix dilutior Oaks, other trees, lower vege tation. (8) Qjercus. (6) Hemiptera: Hctnoptera - Cicadellidae, subfamily Typhlocybinae Alebra albostriella Quercus, Alnus. (3) Alnetoidia alneti Alder, hazel & other trees. (3) Arboridia ribauti Quercus, Acer. (3) Edwardsiana alnicola Alnus, Acer. (3) Edwardsiana avellanae Hazel, elm, sycamore & horse- chestnut. (3) Edwards iana plebeja (Xiercus, Alnus, Betula, Ulmus (Tilia & Corylus ?). (3) Bmpoasca decipiens Lower plants, esp. Urtica some times trees. (3) Bmpoasca vitis Trees & bushes. (3) Eupteryx aurata Nettle, labiates, cowparsnip, burdock, hempagrimony, potato & others. (3) Eurhadina concinna Quercus Fagus, Betula, Notho- fagus, Alnus. (3) 238 Insect species Host plant Ref. Eurhadina pulchella Oak. (3) Kybos b etu licola Betula spp. (3) Lindbergina aurovittata Quercus, Rubus, Fagus, Alnus, Corylus , Betula, Carpinus. (3) Linnavuoriana decempunctata Betula, conifers & gorse. (3) Zygina flammigera Oaks, hawthorn & Prunus spp., conifers, holly & ivy. (3) Zyginidia scuteliaris Grass esp. D. glomerata, Festuca rubra. (3) Holcus. (5) Hemiptera: Homoptera - Delphacidae Cixius nervosus ? trees. (7) Foliferous trees & bushes. (6) Concmelus anceps Juncus. (7) Juncus. (6) Juncus effusus. (5) Criomorphus albomarginatus Grasses. (4) Delphacodes brevipennis Damp places. (7) Delphacodes fairmairei Damp places. (7) Dicranotropis hamata Grasses. 97) Oats, wheat, Phleum, Des- champsia, Agrostis capillaris, H. lanatus, E lytrigia repens, Arrhenatherum elatius, Alopecurus pratensis, Lolium perenne. (6) Holcus spp. (5) Hyledelphax elegantulus Grasses. (7) Deschampsia flexuosa on moors, wood glades with Vaccinium. Festuca rubra. (5) Javesella dubia Grasses. (7) Javesella pellucida Grasses. (7) Cereals, Avena sativa, Lolium perenne. (6) Paraliburnia dalei Grass. (7) Agrostis capillaris. (5) Stenocranus minutus Grasses. (7) Dactylis glomerata. (6) Dactylis glomerata. (5) Hemiptera: Homoptera - Cercopidae Aphrophora alni Trees & bushes. (4) Cercopis vulnerata 7 (4) Neophilaenus lineatus Grasses. (4) Holcus spp., Dactylis glomerata. (5) Philaenus spumarius Wide variety of trees & low p lan ts. (4) Holcus spp., D. glomerata. (5) 239 Insect species Host plant Ref. Coleoptera: Curculionoidea Amalus scortilium Polygonum aviculare, Rumex acetosa, R. obtusifolius. (9 ) Apion aethiops Vicia cracca, V. sativa, V. sepium, Vicia. (9) Apion apricans Trifolium pratense. (9) Apion assimile T. hybridum, T. pratense, Trifolium . (9 ) Apion carduorum Cirsium arvense, C. palustre, Cirsium. (9 ) Apion craccae Vicia cracca, V. hirsuta, V. sativa, V. sepium, Vicia. (9) Apion curtirostre Rumex acetosella, R. acetosa, R. crispus, R. obtusifolius, Rumex. (9 ) Apion dichroum T. hybridum, T. pratense, T. repens, Trifolium. (9) Apion hookeri Matricaria, T. maritimum. (9) Apion hydrolapathi Rumex crispus, R. obtusifolius, Rumex. (9 ) Apion lo ti Lotus comiculatus. (9) Apion marchicum Rumex a ceto sella , Rumex. (9) Apion m eliloti Melilotus alba, M. altissima, M. officinalis, Melilotus. (9) Apion miniatum Rumex crispus, R. obtusifolius, Rumex. (9) Apion nigritarse Trifolium spp. (10) Apion onopordi Centaurea spp., Arctium lappa, Carduus nutans, Onopordum acanthium, Cnicus benedictus. (10) Apion pubescens Trifolium campestre, T. dubium, Trifolium . (9) Apion rubens Rumex a ceto sella , Rumex. (9) Apion sanguineum Rumex acetosella. (9) Apion simile Betula pendula, Betula. (9) Apion tenue Medicago, T. pratense, Trifolium . (9) Apion trifolii T. pratense. (9) Apion virens T. pratense, T. repens, Trifolium . (9) Ceuthorrhynchus assim ilis Raphanus raphanistrum, Cruciferae. (9) Ceuthorrhynchus contractus Raphanus, Capsella. (9) Ceuthorrhynchus erysimi Cruciferae, Capsella bursa- p astoris. (9) Ceuthorrhynchus floralis Capsella bursa-pastoris, Capsella, Cruciferae. (9) Ceuthorrhynchus litura Carduus arvensis. (10) Ceuthorrhynchus molleri Hieracium, Leontodon. (9) Ceuthorrhynchus marginatus Taraxacum officinale, Hypo- chaeris maculata, Crepis virens, Lactuca serriola. (9) Ceuthorrhynchus pyrrhorhynchus Sisymbrium officinale. (9) 240 Insect species Host plant Ref. Ceuthorrhynchus quadridens Raphanus raphanistrum, Cruciferae. (9) Ceuthorrhynchus rugulosus Matricaria, T. maritimum. (9) Deporaus betulae Betula pendula, B. pubescens, Betula, Fagus sy lvatica. (9) Gronops lunatus Spergula arvensis, T. inodorum. (9) Gymnetron pascuorum Plantago lanceolata, Plantago. (9) Hypera arator Spergula arvensis, Stellaria media. (9) Hypera nigrirostris T. hybridum, T. pratense, T. repens, Trifolium. (9) Hypera pedestris Lotus. (10) Hypera plantaginis Lotus uliginosus, Plantago lanceolata, Plantago major, Plantago, T. pratense, T. repens. (9) Hypera postica Lotus corniculatus, Medicago lupilina, Medicago, T. hybridum, T. pratense, T. repens, Trifolium, Vicia sativa, Vicia sepium, Leguminosae. (9) Miccotrogus picirostris T. hybridum, T. pratense. (9) Otiorrhynchus singularis Betula, Fagus sylvatica, poly- phagous. (9) Phyllobius argentatus Betula, Fagus sylvatica, Qiercus, Sorbus aucuparia. (9) Polydrosus cervinus Betula pendula, Betula, Dactylis glcmerata, CMercus, Graminae, Polyphagous. (9) Phyllobius maculicornis Betula, Qoercus. (9) Phyllobius pyri Betula, Sorbus. (9) Phyllobius quadr ituberculatus Polygonum aviculare, Polygonum p ersicaria. (9) Phyllobius urticae Nettles. (10) Phyllobius viridearis Populus tremula, Salix caprea, S alix vim in alis, Ulittus cam pestris. (9) Rhinoncus bruchoides Polygonum persicaria, Poly gonum. (9) Rhinoncus castor Polygonum aviculare, R. a ceto sella , Polygonum, Rumex. (9) Rhinoncus perpendicularis Polygonum. (9) Rhamphus pulicarius Betula pendula, Betula pubescens, Betula. (9) Rhynchaenus fagi Fagus sylvatica. (9) Rhynchaenus rusci Betula pendula, B. pubescens, Betula, Qiercus. (9) Sitona hispidulus Lotus ulignosus, M. lupilina, Medicago, T. pratense, T. repens, Trifolium. (9) Sitona humeralis Lotus, M. lupilina, Medicago, T. hybridum, T. pratense, T. repens, Trifolium, Vicia. (9) References 0 Jy (1932) Joy = 10 VDCXI-ja^Ul^UJNJl—* Hmn (1983) Hyman = L Qen (1960) Quesne Le (1973) = & Solomon Waloff (1981) = Payne & Quesne Le = Suhod Lso, (1959) Leston, & Southwood = L Qen (1969) Quesne Le = (1965) Quesne Le = Osiniso (1983) ssiannilsson O = (1979) & White Hodkinson = oepea Chryscmelidae Coleoptera: atohs polygoni Gastrophysa htdca olivacea Phytodecta rcu ci i t is c Bruchus hlort nemorum modeeri Phyllotreta Hippuriphila transversa Crepidodera ferruginea Crepidodera hortensis Chaetocnema concinna Chaetocnema ta itta v Cassida flaveola Cassida asd rubiginosa Cassida yhu puslus sillu u p Tychius rcu rufipes Bruchus isi p ti Bruchus lo Bruchus tohsns melanogrammus Strophosanus sulcifrons Sitona ioa reqensteinensis Sitona lineatus Sitona Sitona lepidusSitona Insectspecies yiu soais (10) scoparius. Cytisus rse. (9) Grasses. oyou aiuae Polygonum, aviculare, Polygonum bt iolus (9) s. liu sifo tu ob ca (9) icia. V rcfre (9) Cruciferae. opste Gaia. (9) rtica, U Graminae. rtica, Compositae, U Cirsium, ca rca Vii s i , a tiv sa icia V cracca, icia V oc, tc. (9) rtica. U ioica, d ue. (9) (10) arvense. Equisetum Rumex. cpste Gaia. (9) rtica, U Graminae. ioica, d rtica Ccmpositae, U Cirsium, ota (9) (9) Urtica sis, arven Cirsium. Spergula Cirsium ria vulgare, tella arvense, S Cirsium graminae, hostea. tellaria S uru, oyhgu. (9) Polyphagous. Quercus, hnpdu, Polygonum Chenopodium, vclr, oyou, Rumex Polygonum, aviculare, ou cmiult , rflu (10) . Trifolium s, latu icu com Lotus ou cmiult . (9) s. latu icu com Lotus robur, Quercus lvatica, sy yiu soais (9) (9) scoparius. icia. V Cytisus , Trifolium rflu Lgmnse (9) Leguminosae. , Trifolium rtne Tioim, ca (9) Fagus icia. V , flexuosa, Trifolium Deschampsia pratense, ou, eiao T pratense, T. Medicago, Lotus, ou uinss Medicago ulignosus, Lotus ou criuau, T. corniculatus, Lotus lu p ilin a, a, ilin p lu T. otpat . f e R plant Host pratense, 241 24 Z AFPEND IX 2 a) WINT'S METHQO FOR TANNIN ESTIMATION The quantitative estimation of the nutritive Inhibitor content of experimental host plant leaves : the enzyme assay reagent and procedures. The basic starch-amylase reaction 1) REAGENTS USED a) Stock sta rt reagent mixture 50ml. Soluble starch solution (Igm.in 500ml water) 20ml. Salt (NaCI) solution (1% w/w) 20ml. Phosphate buffer, for ph b.b. The buffer solution was made up from two solutions, in the ratio of 3:2 i.e. 150ml. solution 1 and 100ml of solution 2. Solution 1 - 2.269g.KHeFD* in 250ml water. Solution 2 - 4.75g IMasHPO^.. 12HB0 in 250ml water. b) Enzyme Solution An aqueous solution of a-amylase (type 111 —A, from Bacillus subtil is obtainable from Sigma London Chemical Company Ltd.) was prepared at 0.4- mg/ml, 0.3 mg/ml, 0.2 mg/ml and 0.1 mg/ml. c) Iodine indicator solution 5ml. Ia in 5% KI 245ml water d) Tannic Acid Solutions 1g tannic acid in 1 litre Hs0 dilute down to 0.1, 0.05, 0.025 and 0.0067* solutions. 243 2. STANDARD REACTION PROCEDURE 4.5ml of the starch reagent mixture was added to 1ml of each of the four enzyme solutions, held in a water bath at 20 *C. Every 15 seconds, 0.25ml. of each reacting mixture was withdrawn and added to 4ml. of the iodine indicator solution until the end point was reached, as indicated when no change in the colour of the iodine solution was produced by the action of the enzyme-starch. The endpoint was determined by eye in natural daylight against a white background. THE ASSAY 1Q0mg of powdered leaf material was added to 2ml.Ha0. The samples were placed on a mechanical shaker for 1 hour. These were centrifuged at 5000 rpm for 10 minutes and the supernatant was retained. 0.25ml of the supernatant was added to 1ml. of each of the enzyme solutions. These were left for 10 minutes exactly to complex. 4.5ml. of the starch reagent mixture was then added to each enzyme mixture, and the resultant endpoints determined as described above. In order to standardise the method 0.25ml. of each of the tannic acid solutions was substituted for leaf extract and the reaction carried out as before. A control assay was performed alongside the experimental assays, in which water was substituted for the leaf-extract. The end points for this test were then subtracted from those of the leaf extract assays to give the delays in the end point of each enzyme-starch reaction. The delay in seconds at each enzyme concentration was then plotted against tannic acid concentration and used as calibration curves. The equivalent tannic acid concentrations to those found in the leaves were then read o ff the graph. 2 4 ^ r b> KJELDAHL DETERMINATION OF NITROGEN HVI L EAVES Total Nitrogen 1) 60mg of dried material was placed into a Kjeldahl flask. H) 2mls. of cone, sulphuric acid (Nitrogen-free) were added to the flask. 3) A single selenium catalyst tablet was added to the flask. 4) The flasks were placed in a Kjeldahl burner. Initially they were heated gently; the heat was increased when dense white fumes were no longer emitted. 5) The tubes were heated until the digestion was complete, that is when the solution was clear lemon yellow and stayed that way even when shaken. The flasks were allowed to cool. The solution was then made up to 50mIs. with distilled water. 6) A flask with no leaf material added was treated in the same way and served as a blank. 7) 2ml. of the solution was placed in tubes and passed through a Technicon Auto Analyser. 8) 4 standard solutions of nitrogen were made up to concentrations of 1, 5, 10 and 20ppm. 2ml. of each of these were also passed through the Auto Analyser. 9) The peak height of each standard solution was measured on the print out. A calibration curve was drawn on log graph paper. The nitrogen concentration of each sample was read off the calibration curve after calculating peak height. 245 A F F E N D I X ^ Regression analysis of absolute abundance for each insect group in order determine the distribution of the data. PARAMETER ESTIMATES.E T. VALUE P Cicadellidae Intercept -0.2992 0.3083 0.9704 0.3372 Slope 1.4573 0.0947 15.3744 <0.001 Delphacidae Intercept 6.0817x10--B 0.2056 0.3 0.7697 Slope 1.2133 0.1401 8.66 <0.001 Curculionoidea Intercept 2.3857 1.6666 1.43 0.1591 Slope 1.3001 0.0966 13.46 <0.001 Heteroptera Intercept -2.5065 0.5673 4.42 <0.001 Slope 3.5509 0.0144 246.4 <0.001