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5-1978
The Role of Vegetation Architecture in Determining Spider Community Organization
Cynthia L. Hatley Utah State University
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THE ROLE OF VEGETATION ARCHITECTURE IN DETE~~INING
SPIDER COMMUNITY ORGfu~IZATION
by
Cynthia L. Hatley
A thesis submitted in partial fulfillment of the requirements for the degree
of
MASTER OF SCIENCE
in
BIOLOGY ECOLOGY
A~proved:
UTAH STATE UNIVERSITY Logan, Utah
1978 ii
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS • iii
LIST OF TABLES iv
LIST OF FIGURES vi
SUMMARY vii
INTRODUCTION • 1
Plot Description. 4
METHODS 5
Shrubs . 5
Spiders 13
RESULTS 19
Seasonal Variation . 19
Shrubs . 26
Spiders 30
DISCUSSION 53
Seasonal Variation . 53
Shrub Architecture . 55
Species and Guild Diversity . 56
Guilds . 57
Problems and Areas for Future Study 61
REFERENCES 62
APPENDIX 66 iii
ACKNOWLEDGMENTS
I would like to thank my major professor Dr. James A. MacMahon for his assistance throughout this study, and the other members of my commit tee Drs. George E. Bohart and Ivan G. Palmblad for their helpful criti cisms of the manuscript. Eric Zucher and Don Phillips reviewed the manu script, Linda Finchum typed the manuscript and Kim Marshall wrote the data reduction programs. Dr. Willis Gertsch determined the spider species.
Special thanks to G. Murchie Briggs for help in preparation of the manuscript and for constant encouragement. ,Part of the study was made possible by the US/IBP Desert Biome (NSF Grant# GB-32139).
Cynthia L. Hatley iv
LIST OF TABLES
Table Page
1. Spider species included in each guild . 15
2. Correlation coefficients relating seasonal factors and indicies of spider species and guild diversity . 27
3. Correlation coefficients relating seasonal factors and guild relative densities. Guilds are as in Fig. 5. 28
4. Correlation matrix relating shrub architectural parameters . 29
5. Separation of shrubs into groups using shrub photographs 31
6. Comparison of mean weekly spider density in clipped, tied and control shrubs 32
7. Correlation coefficients relating indicies of spider and guild diversity and shrub architectural parameters 33
8. Comparison of weekly guild IV in clipped, tied and control shrubs . 35
9. Comparison of weekly guild relative density in clipped, tied and control shrubs 42
10. Correlation coefficients relating guild density and shrub architectural parameters 43
11. ANOV and LSD calculations comparing species density in the three shrub groups . 45
12. ANOV and LSD calculations comparing resident spider species density in the three shrub groups 46
13. ANOV and LSD calculations comparing spider guild density in the three shrub groups 47
14. ANOV and LSD calculations comparing guild 1 density in the three shrub groups 48
15. ANOV and LSD calculations comparing guild 2 density in the three shrub groups 49
16. ANOV and LSD calculations comparing guild 3 density in the three shrub groups 50
17. M~OV and LSD calculations comparing guild 4 density in the three shrub groups 51 v
Table Page
18. ANOV and LSD calculations comparing guild 5 density in the three shrub groups . 51
19. Mean, maximum and minimum weekly summer temperature and relative humidity in 1974 67
20. Mean, maximum and minimum weekly summer temperature and relative humidity in 1975 68 vi
LIST OF FIGURES
Figure Page
1. Photographs of a clipped shrub taken before and after the foliage density was decreased by clipping 8
2. Photographs of a tied shrub taken before and after the foliage density was increased by tying its branches together 10
3. Photograph of a typical Artemisia tridentata illustrating areas of dense foliage, open foliage and crown . 12
4. Seasonal patterns of spider species diversity (H'), species density (p) and evenness (J') in 1974 and 1975 . 21
5. Seasonal pattern of spider guild IV's in 1974. Guild 1 includes the families Gnaphosidae, Anyphaenidae and Clubionidae; guild 2 includes the subfamily Philodrominae; guild 3 includes the subfamily Misumeninae; guild 4 includes the families Salticidae and Oxyopidae; guild 5 includes the families Linyphiidae, Theridiidae, Dictynidae, Argiopidae and Tetragnathidae 23
6. Seasonal pattern of spider guild IV's in 1975. Spider guilds are as in Fig. 5 25
7. Seasonal comparison of spider guild 1 and 2 IV's in clipped, tied and control shrubs. Spider guilds are as in Fig. 5 37
8. Seasonal comparison of spider guild 3 and 5 IV's in clipped, tied and control shrubs. Spider guilds are as in Fig. 5 39
9. Seasonal comparison of spider guild 4 IV's in clipped, tied and control shrubs. Guild 4 is as in Fig. 5 . 41 vii
SUMMARY
The Role of Vegetation Architecture in Determining
Spider Community Organization
by
Cynthia L. Hatley, Master of Science
Utah State University, 1978
Major Professor: Dr. James A. MacMahon Department; Biology
The relationships between vegetation architecture and spider community attributes were examined in a big sage (Artemisia tridentata) community. Spiders were separated into guilds using similarities of species' hunting behavior. Shrub architecture was experimentally manipulated in the field by either clipping 50% of a shrub's foliage to decrease foliage density or tying together a shrub's branches to increase foliage density.
Temporal patterns of spider species density, diversity (H') and evenness (J') showed midsummer peaks in both 1974 and 1975. Seasonal spider guild trends reflected the temporal prominence of a member species or genus. These temporally abundant species appeared to play a major functional role in this community.
Shrub perturbations resulted in changes in spider species and guild densities. Spider species and guild density in the tied shrubs were significantly higher than that in the clipped or control shrubs sampled.
Spider species diversity, density and guild density were also positively correlated with indicators of shrub volume and shrub foliage diversity.
This suggests that structurally more complex shrubs (tied) can support greater spider species densities and diversity. viii
Spider guild densities and IV's were significantly altered by
changes of shrub architecture. The observed guild distributions . were
in agreement with known hunting behavior and life history data of
the member species.
The data suggest that architectural properties of habitat may be
an important determinant of predatory invertebrate species diversity and distribution. Guild analysis may be useful in examining the roles
of species groups in community studies.
(68 pages) INTRODUCTION
Spatial heterogeneity may be a major factor affecting animal species diversity in a community. Species diversity has been correlated to various measures of habitat physical complexity. MacArthur and MacArthur
(1961) used measures of vertical habitat diversity (Floral Height
Diversity) to explain and predict bird species diversity (MacArthur et al., 1962; MacArthur, 1964). Pianka (1966, 1967) inclucted vertical and horizontal measures of habitat diversity (Plant Volume Diversity) in studies which correlated shrub structure with lizard species diversity in flatland desert communities. The relationship between habitat diver sity and species diversity has also been demonstrated for desert rodents
(Rosenzweig and Winakur, 1969), marine invertebrates (Abele, 1973), spiders (Uetz, 1975) and insects (Murdock et al., 1972). Vegetation structure provides varying types of substrates or microhabitats which are differentially suitable for animal species. The type of substrate on which a species occurs may determine the food sources available to it and also dictates the method in which they are obtained.
Spiders are well suited for ecological studies. As a group they are cosmopolitan, and locally abundant in terms of individuals and taxa.
Their small size permits definition of a community in a small area.
Spiders, as predators, are not coupled to a particular plant species as a food source; vegetation structure may therefore be an important determinant of spider community attributes.
Spider distribution is affected by substrate structure (Barnes and
Barnes, 1955; Duffey, 1962, 1966, 1968; Lowrie, 1948; Uetz, 1975). 2
Coleburn (1974) found that the spatial nature of limestone grikes
affected the patterns of Araneus web distribution. Bulan and Barrett
(1971) found that arachnid density decreased in oak fields after mowing
and remained lower in subsequently burned fields than in unburned fields.
The structure of spider communities has been found to change with plant
succession through changes in spider species density and population
density. In general the proportion of web-builders to hunting spiders
increases towards a climax in vegetation (Lowrie, 1948; Dowdy, 1950;
Chew, 1961).
Studies of vegetation structure with regard to spiders have
included the vertical and horizontal aspects of foliage distribution but rarely the internal qualitative attributes of foliage density. The vertical stratification of spider populations has received the most
study, especially in forest communities (Dowdy, 1950, 1951; Gibson, 1947;
Turnbull, 1960). Turnbull (1960) found that the vertical stratification of spiders in an oak stand was highly developed but not clean cut.
Many spider species move between strata diurnally and/or seasonally
(Muma and Muma, 1949; Turnbull, 1960). Enders (1974) found that orb web spiders chose different vegetation heights at different instars.
MacMahon (in manuscript) found a desert species of Diguetia which placed its web in shrubs at a height corresponding to a break in the vertical temperature profile. Tretzel (1955, in Turnbull, 1973) considers hori zontal and temporal stratification in spiders of primary importance and vertical stratification secondary. Chew (1961) noted a correlation between the presence and abundance of spiders and the level of shrub development. He also found a horizontal separation of several spider 3
species which preferred specific desert shrub species. Uetz (1975)
studying the guild of wandering spiders correlated spatial differences
in species diversity with litter depth and a measure of habitat space.
A functional approach can be developed in community studies by
examining the methods by which organisms exploit their environment.
Functional analysis of community organization has been used in studies
of plant-arthropod associations (Root, 1973), wandering spider commun
ities (Uetz, 1975) and desert mammal communities (MadMahon, 1976).
Species guilds, defined by Root (1967) as "a group of species that exploit the same class of environmental resources in a similar way" can be used to identify functional roles present in a system. This approach considers sympatric organisms as a unit, involved in a competitive
interaction, regardless of taxonomic relationships. Functional organ ization can then be considered independent of the individualistic response a single species may make to local conditions (Gleason, 1939).
Spiders can be arranged into guilds based on similarities in their methods of obtaining food e.g. web builders, running and jumping spiders, and ambushing spiders (Petrusewicz, 1938; Balogh and Laska,
1974, in Turnbull, 1973).
This study investigates the role of spatial heterogenetiy in determining community organization as represented by species and guild attributes. This is implemented through experimental perturbation of shrub architecture in a field situation. The main objectives are:
1. to examine the interaction of vegetation architecture and
spider species diversity; 4
2. to describe the functional organization of spiders in a
shrub community using spider guilds;
3. to examine the role of vegetation architecture in determining
functional organization of spiders in a shrub community.
Plot Description
The study area is located 3.2 km (2 mi) northeast of Logan on the Bonneville Lake terrace of the Bear River Range, 100 m south of the mouth of Green Canyon, Cache County, Utah. Mean annual precipitation is between 38.1 and 43.2 em, mean annual temperature is between 7 and
9°C, and frost-free days average between 100 and 120 (USDA Soil Con servation Service and Forest Service, 1974). The plot is at an elevation of 1477 m and faces southwest (28% slope). Land is used mainly for watershed and wildlife.
The area is dominated by big sage (Artemisia tridentata). Other shrubs present include Purshia tridentata, Gutierrezia sarothrae, and
Chrysothamnus nauseosus. Major herb and forb species include Balsamorhiza sagittata, Wyethia amplexicauli~, Bromus tectorum, and Bromus brizaeformis. 5
METHODS
A one hectare (100 x 100 m) plot was established on the study area. This was divided into twenty-five (20 x 20 m) subplots.
A hygrothermograph (WeatherMeasure) was operated on the plot during the 1974 and 1975 field seasons.
~hrubs
An individual shrub in this study was operationally defined as a shrub mass discontinuous with the foliage of another shrub by 10 -cm or more. Linear measurements of height and minimum and maximum width were recorded for each shrub sampled. These measurements were used to estimate shrub volume and cover. Volume was calculated using the formula for the volume of an oblate spheroid
2 v 4/3nab (1) where a is the linear dimension of the major axis and b is the linear dimension of the minor axis. Cover was estimated using the area of an ellipse
A nab (2) where a and b are as defined above.
In April of 1975 shrubs (Artemisia tridentata) were experimentally manipulated to change their foliage density, in preparation for a second sampling season. The subp-lots were randomly divided into three groups. On each of seven subplots fifty shrubs, chosen randomly, were 6
altered by clipping fifty% of their foliage (Fig. 1). On each of 7
other subplots fifty shrubs chosen randomly were tied up to in~rease
their foliage density (Fig. 2). Eight additional subplots were used
as controls.
Differences in foliage density of sampled shrubs v7ere estimated with the use of Polaroid photographs. Photographs of the shrubs were
taken against a contrasting background gridden in 20 em squares (Fig. 3).
The foliage was subjectively separated into areas of the different
foliage types (dense foliage, open foliage, and crown) and three height
classes (0-40, 41-80 and 81-120 em). Crown was defined as the peripheral vertical branches of a shrub which produce the paniculate infloresence of Artemisia. For each photograph, areas in each category were cut out and weighted on an electrobalance. Since Polaroid film is of constant weight, the percent composition of each foliage type and height class
could be calculated. These values were used in the Shannon-Wiener function (Shannon, 1948):
s H' p.ln (3) - I l i=l to calculate an index of shrub foliage diversity (SFD), where s equals the number of foliage types in each height class (9 possible) and p. l equa 1 s t h e proport1on. o f t h e s h ru b 1n . t h e 1.th f o 1.1age type f or eac h height class. This was an attempt to indicate the patchiness of foliage distribution in a shrub.
The photographs were also used to separate the shrubs into three general groups on the basis of foliage density. Group I includes shrubs composed of greater than 50% open foliage, group II includes 7 Fig. 1. Photographs of a clipped shrub taken before and after
the foliage density was decreased by clipping. Approxi
mately 50% of the foliage was removed by clipping every
other branch. 8
'
Before Clipping
After Clipping 9 Fig. 2. Photographs of a tied shrub taken before and after
the foliage density was increased by tying its branches
together. 10
Before Tying
After Tying 11 Fig. 3. Photograph of a typical Artemisia tridentata taken
against a gridded background. Areas of dense foliage,
open foliage and crown are outlined. 12 open
dense 13 shrubs comp osed of less than 50% open foliage, less than 50% crown and less than 50% dense foliage, and group III includes shrubs with greater than 50% dense foliage.
Spiders in shrubs were sampled in the field with the use of several beating sheets. The shrubs were measured, surrounded with sheets and struck vigorously with a heavy club. Spiders fallen from the shrubs were sorted and preserved in the field.
In 1974, during a ten week period, 108 shrubs (chosen randomly) were sampled.
In 1975, during a 15 week period, 225 shrubs, including 75 each of clipped, tied, and control shrubs were photographed and sampled.
During each week three subplots, one of each perturbation type, were chosen randomly, and five randomly chosen shrubs were sampled on each plot.
Spiders
Spiders present in each shrub sample were identified and counted in the laboratory. Body length (not including spinnerets) was measured and sex determined for each individual. Spider species density, diversity, and evenness were calculated for each sample week in 1974 and 1975.
Species density was the number of species encountered. Species diversity
(Equation 3) was determined where s equals the total number of species and p. equals the proportion of individuals in the ith species. Evenness l (Pielou, 1966) expresses the apportionment of individuals among the species and is calculated by the formula:
H' J' (4) lns 14
where J' equals evenness, H' equals the calculated species diversity
and s equals the number of species.
Immature spiders in early instars usually disperse from the area
where the egg sack was placed. For this reason spider species size
distributions were plotted and used to eliminate these immatures
from the samples and estimate the number of resident spiders. Resident
spider species diversity and evenness were then calculated for each
sample and also used in analysis.
Spider guilds were defined using similarities in predation stra
tegies (Kaston, 1948; personal observation) (Table 1). Spider guild 1
includes the families Gnaphosidae, Anyphaenidae and Clubionidae. These
spiders usually build retreats in shrub foliage and under the bark.
They have poor eyesight and hunt mainly at night. Guild 2 includes
the Philodrominae, a subfamily of the Thomisidae. These spiders are
active runners and move quickly through a shrub running down and
pouncing on prey. Guild 3 includes the Misumeminae, a second subfamily
of the Thomisidae, spiders which sit and wait, ambushing their prey.
The fourth guild includes the Salticidae and the Oxyopidae, spiders
having good eyesight and which are active hunters, running quickly and jumping after prey. Guild 5 is composed of the web-building
spiders including the families Theridiidae, Linyphiidae, Dictynidae, and the Araneidae.
· Guild Importance Values for shrub samples were calculated as in
Curtis and Mcintosh (1951) (IV = relative frequency + relative density + relative dominance). Spider frequency was calculated by the number of
shrubs in which a spider guild occurred out of a sample group. 15
Table 1. Spider species, collected from Artemisia tridentata, included in each guild.
GUILD 1 (nocturnal spiders with generally poor eyesight)
Clubionidae
Chiracanthium inclusum (Hentz)
Anyphaenidae
Anyphaena sp.
Gnaphosidae
Drassyllus nannellus Chamberlin & Gertsch
Herpyllus sp. ·
Poecilochroa montana Emerton
GUILD 2 (active running spiders)
Thomisidae
Philodrominae
Philodromus histrio (Latreille)
Philodromus satullus Keyserling
Philodromus sp. prob. speciosus Gertsch
Thanatus formicinus (Clerck)
Tibellus oblongus (Walckenaer)
Tibellus chamberlini Gertsch
GUILD 3
Thomisidae (ambushing spiders)
Misumeninae
Misumenops asperatus (Hentz)
Misumenops celer (Hentz)
Xysticus cunctator Thorell 16 GUILD 4 (active running and jumping spiders with good eyesight)
Salticidae
Icius similis Banks
Metaphidippus aeneolus (Curtis)
Metaphidippus verecundus (Chamberlin and Gertsch)
Pellenes hirsutus (Peckham and Peckham)
Phidippus johnsoni (Peckham and Peckham)
Sassacus papenhoei (Peckham and Peckham)
Synagales sp ..
Oxyopidae
Oxyopes scalaris (Hentz)
GUILD 5 (web-building spiders)
Linyphiidae
Erigoninae
Front inella communis (Hentz)
Meionet~ sp.
Theridiidae
Dipoena tibialis Banks
Euryopis scriptipes Banks
Enoplognatha ovata (Clerck)
Latrodectus hes_perus Chamberlin and Ivie
Steatoda americana (Emerton)
Theridion neomexicanum Banks
Theridion sp.
Dictynidae
Dictyn~ jdahoana Chamberlin & Ivie
Dictyna completa Chamberlin & Gertsch 17
GUILD 5 cont.
Argiopidae
Aculepeira verae Chamberlin & Ivie
Araneus displicatus (Hentz)
Araneus gemma (McCook)
Argiope trifasciata (Forskal)
Hyposinga singaeformis (Scheffer)
Metepeira foxi Gertsch & Ivie
Tetragnathidae
Tetragnatha laboriosa (Hentz) 18
Relative frequency was determined by the formula:
N. 1 ~- (5) 1 CN
wh ere D~N~ . equa 1 s re 1 at1ve . f requency o f t h e 1.th gu1"ld , N1 • equa 1 s t h e 1 1 frequency of the ith guild, and CN equals the combined frequency of all
guilds. Density was the number of individuals in each guild for a
sample group (perturbation type or week). Relative density was deter~
m1ne· d (E quat1on· 3) wh ere RN . equa 1 s re 1 at1ve· d ens1ty· of the ;th~ gu1"ld , 1 Ni equals the density of the ith guild and CN equals the combined
density of all guilds. Spider biomass was estimated using spider body
length in the formula for the volume of a sphere
V = 4/3r3 (6) where r equals 1/2 the body length of an individual. Volume was
then used to calculate relative dominance. Relative dominance was
similarly determined (Equation 3) where RN. equals relative dominance 1 of the ith guild, N. equals the volume of the ith guild, and CN equals 1 the combined volume of all guilds.
In 1974 spider IV's were calculated weekly for all shrubs combined.
In 1975 spider IV's were similarly calculated weekly for clipped, tied, and control shrubs and for all shrubs combined. 19
RESULTS
Seasonal Variation
A total of 4613 spiders representing 40 species in 11 families were collected from the shrubs sampled (Table 1). The seasonal trend of species diversity, species density and evenness were very similar in both 1974 and 1975 (Fig. 4). Species diversity rose gradually to midsummer peaks and maximum species density values of 24 and 25 species were found in August of 1974 and 1975 respectively. Evenness was rela tively constant throughout the season remaining at approximately 0.80 during both 1974 and 1975. Changes in species diversity are due to changes in species density rather than the equitability.
Temporal patterns of spider guild characteristics are shown in Figs.
5 and 6. Spiders in guild 1 (nocturnal hunting spiders) were collected in low numbers during both sampling seasons, density peaked in July of
1974 and August of 1975. Maximum IV's occurred in August of both years. Guilds 2 and 3 (subfamilies Philodrominae and Misumeminae of the Thomisidae), showed offset peaks in density and IV in 1975.
Guild 2 had highest IV's and densities during August of both seasons.
Guild 3 peaked earlier, reaching greatest IV's and densities during
July of both years. Guild 4, the active hunting spiders, remained high in IV throughout both seasons; population densities peaked during late summer. The web building spiders (guild 5) showed density peaks both early and late in the season. IV's were greatest in June of
1975 and constant in July and August. 20 Fig. 4. Seasonal patterns of spider species diversity (H'),
species density (p) and evenness (J') in 1974 and
1975. 21
(d) AJ.JSN30 0 0 0 0 0 0 t() C\J - 0 I'() (\.] - 0 . y l{)- ;·I ~ : : r<> ·. ·. \' C\J .---r· : - - I - "\/ - I : 0 0 ,...... - (/) \ \ - /\ ..::c- 0) :::r: . . 0) Q) \~ \ \ Q) co ro ...... ,~ - \ · ~ . - -J :r:. . :.J 1'- . ~ 1'- \ I : ~ . w <.0 <.0 ~- I I 1.{) I/~ l- I 10 ~ II ~ I ""'/ I'() \ · ~ . r<> l{) \ ~ \ 1'- . . I . C\J en \·~; ' \\ -~ rr> OJ 0 r() {\j 0 (,r)SS3NN3/\3 9 (,H) Al1SH3/\IO 22 Fig. 5. Seasonal pattern of spider guild IV's in 1974. Guild 1
includes the families Gnaphosidae, Anyphaenidae and
Clubionidae; guild 2 includes the subfamily Philodrominae;
guild 3 includes the subfamily Misumeninae; guild 4
includes the families Salticidae and Oxyopidae; guild 5
includes the families Linyphiidae, Theridiidae,
Dictynidae, Argiopidae and Tetragnathidae. 23
200 -- -GUILD I 1974 h·········· GUILD 2 -·-·- GUILD 3 180 - GUILD 4 --- GUILD 5
160
140
120
100
80 .· . .· .· .· I 60 ' I .· .:··· ...... I , I 40 I , \ :. i /". X... 20 ·.·· .>
o~--~--~----~~----~~~- 3 4 5 6 7 8 9 10 II JULY AUGUST OCT. TIME (weeks) 24 Fig. 6. Seasonal pattern of spider guild IV's in 1975. Spider
guilds are as in Fig. 5. . 25
200 1975 -GUILD I ~········-· GUILD 2 -·-·- GUILD 3 180 --GUILD 4 ---GUILD 5 160
140
120
100
80 \ i'. /-·-. / ·. I > \,' ' /. v ·. ~ 1\ .,/ ·' ·.• I . . r.·~.--- ·... : ·.I 60 I \ "" ..... t·.. \ -...... : - ).. . .. · ...... / .' -- \(I ,, --\ , .. .I ... ,.-,1v \I.\ 40 ··... /-· ...-). : ,, ., ······/..· . :--.... ·. , ·,,/···· .... · 20
0 2 3 · 4 5 6 7 8 9 10 II 12 13 14 15 JUNE JULY AUGUST TIME (weeks) 26
Correlation coefficients relating indices of species diversity and
guild diversity to weekly mean temperature, and percent relative humidity (RH) are given in Table 2. Species diversity in both 1974 and 1975, and species density in 1974, significantly correlated with
temperature. Species density (1975) was negatively correlated with temperature. Guild diversity and evenness (1975) and guild density
(1974) significantly correlated with temperature. Temperature did not significantly correlate with RH in either 1974 or 1975.
Correlation coefficients relating temperature, RH, and guild relative density are given in Table 3. Guild 1 shows no significant correlation with temperature or RH. Guilds 2 and 3 significantly correlate with temperature in 1975. Guild 4 negatively correlates with RH in both 1974 and 1975. Guild 5 negatively correlates with tempe rature and positively correlates with RH in 1975.
Shrubs
Results of correlations relating shrub architectural parameters
(obtained from field measurments and photographs), are given in Table
4. Significant correlations are as follows: height, cover, volume, shrub mass 81-120 em (percent), and SFD are positively correlated to each other; shrub mass 0-40 em negatively correlated with height, cover, volume, mass 41-80 em, mass 81-120 em and SFD; percent dense foliage is negatively correlated to percent open foliage, and negatively correlated with percent crown.
To assess the effect of the field perturbations on shrub architecture, the 1975 shrubs were separated into three groups using Table 2. Correlation coefficients (r) for temperature, humidity and indices of spider species and guild diversity.
Species Guilds
H' J' Density H' J' Density
Mean weekly 1974 . 85 7** .679 . 758* .428 .021 . 761* temperature 1975 .547** .505 .249 .674'~* . 714''<* -.033 .
Mean weekly 1974 -.246 . 366 -.605 .214 .617 -.045 relative humidity 1975 -.205 .190 -.673** -.164 -.113 -.131
·k • 01 < p < • 05 ** p < • 01
N ""-..J Table 3. Correlation coefficients (r) for seasonal factors and guild relative density.
Relative Density
Guild 1 Guild 2 Guild 3 Guild 4 Guild 5 (rtoc turna1) · (running) (ambushing) (jumping) (web building)
Mean weekly 1974 .702 . 685 -.665 .508 -.664 temperature 1975 .016 .747** .576* .078 -.643*
Mean weekly 1974 .646 .237 -.077 -.713* .251 relative humidity 1975 -.228 -.179 .081 -.747** .695**
* .01< p< .OS ** p < • 01
N co Table 4. Correlation matrix of r values for each pair of shrub architectural parameters.
Mass Hass Mass Percent Percent 0-40 41-80 81-120 dense open Height Cover Volume em em ern foliage foliage Crown
Height
Cover . 426'1:
Volume • 523;': . 921•1:
Mass 0-40 em -.561* -. 452'1: -.460*
Mass 41-80 em .159 .022 -.012 -.556*
Mass 81-120 em .550* . 5 24 i< . 561'1: -. 770* -.102
Percent dense -.180 .104 .124 .105 -.110 -.042 foliage
Percent open .138 -.053 -.080 -.004 . 058 -.040 -.897* foliage
Percent crown .068 -.099 -.082 -. 214'1: .099 .179 -.093 -.357*
SFD .598* .480* .480* -. 742* .136 • 783i< -.093 .038 .111
i\: p < • 01 30
foliage density data obtained from the shrub photographs. These
shrub groups exhibited distributions of foliage density in accord with perturbation type (Table 5). Fifty-seven% of the clipped shrubs had greater than 50% open foliage and 71% of the tied shrubs had
greater than 50% dense foliage. The control shrubs were almost evenly
distributed with 43% having greater than 50% dense foliage and 44% having greater than 50% open foliage.
Spiders
Mean weekly spider density for all species was used in the
Wilcoxon ranked sum statistic (a nonparametric analog to a paired "t"
test) to compare clipped, tied and control shrubs (Conover, 1971).
The shrubs were similarly compared for resident spider species and guilds. For all species, resident species and guilds, tied shrubs differed significantly from both clipped shrubs and control shrubs
(Table 6). In both comparisons tied shrubs had greater number of species, resident species and guilds than the clipped and control shrubs. No significant differences were found between clipped and control shrubs in the three comparisons. Correlations between shrub parameters and indices of species and guild diversity are given on Table 7. Spider species diversity, and evenness and density for all spiders and resident spiders show positive significant correlations with shrub height, cover, volume, percent dense foliage, mass 81-120 em, and SFD. Significant negative correlations are found with shrub mass 0-40 em, and percent open foliage. Spider guild density shows significant positive 31
Table 5. Separation of shrubs into groups using shrub photographs.
Group I includes shrubs with greater than 50% open foliage, Group II includes shrubs with less than 50% open foliage, dense foliage and crown, and Group III includes shrubs with greater than 50% dense foliage.
Number of Shrubs
Group I Group II Group III
Clipped 43 (57%) 23 (31%) 9 (12%)
Tied 8 (11%) 15 (20%) 52 (71%)
Control 32 (43%) 10 (13%) 33 (44%) 32 Table 6. Comparison of mean weekly spider density in clipped, tied, and control shrubs. Shrubs were compared using the Wilcoxon Ranked Sum Statistic.
A. ALL SPIDERS
CLIPPED
TIED * n=l4
CONTROL ** n=l4 n=l4
CLIPPED TIED CO~TROL
B. RESIDENT SPIDERS
CLIPPED
TIED ** n=l5
CONTROL ** n=l4 n=l3
CLIPPED TIED CONTROL c. GUILDS
CLIPPED
TIED ** n=lS
CONTROL ** n=l4 n=l4
CLIPPED TIED COl~TROL
* significant at A .10
** significant at A .05
- not significant Table 7. Correlation coefficients (r) for indices of spider species and guild diversity and shrub architectural parameters.
All Species Resident Species Guilds
H' J' Density H' J' Density H' J' Density
Height . 301** .155,'< .321*,'< .276*''< .230** .253** .140* .074 .199**
Cover .178* .064 .206** . 260*,'< .211*7< • 25 9;'cic .023 -.027 .118
Volume .106 .044 .164* . 269*''< .217** .291** .016 -.038 .122
Mass 0-40 em -.171''< - .163''< -.221** -.172* -.180* -.181** -.084 -.105 -.138*
Hass 41-80 em . 054 .078 . 035 .050 .045 .058 .081 .107 .077
Mass 81-120 em .163* .136 .237** .167* .181*,'< .171* .039 .043 .106
Percent dense .157* .074 .096 .139* .151''< .202** .105 .044 .164* foliage
Percent open -.180''< -.089 -.124 -.181** -.155* -.193** -.134 -.064 -.185** foliage
Percent crown .075 ~ 045 .076 .000 .030 .008 .078 . 051 .071
SFD . 2307<* .173* .270* -.172* . 236 7~* . 212*''< .122 .115 .181**
,'c • 01 < p < • 05 •'<* p < • 01 w w 34
correlations with height, percent dense foliage and SFD, and negative correlations with percent open foliage, and mass 0-40 em.
A comparison of the three shrub types by guilds using mean weekly
IV's in the Wilcoxon Ranked Sum statistic is given in Table 8. There are no significant differences among the three shrub types for guild
1 and guild 2 (Fig. 7). Guild 3 shows significant differences between tied shrubs and both clipped shrubs and control shrubs. In most weeks guild 3 had greater IV's in the tied shrubs (Fig. 8). Guild 4 shows significant differences between tied shrubs and both clipped shrubs and control shrubs, and no significant difference between clipped shrubs and control shrubs. For most weeks guild 4 IV's were greater in clipped shrubs and control shrubs than tied shrubs (Fig. 9).
Significant differences were found between clipped shrubs and both tied and control shrubs for guild 5. In most weeks guild 5 had greater IV's in tied and control shrubs than clipped shrubs (Fig. 8).
A second analysis comparing clipped, tied and control shrubs was done using guild relative density in the Wilcoxon Ranked Sum statistic.
The results were in agreement with those obtained using IV's, no differences among shrub types were found for guilds 1 .and 2, and guilds 3, 4, and 5 demonstrated the same significant differences between shrub types (Table 9).
A correlation matrix relating spider guild densities and shrub architectural parameters is given in Table 10. Guild 1 shows no significant correlations with shrub factors. Guild 2 correlates positively with shrub height and SFD, and negatively with percent open foliage. Guild 3 correlates positively with percent dense 35 Table 8. Comparison of spider guild weekly IV's in clipped, tied and con trol shrubs. Shrubs were compared using the Wilcoxon Ranked Sum statistic. Guilds are as in Table 1.
GUILD 1 (nocturnal spiders) GUILD 4 (jumping spiders)
CLIPPED
TIED ** n=l3 n=l5 CONTROL ** n=ll n=l2 n=l5 n=l5 CLIPPED TIED CONTROL CLIPPED TIED CONTROL
GUILD 2 (running spiders) GUILD 5 (web-building spiders)
CLIPPED
TIED ** n=l5 n=l5
CONTROL ** n=l5 n=l5 n=l5 n=l5
CLIPPED TIED CONTROL CLIPPED TIED CONTROL
GUILD 3 (ambushing spiders)
CLIPPED
TIED ** n=l5
CONTROL ** n=l4 n=l5
CLIPPED TIED CONTROL ** significant at A .05
- not significant 36 Fig. 7. Seasonal comparison of spider guild 1 and 2 IV's in
clipped, tied and control shrubs. Spider guilds are
as in Fig. 5. 100 GUILD I ~CLIPPED f:::: :':<::::::::::'1 T IE 0 c:::J CONTROL 50
o~~~--~~~~~~~~--~~~~~~~~~~~~~~
1- > 150 GUILD 2
10 o- r- r-
:~ j ~ ~ ~ · 5 0- § ~ : ~ r- ~ :: ~ ~ ~ ~ ~ ~ ~ ~ '-- ~ r- r- ~ §: ~ § ~ ~ ~ s ~ ~ ~ \ ~ ~ ~ ': ~ ~ ~ ~ ~ ~ ~ ~ ~ ': ~ ~ ~ ~ § ~ \ ~ ~ ~ ': ~ ~ ~ ~ r\~ [1\ ~ ~ § ~ : ~ . ~ 0 . 2 3 4 I 5 6 7 8 9 I 10 II 12 13 14 I 15 JUNE JULY AUGUST s TIME (weeks) 38 Fig. 8. Seasonal comparison of spider guild 3 and 5 IV's in
clipped, tied and control shrubs. Spider guilds are
as in Fig. 5. GUILD 3 IZZZl2'ZJ CLIPPED
1:::'''''""''''''''''''''':::1 TIED 10 c:::::J CONTROL
> GUILD 5
100
50
6 7 II 12 13 JULY AUGUST TIME (weeks) 40 Fig. 9. Seasonal comparison of spider guild 4 IV's in clipped,
tied and control shrubs. Guild 4 is as in Fig. 5. ~CLIPPED GUILD 4 1::::::::::::':::'::: 1 TIE 0 · c:::J CONTROL 200
150 :> t-f 100
50
2 3 4 5 I I I~ I~ JUNE AUGUST TIME (weeks) 42 Table 9. Comparison of spider guild weekly relative density in clipped, tied and control shrubs. Shrubs were compared using the Wilcoxon Ranked Sum -statistic. Guilds are as in Table 1.
GUILD 1 (nocturnal spiders) GUILD L} (j tnnping spiders)
CLIPPED
TIED ** n=l3 n=l5
CONTROL ** n=ll n=l2 n=l5 n=l5 CL I PPED TIED CONTROL CLIPPED TIED CONTROL
GUILD 2 (running spiders) GUILD 5 (web-building spiders)
CLIPPED
TIED ** n=l5 n=l5
CONTROL ** n=l5 n=l5 n=l5 n=l5 CLIPPED TIED CONTROL CLIPPED TIED CONTROL
GUILD 3 (ambushing spiders)
CLIPPED
TIED * n=l5
CONTROL * n=l4 n=l5 CLIPPED TIED CONTROL * significant at A .10
** significant at A .05
- not significant Table 10. Correlation coefficients (r) for guild density and shrub architectural parameters.
Guild 1 Guild 2 Guild 3 Guild 4 Guild 5 (nocturnal) (running) (ambushing) (jumping) (web building)
Height .083 .162* .065 .315** .042
Cover .007 .007 .096 .299** .092
Volume .033 .022 .078 .346** .123
Mass 0-40 em -.022 -.092 ~.051 -.240** -.059
Mass 41-80 em .000 .052 .011 -.028 .075
Mass 81-120 em .025 .070 .053 .309** .014
Percent dense .098 .091 .250** .010 .283** foliage
Percent open -.136 -.145* -.189** -.046 -.243** foliage
Crown .099 .133 -.103 .083 -.051
SFD .086 .157* .096 . 343i~* .042
* . 01 < P < • OS ** p < • 01 44
foliage and negatively with percent open foliage and SFD. Guild 4
correlates positively with shrub height, cover, volume, and mass
80-120 em. Guild 5 correlates positively with percent dense foliage and negatively with percent open foliage.
Shrub groups as described in Table 5 were examined for differences
in spider species and guild parameters. An ANOV and LSD tests were run on the three shrub groups (treatments) using species, resident species and guild densities. Results of these tests are given on fu~OV
Tables 11, 12, and 13. F ratios among shrub groups were significant for all variables. LSD calculations show that group I differs signifi cantly from groups II and III for all treatments. Shrub groups II and
III have greater species, resident species and guild densities.
An ~~OV was also performed for each guild on the three shrub groups (treatments) using guild density. Many of the density values for individual shrubs were zero's, therefore 0.5 was added to the values to alleviate this problem (Sokal and Rohlf, .. 1969). The square root data transformation was performed to make the variances independent of the means, as a Poisson distribution was expected.
The results of this analysis are given in ANOV Tables 14-18.
Significant F ratios were found between shrub groups for guilds 1, 2,
3, and 5. LSD calculations show significant differences between shrub groups I and II for guild 2; between shrub groups I and III for guilds
1, 3, and 5; and between shrub groups II and III for guild 5. Table 11. ANOV and results of LSD calculations comparing spider species density in the three shrub
groups. Shrub groups are as in Table 5.
Shrub No. Mean No. Source of variation df ss MS F Group Shrubs Species
1 Among (shrub) groups 2 95.74 47.87 8.36* I 83 3.74 a
Within groups (error) 222 1270.25 5.72 II 48 5.12 b
Total 224 1366.00 III 94 4.60 b
* F .05[2,222] = 3 · 03 1 Numbers followed by the same letter are not significantly different at the 5% level according to LSD calculations. Table 12. ANOV and results of LSD calculations comparing resident spider species density in the three shrub groups. Shrub groups are as in Table 5.
Shrub No. Hean No. Source of variation df ss MS F Group Shrubs Species
1 Among (shrub) groups 2 43.89 21.95 7.34* I 83 2.22 a
Within groups (error) 222 660.06 2.98 II 48 3.25 b
Total 224 703.96 III 94 3.07 b
* F.05[2,222] = 3 · 03 1 Numbers followed by same letter are not significantly different at the 5% level according to LSD calculations. Table 13. ANOV and results of LSD calculations comparing spider guild density in the three shrub groups. Shrub groups are as in Table 5.
Shrub No. Mean No. Source of variation df ss MS F Group Shrubs Species
1 Among (shrub) groups 2 15.87 7.93 5.90* I 83 2.54 a
Within groups (error) 222 298.35 1.34 II 48 3.06 b
Total 224 314.22 III 94 3.10 b
3 03 * F.05[2,222] = · 1 Numbers followed by the same letter are not significantly different at the 5% level according to LSD calculations. Table 14. ANOV and results of LSD calculations comparing guild 1 density in the three shrub groups.
Shrub groups are as in Table 5. Data transformation was I Y + 0.5 ; original means are reported.
Mean Source of variation df ss MS F Shnub No. Guild 1 Group Shrubs Density
1 Among (shrub) groups 2 1 . 18 0.59 3.42* I 83 0.24 a
Within groups (error) 222 38.42 0.17 II 48 0.62 ab
Total 224 39.60 III 94 0.70 b
* F.05[2,222] = 3 · 03 1 Numbers followed by the same letter are not significantly different at the 5% level according to LSD calculations. Table 15. ANOV and results of LSD calculations comparing guild 2 density in the three shrub groups.
Shrub groups are as in Table 5. Data was transformed as in Table 14.
Mean Source of variation df ss MS F Shrub No. Guild 2 Group Shruhs Density
1 Among (shrub) groups 2 2.27 1.13 4.55* I 83 0.79 a
Within groups (error) 222 55.34 0.24 II 48 1.62 b
Total 224 57.61 III 94 1. 25 ab
* F.05[2,222] = 3 · 03 1 Numbers followed by the same letter are not significantly different at the 5% level according to LSD calculations. Table 16. ANOV and results of LSD calculations comparing guild 3 density in the three shrub groups.
Shrub groups are as in Table 5. Data was transformed as in Table 14.
Mean Shrub No. Guild 3 Source of variation df ss MS F Group Shrubs Density
1 Among (shrub) groups 2 1.40 0.70 3.50* I 83 0.97 a
Within groups (error) 222 44.61 0.20 II 48 1.06 ab
Total 224 46.01 III 94 1.15 b
* F.05[2,222] = 3· 03 1 Numbers followed by the same letter are not significantly different at the 5% level according to LSD calculations.
l..n 0 Table 17. ANOV and results of LSD calculations comparing guild 4 density in the three shrub groups.
Shrub groups are as in Table 5. Data was transformed as in Table 14.
Shrub No. Mean Source of variation df ss MS F Group Shrubs Guild 4 Density
Among (shrub) groups 2 2.68 1.34 I 83 3.07 a
Within groups (error) 222 187.49 0.84 II 48 4.31 a
Total 224 190.17 III 94 3.65 a
ns F.05[2,222] = 3.03 1 Numbers followed by the same letter are not significantly different at the 5% level according to !', LSD calculations. Table 18. ANOV and results of LSD calculations comparing guild 5 density in the three shrub groups.
Shrub groups are as in Table 5. Data was transformed as in Table 14.
Shrub No. Mean Source of variation df ss HS F Group Shrubs Guild 5 Density
1 Among (shrub) groups 2 11.30 5.65 8.86* I 83 1.91 a
Within groups (error) 222 141.64 0.64 II 48 2.68 a
Total 224 152.95 III 94 3.98 b
3 03 * F.05[2,222] = · 1 Numbers followed by the same letter are not significantly different at the 5% level according to LSD calculations.
\..J1 N 53
DISCUSSION
Seasonal Variation
Indices of spider diversity reach maximum values during midsummer
in the big sage community studied. This pattern was also found in
other studies of temperate arthropod communities (Murdock et al.,
1972; Root, 1973; and Uetz, 1975). Since the equitability component
of diversity (J') remained relatively constant through both seasons, the
seasonal pattern of species diversity was attributed to changes in
species density. The seasonal pattern of spider species diversity (H')
accounts for the significant correlations of H' and mean weekly temper
ature. This is probably the result of factors not measured in this
study, namely the seasonal abundance and availability of spider prey.
It has been suggested that communities may be organized by a
characteristic set of functions which determine community structure.
The fulfillment of functions can affect community species composition,
species diversity and the prominent species present. A function may be fulfilled by more than one species creating a redundancy which buffers the effects of perturbations on the community and maintains
community structure (MacMahon, 1976). Since functions may potentially be fulfilled by many different species, within any community the species present may be the result of many successful species additions, sub sequent population growths, and extinctions (Fager, 1968; 1Vhittaker and Woodwell, 1972).
One manner of examining the functional organization of a ·community is to group those species with similar niche dimensions into guilds. If 54
communities are organized by functional roles, individual changes of a
guild member species might be balanced by complementary changes of
another species within that guild. According to this view, guild values
should remain relatively constant within a community (Root, 1973). I
feel that community functions will not remain static over the course
of a season or from year to year, and guild values which reflect
community functions should not be expected to remain constant. The manner in which functions are carried out in any community is probably a dynamic process and is affected by both biotic and abiotic factors.
Spider guild densities and IV's observed in this study do not demonstrate a constancy but fluctuate during the season and between years. The data however do not allow complete examination of the functional roles present in this shrub community. The duration of the study was too short to closely e x amine seasonal and yearly trends, and the guilds established include only the spider predators of this community.
In this study guild trends reflect the temporal prominence
(abundance) of a member species or genus . In guild 1, Chiracanthium inclusum (Hentz) comprises 60.5% of the individuals. Guild 2 is represented by Philodromus histrio (Latreille) (57.5%), and guild 3 by Xysticus cuncator Thorell. (77.3%). Sassacus papenhoei (Peckham and
Peckham) (53.8%) represents guild 4 and Theridion spp. (52.8%) represent guild 5. Weekly IV's of these species significantly correlate with their guild IV's. The variance of guild IV's accounted by the prominent species is as follows: C. inclusum, 77%; ~ · histrio, 29%;
X. cuncator, 89%; S. papenhoei, 30%; and Theridion spp., 52.8%. 55
As seen above most of the seasonal variance in guilds 1 and 3 is accounted for by one species.
Abundant spider species appear to play a major role in the spider guilds studied. Spider guild densities and IV's fluctuate seasonally, reflecting in part reproductive patterns and response to annual weather patterns. The influence of weather is demonstrated by guild correlations with temperature and relative humidity.
Shrub Architecture
It is difficult to assess the complexity of vegetation structure, in a manner which is biologically meaningful. The shrub attributes measured in this study were chosen in an attempt to do this by using experimental manipulation of shrub architecture in a field setting.
As shown in Table 10, shrub photographs indicated that shrub archi tecture was altered by the perturbations and the shrubs exhibited foliage characters fitting their respective perturbation types. Shrubs were segregated into groups to examine the relationship of foliage density to spider parameters.
Shrub values of height, cover, volume, mass 0-40 em, mass 40-80 em, and mass 80-120 em indicate spatial characters of shrub structure
(Table 4). In general values of shrub height, cover, and mass 80-120 em are closely related to shrub volume. Usually a greater diversity of structure is realized in a larger shrub volume, and vertical and horizontal stratification can be better developed. Shrub mass 0-40 em is negatively related to indicators of shrub volume, and shrubs 56
with a large percentage of mass in this class are usually less diverse and have little development of vertical foliage stratification.
The proportion of shrub foliage types in a shrub indicates the quality of substrate available to spiders. Shrubs with large proportions of dense foliage presumably contain more complex substrates. Moreover these shrubs usually contain some open spaces as well. Shrubs with a majority of open foliage usually represent both a decrease in continuous expanses of dense foliage and a decrease in substrate available to spiders.
Shrub crown, defined as the peripheral vertical branches of a shrub which produce an inflorescence, may not be present on all shrubs .
These areas can occur in all height classes and arise from the underlying foliated branches.
The calculation of shrub foliage diversity (SFD) incorporating the distribution of foliage types among the height classes, was an attempt to describe the overall architectural properties of a shrub.
SFD correlated positively with indicators of shrub volume (height, cover, volume, mass 81-120 em) and negatively with mass 0-40 em (Table 4).
This indicates a greater development of structural diversity in larger shrubs.
Species and Guild Diversity
Shrub perturbations did affect changes in spider species and guild densities. Tied shrubs had significantly greater numbers of species, resident species and guilds than the clipped or control shrubs. Indices of spider diversity demonstrated significant positive correlations 57
with the percent dense foliage in a shrub and negative correlations with
the percent open foliage in a shrub. Positive correlations were also
found between indices of spider diversity, indicators of shrub volume,
and shrub foliage diversity (SFD). The ANOV of shrub groups using
spider species and guild densities, yields conforming results. Shrubs
with greater than 50% dense foliage or less than 50% dense foliage,
crown and open foliage combined, have greater numbers of species, resi
dent species and guilds than shrubs with greater than 50% open foliage.
Increased environmental complexity may allow larger numbers of
predatory species to coexist within a given habitat. More types of
substrate, kinds and numbers of prey, and varieties of microhabitat
are available to species within more complex environments. The data
presented indicate that more structurally complex habitats, here generally
represented by tied shrubs, support greater spider species densities
and species diversity. The data also demonstrate that changes in shrub
structure can cause changes in the distribution of spiders in shrubs.
I feel that spatial and architectural properties of habitat structure
can be a very important determinant of species diversity, density and
distribution of small predatory invertebrates in a community.
Guilds
The guilds examined in this study were broadly defined on the basis
of observations of spider hunting methods. The resulting spider guilds
corresponded with general taxonomic groupings. It was assumed that
differences in spider hunting behavior indicated differences in potential prey used, since prey are differentially susceptible to predator capture 58
methods. It was also assumed that each hunting behavior is best suited
to a certain type of habitat structure. For example, web building
spiders require substrate suitable for web attachment and ambushing
spiders require a place for concealment. This study attempted to
elucidate the relationship between species hunting behaviors and spatial
requirements, by analyzing the effects of change of spatial properties
of the habitat on spider guilds in a shrub community.
Guild 1 spiders (nocturnal hunting spiders) were probably less
susceptible to capture than the other diurnal guilds. The smallest
number of spiders captured was obtained from this guild. No signif
icant differences were found between shrub perturbation types for this
guild. However, significantly greater densities of this guild were
found in the shrub group with greater than 50% dense foliage. The
greater densities of this guild found in densely foliated shrubs may
reflect the location of retreats of these spiders in dense foliage.
For example, the retreats of ~· inclusium are constructed between the
leaves of densely foliated branches in Artemisia. Guild 1 spiders
captured were probably knocked from these diurnal retreats.
No significant differences were found between perturbation types
for guild 2 spiders (running spiders). Significantly greater densities were found in the shrub group with less than -50% open foliage, dense
foliage and crown. Guild 2 densities also positively correlate with
shrub height, and SFD, and negatively with percent open foliage.
(Values of shrub height correlate positively with SFD, and shrub crown values correlate negatively with percent open foliage). Philodromus histrio, the most abundant species in this guild (57.5%) is grey- 59
green in color, blending well with the color of Artemisia. Philodromus histrio was frequently observed in all foliated areas of the shrubs.
This may explain the lack of difference between shrub perturbation types observed for this guild. It appears that shrubs with a diversity of foliage types may be more attractive to these spiders. This condi tion can occur in all of the perturbation types.
Guild 3 (ambushing spiders) had significantly higher IV's in tied shrubs than in clipped or control shrubs. ANOV of shrub groups found significantly greater guild 3 densities in shrubs with greater than 50% dense foliage than in shrubs with greater than 50% open foliage. Guild
3 densities also correlate positively with percent dense foliage and negatively with percent open foliage and SFD. These data demsonstrate that shrubs with dense foliage can support higher densities of ambushing spiders.
Spider parameters for guild 4 (jumping spiders) relate positively with indicators of shrub architectural diversity and shrub volume.
Guild 4 IV's were higher in the clipped and control shrubs than in the tied shrubs. Guild 4 densities correlated positively with shrub height, cover, volume, mass 81-120 em and SFD. They correlate negatively with mass 0-40 em. Since these spiders are quick active hunters with excellent vision, shrubs with only dense foliage may obstruct their vision and impair the rapid jumping movements used to capture prey. No differences for this guild were found among the shrub groups. However, these groups are based on foliage density alone and segregate shrubs in a manner which eliminates recognition of shrubs with other structur ally diverse foliage characters. 60
A small proportion of guild 5 spiders (web-building spiders) were
in the family Argiopidae (7.7%), the orb-weaving spiders, which utilize
a more open substrate for web attachment. Most of the web builders
collected construct irregular snare type webs requiring complex
structural support which is available in shrubs with dense branching.
The majority of these spiders were also relatively small in size and built small webs. In the shrub perturbations, guild 5 IV's were signif
icantly greater in tied and control shrubs. These spider densities correlated positively with percent dense foliage and negatively with percent open foliage. In ANOV of shrub groups, guild 5 densities were highest in shrubs with greater than 50% dense foliage. As demonstrated by the data, the density of web-building spiders in a shrub is related to shrub foliage density.
Spider guild densities and Importance Values were significantly altered by shrub architectural changes. The observed guild distribu tions were in accord with known behavior and life histories of the member species. The data demonstrate the correlation between hunting behavior and habitat structure for small invertebrate predators. The data also suggest that guild analysis may be very useful in examining niche dimensions in community studies. 61
Problems and Areas for Future Study
No data were collected on prey abundance, availability, or
utilization by spiders in this study. Changes of shrub architecture
may have caused a subsequent change in prey distribution and abundance.
These factors may strongly affect the distribution of spider guilds.
A similar study examining the effect of vegetation architectural
changes on available prey as well as on predator guilds would yield
information clarifying this relationship.
The relationship between animal and plant species diversity is a confounding issue in many investigations. This study did not examine
the influence of substrate architecture, independent of plant species,
on spider species diversity, distribution and abundance since the
shrub perturbations were performed on only one shrub species, Artemisia
tridentata. The natural variation of architecture and associated
insect faunas of different plant species could determine predator guild distribut ions. Studies which examine similar changes of vegetation architecture using several plant species would be helpful in under standing this relationship. Also comparative guild studies between communities with different species compositions should be conducted. 62
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Cache Valley area Utah, p. 72. Washington: U.S. Govt. Printing
Office 1974
Whittaker, R. H., Woodwell, G. M.: Evolution of natural communities.
In: Ecosystem structure and function (J. Wiens, ed.), pp. 137-156.
Corvallis: Oregon State Univ. Press 1972 66
APPENDIX 67 0 Table 19. Mean, maximum and minimum weekly summer temperature (C ) and relative humidity in 1974.
Sample First Day of Temperature Relative Humidity Week Sample Week
Mean Max. Min. Mean Max. Min.
1 - 6/06
2 _6/26
3 7/14 23 35 16 42 63 18
4 7/21 24 36 13 28 46 15
5 7/28 26 36 17 28 so 8
6 8/04 21 32 13 33 56 12
7 8/11 22 33 10 21 36 7
8 8/18 22 33 10 22 46 6
9 8/25 24 36 12 22 38 18
9/01 20 31 9 29 so 14
9/08 17 28 10 33 46 21
9/15 15 28 6 35 59 16
9/22 --
10 9/29 14 24 6 28 so 9
11 10/06 8 20 0 31 57 7 68 Table 20. Mean, maximum and minimum weekly summer temperature (C 0 ) and relative humidity in 1975.
Sample First Day of Temperature Relative Humidity Week Sample Week Mean Max. Min. Mean Max. Min.
5/13 15 24 6 40 64 24
5/18 8 15 1 54 71 32
5/25 9 17 0 42 67 26
1 6/01 14 23 7 49 72 33
2 6/08 14 23 7 49 72 33
3 6/15 11 18 7 49 65 37
4 6/22 15 22 8 56 74 43
5 6/29 22 31 11 29 63
6 7/06 24 33 15 45 68 24
7 7/14 24 35 15 40 64 22
8 7/21 24 36 13 37 64 17
9 7/28 20 29 10 39 67 17
10 8/04 24 35 13 25 49 11
11 8/11 20 32 12 35 66 14
12 8/18 18 27 11 27 41 1
13 8/25 19 29 10 23 46 9
14 9/01 16 28 5 32 53 15
9/08 18 28 9 32 48 20
9/15 14 24 6 45 66 27
15 9/22 14 25 5 34 52 17
9/29 14 25 4 39 62 21
10/06 10 18 4 43 68 26