MICROHABITAT SELECTION AND REGIONAL COEXISTENCE
IN WATER-STRIDERS (HETEROPTERA: GERRIDAE)
by
JOHN RICHARD SPENCE
B.A., Washington and Jefferson College, 1970 M.S., University of Vermont, 1974
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
in
THE FACULTY OF GRADUATE STUDIES (Department of Zoology)
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA January, 1979
(c) John Richard Spence In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study.
I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of Zool°gy
The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5
Date January 30, 1979 ii
ABSTRACT
This study considers the natural history and ecology of water-strider species occurring on the Fraser Plateau of south- central British Columbia. The overall aim was to assess the effects of spatial heterogeneity on factors controlling the distribution and relative abundance of gerrid species. The relationships among temperature, population dynamics and habitat use were investigated. From a regional perspective, spatial heterogeneity allows species population dynamics to converge in time while keeping them separate in space. ,
Laboratory rearing studies were used to calculate physiological time-scales for developmental processes. Patterns of mating behaviour, fecundity and fertility are described for
Serris comatus and G. pinqreensis in the laboratory and
G- incoqnitus in the field. Egg laying and juvenile growth are shown to be strongly temperature dependent in all species studied. Temperature thresholds for development differ, both among species, and often among stages of a particular species.
Low thresholds recorded for G. pinqreensis can lead to significant growth advantages for this species during early spring. Instar differences seem to be adapted to seasonal temperature regimes experienced by gerrids. Gerris species and instars showed distinct optimum temperatures for survival.
These optima vary with developmental thresholds. It is suggested that species may be best adapted for growth under different temperature regimes.
A method was developed for estimating absolute densities in the field from relative abundance measures using linear regression techniques. Gerrid size and presence or absence of vegetation markedly affect capture rates. No effect of species or type of emergent cover was demonstrated. Availability for capture varies with leg^length in G. buenoi and G. pinqreensis.
This relationship is used to estimate availability constants for other water-strider species.
Field surveys between 1975 and 1977 established that
G. buenoi, G. comatus and G. pinqreensis were the most abundant water-strider species in- the study area. Each of these was strongly associated with a single type of vegetation in the field; G. buenoi with grass/sedge habitats, G. comatus with floating vegetation and G. pinqreensis with bulrush habitat.
Limnoporus dissortis and L. notabilis were commonly encountered on small, temporary ponds, G. incognitus was first taken during
1976 in the study area and small populations are confined to brushy, well-shaded habitats.
G. buenoi, G. comatus and G. pinqreensis are all potentially bivoltine in the study area; Limnoporus spp. are univoltine.
Generation timing varies tremendously among lakes and periods of maximum abundance for each species are not separated in time.
Strong between-lake habitat associations in the field result proximately from habitat fidelity at the time of spring colonization. The tendency of gerrids to overwinter near the iv
mother pond and trial and error habitat selection during spring dispersal enforce habitat fidelity during colonization.
Species distributions within lakes are affected by habitat availability. Habitat preference experiments demonstrate that
G. pinqreensis and G. comatus have active preferences for
emergent cover and open habitats respectively. G. buenoi is a habitat generalist but its distribution can be affected by a
tendency to avoid other species. Smaller stages of each species
are found close to shore and often in areas of dense emergent
vegetation.
Enclosure experiments demonstrated that G. pinqreensis can
exclude G. buenoi and G. comatus from bulrush habitats, which are
most favorable for the growth and development of all species.
Fiftgainsh wheinstan confiner G. dbueno in ithei anr dcharacteristi G. comatus cshowe habitatsd greates. tHabitat weigh-t
specific differences in foraging efficiency among late instars
may help produce the habitat associations observed for these two
species.
Fifth instar G. pinqreensis showed poor survival when
enclosed in freshwater habitats, suggesting the hypothesis that
its distribution is restricted by the presence of surface-feeding
predators other than water-striders. It is suggested that
competition for space, predation, density-independent mortality
and colonization dynamics all interact on the template of spatial
heterogeneity to produce regional patterns of distribution and
abundance. V
TABLE OF CONTENTS
ABSTRACT . . . . ii
TABLE OF CONTENTS ...... V
LIST OF TABLES xi
LIST OF FIGURES xiv
ACKNOWLEDGEMENTS X.vii
CHAPTER I. INTRODUCTION 1
A. Coexistence of species ... A plot 1
B. Water-striders . . . The cast 4
1. Natural history background 4
2. Geographical distributions 9
3. The problem and the approach 11
C. The study area . . . The stage ,. 13
1. Sites . 13 i
2. Weather 20
3. Lakes 21
CHAPTER II. THE EFFECTS OF TEMPERATURE ON WATER-STRIDER
GROWTH AND DEVELOPMENT 31 INTRODUCTION • —• 31
METHODS AND MATERIALS ...... 33
A. Egg production 33
1. Effects of temperature 33
2. Mating behaviour and fecundity in the
laboratory 35
3. Fecundity in the field ...... 37
B. Growth and development 38
1. Development and temperature 38 vi
a. Eggs ...... 38
b. Larvae 38
2. Growth thresholds ... 39
RESULTS ...... 4 0
A. Egg production 40
1. Effects of temperature 40
2. Mating behaviour and fecundity in the
laboratory . 48
3. Fecundity in the field 55
B. Growth and development 59
1. Development and temperature ...... 59
2. Growth thresholds ...... 64
DISCUSSION 69
A. Mating behaviour • • 69
B. Fecundity 70
C. Growth and Development 74
CHAPTER III. DENSITY ESTIMATES FOR GERRIDS 77
INTRODUCTION ...... 77
METHODS 79
A. Seasons, species, and habitats 79
B. Relative abundance estimates 80
C. Absolute abundance estimates 84
D. Test estimates 86
RESULTS 87
A. Differences between seasons and size classes .... 87
B. Differences between habitats • 96
C. Differences between species 96
D. Test estimates 99 vii
DISCUSSION . 100
CHAPTER IV. COMPARATIVE ECOLOGY OF WATER-STRIDERS ON THE
FRASER PLATEAU OF BRITISH COLUMBIA 107
INTRODUCTION 107
MATERIALS AND METHODS 109
A. Lakes and species studied - - 109
1. Field temperatures 109
2. Egg production and alary morphism 113
3. Population dynamics 113
4. Habitats ...... 115
RESULTS . 117
A. Field temperatures .117
B. Egg production and alary morphism ...... 120
C. Population dynamics ...... 125
D. Habitats 140
1. Vegetation ...... 140
2. Surface conductivity .....143
3. Lake permanence 145
DISCUSSION .... 148
A. Life cycles and population dynamics 148
B. Comparative ecology ....152
1. Habitats and timing ...... 152
2. Habitat permanence and adaptive strategies ... 153
3. Habitat and regional coexistence ...... 155
a. Gerrids in British Columbia 156
b. Gerrids in eastern and western North America 158
4. Conclusions ...... 160 viii
CHAPTER V. EXPERIHENTAL ANALYSIS OF MICROHABITAT
SELECTION IN WATER-STRIDERS 161
INTRODUCTION 161
MATERIALS AND METHODS ...... 163
A. Dispersal ... Habitat selection among lakes ... 163
B. Habitat selection within lakes 167
1. Field distributions ...... — 167
a. Habitat differences among species ...... 167
b. Habitat differences within specie's.. 167
2. Laboratory experiments . . . Responses to
artificial habitat structure ...... 168
a. Laboratory conditions and apparatus ...... 168
b. Species tendencies to enter complex habitats 169
c. Selection of artificial habitat mimics ...... 181
d. Habitat structure and foraging success ...... 181
RESULTS ..' , ' 187
A. Dispersal . . . Habitat selection between lakes . 187
1. Immigration 187
2. Emigration ...... 189
B. Habitat selection within lakes 195
1. Field distributions . . 195
a. Habitat differences among species ...... 195
b. Habitat differences within species ...... 197
2. Laboratory experiments . . .Responses to
artificial structure 202
a. Species tendencies to enter complex habitats 202
b. Selection of habitat mimics ...... 210
c. Habitat structure and foraging success ...... 216 ix
DISCUSSION ...... 219
A. Dispersal 219
B. Habitat selection within lakes 222
1. Species differences 222
2. Instar differences 226
3. Species morphology and habitat structure 227
CHAPTER VI. PERSISTENCE, POPULATION PERFORMANCE AND
INTERSPECIFIC COMPETITION ... 229
INTRODUCTION •• --• •-- 229
MATERIALS AND METHODS 231
A. Colonization, persistence and population success 231
1. Field surveys ...... 231
2. Analysis .; .... 233
a. Number of species per lake .... 233
b. Population persistence ...... 233
c. Population success 233
d. Gerrid species diversity vs. plant structural
diversity 234
B. Habitats, growth and competition ',. •. 234
1. Effects on species growth and survival 234
a. Species growth 234
b. Food-fall 237
2. Effects of competition between species ...... 237
RESULTS 238
A. Colonization, persistence and population success 238
1. Number of species per lake 238
2. Population persistence 238
3. Population success 241 X
4.. Gerrid species diversity vs. Plant structural
diversity 248
B. Habitats, growth and competition 248
1. Effects of habitat on species growth 248
2. Effects of competition between gerrids ...... 255
DISCUSSION . .. 261
A. Habitats, diversity and population performance. .. 261
B. Competition and habitat selection 267
C. Evolution and maintenance of habitat preferences 272
D. Summary 273
CHAPTER VII. GENERAL DISCUSSION .. 275
LITERATURE CITED 286
APPENDIX I ...... 300
APPENDIX II .....304
APPENDIX III ...,..,'...308
APPENDIX IV 311 xi
LIST OF TABLES
Table 1. Gerrid distributions in British Columbia ... 10
Table 2. Starting dates of laboratory cultures used to assess the relationship between egglaying and temperature ...... 34
Table 3. Means and standard errors for number of larvae hatched/day-degree/female • 46
Table 4. Analysis of variance for number of larvae hatched/day-degree/female...... 47
Table 5. Total number of eggs laid by female G. comatus and G. pinqreensis in the laboratory .52
Table 6. Average coefficients of variation for daily batch size in Gerris 58
Table 7. Total (egg to adult) development times in days at various temperatures 60
Table 8. Percent of total development time in each stage at 22°C : 61
Table 9. X2 values for differences between growth thresholds of different larval stages ,.. 67
Table 10. Comparison of first instar growth thresholds ... 68
Table 11. Intercepts and standard errors for all summer instars and spring adults 88
Table 12. Analysis of variance of regressions of capture efficiency for various developmental stages of Gerris ... 90
Table 13. Proportionality constants obtained for all summer instars and spring adults ...... 91
Table 14. Proportionality constants and their standard errors for different size classes in two habitats ...... 97
Table 15. Average proportionality constants for size classes of G. buenoi and G. p.ingreensis 98
Table 16. A comparison of test estimates with Gerris population estimates from the literature ...... 105
Table 17. Collection sites of female gerrids used for reproductive dissection ...... 114
Table 18. Breeding condition of gerrid wing-morphs encountered in the Fraser Plateau study area 123 Table 19. Plant species and vegetation structure used to define gerrid habitat classes 141
Table 20. Gerrid species abundance in various habitat categories ....142
Table 21. Gerrid species abundance in various categories of surface water conductivity 144
Table 22. Gerrid species abundance in various categories of lake area 146
Table 23. Gerrid species abundance in temporary and more permanent habitats *... 147 Table 24. Habitat preferences of gerrid species in British Columbia .. 157
Table 25. Chi-sguare values to test for independence of individual position choices on the experimental pools ...174
Table 26. Total numbers of gerrids flying into enclosures (six 24hr periods - May 1977 ., 188
Table 27. Percentage of each species and morph lost from experimental enclosures ...... 190
Table 28. Percentage of animals remaining in enclosures with functional indirect flight muscles ...... 193
Table 29. Occurrence of Gerris species by habitat on two lakes during July, 1976 ...7 196
Table 30. Two-way analysis of variance for the percentage of observations recorded in open habitat sectors ...... 205
Table 31. Means and standard errors of the percentage of total observations made on open habitat sectors ...... 207
Table 32. Analyses of variance for the effects of other species on observed habitat preferences ...... 211
Table 33. Artificial habitats chosen by three Gerris. species in laboratory experiments 215
Table 34. Distributions of long-winged G. incoqnitus and G. pinqreensis on various habitats 225
Table 35. Weights of newly molted fifth instar larvae ....236
Table 36. Number of gerris species recorded per lake 1975-r 1 977 .239
Table 37. Percentages of water-strider populations completing one generation, 1975-1977 240 Table 38. Correlations between net changes in (a) density and (b) biomass for gerrid species during 1977 ...... 243
Table 39. Correlations between net increase of gerrid species and proportions of common habitat types ...... 244 Table 40. Multiple regressions of Gerris population success on habitat variables . 246
Table 41. Mean survival of five gerrids ± standard errors over three day experiments ...... 251 xiv
LIST OF FIGURES
Figure 1. A general life cycle for gerrids of temperate regions. 6
Figure 2. The Becher s Prairie study site.- . 14
Figure 3. An aerial view of the distribution of lakes and the landscape at Becher s Prairie...... 16
Figure 4. The Springhouse study site. 18
Figure 5. Average monthly temperature maxima recorded at the Williams Lake, B. C. Airport. 22
Figure 6. Average monthly precipitation recorded at the Williams Lake, B. C. Airport...... 24
Figure 7. Monthly wind indices recorded at the Williams
Lake, B.C. Airport...... 26
Figure 8. Distribution of surface conductivities ...... 29
Figure 9. Total number of egg batches laid by five gerrids during 15 days 41 Figure 10. Average number of larvae hatched per egg batch
at four constant temperatures. 43
Figure 11. Mating activity, female survival and fecundity 49
Figure 12- Average size of daily egg batches recorded in the laboratory 53 Figure 13. Patterns of oviposition and fertility observed for field populations ...... 56 Figure 14. Survivorship of all larval instars at various , constant temperatures 62
Figure 15. Calculated growth thresholds for all developmental stages 65
Figure 16. A schematic diagram of the standard sampling route. 82
Figure 17. Regressions of absolute density on number of gerrids caught per minute 92
Figure 18. Polynomial regression of availabilty for capture on mesothoracic leg length ...... 94
Figure 19. Areas summed in calculation of physiological time scales. ...111 XV
Figure 20. Physiological time scales for gerrids calculated from field temperatures - .....118
Figure 21. Distribution of reproductive effort among morphs - ....121
Figure 22. Partial population curves for G. buenoi during 1975 ...... 7 126 Figure 23. Partial population curves for G. comatus during 1975 i. .- . 128 Figure 24. Partial population curves for G. pinqreensis during 1975 . . 7.. . 130 Figure 25. Partial population curves for Limnoporus during 1 975. . . . . 132
Figure 26. Total population curves for all water-strider
species during 1975...... 136
Figure 27. Average gerrid biomass per lake during 1975. 138
Figure 28. Field enclosures in floating/submerged vegetation , ...... 164 Figure 29. Four types of artificial habitat in a laboratory pool ...... 170
Figure 30. Apparatus used to generate waves on laboratory pools. 176
Figure 31. Apparatus and habitat configuration used for, testing the effects of wind ?.-•*• ^8
Figure 32. Unique spatial configurations of laboratory h abi tats ...... 182
Figure 33. Starting distribution of Drosophila in laboratory experiments .185
Figure 34. Total number of gerrids disappearing from enclosures in each habitat...... I 191 Figure 35. Within lake spatial distribution observed for various instars .....198
Figure 36. Relationship between depth and density of emergent cover 200
Figure 37. Distribution of gerrids on laboratory pools under three surface conditions. .,..,.203
Figure 38. Effects of conspecific density and the presence of other species 208 xvi
Figure 39. Choices of G. buenoi, G. comatus and G. pinqreensis when offered a range of habitat mimics ...212
Figure 40. Amount of food consumed in each of four artificial habitats. •••• ...... 217
Figure 41. A plot of gerrid species diversity versus plant structural diversity 249
Figure 42. Average weight gains of individual surviving gerrids , .253
Figure 43. Average food fall per square meter ...... 256
Fiqure 44. Average daily mortality rates observed in single-species and three-species experiments in natural habitats...... 258
Figure 45. The relationship between relative abundance of gerrids and relative abundance of their characteristic habitats. .. . . 263
Figure 46. The abundance of all predator/competitors oyer a range of conductivity. 270
Figure 47. Areas used by G. buenoiG. comatus and G. pinqreensis in three-dimensional ecotope space 282 ACKNOWLEDGEMENTS
Thanks to E. Kruger, T- Sentobe, W. and A. Whitecross,
B. Hartwig, H. Balzer, A.Presson and Whitey, there was always something to do in the Chilcotin when hip-boots were off. Good companionship, various medicinal spirits, campfires and starry
Chilcotin night skies made it easier to go get "stuck in the mud" another day. B. Smith helped in the field. I also thank
S. Cannings for Figure 3.
W. Clark, M. Denny, and D. Raworth have offered useful advice. J. Pindermoss and R. Scagel helped with plant identifications. Discussions with C..Whitney about assumptions, doubts and philosophy have been most helpful in precipitating
positive ideas. I thank B. Smith and J. van Reenen for constant stimulation and especially, for understanding the frustrations of an entomologist trying to find a niche in modern ecology.
I have profited greatly from encouraging discussions with
Dr.'s P. A. Larkin, J.H. Myers, J.N.M. Smith and T.R.E. Southwood.
I thank the members of my study committee, Dr.'s A.B. Acton,
N.R. Liley, J.D. McPhail and W.E. Neill for reading, this thesis on short notice and offering constructive advice. Dr. Neill has been a constant source of encouragement and often helped me to recognize interesting segments in the long strings of ideas that I have discussed with him. I thank Dr. D. Holm for his generous assistance in the never-ending task of rearing gerrid food. Neil Gilbert wrote the algorithm used for day-degree
summation and has always been willing to offer and explain xyiii
statistical advice. Interactions with Neil have taught me more than I'd ever hoped to know about asking penetrating biological questions and using statistical methods to help answer them.
My supervisor. Dr. G.G.E. Scudder, has made this entire adventure possible. He suggested the gerrid system and shared his rich storehouse of information about water-strider natural history and the Chilcotin Study area. His editorial efforts have much improved the quality of this presentation. I greatly appreciate his support, interest and encouragement during this project.
I am grateful for the generous support, received for this work from the National Research Council of Canada through an operating grant to Dr. Scudder and from the Faculty of Graduate
Studies through several post-graduate scholarships awarded to me.
My wife, Debbie, has been the supporting cast throughout the study. She has weathered the mud, the frozen fingers, the frustrating long hours, and yet, always managed to muster the
optimism that kept us both sane and together. She has also typed and helped to edit the thesis. Without her unfailing patience and understanding, I'd have gone utterly screamin' crackers!
Debbie and I both thank D. Zittin, B. Webb and S. Harrison
for help with computer programming and FMT. We are also
grateful to J. Miller and C. Whitney for helping to punctuate our
weekly efforts during the "big push". 1
CHAPTER I. INTRODUCTION
A. Coexistence Of Species ... A Plot
Questions of species packing and coexistence are fertile ground for ecologists and evolutionary biologists because they address the central problem of organic diversity. Ecologists have recently become interested in describing and comparing community structure and function (Cody, 1974; Orians and
Solbrig, 1977). The resulting search for emergent, community-
level properties has propelled the development of niche theory
(MacArthur, 1972a; Pianka, 1976) and concepts of species diversity (Pielou, 1975). Evolutionists, on the other hand,
are interested in the origins of diversity, and therefore, view extant patterns as clues for unravelling the dynamic processes that have produced them (i.e. Gould, 1978). Obviously, the
approaches are complementary and their synthesis has led to the new evolutionary ecology (Cody and Diamond, 1975).
Hutchinson (1959) first crystallized the spirit of the
movement for many biologists with a simple, but absorbing
question - "why are there so many kinds of animals?". This
question sprouted from seeds planted when explorer naturalists
from the temperate zone first ventured into the tropics and
recorded a staggering variety of living things. Relative
biotic impoverishment was also observed on islands and isolated
mountains. Numerous hypotheses have been offered to explain
the observed gradients in species diversity [summarized by
Pianka (1966, 1967) and Otez (1974) J, but real understanding 2
will probably depend upon our ability to integrate the effects of several processes in consistent models (Menge and Sutherland,
1976).
The regional coexistence of closely-related species has afforded less mobile biologists with ample opportunity to study factors responsible for the maintenance of organic diversity
(MacArthur, 1965, 1972b). The problem is usually considered as most acute among congeners because of great morphological and behavioural similarity. In fact, Darwin (1859) predicted that competition "will be generally more severe between them ... than between species of distinct genera'1. Niche theory has developed hand in hand with studies of the comparative ecology of similar species coexisting in a small geographical region and laboratory models of population dynamics
(Whittaker and Levin, 1975). Factors controlling species diversity at a regional level are usually viewed in the context
of total resource availability and/or how tightly species are
"packed" into multi-dimensional resource space (MacArthur, 1965,
1972a). The modern approach often centers on the manner in which congeneric species divide up the range of available
resources in order to minimize interspecific competition
(Schoener, 1974a) .
Most of the data feedback, structuring contemporary niche
theory, has come from work with vertebrates, largely from bird
communities. The most notable successes of the theory have
involved prediction of species composition in bird communities
via the methods of niche metrics and the concept of limiting 3
similarity (Cody, 1974; Pulliam, 1975). However, central assumptions of community theory often break down when tested experimentally (Wilbur, 1972; Neill, 1974, 1975; Lynch, 1978).
Few studies of insect coexistence have conformed to quantitative predictions of community theory (see Price, 1975).
This may reflect May's (1973) observation that niche relationships among insects are extremely complex owing to
"intertwining of relevant resource dimensions". It is also probable is that successes have not been reported because modern niche theory assumes the global operation of competition as the most important process driving the evolution of community
structure. Hutchinson (1953, 1957, 1965), Ayala (1970) and
Janzen (1977) have stressed that some insect populations may never reach monospecific or competitive equilibria because insect life cycles and relevant environmental changes occur on the same time scales.
Hutchinson (1975) pointed out that progress in
understanding higher-level ecological relationships depends upon intimate acquaintance with the natural history of the species
involved. As biologists we are challenged, like it or not, to deal with unique and individual characteristics of the systems
that we study (Bronowski, 1973; Elsasser, 1975). Useful theory
in ecology must be pluralistic (May, 1973); strong
generalizations will result from our ability to classify species
into groups with respect to process (Southwood, 1977; Whittaker
and Levin, 1977). Before any species or population can be so
classified we must understand the relevant details of its 4
natural history.
The following investigation was launched from an animal- centered perspective. The overall aim was to provide a balanced account of the processes affecting the coexistence of six water-strider species on the Fraser Plateau of south-central
British Columbia with special emphasis on responses to spatial heterogeneity. A first objective was to establish a detailed baseline of species phenology and natural history in order to construct specific hypotheses about the.processes controlling gerrid distribution and abundance. Some of the emergent ideas were. then tested experimentally in the field and laboratory.
Background information on water-striders and a more specific statement of the problem are provided in the next section.
B. Water-striders . . . The Cast
1. Natural History Background
Water-striders (Heteroptera: Gerridae) occur commonly on
water surfaces around the world (Milne and Milne, 1978),
Inland populations inhabit lakes, ponds, rivers and streams and the genus Halobates has successfully invaded the open ocean
(Andersen and Polhemus, 1976). The family has radiated explosively in the tropics (Andersen, 1975). Temperate faunas are much less complex (Matsuda, 1960) and species are usually common and widespread (Hungerford, 1919; Drake and Harris, 1934
and Brooks and Kelton, 1967). 5
Gerrids are usually considered as opportunistic predators that make quick work of insects trapped on the water surface
(Lumsden, 1949). Special adaptations for "walking on water"
(Andersen, 1976; Bowden, 1976, 1978; Caponigro and Eriksen,
1976) enable water-striders to exploit the surface tension that encumbers their victims. Hunting gerrids are known to respond to both surface vibration (Murphey, 1971a,b; Lawry, 1973) and visual cues as indicators of potential prey (Jamieson, 1973).
Gerrids are the dominant invertebrate predators living on the
water surface. Their unique specializations and great evolutionary success argue that diffuse competition between gerrids and other members of the aquatic community should be
minimal.
Gerrids are hemimetabolous insects; they pass through five
nymphal stadia prior to dispersal and breeding as adults. A general life cycle for temperate gerrids is illustrated in
Figure 1. All active stages inhabit the water surface. Eggs are attached to floating debris and vegetation or laid below the
surface on submerged objects (Brinkhurst,.1960; Matthey, 1975).
Dispersal to new habitats by flight occurs mainly in the spring
(Landin and Vepsalainen, 1977). Pre-reproductive flights may
be extensive in some species. For example, Leston (1956) noted
that Limnoporus rufoscutellatus recolonizes Great Britain
periodically from continental populations. Available evidence
suggests that gerrids overwinter in ovarian diapause (Andersen,
1973; Galbraith and Fernando, 1977) under stones,,logs or in the
leaf litter (Douglas, 1882; Brinkhurst, 1956; Cheng and 6
/
Figure 1. A general life cycle for gerrids of temperate
regions. ADULTS JUVENILE BREEDING DEVELOPMENT LARVAL INSTARS 1-5 OVERWINTERING DIAPAUSE 8
Fernando, 1970; Vepsalainen, 1974a).
Among morphs with functional wings, egg production usually marks the end of the dispersal period because wing muscles are histolysed coincident with gonad maturation in females
(Andersen, 1973; Vepsalainen, 1974a). Adults not destined to breed during the same season that they reach maturity, begin leaving the water surface for overwintering sites as early as mid-July.
The only two rigorous studies of gerrid population dynamics suggest that summer population growth is density dependent in
G. najas (Brinkhurst, 1966) and L. notabilis (Maynard, 1969).
However the sources of mortality have not been clearly defined.
Beginning with Riley (1922) there have been numerous reports of cannibalism among gerrids. Arguments following from these observations have led to the popular idea that gerrid populations are regulated by cannibalism during periods of food shortage (Jarvinen and Vepsalainen, 1976). Evidence for this sort of population regulation is strong for other invertebrate predators (Fox, 1975a), but is largely circumstantial for gerrids.
Predation and parasitism do not appear to have significant effects on gerrid populations. Gerrids harbor a few parasites
(Matheson and Crosby, 1912; Lipa, 1968; Fernando and Galbraith,
1970) but heavy infestations are unusual. Andersen and
Polhemus (1976) note that gerrids have few predators. Macan
(in Brinkhurst, 1965) has noted that gerrids are uncommon 9
components of trout diets. Yellow-headed blackbirds (Orians,
1966) and ducks (McAtee, 1918; Mabbot, 1920) are known to consume gerrids but available data suggests that their effects are minimal. Among invertebrate predators, notonectids, gyrinid beetles (Jamieson, 1973) and dytiscid larvae (Spence, unpublished) are known to take gerrids, but their impacts on field gerrid populations have not been studied.
2. Geographical Distributions
Nine species of water-striders have been recorded from
British Columbia (Scudder, 1977). The general pattern of species distribution in the province is summarized in Table 1.
Detailed distributional records are provided by Spence and
Scudder (1978), Scudder (1977) and Jamieson (1973). Data at hand suggest that there is a large degree of range overlap among most species. The ranges of Prairie and Western species meet in the south-central interior of British Columbia.
In this thesis I shall follow Andersen (1975) and Calabrese
(1977) and consider Gerris s. str. Fabricius and Limnpporus Stal as distinct genera. The two species of Limnoporus recorded from British Columbia are difficult to separate reliably. I have chosen to consider them as a single ecological entity in the following discussions because they are comparatively rare members of the gerrid assemblage on the Fraser Plateau and because no significant ecological differences became apparent during field surveys. TABLE 1
Gerrid distributions in British Columbia
SPECIES KNOWN RANGE
G. buenoi * Widespread G. comatus * Northcentral Interior G. incoqnitus * Southern and Central B.C. Northward along the coast ' G. incurvatus Southern and Central B.C. G. nyctalis Rocky Mountains only G. pingreensis * Fraser Plateau and Northern B.C. G. remigis Widespread in Southern and ~ Central B.C. L. dissortis * Northeastern B.C., Fraser Plateau L. notabilis * Widespread, most common in Southern B.C.
* species considered in this study 11
3. The Problem and the Approach
Jamieson (1973) attempted to show how cannibalism and interspecific predation might interact to produce patterns of coexistence observed among five species of water-striders in southwestern British Columbia. He employed a mathematical model, based on relationships determined in the laboratory, to simulate population growth and gerrid foraging in multispecies assemblages. Temperature thresholds for growth, calculated for first instar larvae, were adequate to predict the sequence of species abundances and the number of generations observed at
Marion Lake. However, results of simulation studies suggest that species coexistence should be perilous for all species except the one able to grow fastest at spring temperatures.
These results are confusing in light of field data that suggest that multispecies assemblages are the rule among gerrids in
British Columbia (Scudder, 1971).
Brinkhurst (1959b), Vepsalainen (1973b) and Calabrese
(1977) have been able to define distinct habitat preferences among geographical assemblages of gerrid species. Although
Jamieson (1973) noted habitat preferences among gerrids in the field, these species characteristics were not modeled. Spatial separation, maintained by such preferences, could easily mitigate the effects of interspecific predation predicted by the model and lead to persistent multispecies assemblages observed in the field.
Because gerrids are opportunistic predator/scavengers. 12
differences in food use are unlikely as general strategies of resource partitioning. Therefore, this study focused on the use of space and the distribution of species in time. Three aspects of gerrid biology were studied in some detail.
(1.) Effects of Temperature on Phenology. Laboratory
studies were undertaken to calculate growth thresholds
for various species and instars (Chapter II) . The
objective was to calibrate physiological time-scales for
mating, oviposition and larval development. Differences
among species time-scales provide the basic mechanism for
seasonal separation of population abundance (Jamieson,
1973) .
(2«) Population Dynamics. A method was developed for
making time-efficient density estimates of water-strider
populations (Chapter III). The method was used to
describe and compare species population dynamics over one
season and to evaluate the significance of seasonal
differences from a regional perspective (Chapter IV).
(3.) Habitat and Microhabitat. Species distributions
were studied among lakes (Chapter V) . Habitat
preferences were studied experimentally at both levels
(Chapter V) as proximate factors leading to distinct
habitat associations. Indicies of population
persistence and performance were used to explore how
natural selection might maintain distinct species
preferences, and assess the importance of spatial
heterogeneity for producing patterns of species
coexistence observed in the field (Chapter VI). 13
C. The Study Area . . . The Stage
1• Sites
Fieldwork described in this thesis was concentrated in the
Cariboo-Chilcotin region of British Columbia. The study sites are located between elevations of 950 and 1Q00 m on the Fraser
Plateau. Surrounding vegetation is native grassland interspersed with stands of lodgepole pine, douglas fir and aspen. Many lakes and ponds are nestled in the rolling topography of these Cariboo Parklands. Topping and Scudder
(1977) and Beil (1970) have summarized information on the geological history of the area and its included lake basins.
Beil (1 970) may also be consulted for an excellent analysis and description of the plant associations of the region.
Two sites were chosen for study; one surrounding
Springhouse, B. C., and the other at Becher's Prairie near Eiske
Creek, B. C. The geographical location of these areas is depicted by Topping and Scudder (1977)., Figure 2 illustrates the main features of the Becher's Prairie site. In excess of
75 lakes and ponds occur in the area which covers approximately
92 km2. Figure 3 illustrates the distribution and spacing of lakes from an aerial perspective. The Springhouse area is shown in Figure 4. The lakes are somewhat less numerous and spread over a wider area (approximately 145 km2) than at
Becher's Prairie. Numbered lakes in Figures 2 and 4 were sampled during this study. A key to the lake numbers is provided in Appendix I. Figure 2. The Becher7 s Prairie study site.. BECHER'S PRAIRIE STUDY AREA 16
Figure 3. An aerial view of the distribution of lakes and the
landscape at Becher's Prairie. 17 Figure 4. The Springhouse study site.
20
Several points of similarity can be noted between the two study areas. Firstly, each includes many lakes which span a broad range of size and thus provide a diverse array of potential habitats. Secondly, the lakes are not isolated so that lake to lake dispersal should be possible for flying insects. Thirdly, most of the lakes and ponds in both areas were sampled. Ponds were ignored only if access was difficult or if two neighboring ponds appeared to provide an identical range of habitats. Differences between the areas include more intensive agricultural use at Springhouse. Also the
Springhouse lakes can be divided into several local groups that are more or less separated by forested areas. Such differences are not considered in this study.
2. Weather
Weather patterns in the Chilcotin-Cariboo region show both historical and year to year variability. For example, Munro
(1945) has documented that even many of. the larger lakes (eg.
Westwick lake) were nearly dry in the , ;(1930's. Although the basic life history parameters of a species may be tuned to the average climate, yearly patterns of distribution and abundance observed in a short term study are likely to reflect the peculiarities of each sampling season. Therefore the weather of sample years should be examined in the light of average conditions.
Figures 5 and 6 present the monthly averages of temperature 21
and precipation respectively, as recorded from April-September of 1975-77, at the William's Lake, B. C. Airport. Average values computed from data gathered during 1941-1970 (Atmospheric
Environment Service, 1975) at the same station are presented for comparison. The study years were cooler and wetter than usual.
Even so, about 10% of the ponds sampled each year dried up during the course of the season.
Wind is an obvious factor expected to affect surface- dwelling insects. Two wind indices are plotted in Figure 7.
Both the "gust" index and the average daily windspeed, peak in
spring and early summer. This is likely to be important for semi-aquatic insects because emergent vegetation is not present to serve as wind baffling during the most critical periods.
3. Lakes
Some of the lakes employed in this investigation have been
studied previously. Topping and Scudder (197?) have classified a representative series of these lakes on the basis of physical- chemical data. Reynolds and Reynolds (1975) have shown that the distributions of aquatic vascular plants are affected by
lake chemistry and ionic profiles. Scudder (1969) presented a
general discussion of the invertebrate fauna in relation to gradients of salinity. Other studies (Cannings and Scudder,
1978; Smith, 1977; Topping and Acton, 1976; Scudder, 1975;
Reynolds, 1974 and Cannings, 1973) have studied the field- biology of various insect species inhabiting these lakes. 22
Figure 5. Average monthly temperature maxima recorded at the
Williams Lake, B. C. Airport. 23 \
©AVERAGE (1941-70)
APR. MAY JUNE JULY AUG. SEPT. MONTH 24
Figure 6. Average monthly precipitation recorded at the
Williams Lake, B. C. Airport. 25
• AVERAGE (1941-70) lOOi A 1975 • 1976 ° 1977
80
z 601 O
,<40| Q_ ^ 20! cr CL
APR. MAY JUNE JULY AUG. SEPT. MONTH 26
Figure 7. Monthly wind indices recorded at the Williams Lake, B. C. Airport. (a.) number of days per month with
gusts greater than 15 knots, (b.) average daily windspeed. 9.
A. AVERAGE NUMBER OF DAYS/MONTH WITH GUSTS > 15 KNOTS (1970-77) ( ® ) ro .£> o> co o ro
ro ,J> o> oo B. AVERAGE DAILY WIND SPEED IN KNOTS (1953-72) (o) 28
Many of the ponds considered in this study, however, have not been studied previously. Appendix I lists all of the ponds sampled during 1975-77 and provides some general information about each lake and the water-strider species that were found on it during this study.
Scudder (1969) has found that conductivity is useful for ordering patterns of species distribution. Figure 8 shows that the distribution of conductivities, as measured from the lakes sampled during summer 1977, is approximately log-normal and spans a broad range. Therefore these lakes afford a natural gradient of physical-chemical conditions that should evoke adaptive responses from plants and animals inhabiting them. 29
Figure 8. Distribution of surface conductivities recorded from
lakes sampled in July-August, 1977- NUMBER OF LAKES 31
CHAPTER II. THE EFFECTS OF TEMPERATURE ON WATER-STRIDER GROWTH
AND DEVELOPMENT
INTRODUCTION
Water-striders are specialized predator-scavengers that
occupy simple two-dimensional habitats, Multispecies
assemblages are common on small ponds where resources are likely
to be limited (Jarvinen and Vepsalainen, 1976) making these
insects convenient subjects for studies of comparative ecology
and coexistence. One view of gerrid population dynamics argues
that selection favors rapid development in order to avoid
cannibalism (Maynard, 1969; Vepsalainen and Jarvinen, 1976) and
interspecific predation (Jamieson, 1973). This argument is
supported by the work of Jamieson (1973) who has shown that
efficiency of prey capture is directly proportional to the size
difference between predator and prey gerrids. Therefore
studies of coexistence among water-striders may well begin by
assessing comparable parameters of species growth and
development.
The effects of temperature on rates of development varies
among insect species and helps to define scenopoetic niche axes
(Hutchinson, 1978) . Growth thresholds and thermal constants
provide useful indices of temperature effects that may be
compared among species and locations (Gilbert et al., 1976).
Although Baker (1971) has pointed out that these parameters are
oversimplifications from a developmental perspective, ecologists 32
consider them as useful indices that can be tuned by natural selection (Campbell et al., 1974; Trimble and Smith, 1978).
This study was undertaken to assess the effects of temperature on growth and development for several gerrid species occurring together on the Fraser Plateau of southcentral British
Columbia. The objective was to quantitatively describe the main events of the gerrid life cycle on a physiological time scale so that species comparisons could be made. 33
METHODS AND MATERIALS
A. Egg Production
1- Effects of Temperature
Overwintered adults of Serris buenoi, G. comatus,
pinqreensis, and Limnoporus spp. were collected from the study lakes during early and mid May 1976. Five.pairs of each species were placed in rearing containers on May 10 and maintained at 22°C. Twenty additional pairs of each Gerris species were established on May 25; five pairs of each were placed in constant temperature chambers held at 10°, 15°, 18.5° and 26°C respectively. Five pairs of Limnoporus sjap. also initiated on May 25 were held at 15°C. All animals were allowed 48 hours to adjust to laboratory conditions before experiments began. The breeding cultures established are summarized in Table 2.
All Gerris cultures were kept in small plastic containers
(7 cm. deep; 9.5 cm. diameter) while Limnoporus were held in large plastic containers (9 cm. deep and 25.5 cm. diameter).
Each culture vessel contained 2 cm. of dechlprinated tap water which was changed every three or four days. Photoperiods were long-day (L:D 14:10 at 26°C and 16:8 at all other temperatures).
Pieces of cork served as Gerris ovipositipn sites and 3/8" plywood blocks (8 cm. x 2 cm.) were provided for Limnoporus.
Each pair of gerrids was given 20-25 frozen Drosophila daily as food. 34
TABLE 2
Starting dates of laboratory cultures used to assess the relation• ship between egg laying and temperature
TEMPERATURE G. buenoi G. comatus G. pingreensis Limnoporus °C
10° May 25 May 25 May 25 —
15° May 25 May 25 May 25 May 25
18. 5° May 25 May 25. May 25
22° May 10 May 10 May 10 May 10
26° May 25 May 25 May 25 35
Experiments with Gerris species lasted a maximum of 15 days at each temperature. Dead males were replaced immediately as they were found. When females died before 15 days had elapsed, the date of death was recorded and observations were terminated for that culture. All Limnoporus experiments continued until the females died. Survival of all females was converted to day-degrees using thresholds for egg development calculated in subsequent experiments.
Cultures were checked daily for the presence of new eqqs.
Oviposition blocks with attached egg batches were transferred to separate cumulative hatching containers for each culture and the date was recorded. The total number of first stage larvae hatching from each culture was divided by the number of dates on which eggs were found in order to estimate the average batch size for each female during the experiment. The total number of larvae hatched was divided by the female survival in day- degrees to calculate the average number of larvae produced per female per day-degree.
2* Mating Behaviour and Fecundity in the Laboratory
Adult G. comatus and G. pinqreensis, collected from the study lakes in September 1977, were kept through the winter in constant temperature cabinets at 5°C. On April 22, 1978, twenty pairs of each species were used to establish room- temperature cultures in large plastic containers (dimensions as above), each containing a single conspecific male . and female. 36
Short sections of birch "stir-sticks" (5 cm. x 1 cm. x 0.2 cm.) supported with small pieces of cork were provided as resting and oviposition sites. Cultures were fed to satiation daily with living vestigial-winged Drosophila.
Laboratory air temperatures were monitored continuously with a Ryan (Model D) recorder for the duration of the experiment. Daily minima and maxima were used to compute physiological time scales based on thresholds for egg development calculated in the next section. Computations were done with the algorithm listed by Frazer and Gilbert (1976).
Each culture was checked twice daily (9:00 AM^12:00 Noon and 3:00-6:00 PM) for mating activity. The number of pairs in copula 'plus the number of coupled pairs without genital contact was recorded as a quantitative index of mating activity for both species. General qualitative observations on mating behaviour were also made.
The number of eggs laid by each female was tallied every second day. Eggs were counted by scanning oviposition blocks until successive counts were repeatable. Use of "stir-sticks" ensured that all eggs were clearly visible. Water was changed, fresh "stir'-sticks" were provided for the adults and eggs were transferred to cumulative hatching chambers for each culture every fourth day.
Dead males were replaced as soon as noticed. Female deaths were recorded and terminated observations for a particular culture. Females were dissected at death and the 37
condition of the indirect flight muscles and the number of chorionated eggs remaining in the reproductive tract were recorded,
3. Fecundity in the Field
Groups of 10-12 female Gerris incoqnitus were collected periodically during the spring of 1978 from a small pond adjacent to 16th Avenue on the U.B.C. Endowment Lands, starting with the first observation of mating pairs on March 27.
Subsequent groups were collected at approximate ten day intervals until the first teneral adults of the new generation appeared on May 23. Females were brought into the laboratory and placed individually in small plastic containers (dimensions as above) filled to a depth of two cm. with dechlorinated tap water. "Stir-sticks" (same dimensions as above) were provided as resting and oviposition sites. Each female was allowed to oviposit for two days in the absence of food. At the end of each oviposition period the gerrids were colors-coded with small dabs of paint (Metron Markers, Metron Optics, Solana Beach,
California) on the prothorax and returned to the site of collection. Females taken during the preceding interval were ignored in all field collections. The eggs laid by each female were counted and held separately until hatching. The total number of first instars hatching was recorded for each batch as an index of number of fertile eggs. 38
B? Growth and Development
1• Development and Temperature
a. Eggs
Egg batches were isolated daily from cultures of field- collected G. buenoi, G. comatus, and G. pinqreensis held at 15°,
18.5°, 22° and 26°C. The number of days required for all individuals of these daily cohorts to hatch was recorded at each temperature. The procedure was repeated for Limnoporus at 22° and 15°C.
b. Larvae
Newly hatched first instar larvae of G. buenoi, G. comatus,
G- pinqreensis, G. incoqnitus and Limnoporus were reared through the. first molt at the constant temperatures used for egg hatching. All gerrids were held individually in small styrofoam "soup cups" (Styrocontainer, Vancouver, B.C., Canada
Cup #108; depth: 6 cm; bottom diameter: 6.5 cm.) containing two cm. of dechlorinated tap water. Larvae from L. notabilis females and those from smaller Limnoporus females (presumably
dissortis) were kept separately through the first instar.
At least 15 first instar larvae of each species were reared at each temperature. Larvae were checked for molting and fed an excess of frozen Drosophila daily.
The general procedure was repeated for all subsequent 39
larval stages of G. buenoi, G. comatus and G. pinqreensis. All stages of G. incognitus were reared at 22°d Limnoporus development times were measured at 15° and 22°C. All rearings are summarized in Appendix II.
At first, second and third stage larvae used were reared from eggs in the laboratory. Additional fourth and fifth stage larvae taken from the field were used for these experiments owing to high mortality during the. last two developmental stages. The number of days required to complete each stage and the mortality incurred at each temperature was recorded.
2- Growth Thresholds
Reciprocals of the development times obtained above express the percent of development completed per day at each temperature. These values were regressed cn temperature for each species and stage for which sufficient data were available.
The resulting X-intercepts estimate the temperature at which measurable growth ceases, i.e. the temperature threshold of development for that stage and species (Gilbert et al., 1976).
Threshold temperatures and their standard errors were calculated for first stage larvae of all species and all other stages of
G. buenoi, G. comatus and G. pingreensis. The methods of calculation are completely described by Campbell et al. , (1974).
The growth thresholds were compared between stages and between species with weighted analyses of variance to account for differences in the accuracy of the estimates (Gilbert, 1973). 40
RESULTS
A. Egg Production
1. Effects of Temperature
Figure 9 shows that the number of egg batches laid by gerrid females is affected by prevailing temperatures. The peaks shown at 22°C for all three Gerris species may be accentuated because younger females were used at that temperature (Table 2) . Even if interpretation is restricted to the four groups started on May 25, the effect of temperature is pronounced and differs from species to species. Egg production was most severely depressed in G. buenoi and G. comatus at the two lowest temperatures; G. buenoi females laid no eggs at 10°C.
G. pinqreensis were least affected by low temperatures.
Comparison of the two Limnoporus groups, although started with animals of different age, is legitimate because females of this species do not carry mature eggs until late May or early June in the study area (Chapter IV) .
The average numbers of larvae hatched per egg batch is compared in Figure 10. No data are presented for 10°C because no eggs hatched after 65 days. The data illustrate that both
G. buenoi and G. comatus lay more eggs per batch with increasing temperature. One-way analysis of variance indicates that the effect of temperature is highly significant for G. comatus
(F=8.90; df=3,18; p«.05). The number of larvae/batch is highly variable among G. buenoi females at low temperatures and 41
Figure 9. Total number of egg batches laid by five gerrids
during 15 days at various constant temperatures. AG. BUENOI
B G. COMATUS
® G. PINGREENSIS
• LIMNOPORUS SPP.
8 IOI
10 15 20 TEMPERATURE 43
Figure 10. Average number of larvae hatched per egg batch at
four constant temperatures. in
X g CQ •s. O 20 UJ <3. comatus X 6 16 I <3. buenot 12 (3. pingreensis UJ 8 < u_ o oc 4 UJ m 15 185 22 26 15 18.5 22 26 15 18.5 22 26 TEMPERATURE - °C 45
the effect of temperature on batch size is much less pronounced
(F=3.29; df=3,17; p=. 052) . Female G. pinqreensis, laid largest egg batches at 18.5°C and, most conspicuously, the number of
larvae per batch does not increase with temperature. No
significant difference was detected among G. pinqreensis batch
sizes with a one-way analysis of variance (F=1.11; df=3,19;
p>.10) .
The mean number of larvae produced per day-degree per
female Gerris at 15°, 18.5°, 22° and 26°C is compared in Table
3. The data show little difference between the upper three
temperatures, but suggest that absolute rates of egg production
fall for G. buenoi and G. comatus at 15°G.
A two-way analysis of variance was performed over the upper
three temperatures. The results are shown in Table 4. There
was no significant effect of temperature over this range.
However, the data indicate that there are between species
differences in larval production rate. The means and standard
errors presented in Table 3 suggest that overall larval
production may peak at 18.5°C in G. pinqreensis, but larger
sample sizes are necessary to establish whether the effect is
statistically significant. 46
TABLE 3
Means and standard errors for number of larvae hatched/day- degree/female gerrid at three constant temperatures
TEMPERATURE SPECIES °C G. buenoi G. comatus G. pinqreensis
Mean±S.E. Mean±S-E. Mean±S.E.
26° 0.65±.064 0.62±.099 0.36±.057
22° 0.53±.038 0-59±.089 0.33±.043
18.5° 0.66±.189 0.58±.128 p.56±.113
15° 0.24±. 102 0.24±. 076 0.31±.081 TABLE 4
Analysis of variance for number of larvae hatched/day- degree/female for three species at three constant temperatures
df S . s. M . S. F P
Temperature 2 0. 097 0. 049 <1 -
Species 2 0. 367 0. 183 3. 53 <.05
Interaction 4 0. 108 0. 027 <1
Remainder 36 1. 873 0. 052 48
2. Mating Behaviour and Fecundity in the Laboratory
Mating behaviour in G. comatus and G. pinqreensis does not
involve elaborate courtship displays. Males in breeding
condition attempt to mount both conspecific females and males,
as well as gerrids of other species. Whether the male achieves
coupling seems to be determined only by how actively the mounted
gerrid resists. Interspecific coupling has been achieved
between several Gerris species in the laboratory during this
study.
Coupled animals remain paired for long intervals, during
which the female may row them about and even feed. With the
completion of sperm transfer the male retracts the aedeagus, but
may remain clasped to the female for some time. Pairs have
remained coupled for as long as four hours in the laboratory.
Individual females and males mate repeatedly throughout
life. Mating behaviour in these species is concentrated during
daylight hours in both the laboratory and the field.
Oviposition generally occurs late in the day and overnight.
The upper histograms of Figure 11 show that mating
continues throughout the period of egg laying. The peak of
mating activity precedes the period of maximum oviposition in
both species, although the effect is most pronounced in
pinqreensis. Neither G. pinqreensis nor G. comatus mates
immediately after overwintering. The fact that no mating
attempts were observed for either species during the first two
days, suggests that males as well as females require some time 49
Figure 11. Mating activity, female survival and fecundity on
physiological time scales in the laboratory. (a.)
G. comatus, (b.) G. pinqreensis. % OF TOTAL EGGS LAID MATING ACTIVITY % OF TOTAL EGGS LAID MATING ACTIVITY (OPEN BARS) (OPEN BARS) (OPEN BARS) ro (OPEN— BARS )r o J> O 0> ro o o J> 'o a> to o o
8 .8 8 S o g 8 8 8 S 8 % FEMALES LAYING NUMBER SURVIVING % FEMALES LAYING NUMBER SURVIVING (DOTTED UNE• O ) (SOLID LINE Q——®) (DOTTED LINE© ®) (SOLID LINE*——•) 51
to achieve full reproductive ma turi ty.
The lower histograms of Figure 11 illustrate the distribution of egg production on the. insect's physiological time scale. Each day repr esented approximately 13.5 day- degrees at laboratory temperatu res. The peak of oviposition occurred between 130 and 16 0 day-degrees for both species,
However, G. comatus survived an d laid eggs slightly longer under
laboratory conditions.
The aver,ag e number of eggs laid by individuals of both species is pr esented in Table 5. The data show that G. comatus
was slightly more fecund than G. pinqreensis. All females
contained man y chorionated eggs at death, although there was a
tendency for older females to carry fewer eggs. The indirect
flight muscle s of all G. comatus females were completely broken
down at dea th and eggs often filled the entire body cavity,
The main caus e of death seemed to be drowning in both species,
Senile gerri ds were unable to stay on the water surface even
after forced periods of "drying out" on paper towels.
Data presented in Figure 11 also show that both mating
activity and egg production decreased before females started to
die. This indicates that egg production" decreases with age.
Figure 12 shows that average daily batch size changed markedly
with day-degree accumulation in both species. Thus the
decreases shown in Figure 11 result from two factors: (1) fewer
females are laying eggs late in the season; (2) each female lays
eggs at a lower daily rate as she ages. 52
TABLE 5
Total number of eggs laid by female G. comatus and G. pinqreensis in the laboratory
SPECIES MEAN±S.E. MAX. . MIN. AVE # CHOEIONATED EGGS AT DEATH
G. comatus 215.0±14.98 354 120 19.1±2.37
G. pinqreensis 185.?±13.20 279 1Q1 22.6±1.84 53
Figure 12. Average size of daily egg batches recorded in the
laboratory for G. comatus and G. pinqreensis. ° CJ. comatus Of 0 (3. pin Qr Gens is I 1 standard errors o 0>2O| Cf> UJ u_ 161 o
!§ 121 CO 8 UJ oc 4 hi 5 54 108 162 216 270 ACCUMULATED DAY-DEGREES 55
3. Fecundity in the Field
Figure 13 shows the pattern of oviposition by G. incognitus taken from the field. Several points should be noted.
Fertility is high, generally greater than 90% throughout the season. However a smaller proportion of females brought in from the field lay daily egg batches than in laboratory populations and, among females that do lay eggs, the average batch size was about one half that recorded for the closely related G. pinqreensis in the laboratory. Although the mean number of eggs laid per female show a pattern similar to that seen in the laboratory, the variances are distressingly large.
A one-way analysis of variance is unable to demonstrate significant differences in average batch size over the season
(F=0.74, df=6,65, p>.10).
Coefficients of variation for daily batch size were compared between laboratory and field populations with one-way analysis of variance. Table 6 lists the means and standard errors of these coefficients., The. analysis indicates that field populations were significantly more variable than laboratory populations with respect to daily batch size
(F=16.68, df=2,26, p<.01). 56
i
Figure 13. Patterns of oviposition and fertility observed for
field populations of G. incognitus. AVERAGE NUMBER OF EGGS/DAY/J ( o ro a CD o ro
CO H OJ >z. o > (0 o m PR I > 3) di o r ro co ro
OJ
<£>
>-< oi ro ro
_ ro ^
% FERTILITY (• •) Ul TABLE 6
Average coefficients of variation for daily batch size in Gerris
SPECIES C.V.±S.E.
G. incoqnitus 0.72±.070 (field)
G. pingreensis 0.411 ±. 037 (lab)""
G. comatus 0T383±.028 (lab) 59
B. Growth and Development
1. Development and Temperature
Total egg to adult development times were estimated by
summing the means recorded for each stage (Table 7). The
specific rates for each developmental stage, and partial data
sets for other temperatures, are listed with standard errors in
Appendix II. Larger gerrids (G. comatus and Limnoporus spp. )
generally required more time to complete development than did
smaller species. Comparison of data for G. pinqreensis and
G. comatus shows that temperature may affect different species
in different ways. In this case G. pinqreensis development is
less retarded by cool temperature than is that of G. comatus.
The percent of total development spent in each stage at v 22°C is recorded in Table 8. The egg and fifth stage were the
longest in all species while the second stage was generally
shortest. However, the first three larval stages require
approximately the same amount of time.
Survivorship of animals hatched in the laboratory at
different temperatures is illustrated in Figure 14. Although
survivorship was generally lower for later instars, a comparison
of "optimum temperatures" (defined by peak survivorships for the
various stages) can be made. Early stages of all three species
had lower optimum temperatures than later stages. Peaks in
survivorship, recorded for the last three stages suggest that
species differences exist. G. pinqreensis had the lowest 60
TABLE 7
Total (egg to adult) development times in days at various temperatures
TEMPERA- SPECIES TURE
°C Gerris Limnoporus
buenoi comatus incoqnitus pinqreensis
15° - 86.9l1.98 - 61.4i3.61
18.5° 43.6l1.26 46.0±1.42 - 42.311.11
22° 35.2i1.56 38.3±1.11 32.6i1.12 32.9±1.18 4Q.9±1.74
26° - 28.5±0,94 - 24.7±0.94 61
TABLE 8
Percent of total development time in each stage at 22°C
STAGE SPECIES AVE
Gerris Limnoporus
buenoi comatus incoqnitus
egg 23.8 27.2 25.1 25.2 26. 9 25.6
1 13.4 12.0 11.1 13. 1 11.5 12. 3
2 11.0 10. 2 11.3 10.3 8.8 10.3
3 12.7 11.4 11.3 10.9 11.2 11.5
4 18. 4 15. 9 15. 7 15. 8 15.9 16.3
5 20.7 23. 3 25.5 24.7 25.7 24.0 62
Figure 14. Survivorship of all larval instars at various
constant temperatures in the laboratory. (a.)
G. buenoi, (b.) G. comatus, (c.) G, pingreensis. 15 20 25 30 REARING TEMPERATURE (°C) 64
optimum temperature and G. comatus had the highest.
2. Growth Thresholds
Growth thresholds for all stadia of G, buenoi, G. comatus and G. pinqreensis are compared in Figure 15. The regression eguations used for each estimation are given in Appendix III.
Weighted analyses of variance show that significant differences exist between thresholds of the five larval stages for all three species. The values of X2 are given in Table 9. The general pattern is that eggs and fifth stage larvae have high growth thresholds while intermediate stages have somewhat lower thresholds for development. The effect is most pronounced for
G. pinqreensis.
Significant differences also exist between species as is illustrated for first stage larvae in Table 10. These data show that first stage larvae of G. pinqreensis have a much lower threshold for growth than other species found in the study area.
The data of Table 10 suggest some evidence for geographical differences. Thresholds tabulated for gerrids on the lower mainland of British Columbia have been taken from Jamieson
(1973). Standard errors are not given for the estimates so statistical comparisons are not possible. However, G. buenoi seems to have a distinctly lower threshold for growth on the
Fraser plateau. 65
Figure 15. Calculated growth thresholds for all developmental
stages of three gerrid species. m<3. buenoi
EGG I 2 3 4 5 DEVELOPMENTAL STAGE
ON cn 67
TABLE 9
X2 values for differences between growth thresholds of different larval stages
SPECIES X2 df p
G. buenoi 16. 24 4 «.01
G. comatus 9.73 4 <.05
G. Pinqreensis 17. 04 4 «-01 TABLE 10
Comparison of first instar growth thresholds
SPECIES INTERIOR LOWER MAINLAND
G. buenoi 8.5±0.66 •12.9 n=100
G. comatus 8.3±0.79 Does not n=126 occur
• £• pinqreensis 3.8±1.62 Does not n=109 occur
G. incognitus 8.9±1.04 9.3 n=57
L. notabilis 9.0±0.86 10.3 n=97
L. dissortis 9.4±1.00 Does not n=52 occur
X2 36.19 ; Not df 5 calculated P <<0.01 69
DISCUSSION
A. Mating Behaviour
Wilcox (1972) has presented the only in-depth analysis of mating behaviour in water-? striders.. He showed that an
Australian species of Rhagadotarsus-engages in elaborate pre- mating displays involving communication by surface waves.
Maynard (1969) and Jamieson (1973) have observed similar behaviour in L. notabilis in southwestern British Columbia and suggested that both male-male and male-female communication is involved. Comparable mating rituals have not been observed in
Gerris and the only rule for males in breeding condition seems to be "catch it and try". The possible role of pheromones in orchestrating this behaviour has not yet been investigated.
Data presented here show that G. comatus and G. pinqreensis mate repeatedly throughout their reproductive lives. This seems to be the rule among temperate Gerris. One possible explanation is that females are unable to store large quantities of sperm. Kaufmann (1971) noted the complete absence of spermathecae in L. rufpscutellatus but this should be substantiated by detailed investigation. Gerrid spermathecae are relatively small and are very easy to miss when the abdomen is distended with eggs. All species considered in this study have spermathecae similar in structure to that described by
Brinkhurst (1960) for G. najas. Frequent mating, then, may serve to keep females fully inseminated and ensure the high fertility observed among G. incoqnitus in the field. Gordon 70
and Gordon (1971) have shown that fertility falls rapidly with decreasing sperm levels in the milkweed bug,
Oncopeltus fasciatus.
Concentration of mating behaviour during daylight hours is consistent with the hypothesis that copulation in Gerris is initiated by visual cues. It also suggests that diurnal predators, hunting by sight, do not exert a strong selective force on populations of spring adults because selection has not minimized time spent in copula or the visibility of mating pairs.
B. Fecundity
Little published information exists concerning fecundity in nearctic Gerris. Labeyrie (1978) stresses that insect fecundity must be considered as a population phenomenon.
Therefore real understanding of selective factors shaping gerrid fecundity patterns awaits publication of comparative data from other populations.
Estimates of maximum fecundity of G. remigis from small montane ponds in Alberta (Matthey, 1975) indicate that this species lays about twice as many eggs as the species studied here, G. remigis is about twice as large as G. comatus or
G« pingreensjs and continues breeding well into summer.
Andersen (1973) reports that the European G. lacustris, which is similar in size to G. comatus and G. pinqreensis, laid up to 250 eggs under field conditions. Ovariole number in temperate 71
Gerris seems to be constant at four per ovary (Brinkhurst, 1960;
Kaufman, 1971; Spence, unpubl.) and therefore, ovariole number
(Price, 1975) is not a good comparative index of gerrid egg production. Available data suggest that there . may be some relationship between fecundity, body size and female longevity.
Fecundity in Gerris has been investigated by two indirect approaches. (1) Most published estimates are based on counts of eggs carried by dissected females. Such counts lead to obvious underestimates because oviposition and egg maturation continue over many days (Matthey, 1975). (2) Jamieson (1973) counted the number of larvae hatching from egg batches accumulated in the laboratory. His estimates for G. incoqnitus and G. incurvatus are about one half of those determined here for G. comatus and G. pingreensis. It is unlikely that species differences are responsible because G. comatus and G. incurvatus as well as G. pingreensis and G. incoqnitus are closely related by morphological criteria (Scudder and Jamieson, 1972; Spence and Scudder, 1978). Two alternative explanations involving methods are possible. (1) Jamieson (1973) did not use females overwintered in the laboratory and it is possible that some eggs may have been deposited before animals were collected. (2)
Counts of larvae hatched in the laboratory are misleading because larvae from later egg batches accumulated in the laboratory are less likely to hatch (Spence, unpubl.).
Results presented above show that a continuously high proportion of eggs hatch throughout the season in field 72
populations of G. incognitus. Therefore neither infertility, food quality nor aging are likely to cause reduced hatching success in natural populations. This information supports
Matthey's (1975) contention that the. problem of fecundity in
Gerris awaits further investigation.
Matthey (1975) found that drowning was the major source of mortality among ovipositing G. remigis . He attributed this to two factors: (1) loss of water-proofing owing to accummulation of debris in the surface hair layers resulting from subsurface oviposition and (2) loss of strength to hold themselves off the water surface as a result of muscle histolysis associated with later stages of oviposition. Observations of this study are consistent with both of these ideas.
Dead Gerris females often carried large batches of eggs in the laboratory, but only one apparently spent animal was found in dissections of 675 females of four species throughout the summer of 1975 (Chapter IV). Thus, there appears to be no significant post-reproductive period in Gerris.
It is established that rates of insect survival and oviposition are affected by temperature (Harris, 1939; Strong and Sheldahl, 1 970). Greenfield and Karandenps (1976) have further shown that both the egg maturation rate and survival of the adult lesser peach tree borer, Synanthedon pictipes, is linearly related to ambient temperature over the range of likely field temperatures. Similarly data from this study show' that 73
larval production/day-degree is approximately the same between
18.5° and 26°C for each of three Gerris species. The proximate biological mechanism is that reduced temperatures lead to both fewer daily egg batches and less eggs per batch, but increase absolute survival times. Sufficient data are not available to estimate exact thresholds of egg maturation by the methods of
Greenfield and Karandinos (1976), but Spence et al., (1978) have shown that the threshold of egg development estimated in this paper allows good prediction of time of first oviposition by
G« pinqreensis in the field.
Thermal constants and development thresholds are commonly built into simulation models of insect population dynamics
(Gilbert et al., 1976). At least one obstacle remains before this can be done for Gerris. Although the general pattern of mean fecundity in the field resembles the patterns seen in the laboratory, the higher variances of field data suggest that additional factors must be considered. Two suggestions can be made that relate to the difficult problem of how insects actually experience temperature in the field. (1) Insects on the same pond may experience significantly different microclimates. (2) Gerrids emerge from overwintering over a period of some weeks and thus several cohorts of breeding animals exist with respect to physiological age. At present few data can be brought to bear on these matters. 74
C. Growth and Development
Vepsalainen (1973a) has reviewed the scant literature on
gerrid development. Most work has considered European species
and there are few data for comparison among populations of North
American Gerris species. Such comparisons provide potentially
interesting information about adaptation. For example Bailer
and Bush (1974) have shown that development times vary among
geographical and host-specific races of the apple maggot,
Rhaqoletis cerasi even though developmental thresholds seem to
be approximately constant (Baker and Miller, 1978). These
results imply that the number of day-degrees required for pupal
development in Rhaqoletis can be adjusted by selection. Bailer
and Bush (1974) show how these adjustments may be viewed as
adaptive tactics.
Temperature thresholds also seem to be adaptive. Campbell
et al., (1974) have pointed out cases of distinct geographical
variation within aphid species. Tentative comparison of data
presented here with those of Jamieson (1973) suggests that
similar patterns exist in Gerris. If coexistence in Gerris is
determined by processes of cannibalism and interspecific
predation as argued by Vepsalainen and Jarvinen (1976) and
Jamieson (1973), we might expect species to adjust their growth constants with respect to both local climate and the growth
parameters of potential competitors. G. buenoi and
S. pinqreensis often co-occur on the same lakes, especially
during the spring (Chapter V) . It is possible that competitive 75
pressure from G. pinqreensis has selected for lower developmental thresholds in G. buenoi. This idea is further supported by the fact that neither G. incognitus nor
L. notabilis have lower growth thresholds on the Fraser Plateau.
Neither of these species co-occur frequently with
G. pinqreensis.
Significant differences have been demonstrated among the growth thresholds of different developmental stages. The consistent pattern observed among three species suggests the following adaptive interpretation. Eggs .develop under water and relatively high thresholds for egg development ensure that hatching is delayed until water temperatures are high enough to buffer low air minima likely during spring. Lower thresholds for early instars lead to rapid development once eggs have hatched. Fifth stage larvae become abundant in the study area around the summer solstice, consequently low thresholds are unnecessary because by mid-June high daily air temperatures are predictable. Data on optimum temperatures for laboratory survivorship show that higher thresholds are also associated with increased survivorship at high temperatures.
Temperature affects the development rates of Gerris species to different extents. These effects can be related to the sequence of appearance of these species in the field.
G. pinqreensis is best adapted for cold temperatures because mating, fecundity and juvenile growth and survival rates are not markedly inhibited by low temperatures. These same parameters are maximized at high temperatures in G. comatus and G. buenoi. 76
however, temperature effects are less striking in G. buenoi.
This observed species ranking, with respect to tolerance of cold
temperatures, corresponds to the order of their appearance and
speed of development in the field (Chapter IV).
The superior . adaptation of G. pinqreensis to low temperatures should lead to habitat preemption if gerrid-gerrid
predation is the main factor limiting species coexistence among
water-striders (Jamieson, 1973). There is some evidence that
this process may help to explain patterns observed in some
habitats (Chapter VI), however as a general rule, nature is not so simple; the fact is that many temperate gerrids do co-occur
regionally and often coexist on the same pond. Other factors
involved in determining the composition of regional water-
strider assemblages are discussed in subseguent chapters. 77
CHAPTER III. DENSITY ESTIMATES FOR GERRIDS
INTRODUCTION
A popular approach for studying species interactions in the
field is comparison of population dynamics. For most animals
the estimation of population size.is a challenging problem in
its own right (Gilbert, 1973). Two types of population
measures are employed by ecologists. Absolute, population
measures lead to density estimates while relative measures
permit only the comparison of relative abundances, given
assumptions of egual sampling efficiency over the range of
comparison (Southwood, 1966). Absolute estimates are generally desirable but relative methods are more widely employed because
they give much better data return per unit effort.
Southwood (1966) discusses the pitfalls of relative
methods. Differences between habitats, species, seasons, and
weather have been shown to affect the efficiency of most
relative measures applied to terrestrial insects. However
guantitative assessment of such effects is rarely available for
particular techniques applied in aquatic communities (Landin,
1976).
Southwood (196 6) suggested that regression analysis might
be used to actually predict more useful absolute estimates from
relative indicies. Although promising for aquatic communities
with relatively uniform habitat structure, the method has not
been developed. Recently however, Landin (1976) showed that 78
there are high correlations between absolute and relative population estimates for near-shore hydrophilid beetles.
The present study documents a high correlation between relative timed-catch sampling and absolute quadrat-count population measures for similar-sized species of Gerris occurring in two structurally different habitats. This association can be used as a basis for predicting absolute numbers from relative measures. Because all life stages operate similarly in overlapping two dimensional habitats, gerrids are ideal subjects for this approach. 79
METHODS
A. Seasons, Species, and Habitats
Most ponds and small lakes in the study area have little emergent or submergent vegetation in early spring when first colonized by overwintering Gerris adults. However, larger lakes with bulrush beds retain a mat of dead growth from the previous year that affords cover for colonizing gerrids.
Spring adults were sampled between 25 May and 6 June, 1975, on ponds with no development of vegetation. At Becher's
Prairie, Crescent was sampled once and Opposite Crescent was sampled on two occasions. In the Springhouse Study area. Grove
Pond was sampled once and Sp 2 was sampled on two dates.
Adults of Gerris buenoi, G. pinqreensis, and G. comatus were encountered in the samples.
Summer adult and juvenile populations were measured on
Boitano Lake and "Sp 2" at approximate ten day intervals starting with the appearance of new vegetation and concluding in early September. These two lakes were chosen because they provided the most uniformly dense stands of bulrush and grass/sedge habitats encountered among the study lakes and because they supported large gerrid populations.
G- pinqreensis was the sole breeding species in the bulrush habitat of Boitano Lake while both G. buenoi and G. pinqreensis bred successfully on Sp 2.
All sampling was done between 10:00 am and 5:00 pm on sunny 80
days. The sampling program was designed to yield both relative and absolute population estimates for each developmental stage over a range of natural densities. The relative estimate was made first on a given sample date with timed-catch sampling.
It was followed immediately by a series of guadrat counts made in an adjacent area of similar habitat structure. All identifications were made according to Scudder (1971), Scudder and Jamieson (1972) and Spence and Scudder (1978).
B. Relative Abundance Estimates
Gerrids avoid areas of wind and wave exposure (Andersen,
1976) and, consequently, a natural sampling zone is well defined by the limits of aquatic vegetation. Before vegetation appeared in the spring, limits were arbitrarily defined by the safety margin of my hip waders (approximately 85 cm depth).
Samples were collected with a long-handled (1.6 m), circular aquatic net (diameter: 26 cm). The net bag (212 AR
Aguatic Net Bag, BioQuip Products, Santa Monica, California) was muslin on the sides with a flat nylon mesh bottom (approximate mesh size: 1mm2).
When timed-catch samples are to be compared the procedure must be standardized as much as possible. Gerrid distributions are patchy and different life stages select different habitats
(Vepsalainen and Jarvinen, 1974; Chapter V) therefore the usual practice of counting standard sweeps was abandoned in favor of sampling over a standard range of habitat in a consistent 81
manner. The standard sampling route is illustrated schematically in Figure 16.
The routes taken on Sp 2 and Boitano Lake described a sguare with sides approximating the width of the vegetated zone of the lake. When employed on other ponds and lakes the procedure is bounded by two extremes. On small, shallow ponds with nearly continuous vegetation the sampling route may include opposite shorelines. When the width of the vegetation zone is less than some predetermined limit the route becomes rectangular. My sampling ritual required that at least two sides of the route were 15 steps (approximately 10.5 m) in length. When the vegetation included several structural types, sampling routes were selected that included all potential habitats.
As the sampling route was transversed, one full surface sweep was taken ahead on alternate sides with every second step.
The time taken to traverse the sample route was recorded on a stopwatch. On occasion sample routes were not completed owing to excessive accumulation of surface debris in the net.
Netted samples were emptied into a round plastic tub (35 cm diameter, 18 cm depth), half full of strained pond water, for field sorting. After gentle mixing to distribute the accumulated vegetation and debris, gerrids were tallied as they floated or swam to the surface. Adults and late instars were tallied and released. Young juveniles were preserved in 70% ethanol and returned to the laboratory for positive 82
Figure 16. A schematic diagram of the standard sampling route. 83
SHORELINE 84
identification. The final counts for each instar were divided by the sample time and recorded as numbers/minute.
C. Absolute Abundance Estimates
Quadrat counts of spring adults were made with a 1m2
sampling box designed and employed for gerrid sampling by
Jamieson (1973). The box was constructed on an aluminum frame
with sides 0.5 m in height and covered with plastic screening.
Two foam floats were attached at each corner. I took samples
by tossing the box 6-7 m ahead of me from random points within the potential gerrid habitat and then counting all gerrids captured within the floating enclosure. Ten guadrats were
counted at each pond on each sample date.. These counts were
averaged to estimate the number of adults per sguare meter of
gerrid habitat for each species encountered.
The 1 m2 box became impractical with the emergence of
vegetation and hatching of early stages for two reasons.
Firstly, thick vegetation (especially bulrushes) delayed or
prevented the box from settling into the water, surface, thus
allowing gerrids to escape. Secondly, high densities of early
instars were encountered so that counting insects stretched
sampling time beyond reasonable limits.
All summer quadrat sampling was done with a 0.5 m2 samplinq
box constructed from 3/16" plexiqlass. Sides of the box were
60 cm high. Because the box was too fragile to throw, samples
were taken by slowly wading to a predetermined sample location, 85
standing still for two minutes, and then dropping, the box at
arms length into the water. Adults and late instar larvae were
counted and released. Early instars were-caught with a small
net and aspirator, preserved in 70% ethanol and returned to the
laboratory for positive identification. At Sp 2 it was j
necessary to clip and remove all vegetation from the enclosed
guadrat before counting gerrids. The number of emergent shoots
was also tallied at Sp 2.
Obvious differences were noted immediately between adult
and early instar distributions. Therefore, in order to obtain
unbiased estimates and quantitative measures of habitat
association (results presented in Chapter V), I took quadrat
samples over a range of lake depths. At Sp 2, 25 m. isopleths
were marked by post and string at depths of 5, 15, 25, 35, and
45 cm. At Boitano Lake which drops off much more steeply,
isopleths were marked at 5, 20, and 35 cm. The isopleth lines
were re-established twice during the summer on Sp 2 owing to
dropping water level. On each sample date two guadrats were
counted along each isopleth at Sp 2 and four were counted along
each isopleth at Boitano Lake. Sample location was established
by picking a two-digit random number less than 35. I then
waded along the current isopleth line that number of steps
before dropping the sample box.
Counts of 10 and 12, 0.5 m2 quadrats were averaged from Sp
2 and Boitano Lake, respectively, on each date. Estimates of
number per sguare meter were computed and recorded for each
stage and species encountered. To allow comparison of spring 86
and summer results, I assume that the two sampling frames and
procedures were equivalent for measuring adult density.
D. Test Estimates
In order to test the generality of relationships discovered in this study, monthly estimates were made for gerrid
populations at six new lakes in the study area starting in May
1976. Habitats with grass/sedge, bulrush, floating (Polygonum sp.) and submergent (Myriophyllum and Ceratophyllum) vegetation were sampled. The lakes employed are listed in Table 16.
Three timed-catch samples were taken at each location at each
sample visit and the average value obtained was used to estimate absolute density. These results were compared with the absolute densities recorded with quadrat sampling in 1975 and
with published absolute estimates from other Gerris populations
to assess the reliability of the method. 87
RESULTS
The results of absolute and relative sampling conducted in
1975 were compared with regression analysis. The final
objective was to predict density from timed-catch samples.
Therefore it is appropriate to treat the absolute measure as the
dependent variable for regression. Both variables were
transformed to the natural log scale.to satisfy assumptions of
the linear regression model. The untransformed data show that
timed-catch samples become relatively less efficient as absolute
numbers increase. Also the reliability of either method falls
off greatly at low densities for all stages. However the function log(x+1.0) satisfactorily transformed the data to
linearity.
A. Differences Between Seasons and Size Classes
The analysis was first performed in the usual manner with
regression equations that included non-zero intercepts. Data for each summer instar (i.e.size class) and spring adults were
considered as separate blocks. The calculated intercepts and
their standard errors are shown in Table 11. Most intercepts
lie close to the origin; five of seven lie within one standard
error. If the two variables are directly proportional, the
intercepts should be zero.
Given the data of Table 11 and the expectation of
proportionality, the analysis was repeated without correction
for the mean (i.e. the regresssion line was constrained to go TABLE 11
Intercepts and standard errors for all summer instars and spring adults
SPRING INTERCEPT STANDARD N ERROR
1st 0.43 0.122 18
2nd -0.06 0. 132 20
3rd 0.01 0. 107 21
4th 0.32 0. 130 20
5th -0.04 0. 151 20
Summer Adults 0.10 0. 118 20
Spring Adults -0.09 0. 160 13 89
through the origin). The within-block correlation between the two measures is very strong (r=0.903; df=131). Table 12 gives the overall analysis of variance obtained. There is no significant curvature but there is significant heterogeneity between the slopes observed in the different blocks.
Table 13 lists the slopes and their standard errors obtained for each block. The slopes are much greater for the first four instars than for fifth stage- larvae and adults.
Higher capture rates observed for larger gerrids probably result from their greater visibility during sampling. Slopes are similar for first and second instars as well as for third and fourth instars suggesting that these data might be pooled in subsequent analysis. The data of Table 13 also show that real differences exist between size classes and seasons. Regression lines for the size classes suggested by data in Table 13 are shown in Figure 17. Differences between size classes seem to be relatively more important than differences between seasons within the adult size class.
Data in Table 13 suggest that the reciprocal of the slope of the regression line (= availability for capture) for each species and instar should vary with some. index of body size.
Availability is plotted against length of meso-thoracic leg in
Figure 18. A second order polynomial equation was fitted to the data by the method of least squares. . The availability for
capture can be adequately predicted ;(r=0.966; df=11) from mesothoracic leg length with the following equation: .
availability = 0.7834 - 0.0910X + 0.0133X2 TABLE 12
Analysis of variance of regressions of capture efficiency for various developmental stages of Gerris
df M.S.
Regression 1 186.68 1556 <-QQ1
Regression within blocks 6 4.46 37.17 <.Q01
Curvature 1 0.02
Remainder 124 0.12 TABLE 13
Proportionality constants obtained for all summer instars and spring adults - regression contrained through the origin (values of N same as Table 11)
Stage Constant S.E.
1st 1.51 0.068
2nd 1.67 0.108
3rd 1.36 0.082
4th 1.29 0.075
5th 0.90 0.057
Summer Adults 0.69 0.057
Spring Adults 0.51 0.065 92
Figure 17. Regressions of absolute density on number of gerrids
caught per minute. (a.) first and second instars,
(b.) third and fourth instars, (c.) fifth instars,
(d.) spring and summer adults. 93 94
Figure 18. Polynomial regression of availabilty for capture on
mesothoracic leg length for all stages of G. buenoi
and G. pingjreensis.
96
B. Differences Between Habitats
The four summer groups suggested by Table 13 were each split into two blocks representing samples from grass/sedge and bulrush habitats respectively. Regression analysis was employed to analyze each group for" significant block differences. The calculated slopes and their S.E. are presented in Table 14. No significant differences exist
between block slopes for any size class. All juvenile stages tend be slightly harder to catch by timed-sampling in bulrushes.
However, this relationship is reversed for adults. Therefore, there is no need to adjust estimates from these two particular habitats for differences in sampling efficiency when predicting absolute abundance.
C. Differences Between Species
Gerris buenoi and G. pinqreensis are very similar in size
throughout development (Scudder and Jamieson, .1972). Separate
regression analyses for each size class were used to assess
possible differences in capture efficiency that might stem from
behavioural differences between species. No significant
differences, were found among the four summer size classes
analyzed. Means and standard errors are given in Table 15. TABLE 14
Proportionality constants and their standard errors for different size classes in two habitats
Size Class Grass/Sedge N Bulrush N
1st and 2nd 1.61±0.129 22 1.53±0.090 16
3rd and 4th 1.37±0.108 24 1.30±0.Q61 17
5th 0.86±0.058 12 0.94±0.Q55 8
Adults 0.68±0.063 12 0.72±0.135 8 98
TABLE 15
Average proportionality constants for size classes of G. buenoi and G. pinqreensis
G. buenoi G. pinqreensis
Size Class Mean S.E. . N Mean S.E. N
1st and 2nd 1.71 0. 14 13 1.50 0.09 25
3rd and 4th 1.51 0.13 14 1.28 0.06 27
5th 0.82 0. 06 6 0.96 0.05 14
Summer Adults 0.63 0.08 6 0.73 0.08 14
======: ======: 99
D. Test Estimates
The test estimates are compared with the range of absolute
estimates used to calibrate the procedures and available
published estimates for other Gerris populations in Table 16.
Data from Vepsalainen (1971) were derived by dividing the
maximum and minimum populations recorded by half the pond area
because Vepsalainen states that the maximum vegetation cover was
50%. Other data derived from absolute density estimates have
been published (Brinkhurst, 1966; Matthey, 1976) but the information presented is insufficient to allow comparison.
Density estimates for specific stages cannot be compared because
no partitioned estimates have been published.
All high estimates reported in Table 16 contained a
substantial proportion of early instars. The estimates of
Jarvinen et al. (1977) are site specific and therefore, should be higher than estimates derived from this study which are averaged over the whole range of potential habitat. The main
point to note is that absolute densities calculated from timed- catch sampling provide density estimates in the same range as reported from more intensive studies that employed several methods of absolute sampling. 100
DISCUSSION
Gerrid population densities have been previously estimated
by several methods. Brinkhurst (1966) and Vepsalainen (1971)
employed mark and recapture methods to follow the seasonal
dynamics of two different European gerrids on small, uniform
habitats. Recently, Jarvinen et al. (1977) have based
estimates of Gerris densities upon the duration of individual
insect visits to a known area of habitat. Matthey (1976) used
direct quadrat counts to determine the density of G. remiqis on
small, montane ponds in Alberta. All.of these methods are
laborious, time-consuming and lend themselves best to studies of
single species occurring on small, open ponds. Only a few
lakes may be sampled adequately within a short sampling
interval. All but the method of direct quadrat counts are
restricted in practice to adults of multispecies communities.
Studies focused at the multispecies level and comparisons
made over a wide range of habitats necessitate the use of
relative abundance measures. Brinkhurst (1959) used timed-
catch sampling to compare gerrid populations , in several habitats. Timed-catch sampling has been employed to assess the
relative abundance of other freshwater (Taylor, 1968; Zimmerman,
1960) and riparian (Andersen, 1969; Spence, 1978) insects over a
range of potential habitats. The reliability of these methods
has been questioned by Andersen (1973) because of suspected differences in sampling efficiency between habitats and seasons.
The present analysis indicates that there is some hope for the continued use of relative methods provided that they are 101
compared with absolute measures to estimate appropriate
correction factors.
Human beings are not perfect sampling machines. Hairston
et al. (1958) cite an unconcious tendency to collect in high
density patches or to select the most obvious animals as
weaknesses of timed-catch sampling. In this study the first
bias was avoided by choosing a sampling route through potential habitats in a consistent manner and without prior knowledge of
prevailing Gerris abundances. The data . presented show that
regression analysis may be used to correct for the fact that larger, more visible animals are collected with greater
efficiency. Generally, biases of "human frailty" should be
minimized, and subject to correction, when sampling follows a
prescribed ritual with respect to potential habitat.
Landin (1976) found a direct relationship between size and capture efficiency among aquatic hydrophilid beetles. He
pointed out that additional factors such as color and behaviour affect timed-catch sampling that involves search, pursuit and capture of individual animals. For such methods, Landin concluded that only classes of animals with similar capture efficiencies should be sampled simultaneously. However, a prescribed routine minimizes pursuit of individuals and the foregoing analysis shows that it is possible to apply different correction factors to estimate the absolute numbers of several classes sampled simultaneously.
The relationship illustrated in Figure 18 can be used to 102
estimate proportionality constants for converting timed-catch
data to absolute density. Therefore, assuming that capture
rates are not affected by differences in species behaviour, density estimation is possible for all four Gerris species in the study area. This pragmatic approach, used in the absence
of specific data, will be used to make all subsequent density
estimates presented.
When dealing with animals outside of the size range
considered in Figure 18, however, caution is necessary. The
regression equation may poorly estimate the capture efficiencies
of larger and smaller animals. For example, consider the two
species of Limnoporus occurring in the study area. Adults and
fifth stage larvae are larger than any stage of co-occurring
Gerris species. Conseguently, these two stages should be more
visible, and hence, more available to sampling. However, their
red-brown coloration contrasts markedly with the grey-black of
Gerris species. Also, my observations suggest that they are
more adept at avoiding the net. How these factors interact to
determine capture rates is unknown at present. Fortunately,
Limnoporus is much less common than Gerris in the study area
and, at these low densities, errors resulting from application
of the Gerris estimator will not seriously distort an overall
view of the water-strider guild.
A surprising result of this analysis is that no significant
differences in sampling efficiency were established between
grass/sedge and bulrush habitats. However, this does not
necessarily mean that habitat differences are unimportant. The 103
seasonal differences observed between spring and summer adults
is probably a reflection of differences in amount of vegetation
cover present. Vegetation can bring about this effect in two
ways: (1) cover makes large gerrids less visible and impossible
to pursue with a single net sweep and (2) increasing cover
diminishes the actual capture rate of gerrids in the path of the
net by providing refuges and increasing net drag. Assuming
that absence of vegetation is sufficient to account for
increasing sampling efficiency in the spring, the presence of
vegetation can reduce the proportionality constant by as much as
26% on the study lakes.
The upper limits of error for summer population estimates
resulting from variable vegetation densities is illustrated by
considering the difference between application of spring (no
vegetation) and summer constants to the highest adult capture
rate recorded in test-estimates made in summer 1976. A density
of 9.28 G. buenoi adults/square meter was calculated at Gerrid
City (Becher's Prairie), using the summer constant derived on Sp
2 and Boitano Lake. Application of the spring constant to the
same data, lowers this estimate by approximately 50% to 4.62/m2.
This is still a relatively high estimate. The best estimate
for this sample must lie somewhere between these two extremes,
because Gerrid City had moderate grass/sedge cover in summer
1976.
Few grass/sedge habitats have vegetation density comparable to that found on Sp 2, but Boitano Lake is representative of a typical Chilcotin bulrush bed. I suspect that presence of 104
vegetation diminishes sampling efficiency to some extent through
interaction of the two effects mentioned above. Field experience suggests that the effects are greatest in dense
grass/sedge or bulrush habitat. Presently, I have no
quantitative basis to compare sampling efficiency across the full range of Gerris habitats encountered. However, the range of estimates presented in Table 16 show that data at hand are
good enough to allow reasonable estimates. Future development
of the method should attempt to adjust estimates in more open
grass/sedge habitats by some function of vegetation density.
Sampling efficiency in habitats of other structure might be
measured and adjusted from the bulrush-grass/sedge baseline by a
similar function. A similar approach has been suggested by
Caughley et al. (1976) for calibrating aerial survey counts of large mammals.
The principal disadvantage of the method in two-dimensional
habitats is the initial work of calibrating the estimation procedure. Individual sets of correction factors will probably be necessary for each worker to accommodate differences in personal sampling style. The problem of habitat differences is difficult and has not been fully overcome by this analysis.
Whether the effort is worthwhile will be determined only from the collective results of studies that attempt it.
The results of this study allow summer density estimates
for gerrids with the proviso that such estimates are interpreted as setting an upper limit to possible densities. In general, the estimates so obtained compare favorably with the range of 105
TABLE 16
A comparison of test estimates with Gerris population estimates from the literature
ABSOLUTE RANGE OF STAGES/ SOURCE METHOD DENSITY SPECIES PRESENT ESTIMATES Gerris
Jarvinen et al. (19 77) |duration of 0.59- arqentatus | | individual no data adult | visits
Jarvinen et al. (1977) I n II 3.1T15. 1 arqentatus adult
Jarvinen et al. (1977) 1 11 1! 37.3-3 62,7 arqentatus 1-4
Vepsalainen (1971) | mark - | 0,61-2.50 odontoqaster | recapture < adult
Sp 2 (this study) | quadrat- | 2.6-64.8 buenoi all | count
Sp 2 (this study) 1 11 0.8-8.2 pinqreensis all
Boitano L.(this study) 1 " 1.8-79.6 I piugreensisj all
TIMED CATCH TEST VEGETATION I ' ' 1 " ' i ESTIMATES-1976 TYPE
Grove Pond | qrass/ 3.1-176.2 buenoi all | sedge
Gerrid City • II 3. 8-75.2 buenoi all
Sp 8 |submerged 3.7-25.3 comatus all
Clear Lake | floating 1. 1-67^ 8 1 comatus all
Westwick Lake | rush 1.5-79.8 1 pinqreensis all
Sapper Lake 1 " 3. 4-75.2 pingreensi s all
J 106
absolute estimates reported by other authors for Gerris and my own quantitative estimates from Sp 2 and Boitano Lake. This general method of population estimation could be applied to many species living in two-dimensional habitats provided that sampling minimizes individual pursuit. It may also work for near-shore aquatic invertebrates that do not actively respond to three-dimensional habitat complexity. Three-dimensional complexity qreatly maqnifies the problem of consistent sampling over the full range of potential habitats. Complicated relative sampling techniques would be necessary, thus eliminating the time-efficiency advantaqes of relative methods. 107
CHAPTER IV. COMPARATIVE ECOLOGY OF WATER-STRIDERS ON THE
FRASER PLATEAU OF BRITISH COLUMBIA
INTRODUCTION
Nine species of water-striders are known from British
Columbia (Scudder, 1977). Most of these show considerable
range overlap in the province, and several species may inhabit
the same small pond. Because gerrids are unspecialized,
opportunistic predators (Lumdsen, 1949) , that are likely to experience periodic resource limitation (Vepsalainen and
Jarvinen, 1976), interesting guestions of species packing are
suggested.
Jamieson (1973) carried out the only whole-season,
multispecies study of gerrid population dynamics, at Marion Lake
in the lower Fraser Valley of British Columbia (U.B.C.
Research Forest, Haney, B.C.). He found differences in life
cycle timing that apparently separated the peak abundances of
several species over the season, and also noted species
differences in microhabitat preference.
The gerrid fauna of the saline lakes on the Fraser Plateau
(Scudder, 1969) offer an interesting comparison with the fauna studied by Jamieson. Quite different from the cool, wet fir-
hemlock forest surrounding Marion Lake, the Fraser Plateau study
area is dry, rolling, grassland dotted with numerous lakes and
sloughs of varying salinity (Chapter I). The gerrid fauna consists of six species, three of which also occur at Marion 108
Lake.
This work was undertaken to acquire basic information about the life cycles, voltinism and habitats of gerrids in central
British Columbia. These data can be compared with results from other studies and used to formulate more specific hypotheses relating to the coexistence of water-strider species. 109
MAMMALS AND METHODS
A. Lakes and Species Studied
Field populations of G. buenoi, G. comatus, G. pinqreensis
and Limnoporus spp. were studied at two sites (Chapter I) on the
Fraser Plateau of south-central British Columbia. Forty-five
study lakes were visited at approximate ten-day intervals
between late May and mid-September. These lakes are listed and
described in Appendix I. Their exact locations are shown on
Figures 2 and 4 (Chapter I).
1. Fie Id Temperatures
Air and surface water temperatures were recorded throughout the summer at both study sites with Ryan (Model D) submersible
temperature recorders (Ryan Instruments Inc., Seattle, Wa.,
U.S.A.). Air temperatures were recorded in the shade, at ground level, near Westwick Lake at Springhouse and at Opposite
Crescent at the Becher's Prairie study site. Surface water temperatures were monitored at small, medium and large lakes at both study sites (Springhouse: Grove Pond, Sp 1 and Westwick
Lake; Becher's Prairie: Opposite Crescent, Clear Lake and Lake
Lye; — lake surface areas provided in Appendix I.)
Field temperatures were used in conjunction with data on growth thresholds from Chapter II in order to compute physiological time scales for gerrids. It is difficult to determine what temperatures should be used in calculations for 110
semi-aquatic animals (i.e. what temperatures are actually
experienced by gerrids in the field). Calabrese (1977) and
Matthey (1976) suggest that water temperatures are most
reliable. However, Jamieson (1973) showed that gerrid body
temperatures rise where animals are exposed to direct sunlight.
In order to correct for the effect of insolation, Jamieson
adjusted day-degree sums by an arbitrary insolation coefficient
that decreased symmetrically on either side of the summer
solstice.
I have adopted a somewhat different approach in these
studies. All physiological timescales presented have been
calculated with a modified version of the algorithm for
temperature summation which was listed by Fraser and Gilbert
(1976). The procedure assumes that water temperature is
constant at the recorded minimum and that gerrids experience
water temperature whenever it is greater than air temperature.
The algorithm then integrates the ambient temperature
experienced by the gerrids, above the threshold, over time
(Figure 19). The method thus allows for actual daily
insolation effects, as measured by recorded air temperatures,
and allows water temperature to buffer the effects of low temperatures when they occur. Spence et al. (1978) have shown that this summation procedure allows good prediction of egg laying and first instar appearance in the field. 111
Figure 19. Areas summed in calculation of physiological time
scales. || AREAS SUMMED
AIR TEMPERATURES 113
2. .Egg Production and Alary Mprphism
During each sampling interval, females of each species were collected and preserved in 70% ethanol. These specimens were
subseguently dissected to assess the state of the reproductive
systems. Female gerrids were taken only from lakes that
supported large populations of a particular species to ensure that species population dynamics were not significantly altered.
The collection sites for female gerrids of each species are listed in Table 17. Limnoporus females were collected at many locations because of their relatively low density throughout the study area. At the time of dissection, each female was classified with respect to wing morph following the criteria of
Vepsalainen (1971) and with respect to the presence or absence of chorionated eggs. Data were grouped into bimonthly intervals for analysis.
3. Population Dynamics
The entire series of study lakes was sampled within five or six days from the beginning of each ten-day sampling interval from late May through mid-September, 1975. Each lake was sampled again during mid-May, 1976, in order to estimate early
spring breeding populations of the following year. At sampling visits, instar-specific abundances were estimated for each species with timed-catch sweeping over a standard sampling route. Details of the sampling procedure are given in Chapter
III. Animals that could be positively identified were tallied and released on site. Early instars were preserved in 70% TABLE 17
Collection sites of female gerrids used for reproductive dissection
SPECIES COLLECTION SITES
G. buenoi Grove Pond Gerrid City
G. comatus Sp 1 Clear Lake Newall Lake
G. pinqreensis Boitano Lake Near Round-up Lake Sapper Lake
Limnoporus Grove Pond Sp 5 Gerrid City Opposite Crescent Centre Arms Pond 115
ethanol for subsequent identification and counting in the
laboratory.
Field and laboratory counts were combined and converted to
numbers of each stage and species caught per minute. These
data were used to estimate absolute densities from the
regression equations developed in Chapter III,
Total gerrid biomass estimates for each lake (expressed as
mg wet weight/m2) were obtained by multiplying calculated
densities times a biomass coefficient for each stage and species
and then summing all values for a particular lake and date.
Biomass coefficients were calculated as the mid-point between
the maximum weights of successive instars of each species.
Maximum weights were determined by rearing cohorts of animals
fed to satiation daily with vestigial-winged Drosophila in the
laboratory. Animals were anesthetized with carbon dioxide and
weighed on the day that the first members of a particular cohort
molted to a new stage. The average maximum weights recorded
for each stage of five species are presented with their standard
errors in Appendix IV.
4. Habitats
The study lakes provide a broad spectrum of potential
habitat for aquatic and semi-aquatic insects. Three habitat characteristics, that seemed important for water-striders, were
measured at each lake sampled regularly in 1975. (1)
Development of aguatic vegetation was noted at each lake, and, 116
by late July, it was possible to classify most lakes with
respect to the dominant type of vegetation-present. If several
vegetation types were presen t in more or less equal abundance
over the area sampled, the 1 ake was classified as "mixed" habitat. (2) Surface water samples were taken at approximate monthly intervals at each lake and conductivity was determined
with a Radiometer CD2 conductiv ity meter; results were corrected to 25°C. (3) Lake size w as used as an indicator of lake permanence in historical time, Another classification more sensitive to variation on sho rter time scales, was provided by considering those lakes that dr ied out completely between May,
1975 and September, 1977 as tem porary.
The total number of each gerrid species collected at each
lake during the 1975 study of p opulation dynamics was tallied separately, as a measure of re lative species abundance. These data were used to construct c ontingency tables in order to compare the effects of the three habitat characteristics on gerrid distributions. 117
RESULTS
A. Field Temperatures
Physiological time scales for gerrids, calculated from
field temperatures recorded in 1975, are presented in Figure 20.
Figure 20a shows the extent of variability between lakes with
respect to a single developmental threshold (8.7°C). Except for data from Westwick Lake, the day-degree accumulations show a
pattern of increase with lake area.
Because the same air temperatures were used for calculating the time scales at each of the two study sites, differences
observed among the three lakes can be attributed solely to variation in surface water temperature. The data suggest that
gerrids on larger lakes experience accelerated physiological time scales owing to an increased capacity of larger water mass
to buffer low nightly air temperatures. Because the effect is cummulative, lake to lake differences increase as the season progresses.
Figure 20b shows the range of physiological time scales experienced by G. buenoi, G. comatus and G. pinqreensig using temperature data from a single lake (Lake Lye, Becher's Prairie site). The data indicate that different instars develop on vastly different time scales in the field owing to the differences in developmental thresholds calculated in Chapter
II. -118
Figure 20. Physiological time scales for gerrids calculated
from field temperatures at Springhouse and Becher's
Prairie. (a.) variability among lakes, (b.)
differences between species and instars. 119
A. PHYSIOLOGICAL TIMESCALES AT SIX LAKES THRESHOLD TEMPERATURE 3 8.7 °C
L.LYE IOOO
CLEAR U SP I 800
WESTWICK L GROVE R 600
OPPOSITE CRESCENT (DRY AUG.7) 400 CO LU LU 200 tr CD LU Q 30 50 70 90 110 130 i B. PHYSIOLOGICAL TIMESCALES AT LAKE LYE USING SEVERAL THRESHOLDS FOR GERRID DEVELOPMENT
10 30 50 70 90 110 DAYS AFTER JUNE 1,1975 120
B. Egg Production and Alary. Morphism
Figure 21 shows the pattern of occurrence observed for
reproductive gerrids during 1975. G. buenoi and G. pinqreensis
were in full breeding condition at the start of this study,
however, some females of G. comatus and Limnoporus spp. had not yet reached reproductive maturity. Females with chorionated eggs were commonly encountered among all four species until the end of July.
Three seasonal patterns of alary morphism were observed among the gerrid species studied. Limnoporus spp. were always fully winged. G. pinqreensis were . most commonly apterous, however some long-winged individuals (approximately 13% of all specimens collected) were encountered among overwintered and first generation populations.^ In late May, 1976, I also collected three micropterous G. pinqreensis from Boitano Lake at the Springhouse site. A common pattern prevailed for G. buenoi and G. comatus. Overwintered populations were entirely long- winged, but a small proportion of micropterous individuals was encountered among the first summer generation. These patterns are summarized in Table 18.
Vepsalainen (1974) has pointed out that, although a continuous spectrum of wing-length is encountered, there are only two functionally different wing morphs among water-• striders: those that can fly at some point in the life cycle
(long-winged forms: macropters) and those that cannot (short- winged forms: apters, micropters, brachypters). Therefore I 121
Figure 21. Distribution of reproductive effort among morphs
over the summer of 1975. Number at the
top of each bar represents the total number of
females dissected during the interval. (a.)
G. buenoi, (b.) G. comatus, (c.) G. pinqreensis. A. G. BUENOI C. G. PINGREENSIS g IOO| ID UJ 80 H LONG-WINGED Q UJ 60
§ 401 o I 20 o
Q Lul \—o . B. G. COMATUS D. LIMNOPORUS SPP UJ if) 100 LO 80 oo < 60 UJ Lu 40 U_ °20
LO — IO CD CD 123
TABLE 18
Breeding condition of gerrid wing-morphs encountered in the Fraser Plateau study area
GENERATION
SPECIES OVERWINTERED 1ST SUMMER 2ND SUMMER
G. buenoi long-winged* long-winged* long-winged micropterous*
G. comatus long-winged * long-winged long-winged micropterous*
G. pinqreensis apterous* apterous* apterous long-winged* micropterous (none found in 1975) long-winged
Limnoporus long-winged* long-winged not^ present
* - indicates morphs found carrying chorionated eggs 124
will distinguish only between long-winged and short^winged
morphs in all subsequent discussion.
Figure 21 and Table 18 indicate how the breedinq
populations of each species were partitioned among the different
wing morphs. These data are based on the dissection of 662
female gerrids. Figure 21 indicates the number of animals
dissected from each two week period and the total number of
females examined for each species.
First generation reproductives of G. buenoi and G. comatus
could be easily distinguished from non-reproductive animals
because they are conspicuously marked by pale-white abdominal and, to a lesser extent, thoracic venters. Ml non-teneral
animals of these two species with reduced,ventral pigmentation carried eggs. For the most part these animals were short-
winged. However, one long-winged G. buenoi female with a pale venter, taken from Grove Pond on July 21, 1975 had histolysed
flight muscles and carried 20 chorionated eggs. Second
generation animals with the dark undersides typical of
overwintered adults, generally carried no eggs. The single exception was one female G. buenoi collected from Grove Pond on
September 9 which carried six chorionated eggs and had fully developed indirect flight muscles.
The general pattern of wing morphs was reversed in
G. .Eiaareensis. Both long and short-winged animals that had overwintered were found in reproductive condition. First
generation long-winged individuals did not breed and left the 125
ponds for winter diapause soon after the adult molt in late June through mid-July. No long-winged G. pinqreensis were collected from any lake after July 24 although they appeared again among the breeding population in the spring of 1976. I was unable to find any morphological marker (i.e. pale venter) to distinguish apterous summer generation G. pinqreensis in breeding condition, from the overwintered population or the newly emerged adults destined for winter diapause. However, the "fresh" appearance of newly molted animals clearly indicated that some first generation G. pingreensis did reproduce in 1975.
Overwintered Limnoporus females in breeding condition survived longer than any of the Gerris species (Figure 21).
However, none of the dissected Limnoporus females that emerged during 1975 carried eggs. Their reproductive tracts were always immature. Therefore, I conclude that all Limnoporus spp. populations were univoltine during 1975.
C. Population Dynamics
Figures 22 - 25 illustrate the partial population curves for each instar of G. buenoi, G. comatus, G. pingreensis and
Limnoporus spp. at the two study sites. Mean densities for each stage are averaged over all lakes where the species was recorded during a given sampling interval. The data are plotted as natural logarithms with points for each interval falling on the median day of the samplng period. The ten-day sampling intervals were too long for accurate tracking of instar-specific populations during the early summer, because 126
Figure 22. Partial population curves for G. buenoi during 1975.
(a.) Springhouse, (b.) Becher's Prairie.
128
Figure 23. Partial population curves for G. comatus during
1975. (a.) Springhouse, (b.) Becher's Prairie.
130
Figure 24. Partial population curves for G. pingreensis during
1975. (a.) Springhouse, (b.) Becher^s Prairie.
132
Figure 25. Partial population curves for Limnoporus during
1975. (a.) Springhouse, (b.) Becher's Prairie. LIMNOPORUS SPR DENSITY- LN (NUMBER/SQ. M. +1.0) PER LAKE
EC I 134
high temperatures promoted rapid development on the daily time- scale. However some general considerations do emerge.
Among the partially bivoltine Gerris species, virtually all stages were present somewhere at each study site throughout the sampling season. Figures 22 - 24 also emphasize that the two breeding generations are not at all. distinct. There were obvious pulses of first instar larvae for G, pinqreensis at both study sites but the distinction was blurred for both G. buenoi and G. comatus. For all bivoltine species, any indication of breeding pulses reflected in first instar abundance is dampened in the curves plotted for later instars.
The data for Limnoporus spp. (Figure 25) shows that the age distribution of contemporary juveniles was also guite broad throughout the summer. This can be explained, in part, by the
fact that overwintered Limnoporus adults survived and bred until late in the season.
The partial population curves suggest that juvenile
mortality was high and probably concentrated among the first four larval instars. This is most apparent for Limnoporus
SPP- (Figure 25) . The open circles plotted for the abundance
of adult gerrids during mid-May 1976 can be compared with the data from summer 1975 as an indication of the relative
magnitudes of winter mortality. The data suggest that
mortality was most severe among overwintering Limnoporus spp.
Among the Gerris species, spring adult densities in 1976 were
generally higher than observed at any point during the summer of 135
1975.
Increased lake to lake variability in instar-specific boundaries is indicated by the relative magnitudes of the standard error bars of Figures 22 - 25, The data reflect the divergence in age distributions between lakes as the season progressed. The relatively large standard errors for the
Limnoporus data result from small sample sizes because individuals of this species were often taken on less than five lakes during a sampling interval.
Figure 26 depicts the overall population curve for each species at each site as the sum of the respective instar means for a given sample interval. Populations of all species started growing earlier at Becher's Prairie and remained ahead of those at Springhouse throughout the summer,
A spring sequence of species population growth was observed at both study sites. G. pinqreensis populations started increasing first and were followed by G. buenoi, G. comatus and
Limnoporus spp. respectively. However, data in Figure 26 demonstrates that the timing differences did not persist throughout the summer in any consistent manner. Separate peaks of abundance for each species did not occur during 1975..
Figure 26 suggests that overall gerrid abundance reached a maximum during July and August. The ecological implications of such peaks may be best considered from the perspective of biomass. In Figure 27, I have plotted the average gerrid wet biomass per square meter, as the sum of biomass estimates for 136
Figure 26. Total population curves for all water-strider
species during 1975. (a.) Springhouse, (b.)
Becher's Prairie. A. SPRINGHOUSE • 6, BUENOI I A & COMATUS
6v6 6/26 7/16 8/5 8/25 9/14 MONTH/DAY, 1975 138
Figure 27. Average gerrid biomass per lake during 1975. (a.)
Springhouse, (b.) Becherfs Prairie.
I 139
A. SPRINGHOUSE
il40
:> 120
CO «oo
808
UJ 60S
ffi 20S Q_ 6/6 6/26 7/16 8/5 8/25 9/14
~ B. BECHER'S PRAIRIE
j_ 140
UJ $ 120
Q IOOI cr LJJ 80
O
_j 60!
^ 4d P.
6/6 6/26 716 8/5 8/25 9/14 MONTH/DAY, 1975 140
all species present for each sampling interval during 1975.
Curves obtained for both sites show that gerrid biomass reached a maximum value between mid-July and mid-August. However, the lake to lake variability in gerrid biomass increased markedly as the season progressed.
D. Habitats
1• Vegetation
The aguatic vegetation of the study lakes was divided into three groups based upon the dominant plant species present and the general structure of the habitat provided for water-
striders. These features are summarized in Table 19.
The major factors affecting gerrids seemed to include the density of emergent cover, and seasonal changes in habitat.
Floating vegetation habitats provide no emergent cover in contrast to grass/sedge and rush habitats. Rush habitats afford emergent cover throughout the season but are generally characterized by less surface^level complexity than mature grass/sedge habitats.
A total of 16,777 individual gerrids were tallied during
1975. The observed ( distribution of gerrid-species is
classified with respect to vegetation-type in Table 20. The
data indicate obvious quantitative associations of each species
with a single type of habitat. G, buenoi and Limnoporus
spp. were collected most often from grass/sedge habitats while TABLE 19
Plant species and vegetation structure used to define gerrid habitat classes. (a.) Grass/Sedge, (b.) Floating Vegetation, (c.) Rush
' CHARACTERISTIC HABITAT EMERGENT COVER PLANT SPECIES STRUCTURE AND SEASONS a. Grass/Sedge
j Beckmannia syzigachne closely-spaced , much 'emergent (Steud.) Fern. thin,emergent cover by Carex sp. stems in early July, Juncus balticus Willd. shallow water; no coyer in Lemna sp. floating early spring; Puccinellia spp. plants often baffles both Scolochloa festucacea present also waves and (Willd.)Link. wind Sium suave Walt. Sparganium auqustifolium Michx.
b. Floating Vegetation
Ceratophyllum floating and/ no emergent demersum L. or submerged1 cover; Myriophyllum vegetation , baffles spicatum L. waves but Polyqonum amphibium L. not wind. Potomoqeton spp. fully Utricularia vulgaris L. developed by early July
c., Rush
Scirpus validus Vahl widely-spaced much emer• Juncus balticus Willd. relatively gent cover thick emergent throughout stems season, baffles both wind and waves 142
TABLE 20
Gerrid species abundance in various habitat categories
HABITAT Gerris Limnoporus TOTALS
TYPE buenoi comatus pinqreensis spp.
Grass/ 5181 531 717 302 6731 Sedge N=13
Floating 694 2637 280 8 3619 N=10
Rush 172 356 3138 3 3669 N=1.1
Open 1 13 1 0 15 N=3
Mixed 812 1089 801 41 , 2743 N=7
Totals 6860 4626 493? 354 16777 ______* N=number in each category 143
G. comatus and G. pinqreensis were strongly associated with floating vegetation and rushes respectively. Data of Table 20 emphasize the relative rareness of Limnoporus on the study
lakes.
In habitats without a single dominant vegetation type
("mixed" in Table 20), relative Gerris species abundance was
much more evenly distributed. However, there was a slight
tendency for G. comatus to predominate. Juvenile gerrids were
never collected from the three lakes visited without aquatic
vegetation (Blake L., Drummond L., Round-up Lake). All animals
recorded at these open habitats were winged adults collected at
the time of spring dispersal.
2. Surface Conductivity
Lakes were classified into three groups with respect to
maximum recorded surface conductivity (0-1000, 1000-4000, >4000
-Hlmhos/cm at 25°C) . Table 21 presents the relationship between
gerrid species abundance and these conductivity categories.
G. buenoi, G. comatus and Limnoporus were strongly associated
with lower conductivities than was G. pinqreensis. The
association is strongest for Limnoporus. Overall gerrid
abundance was also highest at lakes with the lowest
conductivities. 144
TABLE 21
Gerrid species abundance in various categories of surface water conductivity
MAXIMUM SURFACE Gerris Limnoporus TOTALS CONDUCTIVITY -- (Xmohs/cm at buenoi comatus pinqreensis 25°C) 0-1000 N=20 5096 3212 826 . 308 9442
1000-4000 N=18 1721 1201 2703 46 5845
>4000 N= 7 42 213 1408 0 1490
Totals 6860 4626 4937 354 16777
* N=number of lakes in each category 145
3. Lake Permanence
Table 22 shows the relationship between species abundance
and lake size. G. buenoi and Limnoporus were most common on
small ponds while the relative abundances of G. comatus and
£• pinqreensis were greatest on medium-sized and large lakes respectively.
Gerrid abundance •data are partitioned with respect to
whether or not the lake dried out between 1975 and 1977 in Table
23. There is an obvious difference between species with
respect to proportion of the total population encountered in
extremely temporary habitats. Limnoporus spp. invested the
highest proportion of individuals in habitats likely to dry out
during the season. The same tendency becomes less and less
pronounced for G. buenoi, G. comatus and G; pinqreensis
respectively. 146
TABLE 22
Gerrid species abundance in various categories of lake area
AREA Gerris Limnoporus TOTALS (hectares) buenoi comatus pinqreensis
< 2.5 4617 1888 1652 338 8495 N=18
2.5-5.0 1160 2440 1257 8 4865 N = 14
< 5.0 1083 298 2028 8 , 3417 N=13
Total 6860 4626 4937 354 16777
* N=number of lakes in each cateqory 147
TABLE 23
Gerrid species abundance in temporary and more permanent habitats
HABITAT Gerris Limnoporus TOTALS CLASSIFI• CATION buenoi comatus pingreensis
Temporary 2393 1066 77 5 175 4409 N=14 (- 349) (. 230) (.157) (.494)
Permanent 4467 3560 4162 179 12368 N=31 (.651) (.770) (.843) (. 506)
Total 6860 4626 4937 354 16777
* N=number of lakes in each category; number in parentheses represents proportion of all individuals of that species collected in each habitat 148
DISCISSION
A. Life Cycles and Population Dynamics
Andersen (1973) and Vepsalainen (1974b, 1978) have documented a strong association between egg production and pale venters for several European species, but this study is the first report of a similar relationship for North American
gerrids. When established, this correlation greatly simplifies the study of gerrid life cycles because it allows direct estimation of the size of the summer reproductive generation from field data. Subsequent population studies on North
American species miqht well begin with a search for such markers.
Vepsalainen (1971a, 1974a,c, 1978) has developed a model of
the environmental factors that regulate the seasonal timing of
life cycles among temperate gerrids. Present evidence suggests
that nymphs are switched to becoming diapause (-overwintering)
adults if photoperiods begin decreasing before a critical period
during the fourth instar. Thus gerrids that pass through the
critical period before the solstice will reproduce during the
same season.
The results of my investigations are generally consistent
with Vepsalainen*s model. Figures 22 - 24 demonstrate that
fourth instars of G. buenoi, G. comatus and G. pinqreensis were
present at both study sites before the summer solstice of 1975.
Because fourth instars of G. comatus were not common at the 149
solstice, the summer generation of reproductives should have been smaller than in the other two Gerris species. Data in
Figure 21 confirm this expectation for the relative abundance of summer generation reproductives between G. buenoi and
G. comatus. However, reliable comparison with G. pinqreensis
is impossible in the absence of a good morphological marker for summer generation breeders. The partial population curves
(Figures 22 - 24) demonstrate that G. comatus produced a smaller
generation than either G. buenoi or G. pingreensis.
The two Limnoporus species were univoltine at both study
sites and, as predicted by Vepsalainen*s model of diapause control, no fourth instars were collected before the summer i
solstice (Figure 25). This was brought about, in part, because
Limnoporus spp. commenced breeding later than the Gerris species
on the Fraser Plateau. Jamieson (1973) found that Limnoporus
notabilis was also strictly univoltine in southwestern British
Columbia, and suggested that breeding is suppressed until a critical thermal threshold has passed.
Limnoporus notabilis occurs as far south as New Mexico
(Jamieson, 1973), but no data are available concerning the life
cycles and voltinism of more southern populations. However,
L. canaliculatus is known to be multivoltine in southeastern
United States (Calabrese, 1977). It therefore seems that
L. notabilis probably has the potential to produce more than one
generation per year.
It is likely that occasional cool summers at higher 150
latitudes select strongly against a second generation among
Limnoporus populations. The Limnoporus species are the largest
in British Columbia and require the longest time to complete larval development (40+ days at 22°C; Chapter II, Appendix II).
In the case of late summer cold spells, second generation larvae
would have difficulty completing the adult molt and therefore,
could not overwinter (Andersen, 1 973). The extended reproductive life observed for Limnoporus in the field may compensate for predictable univoltinism in British Columbia.
Jamieson (1973) has presented some evidence that Limnoporus is
also more fecund than co-occurring Gerris species.
Although the main features of gerrid life cycles in British
Columbia can be reconciled with Vepsalainen's model, there is
one point that deserves comment. A single long-winged
G. buenoi female taken in September, 1975 carried chorionated
eggs despite its appearance as a typical, dark-ventered diapause
individual. Also, Figure 24 shows a pulse of first instar
G? pingreensis at Springhouse in September, 1975 owing to the
sudden appearance of first instar larvae at Westwick Lake.
Jamieson (1973) noted a similar late season pulse for G. buenoi
at Marion Lake in South-western British Columbia.
Vanderlin and Streams (1977) have shown that diapause may
be broken by cool autumn temperatures in Notonecta even though
the primary control is exercised by photoperiod. It is
possible that analagous effects of cold temperatures may explain
the late season reproduction observed among gerrids in British
Columbia. Andersen (1973) has reported partially disintegrated 151
oocytes in G. lacustris collected late in the season in Denmark.
This suggests that oosorption may occur before winter diapause, allowing some gerrids to lay eggs during successive seasons.
Even though photoperiod regulates the induction of diapause
in individual gerrids, the percentage of the population that
breeds is ultimately controlled by spring temperatures. Warm springs will lead to larger second generations because of
accelerated rates of juvenile development (Chapter II).
Several authors (Andersen, 1973; Jamieson, 1973; Vepsalainen,
1974a) have commented on the pronounced effect of field
microclimates. In fact, the lake to lake variability on gerrid
physiological time scales probably accounts for one of the most
puzzling field observations made during , this study; as the
season progressed, lake to lake variability in species age
distributions became startling. On some -lakes there was no
evidence that summer generation reproductives were produced by
any species. The gerrid physiological time scales presented in
Figure 20a show that as much as a two-fold difference may exist
in accumulated day-degrees by the critical solstice period.
Differences in growth thresholds between instars
demonstrated in Chapter II make a simple graphical
representation of population dynamics impossible even for a
single species. Subsequent progress in the study of gerrid
species interaction, from the perspective of comparative
population dynamics, awaits the development of adequate
simulation models. These must incorporate the complexity
introduced by different growth thresholds and local temperature 152
variation if we are to believe the results.
B. Comparative Ecology
1. Habitats and Timing
This study has demonstrated that there are significant ecological differences between the gerrid species coexisting on the Fraser Plateau. The most obvious differences revolve around habitat.
The data of Figures 22 - 26 also revealed pronounced differences between species in the timing of spring appearance and the initiation of breeding. However, subsequent peaks of
abundance overlap greatly in time among all four species
studied. In contrast, Jamieson (1973) found differences in
life cycle timing that seemed to result in seasonal separation
of gerrid species occurring on Marion Lake in South-western
British Columbia.
Timing differences may come into play when gerrids face an
environmental background of widely-spaced, permanent habitats as
encountered in Jamieson's study area (U.B.C. Research Forest).
Under these conditions interlake dispersal would entail high
risks (Vepsalainen , 1978) and selection operating to segregate
species would be most intense with respect to intralake factors.
However, the ontogenetic programs governing life cycle timing
are tuned to guarantee species persistence in a seasonal
environment (Vepsalainen, 1974a, 1978; Jarvinen and Vepsalainen, 153
1976) and species growth rates are determined by yearly temperature regimes. With these constraints, it is doubtful that timing can be reliably adjusted to accommodate for interlake differences in species composition, especially if there is moderate year to year variation in climate.
It is not surprising, then, that Jamieson (1973) also found
striking differences in microhabitat preference among the
species that he studied. In fact, most lakes and ponds with several gerrid species afford diverse mosaics of habitat
structure.
2. Habitat Permanence and Adaptive Strategies
Vepsalainen and his co-workers (summarized in Vepsalainen,
1978) have explored the relationships between life cycle timing,
alary dimorphism and habitat with theoretical models and field data. In general they have been concerned with the genetic
strategies that individual species employ to deal with seasonal
variation. Results of their work suggest a strong association
between the tendency to occupy temporary habitats and the retention of flight ability. In more permanent habitats,
natural selection leads to monomorphism for flightlessness
because the cost of lost dispersers outweighs the gain from
colonizing new habitats.
Field data presented here provide some independent
confirmation for Vepsalainen's predictions. G. pinqreensis,
the only species studied with a dominant proportion of short- 154
winged individuals, occupies the most permanent habitats. The two species of Limnoporus on the Fraser Plateau are monomorphic for long wings and show the strongest association with temporary habitats. G. buenoi and G. comatus have a seasonal polyphenism
(cf. Shapiro, 1976; Vepsalainen, 1978) dominated by long-winged animals and make intermediate investment in temporary habitats.
Table 23 shows that all Gerris species considered here place the bulk of their reproductive effort in habitats that are relatively permanent over the short run. However, all species
retain the long-winged phenotype, contrary to theoretical
predictions of Jarvinen (1976) and Vepsalainen (1978). Even
among G. pingreensis more than 10% of the individuals collected
between 1975 and 1977 were long-winged.
Recently Vepsalainen (1978) has suggested that drought
intervals of "tens to hundreds" of years may be necessary to
make gerrid habitats permanent in the evolutionary sense.
Munro (1945) reported that many of the lakes studied here were
dry in the 1930*s. Therefore the high percentage of long-
winged individuals encountered in the study area suggests that
50 year drought intervals are sufficient to render gerrid
habitats temporary.
Two other factors may reduce the effect of natural
selection against the long winged morph. Firstly, interlake
distances are relatively small and the intervening grassland
does not obscure flight paths between lakes (Figure 2 and 4,
Chapter I). Therefore risks involved in lake to lake dispersal 155
should be small. Secondly, yearly rates of population extincton are surprisingly high (Chapter VI) and thus frequent recolonization of abandoned sites is advantaqeous because habitats in the study area are heterogeneous in time (especially the balance between grass/sedge and floating vegetation) as well as in space.
3. Habitat and Regional Coexistence
The three habitat classifications employed in Tables 20
23 are not independent. Reynolds and Reynolds (1976) showed that the distributions of aquatic angiosperms in these lakes are strongly influenced by conductivity. Bulrushes (Scirpus validus) become more common with increasing salinity. The most saline lakes are also the largest and most permanent. In contrast, grasses, sedges and floating aquatics abound in the
small, freshwater ponds created anew ;each sprinq by meltinq snow. Submerged plants, (especially Myriophyllum and
Ceratophyllum) predominate over the mid-range of conductivities encountered among the study lakes. These strong correlations plus comparison of the species separations obtained in Tables 20
23 argue that species-specific responses to habitat structure have been a major theme in the evolution of guild structure among gerrids occurring on the Fraser Plateau.
Other regional assemblages of water-striders have been characterized by distinct habitat associations (Brinkhurst,
1959b; Vepsalainen, 1973; Jamieson, 1973; Calabrese, 1977).
Several species occurring in central British Columbia have been 156
studied elsewhere so some comparisons are possible.
a. Gerrids in British Columbia
Table 24 presents data currently available on gerrid habitat preferences in British Columbia. (Data from the lower
mainland are from Jamieson (1973) and from my own records).
For completeness I have also included data collected for
_.- incoqnitus on the Fraser Plateau during 1976 and 1977.
Species that occur on both the Lower Mainland and the Fraser
Plateau show virtually identical habitat preferences in both
regions.
Distributions of G. comatus and G. incurvatus abut, but
apparently, do not overlap in British Columbia; to date both
species have not been recorded from the same lake (Scudder,
1977). Although G. incognitus is generally confined to
Southern B.C. the known distribution shows no clear relationship
to obvious physiographic factors. The two species frequent the
same habitats; both are associated with floating vegetation.
Thus it is possible that interspecific,competition explains the
distribution of these two species in the province. The
distributions and habitat preferences of these two species
should be examined in detail in south-central B.C. (between
Clinton and Kamloops) .
The only cases of significant habitat overlap within
regional guilds occurs between G. buenoi and the genus
Limnoporus - respectively, the "big and little" of British TABLE 24
Habitat preferences of gerrid species in British Columbia
SPECIES LOWER MAINLAND FRASER PLATEAU STUDY AREA
G. buenoi inshore, clean water grass/sedge habitat surfaces; thick emergent vegetation
G. comatus does not occur floating vegetation
G. incognitus inshore, cluttered shaded, often very water surfaces temporary habitats; under dense willows and alders
pinqreensis does not occur rush habitat
G. incurvatus offshore, Potomoqeton not present beds, lily pads
Limnoporus inshore, clean water grass/sedge habitat, (L. notabilis surface, grassy areas temporary ponds only on LML) 158
Columbia gerrids. The ratios of body length between G. buenoi
and either Limnoporus notabilis or Limnoporus dissortis are
approximately 2.0 over all stages of development. This is much
greater than the values of 1.2-1.3 suggested by Hutchinson
(1959) as generally sufficient to permit complete habitat
overlap through differential selection of available food sizes.
Schoener (1974b) and Istock (1977) reported similarly high size
ratios between co-occurring species of Anolis lizards and
waterboatmen (Corixidae) respectively. Like water-striders,
populations of these animals have distinct size/age
distributions,
Werner (1977), working with sunfish, suggested that
elevated size ratios may result when species that partition food
resources are divided into several size classes. In such cases
the distributions of prey size used by each species (niche
width) are increased by significant between size class
components. It should be possible to extend this analysis to
water-striders and determine if size differences between
G. buenoi and Limnoporus are important to the coexistence of
these species in the same habitats.
b. Gerrids in Eastern and Western North America
Calabrese (1977) studied the habitat associations of water-
striders in Connecticut. Three species of the central British
Columbia gerrid fauna also occur in Connecticut (G. buenoi,
G. comatus, and L. dissortis). However, straightforward
comparisons with Calabrese*s data are difficult to interpret 159
because only qualitative data based on adult presence or absence are presented.
There is one interesting qualitative contrast between the habitat preferences recorded for G. comatus in British Columbia and Connecticut. Calabrese's (1977), data show a significant tendency for G. comatus to avoid habitats with submerged vegetation, but in central British Columbia five lakes dominated by submerged vegetation contributed one third (32.2%) of the individuals recorded in Table 20 from habitats with floating vegetation.
One hypothesis to explain this contrast involves regional differences in the gerrid fauna. Although Calabrese's data do not indicate significant positive responses to submerged vegetation, G. alacris, G. arqenticpllis, G. marginatus and
L. canaliculatus all occurred with fair freguency in areas of submerged vegetation. In Eastern North America all of these species are larger than G. comatus (Blatchley, 1926).
Because body length is directly proportional to leg length in gerrids (Matsuda, 1961), effective stride length also increases with body length. Therefore, we can surmise that larger gerrids should be able to move faster and more efficiently in habitats unencumbered by surface vegetation.
The larger species in Connecticut should therefore have a significant advantage over G. comatus in habitats of submerged vegetation.
It is interesting to note that G. comatus collected on the 160
Fraser Plateau (9.94 ± .085, n=30) are generally larger than those from Eastern North America (8.09 ± .098, n=5) . (Data for eastern G. comatus have been taken from Blatchley, 1926; Drake and Harris, 1934; Deay and Gould, 1936; Cheng and Fernando,
1970; and Calabrese, 1974. Where ranges of body length were given, the median was used in the calculations.). These data suggest that morphological adaptation to habitat structure is possible among water-striders.
4. Conclusions
The fascinating study of how spatial heterogeneity contributes to the evolution of pattern in nature must remain a highly speculative matter until we .develop solid habitat classifications (Southwood, 1977). These classifications must attempt to reflect the environment as perceived by the animal
(Wiens, 1976; Janzen, 1977). Quantitative data on species distributions and regional comparisons can help to assess the usefulness of our classifications.
In this chapter, I have shown that the distribution and relative abundance of gerrid species on the Fraser Plateau are most sensitive to habitat structure and that the most conspicuous differences in species natural history revolve around the use of potential habitat. The.next chapters explore how active habitat selection contributes to these natural patterns and the extent to which population success depends upon the background mosaic of habitat structure. 161
CHAPTER V. EXPERIMENTAL ANALYSIS OF MICROHABITAT SELECTION IN
WATER-STRIDERS
INTRODUCTION
Indications that species prefer particular habitats often emerge during surveys to assess their distribution. Habitats are classified with respect to environmental variables and the survey data are grouped in contingency tables for analysis with the chi-sguare statistic (Taylor, 1968; Streams and Newfield,
1972; Fleetwood et al., 1978). Vepsalainen (1973b) and
Calabrese (1977) analysed data collected on species presence and absence to show that various gerrid species could be associated with constellations of habitat features.
Similar analyses of species presence among gerrid populations on the Fraser Plateau of South-central British
Columbia suggested that the study lakes were relatively homogeneous with respect to species composition. However, when species abundances were compared in Chapter IV, clear patterns emerged. These kinds of analyses lead to habitat associations; experiments are necessary to establish that species exercise habitat preferences (Klopfer, 1 969).
The experimental approach asks if a species will respond positively to a particular habitat type when confronted with a simultaneous choice of two or more habitats. This method has been commonly used to investigate substrate preferences of benthic invertebrates (summarized in Meadows and Campbell, 162
1972). Some investigators (i.e. Madsen, 1968; Lock, 1975;
Higler, 1975) have demonstrated species preferences that could account for natural distributions, while in other cases
(i.e. Cummins and Lauf, 1969; Gale, 1971; Dodson, 1975) the observed preferences indicated that other processes were responsible. -
In this chapter I discuss the results of field work and experiments undertaken to assess the possibility that active habitat preferences help account for gerrid distributions observed on the Fraser Plateau. 163
MATERIALS AND METHODS
A. Dispersal ... Habitat Selection Among Lakes
Field enclosures were placed at Sp 6, Sp 8 and Westwick
Lake in early May, 1977. Each of these lakes was characterized by relatively homogeneous vegetation structure, representing grass/sedge, floating vegetation and rush habitats,
respectively.
Enclosures were open at the top and were constructed as follows. Side panels (2.0 x 0.5 m), made of heavy gauge, clear
plastic sheets stretched on a wooden frame, were bolted to
cornerposts. Corners were sealed by strips of foamrubber
compressed in corner joints. Four enclosures were set in
approximately 50 cm. of water at each lake and corner posts were
pushed into the lake bottom until only 15 - 20 cm. of each side
panel remained above the waterline (Figure 28).
Overwintered adults of G. buenoi, G. comatus,
G. pinqreensis and Limnoporus spp. collected from other lakes at
Springhouse and Becher's Prairie were used in these experiments.
Sixty individuals of each species were divided among three
groups of twenty animals. Gerrids of each group were color-
coded with a small dab of fluorescent paint (Metron Markers,
Solana Beach, Ca., U.S.A.) on the prothorax and on at least one
mesofemur. These animals were, held overnight at field
temperatures without food. The next morning 20 marked animals
of each species (sex ratio 1:1) were placed separately in one of 164
Figure 28. Field enclosures in floating/submerged vegetation
at Sp 8 in late July, 1977. 165 166
the enclosures at each of the three lakes. Four apterous
_.- pinqreensis which had been similarly marked and handled, were
also added to each enclosure as controls.
Approximately twenty-four hours later populations in each
enclosure were censused and the following data were recorded:
(1) number and species of immigrant (unmarked animals) , (2)
number of emigrants (20 minus the number of marked animals
remaining) and (3) number of apterous G. pinqreensis remaining in each cage.
This experiment was repeated six times during May, 1977 (8-
9/5, 10-11/5, 13-14/5, 14-15/5, 22-23/5 and 23-24/5).
Completely new populations of potential emigrants were added on
May 8, 12 and 19. Lost animals were replaced in each enclosure
on the other three dates'. Animals removed from enclosures on
May 10 and May 24 were preserved for dissection to determine the
condition of the indirect flight muscles.
In order to monitor the potential dispersal of summer
reproductives, enclosures were checked weekly for immigrants
between July 12 and August 8, 1977. 167
B. Habitat Selection Within Lakes
1- Field Distributions
a. Habitat Differences Among Species
Two lakes (Opposite Near Round-up Pond, Becher's Prairie and Sp 1, Springhouse), composed of distinct patches of several habitat types, were chosen for more intensive analysis during
July 1976. Separate collections, lasting five minutes, were made within patches of all habitats occurring on both lakes.
An attempt was made to collect every gerrid encountered while moving slowly through each habitat patch. All gerrids collected were identified on site or preserved in 70% ethanol for subseguent dissection. Visual estimates of the percent cover provided by each type of vegetation were made at both lakes.
b. Habitat Differences Within Species
Throughout the summer of 1975 guadrat-sampling was carried out at Sp 2 and Boitano Lake on a regular basis. The methods are fully described in Chapter III. Each quadrat sample was assigned to a mean depth thus providing a relative index of distance from shore. At Sp 2 vegetation density was also measured for each sample by counting the number of emergent grass shoots per each 0.25 square meter quadrat. The total numbers of all instars collected of G. buenoi at Sp 2 and
_.. pingreensis at Boitano Lake were partitioned according to 168
depth for analysis.
2. Laboratory Experiments . . . Responses to Artificial Habitat Structure
Three types of experiments were run to investigate the
degree to which field distributions can be explained by species
preferences for simple, structural characteristics of habitat.
The following relationships were investigated: (1) the effect of
species and stage on tendencies to enter complex habitats, (2)
species preferences when offered a range of natural habitat
mimics and (3) the effects of habitat structure on adult
foraging ability. The first section below describes the
laboratory habitats and sections b, c and d describe the methods
used for each of the three blocks of experiments listed above.
Field collected gerrids were used in all experiments with
adults. First instar larvae were hatched from eggs laid in the
laboratory.
a. Laboratory Conditions and Apparatus
Two plastic wading pools, 125 cm. in diameter, were employed in the following experiments. Pools were filled to a depth of 12-14 cm. with dechlorinated tap water and all experiments were run after water temperature had equilibrated to room temperature (20 - 22 ° C) . Lighting was from overhead fluorescent lamps.
The bottom of each pool was equipped with bolts and fitted 169
so that four plywood (1/2") "habitat sectors" of equal area could be attached with winq-nuts. Habitat sectors were painted a uniform qrey with Rustoleum enamel. Four types of artificial habitat are illustrated in Figure 29. They were constructed as follows:
(1.) Artificial rush ("rush"). Fifty ml. disposible
glass pipets were inserted upside-down into holes drilled
at the corners of a 4.0 cm. grid.
(2.) Artificial .grass ("grass") . Plastic drinking
straws (20cm. in length, 0.4 cm. in diamter) were painted
green and inserted in alternate 10 x 10 cm. patches of
sparse and dense areas. The sparse areas had straws at
the corners of 2 cm. grids; the dense areas at the
corners of 1 cm. grids.
(3.) Artificial smartweed ("floating"). Five styrofoam
strips (4.0 x 3.0 x 0.3 cm.) were sewn together into
rosettes. Each rosette was approximately 10 cm. in
diameter. Rosettes were affixed to "floating" habitat
sectors at the intersection points of a 10 cm. grid.
(4.) No vegetation ("open"). These habitat sectors were
simply painted grey; no artificial complexity was added.
b. Species Tendencies to Enter Complex Habitats 170
Figure 29. Four types of artificial habitat in a laboratory
pool used for preference experiments. Starting
at the bottom left-hand corner, and proceeding
clockwise: "floating", "grass!1, "rush" and "open"
habitat. 171 172
(1.) Basic Procedure
The same basic procedure was used in all of the following experiments to investigate species responses to habitat complexity. A set number of gerrids were dropped onto the middle of an experimental pond containing a particular configuration of artificial habitats. These animals were allowed 10 - 15 minutes to distribute themselves and then the positions of all animals were recorded at five successive observation intervals. In order to allow for greater adult mobility, observation intervals were set at one minute for adults and two minutes for first instars. The sums of observations recorded in each artificial habitat over these five observations were used as the basic data for analysis. Thus the tendencies to select and to remain in a particular habitat were considered simultaneously.
Food was provided ad libitum to all experimental animals before use. No food was available on the experimental ponds.
Sex ratios in all adult experiments were 1:1.
(2,) Experiments
(a.) Some Preliminaries
Intraspecific attraction or some other mechanism resulting in clumped distributions would confound interpretation of results derived from group testing. Therefore it was necessary to ensure that the position choices of individual gerrids were 173
independent. In-order to test the hypothesis of independence
for each species, the distribution of twenty groups of ten
animals (i.e. n=200 for each species) oyer four open habitat
sectors was compared with random expectations calculated from
the binomial distribution. The observed distributions were
tested for goodness of fit by calculating chi-square values for each species (Table 25) .
These data show that it is appropriate to ignore the
possible effects of clumping in the following experiments. The
data for G. comatus suggest a weak tendency for clumping, but in
all other species the recorded distributions were clearly not
contiguous. If individual interactions lead to more uniform
spacing, the results of subseguent habitat preference
experiments will be only more conservative.
The following experiments investigated species-specific
tendencies to enter structured habitats when offered a paired
choice between egual areas of emergent cover (artificial rush
habitats) and open habitat sectors (two contiguous sectors of
each) .
(b.) The Effect of Surface Conditions
All experiments were run with groups of ten animals of a
single species. Ten such groups of G. buenoi, G. comatus,
G, incoqnitus, G. pinqreensis and Limnoporus spp. were tested
for each of three surface conditions. Surface conditions were
as follows: TABLE 25
Chi-sguare values to test for independence of individual position choices on the experimental pools
SPECIES X2,df=3 P
Go buenoi 1.40 >> . 05
G. comatus 6.52 > . 05
G. pingreensis 1.64 >> . 05
Limnoporus 1.56 >> . 05 175
1- No surface disturbance was generated.
2. Waves. Waves were produced by the action of a
polyvinyl cylinder (length: 16 cm.; diameter: 3.8
cm.) suspended from a pulley at the center of the
experimental pool. The cylinder was attached to
the rotating arm of an electric motor and was
thereby alternatively raised and lowered between 55-
65 times per minute. This movement generated
concentric waves, 1-2 cm. in height, that spread
outward from the center of the pond. (Figure 30)
3* Wind. An Electrohome cabinet fan with 12"
blades was placed at an opening cut in the side of
the pool and tipped forward until it was angled at
approximately 60° to the water surface. The fan
was centered on the line separating the two
contiguous sectors of the open habitat from the rush
habitat sectors (Figure 31). The fan ran at top
speed for the duration of the, experiment. A strong
surface current (12.17 cm./sec.), presumably
reflecting surface wind, was generated through the
center of the pond, perpendicular to the fan.
(c.) The Effect of Stage
Ten groups of ten first instar larvae of G. buenoi,
G. comatus, G. incoqnitus, G. pinqreensis and Limnoporus spp. were tested on pools with the same habitat confiquration as used above. The preferences recorded were compared 176
Figure 30. Apparatus used to generate waves on laboratory
pools. 177 178
Figure 31. Apparatus and habitat configuration used for testing
the effects of wind on gerrid habitat preferences.
Gerrids on the pool are Limnoporus notabilis, the
largest species tested. •MI333!!!!!'JI!1I!H railttllttl 1 80
respectively with those of adults on calm surfaces as determined
in the previous experiments.
(d.) The Effect of Other Species
Twenty groups of twenty adults and twenty groups of ten adults were tested as above for G. buenoi, G. comatus and
G- pinqreensis. These data were used to estimate how increasing density might affect species distribution on the
experimental pools. In addition twenty groups, containing ten
individuals of each of two species, were tested for each two-
species combination of G. buenoi, G. comatus and G. pinqreensis.
(e.) The Analysis
Analyses of variance were used to interpret the results of
all experiments in this section. For the experiments
concerning species interactions, the original freguencies were
weighted by the total number of observations which varied
necessarily with density (i.e. variances were estimated as chi-
sguares). Analyses of the other experiments with chi-sguare
would involve the interpretation of three-way contingency
tables, with associated problems of non-orthogonality. To
avoid this problem in the analyses of two-way experiments
(i.e. species and surface condition; species and stage),
unweighted analysis of variance was used on the original
freguencies. This is appropriate, in this case, because all
weights are the same (50 observations per run). 181
c. Selection of Artificial Habitat Mimics
G. buenoi, G. comatus and G. pinqreensis were allowed to choose between equal areas of all four types of artificial habitat (grass, rush, floating and open sectors) during this experiment. Trials were run using all three unique (excluding mirror images) spatial configurations possible with four different sectors (Figure 32).
Preferences of eight groups of each species were assessed independently on each configuration of habitat sectors. Each run was started by placing ten adult gerrids in the middle of a particular habitat sector, the positions of all animals were recorded after 15 minutes. Groups were started twice in each type of habitat sector for a given configuration. Therefore 24 groups of ten gerrids were tested for each species.
The observations on final resting place were collected into a 3x4 contingency table (species x habitats) and analyzed for heterogeneity by calculation of chi-square,
d. Habitat Structure and Foraginq Success
The.objective of this experiment was to determine the effect of habitat structure on the foraging ability of
G. buenoi, G. comatus and G. pinqreensis. In order to delimit equal foraging areas for each laboratory habitat, the four sectors of the experimental pools were partitioned from one another with sheets of opaque black plastic extending from the bottom of the pool to a height of 15 cm, above the water 182
Figure 32. Unique spatial confiqurations of laboratory habitats
used in preference experiments. j "FLOATING" "OPEN" I "GRASS" " "RUSH" 184
surface.
Six frozen vestigial-winged Drosophila, previously weighed to ± .002 mg. (Mettler Microgramatic Balance, Mettler
Instruments, Inc.) were distributed uniformly in each sector as illustrated in Figure 33. A single adult water-strider, previously starved for 48 - 60 hours to induce maximum hunger
(Jamieson and Scudder, 1977), was immediately introduced at the center of the sector (Figure 33) and allowed to forage for 90 minutes.
At the end of the allotted foraging period the six flies were collected from each sector, dried to a constant weight and reweighed. The predicted dry weight was calculated by multiplying the original wet weight by a coefficient
(0.257±.002) calculated by drying and reweighing 20 groups of
Drosophila reared under the same conditions. This coefficient was adjusted to account for weight lost on the water surface by methods described by Jamieson (1973). The difference between this predicted dry weight and the actual measured dry weight was taken as the amount consumed during the foraging period.
The experiment was repeated for ten animals of each species in all four types of artificial habitat. 185
Figure 33. Starting distribution of Drosophila in laboratory
experiments to measure foraging efficiency in
various habitats. o STARTING POINTS FOR DROSOPHILA
STARTING POINT FOR GERRIDS 187
RESULTS
A- Dispersal ... Habitat Selection Between Lakes
1. Immigration
The total numbers of each species, captured in the field
enclosures at Sp 6, Sp 8 and Westwick Lake, are recorded in
Table 26. The overall capture rate was low (0.11
gerrids/sguare meter/day) , however, it is apparent that all
three Gerris species were dispersing during May, 1977. More
G. buenoi were taken at Sp 6 (grass/sedge) than at either
Westwick Lake (rush) or Sp 8 (floating) suggesting that
grass/sedge habitat was most attractive to flying individuals of
that species. The limited data do not permit such comparisons
for the other species.
Air temperature had a pronounced effect on capture rate.
Eighty-five percent of the captures occurred during the three
days (May 8, 9, 23) when maximum air temperatures exceeded 14°C
at Westwick Lake.
Only one G. comatus female was caught during the summer
trapping period (Sp 6, July 22, 1977). It had a dark venter,
typical of diapause adults, and the reproductive system was
immature. These observations suggest that most lake to lake
dispersal occurred during early spring periods of warm weather. TABLE 26
Total numbers of gerrids flying into enclosures (six 24hr periods - May 1977)
HABITAT
SPECIES Sp. 6 Sp 8 Westwick TOTALS grass/sedge floating rush
G. buenoi 15 6 3 24
G. comatus 2 4 1 7
G. pinqreensis 0 1 1 2
Limnoporus 0 0 0 0
Totals 17 11 33 189
2. Emigration
Table 27 shows the percentage of each species and wing- morph that disappeared from the stocked enclosures during the six, one-day experiments. The rate of disappearance was higher for macropters of all species than for G. pingreensis.
Therefore, loss from the enclosures can be used as a relative index of species tendencies to abandon habitats.
Figure 34 shows the number of each species disappearing from the three habitats. A common pattern appeared for
G. buenoi, G. comatus and Limnoporus spp.; they flew most from rush and least from grass/sedge habitats. In contrast the highest disappearance rates for G. pingreensis were observed from the floating habitat.
A surprisingly small percentage of potential emigrants actually flew (7.92% over all species and experiments), even though gerrid densities in the enclosures were 2-3 times the ambient population densities at the three lakes. This is partially explained by data in Table 28 which show that 20-30% of the Gerris species stocked had non-functional indirect flight muscles by the end of May. This decrease in the percentage of the population with functional flight muscles is brought about by the histolysis of indirect flight muscles that occurs coincidently with egg production in female gerrids (Andersen,
1973).
Comparison of data in Tables 27 and 28 demonstrates the most actively dispersing gerrids (Limnoporus spp, and TABLE 27
Percentage of each species and morph lost from experimental enclosures (six 24hr, periods - May 1977)
SPECIES AND TOTAL NUMBER % LOST ,WING MOEPH TESTED
G. buenoi 360 5. 8% macropters
G. comatus 360 4.4% . macropters
G. pinqreensis 360 10. 8% macropters
Limnoporus 360 10.. 5% macropters
G. pinqreensis 288 2.8% apters 191
Figure 34. Total number of gerrids disappearing from enclosures
in each habitat. 281 • GRASS/SEDGE FLOATING 24
BULRUSH
20
g or
£121 cr UJ CD 2 81 g f2 41
G. BUENOI G. COMATUS G PINGREENSIS LIMNOPORUS SPR CONTROLS
(APTER0US Q. PIN GREEN 818) TABLE 28
Percentage of animals remaining in enclosures with functional indirect muscles and the sex ratios of emmigrating gerrids
% OF "NON-EMIGBANTS" | SEX-RATIO OF WITH FUNCTIONAL | "EMIGRANTS" SPECIES FLIGHT MUSCLES |
May 10 May 24 | Male Female
G. buenoi 93.1% 70.1% | 12 6 n=58 n=56 |
G. comatus 96.4% 75.7% | 9 5 n=56 n=58 |
G. pinqreensis 98.2% 81.5% | 20 16 n=57 n=54 |
Limnoporus 100% 100% | 21 16 n=51 n=51 | 194
G. pinqreensis) also showed the highest retention of flight muscles during the experiment. Observed flight muscle histolysis was restricted to females. This probably accounts for the greater proportion of male emigrants among G. buenoi and
G. comatus. However if only data for males are considered.
Table 28 confirms the tendency for greater dispersal among
G. pingreensis and Limnoporus spp.
The relative species abundances observed among immigrants
(Table 26) and emigrants (Table 28) are strikingly different.
The disproportionate representation of G. buenoi may be explained because it is the most common species at the
Springhouse site (Chapter IV). The effect of relative abundance should be accentuated because two thirds of the trapping effort (Sp 6 and Sp 8) centered among a series of small ponds where G. buenoi was the dominant species. In fact the traps at Sp 6 and Sp 8 accounted for 80% of the G. buenoi captured.
The poor representation of G. pinqreensis and Limnoporus spp. among immigrants may reflect their relatively low abundances in surrounding populations. Less than 15% of all
G- Pinqreensis collected at Springhouse between 1975 and 1977 have been long-winged and so the population of potential migrants is low in comparison to G. buenoi and G. comatus.
Limnoporus spp. are the least abundant water-striders at
Springhouse and are also strongly associated with small, very temporary ponds (Chapter VI). Therefore the absence of
Limnoporus among captured immigrants; may result from a 195
combination of rareness in natural populations and active avoidance of larger ponds and lakes.
B. Habitat Selection Within Lakes
1. Field Distributions
a- Habitat Differences Among Species
Table 29 shows the gerrid species collected from various habitats on Opposite Near Bound-up and Sp 1 during July, 1976.
No Limnoporus were collected so analysis is restricted to the three Gerris species. The high chi-sguare values computed for both tables demonstrate that species occurrence was not random with respect to habitat. Habitat associations at Opposite Near
Round-up and Sp 1 were as follows: G. buenoi - grass/sedge,
G. comatus - floating+open, G. pingreensis - rush. In fact, peak abundances for each species occurred in the same habitats as suggested by the lake surveys discussed in Chapter IV.
These data show that gerrid species occurring on the same pond are separated in space.
Comparison of data on percent cover with percent of total catch for G. comatus and G. pingreensis suggest that these species exhibit active habitat preferences. However, the catch of G. buenoi tracks the percent composition of the various habitats almost exactly, and implies that this species is more of a habitat genera list. 196
TABLE 29
Occurrence of Gerris species by habitat on two lakes during July, 1976 a. Opposite Near Round-up
VEGETATION TYPE G. buenoi G. comatus G. pinqreensis (percent cover)
Grass/Sedqe 163 7 41 (55%) (55%) (33%) (33%) Floatinq 27 6 , 15 (10%) (9%) (29%) (12%) Rush 104 4 53 (30%) (35%) (19%) (43%) Open Water 18 4 15 ( 5%) (6%) (19%) (12%)
• i X2 = 27.21; df=6; p<.01 b. Sp 1
VEGETATION TYPE G. buenoi G. comatus (percent cover)
Grass/Sedge 69 46 (60%) (86%) (23%) Floating 10 145 (10%) (13%) (72%) Open Water 1 10 (30%) (1%) (5%)
X2 = 95.07; df=2; p<.001 197
b. Habitat Differences Within Species
Figure 35 illustrates the spatial distribution observed for various instars of G. buenoi and G. pinqreensis collected in quadrat samples at Sp 2 and Boitano Lake, respectively.
Although all stages were encountered at each depth sampled, there was a marked separation of instars 1-3 from 4-adult at both lakes.
Mean depth at Boitano Lake and Sp 2 was related to at least two additional habitat parameters of probable significance to gerrids: distance from shore and vegetation density. At Sp 2 the deepest samples were as much as 35 meters from shore during
June, at Boitano Lake samples were taken over a 15 m range from shoreline. At both locations the deepest samples marked the limits of the vegetation zone (i.e. potential gerrid habitat) during most of the season. At Sp 2 it was also possible to predict vegetation density as an inverse function of mean depth
(Figure 36) .
Data presented in this section show that there was significant spatial separation among the various size classes of both G. buenoi and G. pinqreensis. Younger instars occurred more frequently near shore at both lakes studied and in areas of higher vegetation density at Sp 2. 198
Figure 35. Within lake spatial distribution observed for
various instars during 1975. (a.) G, buenoi at
Sp 2, (b.) G. pingreensis at Boitano Lake. A.G.BUENOI AT SP2
LO _ 50 0_ UJ 40 03 __ LU 30 t LU 20 P 10 UJ Z •2 •2 ^1 1-2 3 4 5 A . F 3 4 l o B. G. PINGREENSIS AT BOITANO LAKE LU h- O 60 LU a _J o_J 50 o LU O 40
30 X o < 20 LU Lu O 10 1-2 1-2 1-2 5 CM 20 CM 35 CM DEPTH 200
Figure 36. Relationship between depth and density of emergent
cover at Sp 2 during 1975. 201
5 15 25 35 45 55 DEPTH (CM) 202
2. Laboratory Experiments . .. . Responses to Artificial
Structure
a. Species Tendencies to Enter Complex Habitats
(1 •) The Effect of Surface Conditions
Figure 37 compares species tendencies to enter complex habitats over a range of surface conditions. All species showed increased preference for artificial rush sectors in response to wind and waves. The effect of surface wind was more severe than the effect of waves.
There were significant differences in tolerance among species. The two largest water-striders, G. comatus and
Limnoporus, preferred open habitat sectors when the surface was calm and were least affected by wind and waves. G. buenoi, the smallest species tested, showed no active preference between sectors under calm conditions but preferred the rush areas with the addition of surface disturbance. Both G. incoqnitus and
2f pinqreensis always preferred the rush sectors, but preferences of G. incoqnitus were much less affected by wind and waves.
These differences are clearly illustrated by a two-way analysis of variance performed on the percentage data of Figure
37 transformed to arc sins. Table 30 presents the results of this analysis. The interaction between species and surface condition was significant showing that the tolerance to surface 203
Figure 37. Distribution of gerrids on laboratory pools under
three surface conditions. (a.) G. buenoiy (b.)
G. comatus, (c.) G. incognitus, (d.) G. pingreensis
(e.) Limnoporus. A. G. BUENOI B. G. COMATUS 70
60
50
40
30 CO 20 cr 10 o LU CO C. G. INGOGNITUS D. G. PINGREENSIS 50 i* 40 CD < 30
LLI 20 CL 10 O
E. LIMNOPORUS SPP. 80 CO __: 70 o CALM 60 WAVES i 50 cr WIND LLI 40 CO STANDARD CO 30 ERRORS O LL. 20 O 10 TABLE 30
Two-way analysis of variance for the percentage of observations recorded in open habitat sectors.
SOURCE df S..S . M.S. F P Spec ies 4 118. 43 29. 61
Surface conditions adjusted for species 2 220. 19 110.09 24. 42* <. 010
Species adjusted for surface conditions 4 127. 56 31. 89 6.78 * <. 025
Surface conditions 2 211. 06 105.53
Interactions 8 37. 56 4.70 9. 77 <. 001
Remainder 164 78. 09 0. 48
* tested over interactions M.S. 206
disturbance depends upon the species considered. However, both main effects absorbed significantly more of the observed variance than the interaction. Therefore, I conclude that all gerrid species tested actively avoid wind and waves by seeking out sheltered habitats but that the extent of the effect varies from species to species.
(2.) The Effect of Stage
Table 31 compares the habitat preferences of first instars and adults among the five species tested. The total range of preferences exhibited by all first instars is smaller than observed for adults. Also, means for first instars of all species (except G. comatus) are closer to-50% than for adults.
These data suggest that early instars exhibit less active preference than adults on the scale of this experiment, or that first instars use different cues than adults when choosing habitats.
(3.) Effects of Other Species
The responses of G. buenoi, G. comatus and G. pinqreensis to doubling conspecific density, and to the presence of other species are compared in Figure 38. The average percentage of each species choosing open water sectors remained roughly constant over both conspecific densities tested. Therefore, it was appropriate to pool the chi-sguare values computed from species distributions at both densities with those observed in all two-species experiments to calculate a residual variance for TABLE 31
Means and standard errors of the percentage of total observations made on open habitat sectors under calm surface conditions
SPECIES INSTAR First (n=10) Adult (n=20)
G. buenoi 51.8±4.26 57.0±3.30
G. comatus 38.6±4.29 63.6±2.49
G. incognitus 31.6±5.17 22.8±2.Q3
G- pingreensis U3.6±5.21 37.7±2.68
Limnoporus 6H.0±U. 12 76.2±3.03 208
Figure 38. Effects of conspecific density and the presence of
other species on habitat preferences. (a.)
G. buenoi, (b.) G, comatus, (c.) G. pinqreensis. EFFECTS OF CONSPECIFICS EFFECTS OF OTHER SPECIES
A. 6. BUENOI
60
40
20
WITH WITH B. G. COMATUS G. COMATUS G, PINGREENSIS 80
60
UJ to 40
CO 9 £ 20 UJ (9
WITH WITH 6.BUENOI G, PINGREENSIS 60 C. G. PINGREENSIS
UJ a. 40 UJ CD < £ 20 <
WITH WITH 10/POOL 20/POOL 6. BUENOI G. COMATUS DENSITY OTHER SPECIES PRESENT 210
testing the effects of heterospecific individuals.
Analyses of variance for heterospecific effects are given in Table 32. G. pinqreensis was unaffected by the presence of other species, but both G. buenoi and G. comatus showed significant responses. G. comatus responded to G. pinqreensis and G. buenoi with slightly increased preference for open water sectors. Although the observed responses were consistent.
Figure 38 demonstrates that the effects were too small to have any ecological significance.
On the other hand G. buenoi showed a strong tendency to avoid both other species. The effect is underscored because
G. buenoi distributions shifted in opposite directions as if accomodating for the active habitat preferences shown by
G. comatus and G. pingreensis for open habitats and emergent cover respectively. Figure 38 shows that the presence of heterospecific individuals affected the average response of
G. buenoi by about 10% in both directions.
b. Selection of Habitat Mimics
Figure 39 shows the overall distributions of G. buenoi,
G. comatus and G. pinqreensis when offered a choice among four types of artificial habitat structure in the laboratory. The significant chi-sguare value, calculated from these data arranged in a 3 x 4 contingency table (species x habitat), shows that preferred habitats varied among the three species
(X2-35. 98; df=9; p<.001). TABLE 32
Analysis of variance for the effects of other species on observed habitat preferences: a.- G. buenoi, b. G. comatus, c. G. pingreensis. a. G. buenoi
SOURCE df S. S. M. S- F P Alone vs. mixed 1 2. 13 2. 13 0..6 4 >.. 10 Between species 1 51. 89 51. 89 15.,7 2 <..00 5 Residual 77 253. 82 3. 30 b. G. comatus.
SOURCE df S. S. M. S. F P
Alone vs. mixed 1 12. 33 12. 33 6., 46 <•,02 5 Between species 1 0. 79 0. 79 0..4 1 • >., 10 Residual 77 146. 99 1. 91 c. G. pingreensis
SOURCE df S. S. M. S. F P
Alone vs. mixed 1 1. 28 1. 28 0.,6 0 >..1 0 Between species 1 4. 25 4. 25 2..0 0 >..1 0 Residual 77 163. 19 2. 12 212
Figure 39. Choices of G. buenoi, G. comatus and G. pinqreensis
when offered a range of habitat mimics in the
laboratory. "FLOATING?*
"OPEN"
"GRASS"
"RUSH"
G. BUENOI G. COMATUS G. PINGREENSIS 214
Species preferences can be dissected by examining more detailed data from each experiment as provided in Table 33.
The chi-sguare values calculated for G. buenoi and G. comatus show that the habitat selected depended on where the experiment was started. The tendency to remain in the initial habitat was very strong for G. buenoi and there was no obvious pattern of habitat selection among animals that did move. Except when started in grass habitats individuals of G. comatus changed habitats more frequently than G. buenoi. The data also suggest that those G. comatus which did move preferred habitats without emergent cover. G. pinqreensis, on the other hand, actively sought out sectors affording emergent cover, regardless of where the experiment was started.
These results coincide with the species preferences suggested by field data. G. comatus and G. pingreensis displayed active habitat preferences for open habitats and emergent cover respectively; G. buenoi showed no obvious habitat preferences. The important point is that artificial mimics of natural habitat structure are sufficient to evoke strong responses, even over the relatively small scale of these laboratory experiments. TABLE 33
Artificial habitats chosen by three Gerris species in lab• oratory experiments: a. G. buenoi, b. G. comatus, c- _.. pinqreensis a. G. buenoi
HABITAT | HABITAT AT START |
, --| X2 P AT FINISH | Floating Open Grass Rush I
Floating | 35 14 7 7 I Open | 11 30 9 2 I 178. 1 <. 001 Grass | 7 5 34 3 I df=9 Rush | 7 11 10 48 I b. G, comatus
HABITAT | HABITAT AT START I X2 I P AT FINISH | Floating Open Grass Rush I
Floating j 24 17 8 11 I Open | 20 18 13 18 I 56.62 <. 001 Grass | 6 11 34 9 I df=9 Rush | 10 14 5 22 I c. G. pingreensis
HABITAT | - HABITAT AT START I X2 P AT FINISH | Floating Open Grass Rush !
Floating | 12 9 10 10 ' I Open | 9 5 8 10 l 9,224 0. 42 Grass |-17 19 25 13 I df =9 Rush | 22 27 17 27 i 216
c. Habitat Structure and Foraging Success
Figure 40 shows the amount of food consumed by G. buenoi,
G. comatus and G. pingreensis during egual periods of foraging in the four artificial habitats. G. comatus was the only species to show clear statistical differences among the habitats
(one-way analysis of variance: F=6.27; df=3,36; p<.025) . The data of Figure 40 show that G.. comatus was at a severe disadvantage in grass but did relatively better than G. buenoi or G. pingreensis in both habitats without emergent complexity.
These results suggest an explanation for the curious tendency of
G. comatus to remain in grass habitat during the previous experiments. Because G. comatus has the longest legs of the species tested it may actually be trapped by dense, emergent vegetation.
The means presented in Figure 40 suggest that
£. pinqreensis fared slightly better in emergent vegetation than in open areas. However analysis of variance demonstrates the absence of statisticaly significant differences among habitats in the data on hand (F=2.07; df=3,36; p<.10). Bartlett^ test indicates that the variances for G. buenoi were too heterogeneous to allow statistical comparison (probability of homogeneity approximately equal to .01). However the average performance of G. buenoi was similar in all habitats except artificial floating vegetation, where food intake was slightly depressed. 217
Figure 40. Amount of food consumed in each of four artificial
habitats. (a.) G. buenoi, (b.) G. comatus, (c.)
G. pinqreensis. A. 6. BUENOI
300
200
CO _> < 100 Q_ C9 O QT O B. G. COMATUS § 400 CO LU FLOATING h- 300 lllilliijjjil!1!!!! "OPEN"
_> "GRASS" 200 O "RUSH" G) Ill I i! STANDARD 100 ERRORS __: LU h<- LU C. G. PINGREENSIS IN f 300 219
DISCUSSION
A• Dispersal
The experiments reported here were designed to investigate the .possibility that gerrids actively select breeding habitats by flight. Therefore, I shall not discuss summer flights to overwintering habitats made by diapause individuals. The scanty information available on selection of overwintering sites is summarized by Riley (1919), Brinkhurst (1956) and Landin and
Vepsalainen (1977).
The seasonal distribution of captured immigrants indicates that the spring is the major period of interlake dispersal for gerrids on the Fraser Plateau. In addition, data presented in
Chapter IV show that summer generation reproductives are generally short-winged and thus incapable of flight. The work of Vepsalainen (1974b, 1978) suggests that a summer dispersal period should be found among more southern populations of these same species. Vepsalainen (1974a, 1978) has proposed a model to explain how genotypes and environment interact to ensure the production of long-winged summer reproductives where conditions are appropriate. The predictions of his model fit most of the available data for European gerrid populations.
In south-central British Columbia, short summers select against mid-season dispersal because gerrids that develop flight muscles delay investment in the reproductive system [i.e. the oogenesis-flight syndome of Johnson (1969)]. Andersen (1973) 220
showed that flight muscle development can postpone egg-laying by as much as two weeks in G. lacustris. Such delays would seriously shorten the period available for larval development on the Fraser Plateau.
Spring flight was markedly affected by temperature and reproductive condition in this study. Most gerrids flew during days when the maximum air temperature exceeded 14°C. This is in good agreement with data of Landin and Vepsalainen (1977) which suggest a threshold of 12-13°C for flight among spring populations of G. arqentatus in Sweden, Landin and Vepsalainen also found that female gerrids, taken at reflection traps, had completely immature reproductive tracts. This observation suggests that the condition of flight apparatus is a poor indicator of flight potential and may thus help to account for the relatively small proportion of animals that flew from the stocked enclosures.
These data show that gerrid flight patterns are sporadic; they will be difficult to study guantitatively in the field.
Bursts of flight activity seem to be the rule. For example,
1.32 gerrids per sguare meter per day were captured during test runs at Westwick Lake on April 23 and 24, 1977. It is likely that most dispersal occurs as gerrids leave overwintering sites during the first few hot days each spring. The subseguent timing and extent of dispersal each year will vary with prevailing weather patterns, Nevertheless major redistributions of gerrid species are possible each spring.
Because such redistributions are not characteristic of 221
populations on the Fraser Plateau (Chapter VI), colonizing individuals must exercise some preference in choosing habitats.
Data on immigration show that G. buenoi flew most frequently into Sp 6. However, it is doubtful that this reflects an active choice of grass/sedge habitat because there was little development of emergent vegetation at Sp 6 until late
May. Temperate gerrids fly exclusively during daylight in the spring, and seem to rely upon reflection from water surfaces as landing cues (Landin and Vepsalainen, 1977).
A more likely explanation of the disproportionate catch of
G. buenoi at Sp 6 is that most gerrids seek overwintering sites near the pond where they developed and therefore have high probability of recolonizing the same habitat on initial spring flights.
The emigration data suggest that species have different tendencies to abandon habitats. G. buenoi, G. comatus and
Limnoporus disappeared most frequently from enclosures in the rush habitat at Westwick Lake while G. pingreensis showed a very strong tendency to leave Sp 8. The exact mechanisms leading to these observations cannot be specified at present, but it is clear that cues emanate from the habitat and do not necessarily involve species interaction. The observations are consistent with the observed abundance of G. buenoi and G. comatus in many rush habitats from April to mid-May (1976 and 1977) and the subseguent failure to collect larvae of those species from the same locations later in the year. 222
The distinct preferences observed among emigrating individuals suggest that spring colonization proceeds as a trial and error process during which adults may sample several habitats. However, with present data there are few indications of what cues spring adults are using to choose sites for breeding.
B. Habitat Selection Within Lakes
1- Species Differences
Data presented in this chapter demonstrate that the gerrid species considered show distinct responses to habitat. These responses separate species co-occurring on the same lake and thereby reduce the potential for interspecific competition during periods of food shortage.
The responses observed to artificial habitat mimics in the laboratory confirm that species choices are iased upon simple structural characteristics of habitat, the exact composition of the aquatic vegetation seems to be relatively unimportant to gerrids. . Similar responses to habitat structure have been recently demonstrated among aquatic invertebrates (Macan and
Kitchinq, 1972, 1976) and lycosid spiders hunting on plants
(Greenquist and Rovner, 1976).
Species responses to emerqent cover were strongly influenced by the action of wind and waves. Among the Gerris species, G. comatus was most tolerant of surface disturbance. 223
This coincides with the preference expressed by G. comatus for relatively open habitats. G. pingreensis, on the other hand, showed the strongest negative responses to surface disturbance.
In the laboratory and in the field G. pingreensis seeks out habitats that afford emergent cover.
G. pingreensis populations are largely confined to bulrush habitats (Chapter IV), even though laboratory preferences suggest that grass/sedge habitats should also be used. Figure
7 (Chapter I) shows that the effects of wind are most severe during April and May on the Fraser Plateau. Because grass/sedge habitats provide no emergent cover in the early spring, G. pinqreensis colonizing these habitats may suffer heavy mortality owing to their poor ability to tolerate surface disturbance.
All available evidence suggests that G. buenoi has no active habitat preference. However G. buenoi avoided both
G. comatus and G. pingreensis on the laboratory pools. Thus it appears that G. buenoi may be concentrated in grass/sedge habitats (Chapter IV) by default. Grass/sedge areas provide emergent cover not generally occupied by the other two common
Gerris species. This habitat separation could be further enforced if G. buenoi flies more freguently in the presence of other species. Dodson (1975) has also suggested that avoidance mechanisms explain the distinct field distributions of two corixid species that showed the same habitat preferences in laboratory experiments. 224
incoqnitus preferred emergent coyer to open water in laboratory experiments. This species is rare in the study area; it first appeared as migrant, long^winged individuals during the warm spring of 1976. although laboratory preferences of G. incoqnitus and G. pinqreensis are similar, little habitat overlap occurs on the study lakes.
G. incoqnitus is encountered almost exclusively in "brushy" habitats beneath alder and willow brushes and on small ponds completely surrounded by forest.
A comparison of collection sites for lonq-winged individuals of G. incognitus and G. pinqreensis suggests a mechanism to account for the observed distributions (Table 34).
G. incoqnitus dispersers appear to land preferentially in brushy habitats while G. pinqreensis does not usually colonize them.
Because I have commonly collected G. incoqnitus from bulrush and cattail (Typha sp.) habitats on the lower mainland of British
Columbia, it is possible that interactions between sympatric populations of G. incognitus and G. pinqreensis have led to habitat restriction in G. incoqnitus on the Fraser Plateau.
Further work is needed to clarify the exact processes involved.
Laboratory preferences recorded for Limnoporus spp. are remarkably different from the observed field distributions. In the laboratory Limnoporus adults avoid areas of emerqent cover, but in the field Limnoporus spp. are strongly associated with grass/sedge habitats. The long legs of the last three instars of Limnoporus spp. must interfere with their ability to move among dense vegetation. 225
TABLE 34
Distributions of long-winged G." incoqnitus and G. pingreensis on various habitats durinq 1976-77
HABITAT % OF LONG-WINGED INDIVIDUALS ENCOUNTERED | G. incoqnitus n=76 | G. pinqreensis n=148
Rush 0% 53.4%
Brush 85.5% 7.4%
Other 14.5% 39.2% 226
Two factors may help account for the restriction of
Limnoporus spp. to small ponds in the . field (1) Limnoporus spp. start reproducing later than all Gerris species on the
Fraser Plateau and therefore survival of early instars may depend on the presence of dense vegetation as a refuge against predation by larger gerrids. Spatial refuges have been shown to be important for circumventing excessive cannibalism in the backswimmer, Notonecta hoffmanni Fox, . 1975b). (2) Although
Limnoporus spp. have longer legs than other gerrids they must support more weight per unit leg length (Spence, unpublished).
Maynard (1959 (1969) has noted the poor survival of the larger stadia of Limnoporus spp. during storms. Therefore survival of the larger stadia of Limnoporus spp. may be maximized by seeking out habitats that strike a balance between gerrid maneuverability and protection from wind and rain. In the study area small, sheltered ponds probably afford the best compromise.
2. Instar Differences
This study and the work of Vepsalainen and Jarvinen (1974) suggest that space is further partitioned among the various developmental stages of water-strider populations. Habitat preferences observed in the laboratory were less marked among larvae than among adults. In the field, early instars occurred most often in sheltered near-shore areas. Matthey (1974) also observed that young stages of G. remiqis were found only in 227
near-shore habitats. The data suggest that spatial overlap should be greatest among the early instars of co-occurring species.
Jamieson (1973) showed that pronounced size differences between predator and prey are required for interspecific predation and cannibalism to be effective among gerrids.
Therefore the spatial separation observed between size classes should minimize gerrid-gerrid predation in natural habitats.
It seems unlikely that gerrid populations on the Fraser Plateau can be regulated in a strict density dependent manner by either cannibalism or interspecific predation.
3- Species Morphology and Habitat Structure
The experiments to compare adult foraging abilities show that gerrid leg length is associated with foraging success in certain types of habitat structure. In dense emergent vegetation, relatively short legs maximize.success at finding food. There must always be a trade-off between maneuverability and locomotory efficiency. Longer legs increase loc.omotory efficiency (Andersen, 1976) and, in the case of G. comatus, seem to allow more effective foraging in habitats covered by floating vegetation.
Werner and Hall (1977) argue that morphological differences between the bluegill and the green sunfish lead to differences in foraging ability that depend upon the habitat background.
Feeding morphology and behaviour are often linked with habitat 228
structure and this relationship seems to guide the evolution of community structure in fish (Keast and Webb, 1966; Werner,
1977) .
In similar fashion, the evolution of niche relationships among Gerris species on the Fraser Plateau seems to have focused on the relationship between habitat structure and species maneuverability. Because water-striders are unspecialized predator-scavengers (Lumsden, 1949; Matthey, 1974), the main theme in their ability to coexist is specialization %o habitat structure.
The relationships presented in this chapter suggest that the population performance of gerrid species should show marked differences among habitats of different structure. In the next chapter I discuss evidence for such differences and assess the relative importance of habitat structure in producing them. 229
CHAPTER VI. PERSISTENCE, POPULATION PERFORMANCE AND
INTERSPECIFIC COMPETITION
INTRODUCTION
The development of modern competition theory, spear-headed by the work of Hutchinson (1957, 1965), MacArthur (1968, 1972a) and May (1973), has provided an important paradigm for evolutionary ecology. Techniques of niche metrics, recently summarized by Pianka (1976), have organized field work by allowing quantitative expression of resource partitioning thought to explain the coexistence of simialar species (Shoener,
1974a). These niche differences are often assumed to result and persist through the action of interspecific competition.
The mechanism of competition is often inferred from natural history data, but the action of ccmpetiton is rarely demonstrated experimentally in natural communities. Recent papers by Heck (1976) and Weins (1977) argue that we should not be content with patterns developed on this scale of investigation.
Paine (1966) and Connell (1975, 1978) have offered convincing demonstrations that predation can also allow the coexistence of similar species by keeping competitor populations below levels permitted by ambient resources. The action of predation has been clearly demonstrated experimentally, but studies to assess the presence of resource partitioning in these same communities are usually dismissed a priori as irrelevant.
In contrast, Connell (1961) has provided one of the few 230
convincing demonstrations of competition in nature with his classic study of barnacle coexistence. '
Heated discussions, based on data accumulated by the
"competition" and "predation" schools, often attempt to decide whether competition, predation or some other natural process is most important in promoting high species diversity. Menge and
Sutherland (1976) however have suggested that "either^or" arguments miss the point, and that the predation and competition hypotheses are probably complementary.
Empirical studies, launched from an animal-centered perspecitve, offer some hope of unravelling nature's complexity in specific communities. We should expect the relative impact of various processes to vary from system to system and perhaps from taxon to taxon. Single factor explanations may often oversimplify natural situations and, in complicated questions of community structure, powerful generalization will probably elude us (Whittaker and Levin, 1977).
In preceding chapters, I have shown that water-striders on the Fraser Plateau partition resources along the habitat dimension. In this chapter, I examine pattens of species persistence and population performance and ask if interspecific competition can account for them. I also assess the potential impact of other ecological processes on the distributions and relative abundance of these gerrid species. 231
MATERIALS AND METHODS
A. Colonization, Persistence and Population Success
1. Field Surveys
Yearly surveys were conducted at Springhouse and Becher's
Prairie to determine which gerrid species colonized and produced at least one generation over a representative series of lakes.
Data were collected for 60 lakes in 1975, 74 lakes in 1976 and
103 lakes in 1977.
Sampling methods differed slightly from year to year.
Species colonization and success was easily determined from data for the 45 lakes sampled regularly throughout 1975 (Chapter IV).
Fifteen additional lakes were sampled once in late May for colonists and once or twice during July and early August for the new generation. Sampling entailed 2-10 minutes (depending upon lake size) of intensive sweeping over the full range of habitats available at the lake. A new generation was recorded as present when a single fifth instar or new adult was collected.
If only fourth or earlier instars were collected for any species, the lake was revisited two or three weeks later.
Similar spring and summer surveys were carried out at all lakes sampled in 1976.
Quantitative data on species abundances were collected in
1977. The standard sampling method developed in Chapter III was employed and the resulting data were converted to density 232
estimates. Three samples taken at each lake and were used to calculate an average density value for spring and summer populations of every gerrid species present.
Variables suspected to affect gerrid population growth were recorded for each lake visited in 1977 as follows:
(1) Invertebrate Predator/Competitors. Other inverte•
brates feed at the water surface and are known to prey
upon water-striders. Many of these predators were
collected while sampling for gerrids. During the summer
1977 lake survey the total numbers of backswimmers
(Notonecta) , dytiscid (Aeilius, DytiscusV larvae,
whirligig beetle (Gyrinus) adults, and pisaurid spiders
collected were recorded at each lake. These data were
converted to numbers of individuals caught per minute.
(2) Habitat Availability. Potential gerrid habitat
was divided into three classes depending upon vegetation
structure: grass/sedge, rush and floating vegetation plus
open water (cf. Chapter IV) . At the summer sampling
visit, two observers made independent estimates of the
proportion of each habitat type sampled at each lake and
the average estimate was recorded.
(3) Surface Conductivity. Water samples were
collected during the summer and conductivity was
determined with a Radiometer CD-2 conductivity meter;
results were corrected to 25°C.
(4) Lake Area. Lake areas were determined plani-r
metrically from aerial photographs. 233
(5) Tree Shelter. Trees and bushes growing along lake
margins provide some protection against the effects of
wind. The percentage of the lake margin sheltered by
trees and bushes was determined from aerial photographs.
2. Analysis
a. _ Number of Species per Lake
The average number of species per lake and its variance were calculated for spring and summer data from 1975-1977.
Variance to mean ratios were calculated to assess the pattern of species distribution over the lakes surveyed. The method of calculation is summarized by Andrewartha (1961).
b. Population Persistence
The proportion of spring populations that produced
subsequent generations was calculated for each species in 1975,
1976 and 1977. This index of species persistence was compared
among species.
c. Population Success
All density estimates from 1977 were converted to natural
logarithms. The difference between ln(spring density+1) and
ln(summer density+1) was taken as one index of population
success. Correlation and multiple regression analyses were
used to determine which of the lake characteristics were most 234
strongly associated with population success in the field.
Analyses were repeated with biomass estimates calculated as in
Chapter IV. :
d« Gerrid Species Diversity vs. plant Structural Diversity
The Shannon-Wiener index of diversity (H', Pielou, 1975) was calculated for water-strider species and gerrid habitat present at each lake. Regression analysis was used to ascertain the relationship between the two measures.
B. Habitats, Growth and Competition
1- Effects on Species Growth and Survival
a. Species Growth
Twelve small circular enclosures (0.217 sg. meters) were established at Sp6, Sp8 and Westwick Lake in relatively homogeneous stands of grass/sedge, floating
(Ceratophyllum/Myriophyllum) and bulrush vegetation, respectively (see Table 19, Chapter IV). Enclosures were clear, polyvinyl acetate rings (25 cm. high), open at the top and bottom. Each ring was stapled to four exterior support
posts and adjusted to extend 5-10 cm. beneath the surface.
Fourth instars of G. buenoi, G. comatus and G. pinqreensis were.collected from lakes at Springhouse and Becher's Prairie
and used to establish mass cultures of each species maintained 235
at field temperatures and photoperiod. Gerrids were fed liberally with vestigial-winged Drpsophila and insects captured in a light trap near Westwick Lake. Cultures were checked several times daily for newly molted fifth stage larvae which were immediately isolated and held up to two days without food in small styrofoam cups.
Twenty-five teneral fifth instars of each species were killed with ethyl acetate, dried at 90°C and weighed with a
Mettler Microgrammatic Balance (Mettler, Instruments, Inc.) accurate to ±0.00 2 mg. The average weights recorded and their standard errors are given for each species in Table 35. These weights were used as starting weights for each species in subsequent experiments.
Field experiments were started by placinq five unfed fifth instars of a single species in an enclosure at each lake.
Three days later, all survivors were collected, killed, dried and weighed. Individual weight gains were calculated by subtracting the average initial weight (Table 35) from the average final weight recorded in each experiment. Ten groups of five animals were run at each lake for G. buenoi,' G. comatus and G. pinqreensis between July 23 and August 22. TABLE 35
Weights of newly molted fifth instar larvae
SPECIES AVERAGE WEIGHT S.E.
G. buenoi 0.882 0.0227
G. comatus 1.221 0.0195
G. pingreensis 0.997 0.0193 237
b. Food-fall
Insects falling onto the water surface at each lake were collected in round plastic containers (0.123 sq. meters) as a measure of food availability. Each food-^fall trap contained a dilute solution of formalin. Three traps were run at each lake during each gerrid growth experiment. The collected insects were dried and weighed; the results were converted to mg. per sguare meter per day.
2. Effects of Competition Between Species
The large 4.0 sguare meter, enclosures described in Chapter
V were bisected diagonally with a sheet of heavy gauge plastic to create 2.0 sg. meter compartments. Fifteen field-collected fourth instars, each, of G. buenoi, G. comatus and
£• pinqreensis, were introduced into four of these enclosures at
Sp 6, Sp 8 and Westwick Lake. Thus a total of 45 gerrids was added to each compartment (22.5 per sg. meter), and these experiments were run at the same gerrid density as the monospecific experiments reported above. Competition experiments were terminated after six days when all surviving gerrids were collected and identified. 238
RESULTS
A. Colonization, Persistence and Population Success
1. Number of Species per Lake
Table 36 presents the average number of water-strider species colonizing and reproducing per study lake between 1975 and 1977. The higher means for 1976 and 1977 reflect the establishment of G. incognitus populations in the study area during Spring, 1976. Over the three year study there was a consistent loss of about one species per lake during the season.
The low variance to mean ratios indicate that gerrid species are not randomly distributed over the study lakes. In fact the number of gerrid species collected per lake tends to be remarkably uniform during both spring and summer. These data imply that spring dispersal is egually effective at mixing species among lakes, and that processes leading to local species extinction operate with similar efficiency across all lakes
studied.
2. Population Persistence
The average percentage of spring populations that left
offspring between 1975 and 1977 is presented for all species
studied in Table 37. Persistence was .clearly highest for
G. buenoi and lowest for G. incoqnitus; G. comatus,
Pingreensis and Limnoporus showed intermediate and 239
TABLE 36
Number of gerrid species recorded per lake 1975-1977
| SPRING | SUMMER
YEAR j Mean Nunber Variance: |Mean Number Variance: | Mean Mean I Spp./Lake Ratio j Spp. ./Lake Ratio
1975 3.03 .204 j 1.75 .442 N=60
1976 3.31 .248 | 2.28 .445 N=74
1977 3.50 . 280 j 2.51 .321 N=103 240
TABLE 37
Percentages of water-strider populations completing one generation during each year, 1975-1977
% OF COLONIZING POPULATIONS COMPLETING AT AT LEAST ONE GENERATION SPECIE S ~ f Including dry-outs Excluding dry-outs Mean S. E. Mean S. E.
G. buenoi 76.5 6. 73% 84. 1 8. 03%
G. comatus 57. 1 5. 37% 62.8 6.28%
G. pin greens is 59.1 0. 77% 61.0 4. 60%
G. incognitus 15.4 1. 20% 25.0 0.00%
Limnoporus 52.3 2. 06% 62. 5 2. 10% 241
approximately equal persistence.
Two main points emerge from these data. Firstly, annual rates of species extinction were relatively high, even when populations on temporary ponds are omitted from the analysis.
However, it is important to note that most species extinctions occurred on lakes that were sparsely colonized. Secondly,
G. buenoi, G. incognitus and Limnoporus spp. were most strongly affected by habitats drying out. This probably results from their association with small ponds and temporary habitats
(Chapter IV) .
The very low success rate of G. incognitus may explain why this species has not been consistently recorded from the study lakes despite intensive fieldwork in the area (Scudder, 1971,
1977). A combination of low spring density and preferential colonization of temporary habitats (Chapter V) may exclude
_.- incognitus from the study lakes during some years.
3- Population Success
Net population change, calculated for each lake as in equations 1 and 2, provide an index of species success:
net density = ln(summer density+T) - ln (spring density+1) (1) change
net biomass = summer density - spring biomass (2) change
The pair-wise correlations among population success of all
water-strider species studied during 1977 are presented for both 242
density and biomass in Table 38.
With the exception of a weak, positive correlation between
density changes of G. incognitus and Limnoporus spp., there were
no significant relationships between particular pairs of gerrid
species. These data suggest that pair-wise species
interactions did not affect species success in a consistent
manner from lake to lake. Therefore, it-is appropriate to lump
the net changes of all co-occurring gerrid species in subsequent
analyses of general factors associated with the population
success of particular species.
Table 39 lists the correlation coefficients between the
observed density changes of each species and the proportion
(transformed to arc sins) of three habitat classes available at
each lake. The only significant positive correlations are
those of G. buenoi, G. comatus and G. pinqreensis with the
relative abundance of their usual habitats (grass/sedge,
open/floating and bulrush, respectively; cf. Chapter IV). In
addition success in each of these species was negatively
correlated with the proportions of the remaining two habitats.
Negative correlation coefficients were significant for
G. comatus and G. pingreensis in grass/sedge habitat and for
G. buenoi in bulrushes. Success in G. incognitus and
Limnoporus spp. was not related to the abundance of any habitat
class. The same pattern held for changes in biomass.
Stepwise multiple regression was used to assess the
combined ability of six variables to predict population success 243
TABLE 38
Correlations between net changes in (a) Density and (b) Biomass for gerrid species during 1977 a. Density
Gerris Limnoporus
buenoi comatus pingreensis incognitus
G. buenoi 1. 000 — — . — - G. comatus . 102 1.000 - - G. pingreensis . 068 .060 1.000 - G. incognitus . 082 -.032 .047 1 .00- 0 - Limnoporus . 009 . 064 .018 .275* 1.000 b. Biomass
G. buenoi 1.000 — — G. comatus -. 060 1.000 G. pingreensis .053 .012 1.00- 0 - - G. incognitus -. 064 .040 .090 1.000 - Limnoporus -. 122 .066 -.093 -.092 1.000
* significant, p<.05 TABLE 39
Correlations between net increase of gerrid species and proportions of common habitat types
SPECIES GRASS/SEDGE RUSH OPEN S FLOATING
G. buenoi .417* 366* -.083
G. comatus -.273* 128 .437*
G. ping reensis -.344* 515* -. 152
G. incognitus -.059 205 .143
Limnoporus -.085 101 -.012
• significant, p < .05 245
for each gerrid species. The significance level necessary for inclusion or exclusion of variables at each step was set at .05.
Only lakes with gerrids present at the summer sample were considered in the analysis; sufficient data were available from a total of 86 lakes.
Table 40 shows the order of inclusion for all significant variables for each Gerris species based on data for net density changes. The partial correlations of each significant variable
with species success at the final step are given in parentheses.
None of the measured variables allowed significant prediction of
success for Limnoporus spp.
The best predictors of success for G. buenoi, G. comatus and G. pinqreensis are the respective proportions of their characteristic habitat. An inverse relationship with
conductivity afforded the best single-variable prediction of
success for G. buenoi. However, the qreatest partial correlation was obtained between G, buenoi success and the
proportion of grass/sedge habitat in the final eguation.
Success in G. buenoi and G. pinqreensis was also positively
related to the overall success of other co-occurrinq waters
striders ; no species was siqnificantly depressed by the success
of the other gerrids. These data suggest that interspecific
competition alone cannot account for the observed patterns of
distribution and abundance in ecological time. In general,
patterns of distribution and abundance were not produced as a
direct result of differential species success, but resulted from TABLE 40
Multiple regressions of Gerris population success on habitat variables a. Order of inclusion and partial correlation coeffi• cients in final eguation
Gerris VARIABLE | buenoi comatu s pinqreensis incoqnitus
Net Change of | 3 n.s. 2 n.s. other gerrid | (. 304) (• 303) spp. , 1
Proportion of | 2 1 1 Favoured Habi-| (.401) (.452) (. 570) tat Present | (arc sin) |
Summer | 1 n.s. n. s. n.s. Conductivity j (-.319)
Abundance of | n. s. 2 n.s. n.s. Predator/ | (.406) Competitors |
Lake Area | n. s. n.s. n. s, .. n. s.
Proportion of | n. s. 3 n« s. 1 Margin with | (-.248) (-.331) Tree Shelter | (arc sin) |
n.s. = not significant (p >. 05)
b. Efficiency and significance
1 buenoi comatus pingreensis incognitus
Multiple r | .580 .342 .333 .109
d.f. 1 3. 83 3.83 2.84 1.85
F | 14. 00 14.40 20.95 10.44 ,
P 1 <.Q01 <.001 <. 001 <.002 247
habitat fidelity at the time of spring colonization.
G. comatus exhibited greatest success where other predator/competitors (mostly Notonecta spp.) were abundant.
There were no significant negative associations between gerrid species success and the abundance of potential invertebrate
predators suggesting that selective predation cannot provide a
general explanation of gerrid species . .success on the Fraser
Plateau. Predation does not consistently promote high gerrid species diversity in the manner described by Paine (1966), because real losses in gerrid species were recorded during the
season at each lake.
The only significant variable for G. incognitus was the
proportion of the lake margin surrounded by trees. The correlation is negative because of the generally poor success of
all G. incognitus populations during 1977-
When the analyses were repeated with biomass data only the
variables representing habitat structure were included in the
regression eguations for G. comatus and G. pinqreensis. A
combination of summer conductivity and proportion of grass/sedge
habitat allowed the best predictions for G. buenoi. None of
the variables could be included for G. incoqnitus or Limnoporus
spp. These data strengthen the conclusion that habitat
structure is the most reliable predictor of gerrid distribution
and abundance on the Fraser Plateau,
Although significant regressions were obtained, the final
multiple regression eguations explained only a small amount of 248
the variance observed in field population success (Table 40-B).
None of the variables measured in this study allow accurate prediction of gerrid population success as defined above.
1« Gerrid Species Diversity vs. Plant Structural Diversity
Figure 41 is a scatter plot of gerrid species diversity versus plant structural diversity using data from 90 lakes in
1977- There is no significant relationship between the two indices (r=. 136; df=90; p>>.05). These data show that water-
strider species diversity cannot be predicted from the diversity
of their characteristic habitats.
B. Habitats, Growth and Competition
1• Effects of Habitat on Species Growth
The average survival of five gerrids over the set of three-
day growth experiments at each lake is presented in Table 41.
All species survived best in the bulrush habitat of Westwick
Lake. G. pinqreensis showed markedly reduced survival in
grass/sedge and floating vegetation habitats. G. buenoi
survived better than G. comatus in both grass/sedge and floating
habitats. The mean survivorship of G. comatus was slightly
higher in floating vegetation than in grass, although more data
are needed to establish statistical significance.
Gerrid remains found floating at the surface indicate that
at least 15-20% of the observed mortality can be attributed to 249
Figure 41. A plot of gerrid species diversity versus plant
structural diversity during July-August, 1977. • 1.5 9 9 0 0 9 9 9 0 >- • 9 9 o tz © O • © 9 CO 6 0 9 9 • 9 00 > 0 0 • O e o o o CO o UJ • 0 • • 9 9 9 0© • 0 Q_ 9 . 0 CO o 0 o 9 e ' Q 9 9 tr o • 9 0 or 9 9 9 0 • UJ o9 • 9 e e .0.5 1.0 1.5 2.0 PLANT STRUCTURAL DIVERSITY (H1) o TABLE 41
Mean survival of five gerrids ± standard errors over three day experiments in three habitats
SPECIES GRASS/SEDGE RUSH FLOATING
G. buenoi 3.8±0. 33 4. 6±0.22 3.2±0.39
G. comatus 1.8+0.49 4.7±0.13 2.2±0.33 252
invertebrate predation. Predation by Notonecta spp. and/or other gerrids accounted for the largest proportion of the remains. Predators with chewing mouthparts (most probably dytiscid beetle larvae) accounted for a smaller proportion of the gerrids eaten.
Figure 42 illustrates the average weight gain recorded per
surviving individual at each lake. All species experienced
maximum weight gains in rush habitat. The performance of
G. buenoi was identical in rush and grass/sedge habitat but it did relatively poorly in floating vegetation. G. comatus
performed equally in grass/sedge and floating habitats.
Statistically significant differences could not be detected in
weight gains of G. pingreensis among the three habitats with a
one-way analysis of variance (F=3.03; df=2,22; p<.07).
Data in Figure 42 indicate that all three species showed
similar absolute weight gains in grass/sedge habitat. However,
G. buenoi gained significantly less weight in both bulrushes and
floating vegetation than did either of the two large species.
This suggests that G. buenoi is able to sustain maximum growth
rates in grass/sedge habitat while growth rates for
G. pingreensis and G. comatus fall. In floating vegetation,
however, absolute weight gains recorded for G. buenoi were
smaller than those of the two larger species.
It is clear that the best performance observed for all
species in bulrush habitat was not simply the result of greater
absolute food availability (Figure . 43), Generally, surface 253
Figure 42. Average weight gains of individual surviving gerrids
over three days in natural habitats. (a.)
G. buenoi, (b.) G. comatus, (c.) G. pingreensis. 254
A. & BUENOI
1000
800
600 >- cr 400 Q 200 co
< B. G. COMATUS BULRUSH HABITAT (WESTWICK L.) 1400 GRASS/SEDGE HABITAT (SP 6) s I200i o FLOATING/OPEN HABITAT (SP 8) 1000 STANDARD ERRORS or 800 i 600 > cr 400
CO 200
< o C. G. PINGREENSIS
X 1200 (3 LU 1000 800
LU 600 CD < 400 cr LU 200 255
food-fall was greatest in grass/sedge and floating vegetation.
The possible effects of resource depletion by other surface- feeding invertebrates was not considered in these experiments.
Data of Figure 43 also emphasize that food-fall is very patchy in time and space.
2. Effects of Competition Between Gerrids
Figure 44 presents the daily mortality rates calculated for
fourth and fifth instars of each species in three-way competition experiments and in the single-species growth
experiments. Comparison of these data suggest that the
presence of other species significantly increased the mortality
rates observed for all species in bulrushes. The effect is least pronounced for G. pingreensis. Only the mortality rate
of G. buenoi was significantly increased by competitive effects
in grass/sedge habitats. However, both G. pingreensis and
G. buenoi suffered significantly higher mortality in competition
experiments run in floating vegetation,
Although competition and single-species experiments were
started at the same initial density, precise quantitative
interpretation of these data is clouded because gerrids molted
during the three-species experiments. It was not possible to
rear and handle enough fifth instar larvae, under field
conditions, for both sets of experiments. If molting
significantly increases vulnerability in the competitive
situation, mortality rates may be disproportionately elevated in
the competition experiments, 256
Figure 43. average food fall per sguare meter during single-
species growth and three-species competition
experiments in the field. BULRUSH HABITAT, WESTWICK L.
GRASS/SEDGE HABITAT, SP 6
FLOATING HABITAT, SP8
STANDARD ERRORS
[4i
JULY 23 JULY 26 JULY 31 AUGUST3 AUGUST9 AUGUST 19 STARTING DATE FOR THREE-DAY GROWTH EXPERIMENT 258
Figure 44. Average daily mortality rates observed in single-
species and three-species experiments in natural
habitats. (a.) G. buenoi, (b.) G. comatus, .(c)
G. pingreensis. 259
20| A. G. BUENOI SINGLE-SPECIES EXPERIMENTS THREE-SPECIES COMPETITION rh EXPERIMENTS | STANDARD ERRORS
B. G. COMATUS
LU LO rh Q U_ O 0.5
UJ CO C. fi PINGREENSIS Z 2.0
UJ
<2
> < 1.0 0.5
GRASS/SEDGE FLOATING BULRUSH SP 0 WESTWICK L. SP 6 260
Fortunately, it is the patterns of species mortality among the three habitats that are of real interest, and they are relatively clear. Presence of other species affects G. buenoi least in grass/sedge and G. pinqreensis least in bulrushes.
_.- comatus showed similar mortality in competition experiments in all three habitats. Because these patterns differ markedly from those observed in single species experiments, I conclude that competitive interactions can affect the success of these three species, differentially according to habitat. The data suggest that each species should be favoured in its characteristic habitat if forced into a competitive situation at
relatively high densities. The exact mechanisms remain to be experimentally elucidated. 261
DISCUSSION
A. Habitats, Diversity and Population Performance
Data presented in this chapter emphasize that water- striders must deal with an extremely heterogeneous mosaic of potential habitats on the Fraser Plateau. Population performance varies both among lakes, and' among habitat patches within a lake. Therefore, models, like those of Jarvinen
(1976), that address evolutionary guestions about gerrids, sacrifice considerable realism by assuming uniformity of water- strider habitats. The variance of habitat favourablity must be considered. Southwood (1977) has discussed several examples of how variable favour ability influences the dynamics of insect populations. Grant (1975) has demonstrated similar differences among habitats occupied by microtine rodents.
A combination of habitat effects and species interaction undoubtedly influence the surprisingly high rates of population extinction observed in this study, but the precise mechanisms involved are not yet clear. Assessment of species loss from discontinuous spring and summer samples may confound genuine extinction with migration from unfavourable habitats. The continuous population data available from 1975 suggest that both processes are involved. Larvae of all colonizing species were usually collected at each lake, but not always in proportions expected from the relative abundances of spring adults.
Vepsalainen (1973b) has also noted a consistent loss in 262
gerrid species per locality in Finland, It is interesting that the average number of species reported per lake in southern
Finland is considerably lower than those reported for spring populations on the Fraser Plateau (2.26 vs. 3.28 species per lake), even though Vepsalainen considered distribution records for seven species. This suggests that Finnish localities are more effectively isolated than those studied in central British
Columbia, and helps to account for the more general occurrence of short-winged forms and dimorphic strategies among Finnish
gerrids J. summarized in Vepsalainen (1978)],
Population success rates observed for G. buenoi, 6. comatus
and G. pingreensis are generally concordant with the habitat
preferences demonstrated experimentally in Chapter IV.
G. buenoi, the habitat generalist, was more persistent in all
three years than either G. comatus or G. pingreensis which are
habitat specialists.
At a more detailed level of population performance,
however, habitat structure alone was insufficient to accurately
predict species success. This is clarified by reference to
Figure 45 where the relative abundance .of each species during
summer 1977 is plotted against the percentage of its
characteristic habitat available on each lake (both variables
are transformed to arc sins). Patterns of success varied
between G. buenoi and the two habitat specialists. Where there
was a high proportion of grass/sedge habitat, G. buenoi did
consistently well, but when grass/sedge habitat was rare the
proportion of G. buenoi collected was highly variable. The 263
Figure H5. The relationship between relative abundance of
gerrids and relative abundance of their
characteristic habitats. (a.) G. buenoi, (b.)
G. comatus, (c.) G. pinqreensis. A. G. BUENOI 88
o z UJ 68 CO • • • §48 © • • • 8 • J ' "# *
28
to
1 8 ARC SIN (PROPORTION GRASS/SEDGE PRESENT) coi88 B. G. COMATUS 3 UJ O o 68 •• • < o Q e>l Q48 fe CD o
< o28 o_ CO 2 CO yj 8 o o 5 UJ Q_ ARC SIN (PROPORTION FLOATING/OPEN PRESENT) CO C. G. PINGREENSIS Q m (T 68 UJ
48
• 28
? 8 CoO cr < 8 18 28 38 48 58 68 78 88 ARC SIN (PROPORTION OF BULRUSHES PRESENT) HABITAT ABUNDANCE 265
pattern is exactly reversed for G. comatus and G. pingreensis; as the proportions of characteristic habitat increased the
relative abundances of both species became more and more
indeterminate. Simple explanations do not suffice; although
resource partitioning with respect to .habitat structure is
demonstrably important, other factors must interact to produce
natural patterns.
Strong evidence that gerrids segregate along habitat
dimension led me to expect a positive relationship between
habitat complexity and gerrid species diversity.. Since the
seminal work of MacArthur and MacArthur (1961), there has been a
deluge of papers claiming to demonstrate clear-cut, causal
relationships between species diversity and habitat diversity
among all sorts of animals. However, the evidence is often
equivocal (Murdoch et al., 1972) and, the addition of
gualifications (e.g. Gorman and Karr, 1978; Andersen, 1978) may
reduce the generalization to something like "the relationship is
strong except where it isn't".
In this study no general relationship between gerrid
diversity and habitat diversity was found. Tomoff (1974) and
Vuilleumier (1972) have found that foliage height diversity is
not always the best predictor of bird species diversity. Allan
(1975) noted the absence of a predictive relationship between
habitat complexity and species diversity among stream insects,
even though he also presented strong evidence for active habitat
preferences. He showed that habitat complexity was fairly
constant over the stream gradient and suggested that species 266
diversity was insensitive to fine-scale variation between habitats. ; -
A similar explanation cannot apply to water-striders on the
Fraser Plateau, because the mosaic of habitat structure varies considerably among lakes. The generalization probably fails, in this case, because habitat variation is superimposed on
variable patterns of resource availability, gerrid colonization, density independent mortality and interspecific competition.
Vepsalainen and Jarvinen (1976) have summarized the
arguments for resource limitation in water-striders. Figure 43
and other data (Spence, unpublished) show that food resources
are extremely patchy in time and space. In general, food
availability falls after mid-summer, but gerrid numbers remain
high until late August (Chapter IV). Interspecific competition
among insects with seasonal life cycles is often "transient"
(Istock, 1967, 1973; McClure and Price, 1975), reflecting such
variation in resource availability. Transient competition
among gerrids could permit elevated species diversity.
Gerrid species diversity could be lower than expected on
some lakes owing to colonization failure and density independent
mortality. Only careful experiments and detailed study of
natural populations will help explain the scatter observed in
Figure 41. Explaining away the points that do not fit, a
posteriori, is not likely to be instructive. 267
B. Com petition and Habitat Selection
Results of the enclosure experiments indicate that interspecific competition can affect the distribution and
relative abundance of G. buenoi, G. comatus and G. pinqreensis
on the Fraser Plateau. However, the natural processes leading
to observed distributions are complex.
Bulrushes clearly afford the optimum habitat for all three
species, but only G. pingreensis regularly occurs there (Chapter
IV), often in virtually monospecific populations. Competitive
experiments demonstrated that G. pingreensis has the competitive
edge over similar-sized G. buenoi and G. comatus in bulrush
habitats. With respect to the habitat axes then,
_.• pingreensis occupies a narrower, realized niche that is
"included" (Miller, 1964) within the broader realized niches of
G. buenoi and G. comatus.
Hutchinson (1978) pointed out that "the adaptation of the
species occupying the best, or included, part of the.niche is
likely to evolve innate behavioural dominance", Gerrids are
known to prey on other gerrids in laboratory cultures and in the
field (Maynard, 1969; Jamieson, 1973; Vepsalainen and Jarvinen,
1976), and at least part of the mortality observed in the
enclosure experiments resulted from cannibalism and
interspecific predation. Jamieson (1973) demonstrated that
gerrid species show different levels of aggression. Laboratory
observations suggest that G. pingreensis is, in fact, the most
aggressive of the three species considered here (Spence, 268
unpublished). Istock (1966, 1967) has found that such differences in innate aggressiveness help to explain natural distributions of whirligig beetles' (Dineutes) .
Jamieson's (1973) laboratory studies,indicated that size difference was an important component of success in gerrid- gerrid predation. The lower growth thresholds of
_.. pingreensis should lead to considerable size advantages for this species during the first generation. However, this does not seem to be a necessary condition for the dominance of
G. pingreensis in bulrushes because both cannibalism and interspecific predation occurred among similar-sized gerrids in enclosures. Fox (1975b) showed that molting backswimmers
(Notonecta) were especially vulnerable to cannibalism. Thus, the lower survival rates observed among all species may have resulted because gerrids molted during competition experiments,
Results of single-species growth experiments demonstrate that competition from other gerrids is unnecessary to explain the low success of G. pingreensis in grass/sedge habitats.
Furthermore, because the enclosures effectively eliminated surface disturbance, the potential biomechanical disadvantage of
G. pingreensis, noted in Chapter V, is not totally responsible for. the absence of G. pinqreensis from grass/sedge areas.
Two explanations can be offered in the face of present data. Firstly, greater aggressiveness of G. pingreensis may have led to higher rates of cannibalism in single species
enclosures at Sp 6 because foraging became more difficult in 269
dense emergent vegetation. However, in one half of the single species experiments in grass/sedge habitat, no G. pinqreensis fifth instars survived for the entire three-day period suggesting that mortality factors other than cannibalism were operating,
A second, more likely, hypothesis is that G. pinqreensis is more vulnerable to competition and/or predation from other surface feeding predators. Total predator/competitor abundance during 1977 was inversely correlated with In (conductivity), (r=
-.322; p<.01) as is shown in Figure 46. Even though there was no significant correlation between the percentage of grass/sedge habitat and total predator/competitor abundance (r= .018; p>>.05), grass/sedge vegetation is most common on extremely fresh, snowmelt ponds.
Pilot experiments suggest that fifth instar G. pinqreensis are more vulnerable to notonectid predation than are similar- sized G. buenoi or G. comatus. This hypothesis is particularly attractive because it also helps to explain the strong positive association between population success in G. buenoi and lakes with low surface conductivity.
G. comatus appears to have the largest realized niche of these three species. Foraging success of late instars and adults is reduced by dense emergent cover (Chapter V), but this factor should not often be important in the grass/sedge habitats studied on the Fraser Plateau. The relatively late appearance and reproduction, observed for G. comatus in the spring (Chapter 270
Figure 46. The abundance of all predator/competitors over a
range of conductivity. 271
_ Si 25 _I UJ & LU
Eo 15 <
b 10 Q_ _> • • » o • • • • e * 9 • % o Q o >2c 5 6 7 8 9 Q_ 10 LN (SUMMER CONDUCTIVITY) 272
IV), will lead to temporal co-occurrence of its early instars with larger size classes of G. buenoi and G. pinqreensis.
Perhaps G. comatus is restricted to floating vegetation habitats by a combination of late spring appearance and lower innate aggressiveness.
C. Evolution and Maintenance of Habitat Preferences
Habitat shifts (Shoener, 1974b; Diamond, 1978) have become increasingly common explanations for coexistence via resource partitioning. Werner and Hall (1976, 1977) have shown that the mechanism is largely behavioural among coexisting species of sunfish, and thus allows short-term adjustments to seasonal patterns of resource availability. Data presented in Chapter V show that G. buenoi may actually alter its distribution within a pond in response to the presence of other gerrids. However, short-term behavioural plasticity alone cannot account for the distributional patterns observed on the Fraser Plateau.
My studies suggest that most spring adults recolonize the
same ponds where they emerged during the previous summer.
Often this will guarantee them association with appropriate
habitats. Selection has produced behavioural preferences for
certain habitat characteristics which explain patterns of
intralake distribution. Preferences seem to be based upon
matching morphology with habitat structure to allow efficient
foraging (Chapter V). The possible relationship between
habitat preference and food distribution (Werner and Hall, 1976)
is not likely to be important among generalist predators like 273
gerrids.
A smaller proportion of overwintered adults make more extensive colonization flights and. perhaps, settle on lakes that do not contain the appropriate habitat for the species.
Some evidence suggests that such gerrids can fly again in a trial and error attempt to locate preferred habitats (Chapter
V). Data presented here indicate that gerrids which breed in uncharacteristic habitats pay an evolutionary penalty of reduced reproductive success if resources become: limiting and other species are present. Although habitat structure provides only a fair prediction of guantitative reproductive success, it is apparently the best clue available to gerrids on the Fraser
Plateau.
D.. Summary
Interspecific competition from G. pingreensis can exclude
G. buenoi and G. comatus from bulrush habitats on the Fraser
Plateau. Contrasting habitat use by G. buenoi and G. comatus
may be explained as a result of differential population
performance in grass/sedge and floating vegetation. It seems that G. pingreensis is rare in freshwater habitats because of
competition and/or predation from other surface-feeding insects
and possibly, owing to the absence of sheltered microhabitats
during the early spring. These . processes have led to and
enforced the habitat associations and preferences documented in
earlier chapters. However, these mechanisms operate on an
unpredictable background of colonization dynamics and density 274
independent mortality, and therefore, habitat preferences allow
only fair prediction of population performance over a range of
lakes. The simultaneous operation of many processes, could
easily keep gerrids from maintaining equilibrium populations and
may be responsible for the observed persistence of G. incoqnitus
and Limnoporus on the Fraser Plateau. Therefore competition,
predation and environmental heteroqeneity in space and time are
all important aspects of qerrid coexistence in central British
Columbia. 275
CHAPTER VII. GENERAL DISCUSSION
The preceding chapters lay a foundation for studying the comparative dynamics of gerrid populations in the field. This
study has emphasized how natural patterns of habitat variation
can affect the species composition of individual lakes bounded
within a small geographical area. Such variation provides a
template for population responses (Southwood, 1977), which are
driven by climate and natural processes of competition,
predation, density-independent mortality and colonization
dynamics. Through interaction of these factors, each lake
develops an individual roster of species and their abundances.
The first challenge for the biologist is to compress data
from individual lakes into a framework of more general patterns
that imply similar operation of natural processes. These
classifications will always be more or less imperfect owing to
the underlying individuality of each lake. The next job, only
started here, is to dissect the various: processes affecting
population responses experimentally, and thus, explain the
natural discontinuities that allowed us to first perceive the
patterns.
The initial season of field work established strong
quantitative associations between gerrid species and particular
lakes characterized by dominant vegetation structure.
Subsequent work showed that the marked associations resulted
from differences in species growth and survival as well as
remarkable habitat fidelity at the time of spring colonization. 276
Habitat fidelity seems to be most important for producing patterns in the field; it results from the apparent choice of overwintering sites near the mother pond and, to a lesser extent, active habitat selection during the dispersal period.
For gerrids, however, habitat classifications based upon vegetation structure are not strictly between-lake, variables.
Several structurally different habitats often occur on the same lake and gerrid species adjust their distributions in response to such variation (Chapter V). It was demonstrated that habitat associations observed for G. pinqreensis and G. comatus are paralleled by stronq preferences for emergent cover and lack of it, respectively. Field observations and experiments
established that G. buenoi is a habitat generalist, but that its distribution within a lake can be affected by a tendency to
avoid other species. Although the adults and late instars of
co-occurring gerrid species show distinct separation in space,
the first three instars are often found together in sheltered
areas near the lake margin. Thus it is appropriate to consider
responses to structural characteristics of habitat as part of
the intracommunity role or niche of each species and instar
(Whittaker et al., 1975, 1973). MacArthur (1958) used similar
positional criteria in his classic study of niche division among
warblers coexisting in coniferous forests of the northeastern
United States.
Whittaker et al. (1975, 1973) argue that the terms
"niche" and "habitat" should be restricted to species responses
to intra- and intercommunity variables, respectively. This 277
distinction is compelling from a theoretical viewpoint. It
permits us to separate species packing (alpha diversity) from
the effects of changing environmental backgrounds (beta
diversity). However, the argument begs for a clear,
unambiguous definition of "community" that can accomodate
differences in scale that are relevant to the organisms
involved.
Other authors (e.g. Kulesza, 1975; Rejmanek and Jenik,
1975) argue that the distinction between "niche" and "habitat"
is not often clear in nature. Results of the present study
suggest that it would be unrealistic to separate gerrid niches
and habitats in an evolutionary context. Whether or not a
particular lake (i.e.."habitat") is suitable for a given water-
strider species, seems to depend almost completely upon the
presence of its characteristic microhabitat defined by
vegetation structure (i.e. "niche", as argued above). Tight
linkage between niche and habitat is likely to be common among
insect species. Because they have relatively little control
over their environment, insects have often become extreme
habitat specialists.
The ultimate context of species evolution is response to
the total array of niche and habitat variables. Whittaker et
al. (1975, 1973) argue that we should consider this
combination of variables as "ecotope". I follow this
convention in subseguent discussion.
Experimental evidence presented in Chapter VI suggests that 278
pinqreensis preempts the most favorable gerrid ecotopes on the Fraser Plateau- These are bulrush beds, generally located on larger lakes toward the upper end of the conductivity range.
Bulrush beds provide emergent cover during both spring and summer, and their presence is a good predictor of habitat permanence for water-striders. Also lakes with higher conductivities tend to be more productive (Orians, 1966) which may lead to greater food availability for gerrids. . Because both G. buenoi and G. comatus exhibited highest survival and growth when confined alone in bulrush habitat, I conclude that the ecotope of G. pinqreensis is included within that of the other two species.
Mortality rates for G. pinqreensis were much lower than those observed for either G. buenoi or G. comatus in three- species competition experiments in bulrush habitat. This suggests that G. pinqreensis is the competitive dominant in bulrushes; the most likely mechanism is greater success under conditions leading to interspecific predation, a form of interference competition for space (Milne, 1961). , Size differences were not a factor in competition experiments but
laboratory observations suggest that G. pinqreensis is the most
aggressive of these three Gerris species. I suspect that the
more robust morphology of G. pinqreensis leads to greater kill
and escape efficiency, but these possibilities have not been
tested experimentally.
Among natural populations, G. pingreensis should gain
additional advantages because it commences reproductive activity 279
earliest in the spring (Chapter IV; Spence et al., 1978) and the low temperature thresholds of early instars (Chapter II) promote rapid growth. Thus early seasonal timing should lead to distinct size advantages for G. pingreensis. Jamieson (1973) has established that kill efficiency increases with size difference between predator and prey gerrid. However, the fact that different size classes . tend to occupy different microhabitats should diminish the overall effect of size difference to some extent in the field.
Whatever the mechanism, competitive superiority of
_.- pinqreensis in bulrushes has apparently selected for ecological characteristics to ensure that G. buenoi and
G, comatus make minimal population investments in bulrush habitats. However, it seems that these ecotope boundaries are
tested yearly. As an example, I cite the gerrid population
histories observed between 1975 and 1977 at Long Lake and Barnes
Lake.at Becher's Prairie. These two lakes are at the upper end
of the conductivity scale (Appendix I) and the only gerrid
habitats are small (30-50 m2) patches of bulrush. In 1975
dense, monospecific populations were observed throughout the
season at both lakes. Populations seemed to decline during
1976, probably as a result of the intensive sampling program in
1975. During May 1977 only a few overwintered G. pinqreensis
were found at both lakes, but a fair number of immigrant
G. buenoi and G. comatus were also taken at Long Lake. During
the summer survey, no G. pinqreensis were collected at either
lake but G. buenoi and G. comatus were common at Long Lake. It 280
is tempting to suggest that the reduction of the G. pingreejisis population at Long Lake allowed establishment of the other
Gerris populations during 1977.
It is obvious, that G. pinqreensis is severely disadvantaged outside of bulrush habitats. The marked intolerance of G. pingreensis to surface disturbance (Chapter V) may restrict it to bulrush beds during spring colonization.
When this species is collected elsewhere in the spring, it is almost invariably on lakes that provide some cover among overhanging brush or emergent grass stalks in flooded meadows.
Single-species growth experiments, however, demonstrated that additional factors must be involved. These have not yet been experimentally verified. An interesting hypothesis is that G. pingreensis suffers disproportionately from the presence of other invertebrate predators feeding at the water surface.
These potential predator/competitors are most abundant in lakes with low conductivity.
Poor growth and survival of G. pinqreensis in freshwater habitats leads to diminshed recruitment into the pool of qerrids overwinterinq locally. Most G. pinqreensis are apterous and
are thus incapable of regular, efficient dispersal. By and large, these gerrids re-colonize the same pond on which they
became adults. Therefore, the proximate cause of low
Pingreensis abundance in freshwater habitats is rigid habitat
fidelity at colonization. Data in Table 34 show that long-r
winged G. pingreensis also settle most freguently in bulrushes. 281
It is clear that selection has acted upon G. pingreensis to enforce minimum population investment in freshwater habitats.
The ecotope boundaries between G. buenoi and G. comatus are less rigid than those with G. pinqreensis. Data presented in
Chapter V suggest that habitat use by these two species may be a function of matching foraging efficiency with morphology among late instars. A comparison of weight gains recorded in single- species growth experiments demonstrated that fifth instar
G. buenoi were significantly less efficient at foraging in floating vegetation habitats than were fifth instar G. comatus
(Chapter VI). The extreme patchiness of gerrid food resources in space and time virtually guarantees that populations will be faced with periodic food limitation each season. It is also likely that high temperature optima (Chapter II) and the exceptional tolerance of surface disturbance recorded for
G. comatus in the laboratory (Chapter V) confer some adaptive advantage in habitats without significant development of emergent cover. The survey data presented in Chapter VI show that the best predictors of population performance in G. buenoi and G. comatus were the relative abundances of grass/sedge and floating vegetation habitat, respectively.
Figure 47 summarizes the qualitative ecotope relationships
among G. buenoi, G. comatus and G. pinqreensis, as discussed
above, in three dimensions. G. incognitus and Limnoporus
spp. are not readily placed within the ecotope space defined by
axes of Figure 47, but all available data suggest that they are
extreme habitat specialists,. G. incognitus colonizes 282
Figure 4 7. Areas used by G. buenoi, G. comatus and
G. pinqreensis in three-dimensional ecotope space. 233
G buenoi
CI G comatus
U G pingreensis 284
protected, brushy habitats almost exclusively (Chapter V) and
Limnoporus spp, are strongly associated with very small, temporary ponds (Chapter IV). Limnoporus spp. frequently co- occur with G. buenoi and, because of the great size difference among these species, there is some suggestion that trophic niches are separated on the basis of prey size.
The overriding conclusion of this investigation is that the number of gerrid species co^occurring in a small geographical region is largely a function of the number of distinct ecotopes available . Strong habitat associations are most important on the Fraser Plateau, and seem to be maintained through both interspecific competition and predation by other surface-feeding insects. However, patterns of distribution and abundance are fuzzy because yearly temperature regimes and weather patterns influence food availability, rates of growth, density- independent mortality and the extent of spring colonization.
The initial assumption that gerrid guild structure was unlikely to be influenced by "diffuse competition1' (MacArthur,
1972a) needs to be re-examined because it is now clear that
other surface-feeding invertebrates may affect the distribution and relative abundance of water-strider species. Further study of gerrid coexistence must consider the entire assemblage of
surface-feeding predators. Field experiments are possible and,
if successfully coupled to solid models of gerrid population
growth and habitat occupancy, a fascinating "bird's nest" of
ecological relationships is available for study. 285
The dynamic inter-play of processes that control gerrid distribution and abundance will not be successfully captured by simple, deterministic models. Stochastic.variation in yearly climate drives gerrid population growth through direct effects on dispersal, reproduction and development. Weather patterns also orchestrate the development of aquatic vegetation and patterns of gerrid food availability. These in turn,, exert profound effects on species growth and survival. Under such conditions it is doubtful that water-strider populations are freguently in eguilibrium.
As Hutchinson (1953) first pointed out, non^eguilibrium explanations of coexistence based on environmental heterogeneity in space and time are often appealing for insect species.
However, the only reliable route for predicting the effects of environmental change for such species is setting detailed information about the mechanism of natural processes against a background of relevant natural history. 286
LITERATURE CITED
Allan, J. D. 1975. The distributional ecology and diversity of benthic insects in Cement Creek, Colorado. Ecology 56: 1040-1053.
Andersen, J. 1969. Habitat choice and life history of Bembidiini (Col., Carabidae) on river banks in central and northern Norway. Norsk, ent. Tidsskr. 17: 17-65. Andersen, N. Moller 1976. A comparative study of locomotion on the water surface in semiaquatic bugs (Insecta, Hemiptera, Gerromorpha). Vidensk. Meddr. Dansk. Naturn. Foren. 139: 337-3 96.
Andersen, N. Moller 1975. The Limnogonus and Neogerris of the old world with character analysis and a reclassification of the Gerrinae (Hemiptera: Gerridae) . Entomologica Scandanavica 8: supplement #7, 96 pp.
Andersen, N. Moller. 1973. Seasonal polymorphism and developmental changes in organs of flight and reproduction in bivoltine pondskaters (Hemiptera: Gerridae). Ent. Scand. 4: 1-20.
Andersen, N. Moller and J.T. Polhemus. 1976. Water-striders (Hemiptera: Gerridae, Veliidae, etc.) In L. Cheng (ed.). Marine Insects. Elsevier, Amsterdam pp 187-224.
Anderson, J. M. 1978. Inter- and intra-habitat relationships between woodland Cryptostigmata species diversity and the diversity of soil and litter microhabitats. Oecologia (Berl.) 32: 341-348.
Andrewartha, H. G. 1961. Introduction to the study of animal populations. Univ. of Chicago Press, Chicago, Illinois. pp. 187-192.
Ayala, F. 1970. Competition, coexistence and evolution. In M. K. Hecht and W. C. Steere (eds.). Essays in Evolution and Genetics in Honour of Theodqsius Dobzhansky ... A Supplement to Evolutionary Biology, Appleton-Century- Crofts, New York. pp. 121-158.
Baker, C, R. B. 1971. Egg and pupal development of Spilosoma lubricipeda in controlled temperatures. Ent. exp. & appl. 14:^15-22. ~
Baker, C. R. B. and G. W. Miller. 1978., The effect of temperature on the postdiapause development of four geographical populations of the european cherry fruit fly (Rhagoletis cerasi). Ent. exp. & appl. 23: 1-13. 287
Beil, C. E. 1970. The plant associations of the Cariboo^- aspen-lodgepole pine-Douglas-fir Parkland Zone. , Ph.D. Thesis, Department of Botany, Univ of British Columbia. 342 pp.
Blatchley, W. S. 1926. Heteroptera or true Bugs of eastern North America. The Nature Publishing Company. Indianapolis, Indiana. pp. 967-987.
Boiler, E. F. and G. L. Bush. 1974. Evidence for genetic variation in populations of the European Cherry Fruit Fly, Rhaqoletis cerasi (Diptera, Tephritidae) based on physiological parameters and hybridization experiments. Ent. exp. & appl. 17: 279-293.
Bpwdan, E. 1978. Walking and rowing in the waterstrider, Gerris remigis. 1. Cinematographic analysis of walking, J. Comp. Physiol. A. 123-143. Bowdan, E.. 1976. The functional anatomy of the mesothoracic leg of the waterstrider, Gerris remigis Say (Heteroptera). Psyche 83: 289-303.
Brinkhurst, R. 0. 1966. Population dynamics of the large pond-skater Gerris najas DeGeer (Hemiptera-r Heteroptera).. J. Anim. Ecol. 35: 13-25.
Brinkhurst, R. 0. 1965. Studies on the population dynamics of the water-strider Gerris na-jas DeGeer, Mitt. Internat. Verein. Limnol. 13: 36-39.
Brinkhurst, R- 0. 1960. Studies on the functional morphology °f Gerris na-jas Degeer (Hem. Het. Gerridae) . Proc. Zool. Soc. London, 133: 531-559.'
Brinkhurst, R. 0. 1959a. Alary polymorphism in the Gerroidea (Hemiptera-Heteroptera). J. Anim. Ecol. 28: 211-230.
Brinkhurst, R. 0. 1959b. The habitats and distribution of British Gerris and Velia species. J. Soc. Brit. Ent. 6: 37-44.
Brinkhurst, R. 0. 1956. Some Gerroidea taken in hibernation. Ent. mon. Mag. 92:374.
Bronowski, J. 1973. The Ascent of Man. Little, Brown and Co., Boston. 448 pp.
Brooks, A. R. and L. A. Kelton. 1967. Aguatic, and semiaguatic heteroptera of Alberta, Saskatchewan, and Manitoba (Hemiptera) . Mem, ent. Soc. Canada, No. 51, 92 pp. 288
Calabrese, D. M. 1977. The habitats of Gerris F. (Hemiptera: Heteroptera: Gerridae) in Connecticut. Ann, ent. Soc. Amer. 70: 977-983.
Calabrese, D. M. 1974. Keys to adults and nymphs of the species of gerris Fabricius occurring in Connecticut. Mem. Conn. Ent. Soc. p 227-266.
Campbell, A., B. D. . Frazer, N. Gilbert, A. P. Gutierrez, M. MacKauer. 1974. Temperature requirements of some aphids and their parasites. J. .A£PjL. Ecol. 11: 431-438.
Cannings, R. A. 1973. An ecological study of some of the Chirpnomidae inhabiting a series of saline lakes in central British Columbia with special reference to Chironomus tentans Fabricius. M.Sc. Thesis, Dept. Of Zoology, Univ."" of British Columbia, Vancouver. 142 pp.
Cannings, R. A. and G. G. E. Scudder, 1978. The littoral Chironomidae (Diptera) of saline lakes in central British Columbia. Can. J. Zool. 56: 1144-1155,
Caponigro, M. A. and C. H. Eriksen.. 1976. Surface film locomotion by the water-strider, Gerris remigis Say. Amer. Midi. Nat. 95: 268-278.
Caughley, G., R. Sinclair and D. Scott-Kemmis. 1976,. Experiments in aerial survey. J. Wildl. Manage. 40: 290-300. -
Cheng, L. and C. H. Fernando. 1970. The water-striders of Ontario (Heteroptera: Gerridae). Life Sci. Misc. Publ., R. Ont. Mus. 23 pp. Cody, M.L. 1974. Competition and the structure of bird communities. Princeton Univ. Press, Princeton, N.J. 318 pp.
Cody, M. L. and J. M. Diamond (eds.). 1975. Ecology and Evolution of Communities. Belknap Press of Harvard Univ., Cambridge, Mass. 549 pp.
Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science 199: 1302-1309.
Connell, J. H. 1975.. Some mechanisms producing structure in natural communities: a model and evidence from field experiments. In M. L. Cody and J. M. Diamond (eds,), Ecology and Evolution of Communities, Belknap Press of Harvard University, Cambridge, Mass, pp, .460-490.
Connell, J. H. 1961. The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus Stellatus. Ecology 42: 710~723. 289
Cummins, K. W. and G. H. Lauff. 1969. The influence of substrate particles on the microdistribution of stream macrobenthos. Hydrobiolqgica 34: 145-181.
Darwin, C. R. 1859. On the origin of species by means of natural selection. John Murray, London (reprinted by Modern Library, Random House, N. Y.). pp. 60-62,
Deay, H. 0. and G. E. Gould. 1936. The, Hemiptera of Indiana, I. Family Gerridae. Amer. Midi. Hat. 17: 753-769.
Diamond, J. 1978. Niche shifts and the rediscovery of interspecific competition. Amer, Scientist 66: 322-331.
Dodson, V. E. . 1975. The distribution and habitat separation of three corixids (Hemiptera: Heteroptera) in western Colorado. Freshwater Biology 5: 141-150.
Douglas, J. W. 1882. Note on Gerris lacustris hibernating far from water. Entomol. mon. Mag. 19:20.
Drake, C. J., and H. H. Harris. 1934. The Gerrinae of the Western Hemisphere (Hemiptera) . Ann, Carnegie Museum vol. XXXIII : 179-240.
Elsasser, W. M. 1975. The Chief Abstractions of Biology. North-Holland Publ. Co., Amsterdam. 251 pp.
Fernando, C. H. and D. Galbraith. 19 70. A heavy infestation of gerrids (Hemiptera-Heteroptera) by water-mites (Acarina-Limnocharidae). Can. Ji Zool. 48: 592-594.
Fleetwood, S. C., C. D. Steelman and P. E. Schilling. 1978. The association of certain permanent and floodwater mosquitoes with selected plant species in Louisiana coastal marshes. Envir. Ent. 7: 300-305.
Fox, L. R. 1975a. Cannibalism in natural populations. Ann. Rev. Ecol. Syst. 6: 87-106.
Fox, L. R. 1975b. Factors influencing cannibalism, a mechanism of population limitation in the predator Notonecta hoffmanni. Ecology 56: 933-942.
Frazer, B. D. and N. Gilbert. 1976. Coccinellids and aphids. J. entomol. Soc. Brit.. Col. 73: 33-56
Galbraith, D. F. and C. H. Fernando. 1977. The life history °f Gerris remigis (Heteroptera: Gerridae) in a small stream in southern Ontario. Can. Ent. 109: 221-228.
Gale, W. F. 1971. An experiment to determine substrate preference of the fingernail clam, Sphaerium transversum (Say). Ecology 52: 367-370. 290
Gilbert, N. 1973. Biometrical Interpretation. Clarendon Press, Oxford. 125 pp.
Gilbert, N., A. P. Gutierrez, B. D. Frazer and E. E. Jones. 1976. Ecological Relationships.. W.H. Freeman and Co., San Francisco, Ca. 157 pp. Greenfield, M. D. and M. G. Karandinos. 1976. Fecundity and longevity of Synathedon pictipes under constant and fluctuating temperatures. Environ. Ent. 5: 883-887.
Greenguist, E. A. and J. S. Rovner. 1976. Lycosid spiders on artificial foilage: stratum choice, orientation preferences, and prey-wrapping. Psyche 83: 197-210.
Gordon, B. R. and H. T. Gordon. 1971. Sperm storage and depletion in the spermatheca of Oncopeltus fasciatus. Ent. exp. S appl. 14: 42 5-43 3. '
Gorman, 0. T. and J. R. Karr. 1978. Habitat structure and stream fish communities. Ecology 59: 507-515.
Gould, S. J. 1978. Senseless signs of history. Natural History 87 (10): 22-28.
Grant, P. R. 1975. Population performance of Mic£otus pennsylvanicus confined to woodland habitat, and a model of habitat^occupancy. Can. J. Zool. 53: 1447-1465.
Hairston, N. G., B. Hubendick, J. M. Watson and L. J. Oliver. 1958. An evaluation of techniques used in estimating snail populations. Bull. Wld. Hlth. Org. 19: 661-672.
Harries, F. H. 1939. Some temperature coefficients for insect oviposition. Ann. ent. Soc. Amer. 32: 758-776.
Heck, K. L. Jr. 1976. Some critical considerations of the theory of species packing. Evol. Theory 1: 247-258.
Higler, L. W. G. 1975. Reactions of some caddis larvae (Trichoptera) to different types of substrate in an experimental stream. Freshwater Biology 5: 151-158-
Hungerford, H. B. 1919. The biology and ecology of aquatic and semiaquatic Hemiptera. Kans. Univ. Sci. Bull. II: 106-121
Hutchinson, G. E. 1978. An Introduction to Population Ecoloqy. Yale Univ. Press, New Haven, Conn. 260 pp,
Hutchinson, G. E. 1975. Variations on a theme by Robert MacArthur. In M. L. Cody and J. M. Diamond, (eds.), Ecoloqy and Evolution of Communities, Belknap Press of Harvard University, Cambridge, Mass, pp 492-521. 291
Hutchinson, G. E. 1965. The Ecological Theater and the Evolutionary Play. Yale Univ. Press., New Haven, Conn. 139 pp.
Hutchinson, G. E. 1959. Homage to Santa Rosalia or why are there so many kinds of animals? Amer. Nat. 93: 145-159.
Hutchinson, G, E. 1957. Concluding remarks. Cold Spring Harbor Symp. Quant. Biol. 22: 415-427.
Hutchinson, G. E. 1953. The concept of pattern in ecology. Proc. Acad. Nat. Sci. Philadelphia 105: 1-12.
Istock, C. A. 1977. Logistic interaction of natural populations of two species of waterboatmen,. Amer. Nat. 111: 279-287. Istock, C.A. 1973. Population characteristics of a species ensemble of waterboatmen (Corixidae). Ecology 54: 535- 544.
Istock, C. A. 1967. Transient competitive displacement in natural populations of whirligig beetles. Ecology 48: 9 29-93 7.
Istock, C. A. 1966. Distributions, coexistence and competition in whirligig beetles. Evol. 20: 211-234.
Jamieson, G. S. 1973. Coexistence in the Gerridae. Unpubl. Ph.D. Thesis, Dept. Of Zoology, University of British Columbia, Vancouver, Canada. 255 pp.
Janzen, D. 1977. Why are there so many species of insects? Proc. XV Int. Cong. En to mol. , Washington, D. C. pp.84-: 94. ~ ' Jarvinen, 0. 1976. Migration, extinction, and alary morphism in water-striders (Gerris Fabr.). Ann. Acad. Sci. fenn. A, IV Biologica 206: 1-7. I
Jarvinen, 0., M. Nummelin and K. Vepsalainen. 1977. A method for estimating population densities of waterstriders (Gerris). Notulae Entomologicae 57: 25-28.
Jarvinen, 0. and K. Vepsalainen. 1976 Wing dimorphism as an adaptive strategy in water-striders (Gerris). Hereditas 84: 61-68.
Johnson C. G. 1969. Migration and dispersal of insects by flight. Methuen, London. 763 pp.
Kaufmann, T. 1971. Ecology, biology, and gonad morphology of Gerris rufoscutellatus (Hemiptera: Gerridae) in Fairbanks, Alaska. Amer.' Mid. Nat. 86: 407-416. 292
Keast, A. and D. Webb. 1966. Mouth and body form relative to feeding ecology in the fish fauna of a small lake, Lake Opinicon, Ontario. J. Fish. Res. Bd. Can. 23: 1845- 1874.
Klopfer, P. H. 1969.. Habitats and Territorities .... A Study of the Use of Space by Animals. Basic Books, Inc., New York. 117 pp.
Kulesza, G. 1975. Comment on "niche, habitat and ecotope" Amer. Nat. 109: 476-479.
Labeyrie, V. 1978. The significance of the environment in the control of insect fecundity. Ann. Rev. Ent. 23: 69- 90. Landin, J. 1976. Methods of sampling aguatic beetles in the transitional habitats at water margins. Freshwater Biology 6: 81-87.
Landin, J. and K. Vepsalainen. 1977.. Spring dispersal flights of pond-skaters Gerris spp. (Heteroptera). Oikos 29: 156-160.
Lawry, J. V. Jr. 1973. A scanning electron microscopic study of mechanoreceptors in the walking legs of the water- strider, Gerris remigis. J. Anat. 116: 25-30.
Leston, D. 1956. The status of the pondskater Limnoporus rufoscutellatus in Britain. Entompl. mon. Mag. 9_: 189- 193. Levins, R. 1968. Evolution in Changing Environments. Princeton Univ. Press, Princeton, N.J. 120 pp.
Lipa, J. J. 1968. Some observations on flagellate parasites of hemipterans Corimelaena, Eusehistus, Gerris, Leptocpris, and Oncopeltus in the United States. Acta Protozoal. 6: 59-68.
Lock, M. A. 1975. An experimental study of the role of gradient and substratum in the distribution of two stream-dwelling triclads, Crenobia alpina (Dana) and Polycelis felina in North Wales. Freshwater Biology 5: 211-226.
Lumsden, W. H. R. 1949. A note on the ecology of Gerris najas. Entomol. mon. Mag. 85: 169-173.
Lynch, M. 1978. Complex interactions between natural coexploiters - Daphnia and Ceriodaphnia. Ecology 59: 552-564. 293
MacArthur, E. H. 1972a. Geographical ecology, patterns in the distribution of species. Harper and Row, Publ., N.Y. 269 pp. -
MacArthur, R. H. 1972b. Coexistence of species. In. J. A. Behenke (ed.), Challenging Biological Problems, Oxford University Press, New York. pp 253-259.
MacArthur, R. H. 1968. The theory of the niche. In,. E.C. Lewontin (ed.). Population Biology,and Evolution, Syracuse University Press, Syracuse, New York. pp. 159- 176.
MacArthur, E. H. 1965. Patterns of species diversity. Biol. Rev. 40: 510-533.
MacArthur, R. H. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39: 599- 619.
MacArthur, R. H. and R. Levins. 1964. Competition, habitat selection, and character displacement in a patchy environment. Proc. Nat. Acad. Sci. U.S.A. 51: 1207- 1210.
MacArthur, R. H. and J. w. MacArthur. 1961. On bird species diversity. Ecology 42: 594-598.
McAtee, W. L. 1918. Food habits of the Mallard ducks of the United States. U. S. Dept. Agric. Bull. ?20, p 27.
McClure, M. S. and P. W. Price. 1975. Competition among sympatric Erythroneura leafhoppers (Homoptera: Cicadellidae). Ecology 56: 1388-13 97.
Mabbot, D. C. 1920. Food habits of seven species of American shoal-water ducks. U. S. Dept. Agric. Bull. 862, pp 47- 59.
Macan, T. T. and A. Kitching. 1976. The colonization of sguares of plastic suspended in midwater. Freshwater Biology 6: 33-40. Macan, T. T. and A. Kitching. 1972. Some experiments with artificial substrata. Verh. Internat. Verein. Limnol. 18: 213-220. i Madsen, B. L. 1968. A comparative ecological investigation of two related mayfly nymphs. Hydrobiologica 31:, 337- 349. T Matheson, R. and C. R. Crosby. 1912. Aguatic Hymenoptera in America. Ann. ent. Soc. Amer. 5: 65-70, 294
Matsuda, 8- 196 1. Studies of relative growth in Gerridae (Hemiptera: Heteroptera). J. Kansas ent. Soc. 34: 5-17.
Matsuda, R. 1960. Morphology, evolution and classification of the Gerridae (Hemiptera - Heteroptera). Kans. Univ. Sci. Bull., 41: 25-632.
Matthey, W. 1976. Observations sur l'ecologie de Gerris remigis Say (Heteroptera): duree du developpment larvaire et colonisation de differents types d'etangs dans les Montagnes Bocheuses Canadiennes. Revue Suisse Zool. 83: 405-412. ~
Matthey, W. 1975. Observations sur la reproduction de Gerris remigis Say. (Hemiptera: heteroptera). Bull. Soc. Entomol. Suisse 48: 193-198.
Matthey, W. 1974. Contribution a l'ecologie de Gerris remigis Say sur deux etangs des Montagnes Rpcheuses. Bull. Soc. Ent. Suisse 47: 8 5-95. May, R. M. 1973. Stability and Complexity in Model Ecosystems. Princeton Univ. Press, Princeton, N.J. 235 PP. Maynard, K. J. 1969. A population study of waterstriders (Gerridae: Hemiptera) in Marion Lake, B.C. M.Sc. Thesis^ Dept. Of Zoology, University of British Columbia, Vancouver.
Meadows, P. S. and J. I. Campbell. 1972. Habitat selection by aquatic invertebrates. Advances in Marine Biology 10: 271-383. '
Menge, B. A. and J. P. Sutherland. 1976. Species diversity gradients: synthesis of the roles of predation, competition and temporal heterogeneity. Amer. Nat. 110: 351-369.
Miller, R. S. 1964. Ecology and distribution of pocket gophers (Geomyidae) in Colorado. Ecology 45: 256-272.
Milne, A. 1961. Definition of competition among animals. In F.L. Milthorpe (ed.), Mechanisms in Biological Competition, Symp. Soc. Exp. Biol. 15. Cambridge, Cambridge Univ. Press. pp. 40-61. ,
Milne, L. J. and M. Milne. 1978. Insects on the water surface. Sci. Amer. 238 (4): 134-143.
Munro, J. A. 1945. The birds of the Cariboo parklands, British Columbia. Can. J. Res., Sect. D. 23: 17-103. 295
Murdoch, W. W.,* F, C. Evans and C. H. Peterson. '1972. Diversity and pattern in plants and insects. Ecology 53: 819-829.
Murphey, R. K. 1971a. Motor control of orientation to prey by the waterstrider, Gerris remigis. Z. Vergl. Physiol. 72: 150-167. Murphey, R. K. 1971b. Sensory aspects of the control of orientation to prey by the waterstrider, Gerris remigis. Z. Vergl. Physiol. 7 2: 16 8-185.
Neill, W. E. 1975. Experimental studies of microcrustacean competition, community composition and efficiency of resource utilization. Ecology 56: 809-826.
Neill, W. E. 1974. The community matrix and interdependence of the competition coefficients. Amer. Nat. 108: 399- 4 08.
Orians, G. H. 1966. Food of nestling yellow-headed blackbirds, Cariboo Parklands, British Columbia. Condor 68: 321-337. Orians, G. H. and 0. T. Solbrig (eds.). 1977. Convergent evolution in warm deserts. (US-IBP Synthesis Ser.: No 3). Academic Press, New York. '
Paine, R. T. 1966. Food web complexity and species diversity. Amer. Nat. 100: 65-75.
Pianka, E. R. 1976. Competition and niche theory. in R. M. May (ed.), Theoretical Ecology, principles and applications. Blackwell Scientific Publications, Oxford. pp. 114-141.
Pianka, E. R. 1 967. On lizard species diversity: North American flatland deserts. Ecology 48: 333-351.
Pianka, E. R. 1966. Latitudinal gradients in species diversity: a review of concepts. Amer. Nat. 100: 33-46.
Pielou, E. C. 1975. Ecological Diversity. John Wiley and Sons, N.Y., 165 pp.
Price, P. W. 1975. Insect Ecology. John Wiley and Sons, New York. 514 pp.
Pulliam, H. R. 1975. Coexistence of sparrows: a test of community theory. Science 189:474-476.
Rejmanek, M. and J. Jenik. 1975. Niche, habitat, and related ecological concepts. Acta Biotheoretica 24: 100-107. 296
Reynolds, J. D. 1974* Aspects of the ecology of two species of Cenocorixa (Corixidae: Hemiptera) in allopatry and sympatry. Ph.D. Thesis, Dept. Of Zoology, Univ. of British Columbia. 280 pp.
Reynolds, J. D. and S. C. P. Reynolds. 1975. Aquatic angiosperms of some British Columbia saline lakes. Syesis 8: 29 1-295.
Riley, C. F. C. 1922. Drought and cannibalistic responses of the water-strider, Gerris marginatus Say. Bull. Brooklyn ent. Soc. 17: 7 9-87.
Schoener, T. W. 1974a. Resource partitioning in ecological communities. Science 185: 27-39,.
Schoener, T. W. 1974b. Competition and the form of habitat shift. Theor. Pop. Biol. 6: 265-307.
Scudder, G. G. E. 1977. An annotated checklist of the aguatic Hemiptera (Insecta) of British Columbia. Syesis 10: 31-38. Scudder, G. G. E. 1975. Field studies on the flight muscle polymorphism in Cenocorixa (Hemiptera: Corixidae). Verh. Internat. VereinT Limnol. 19: 3064-3072.
Scudder, G. G. E. 1971. The Gerridae (Hemiptera) of British Columbia. J. ent. Soc. Brit. Columbia 68: 3-10.
Scudder, G. G. E. 1969. The fauna of saline lakes on the Fraser Plateau in British Columbia. Verh, Internat. Verein. Limnol. 17: 430-439.
Scudder, G. G. E. and G. S. Jamieson. 1972. The immature stages of Gerris (Hemiptera) in British Columbia. , J. ent. Soc. Brit. Columbia 69: 72-7 9.
Shapiro, A. M. 1976. Seasonal polyphenism. Evol. Biol. 9: 259-334.
Smith, B. P. 1977. Water-mite parasitism of waterboatmen (Hemiptera: Corixide). Unpubl. M.Sc, Thesis, Dept. of Zoology, Univ. of British Columbia, Vancouver, Canada. 117 pp.
Southwood, T. R. E. 1977. Habitat, the templet for ecological strategies? J. Anim. Ecol. 46: 337-365.
Southwood, T. R. E. 1966. Ecological methods with particular reference to the study of insect populations. Chapman and Hall, London. 391 pp. 297
Spence, J. R. 1978. Riparian carabid guilds - a spontaneous question generator. In. Erwin, T. L., G. E. Ball, D- R. Whitehead and A. L. Halpern, (eds.), Carabid Beetles: Their Evolution, Natural History and Classification (Proc. 1st Int. Symp. CarabidologySmithsonian Inst. , Washington, D. C, August 21, 23 and 25, 1976), W. Junk b.v., Publishers, The Hague.
Spence, J. R. and G. G. E. Scudder. 1978. Larval taxonomy and distribution of Gerris pinqreensis and G. incognitus (Hemiptera: Gerridae) in British Columbia. J. ent. Soc. Brit. Columbia (in press).
Spence, J. R., D. Hughes Spence and G. G. E. Scudder. 1978. Submergence behaviour in Gerris: underwater basking as an adaptive tactic. Submitted manuscript. Streams, F. A. and S. Newfield. 1972. Spatial and temporal overlap among breeding populations of New England Not one eta. 0 niy. Conn. Occas. Pap.. Biol. Sci. Se r. 2: 139-157.
Strong, F. E. and J. A. Sheldahl. 1970. The influence of temperature on longevity and fecundity in the bug Lygus hesperus (Hemiptera: Miridae). 'Ann, ent. Soc. Amer. 63: 1509-1515. ~'
Taylor, 0. R. Jr. 1968. Coexistence and competitive interactions in fall and winter populations of six sympatric Notonecta (Hemiptera, Notonectidae) in New England. Uniy. Conn. Occas. Papers (Biol. Sci. Series) 1: 109-139.
Tomoff, C. S. 1974. Avian species diversity in desert scrub. Ecology 55: 396-403.
Topping, M. S. and A. B. Acton, 1976. Influence of environmental factors on freguencies of certain chromosomal inversions in Chironomus tentans Fabr. Eyol. 30: 170-179. " \
Topping, M.S. and G. G. E. scudder. 1977- Some physical and chemical features of saline lakes in central British Columbia. Syesis 10: 145-166.
Trimble, R. M. and S. M. Smith. 1978. Geographic variation in development time and predation in the tree-hole mosguito, ToxorhYnchites rutilus septentrionalis (Diptera, Culicidae) . Can. J. Zool. 56: 2156-2165.
Dtez, G. 1974. Species diversity: a review. The Biologist 56: 111-129. 298
Vanderlin, R. L. and F. A. Streams. 1977. Photoperiodic control of reproductive diapause in Notonecta undulata. Envir. Ent. 6: 258-262.
Vepsalainen, K. 1978. Wing dimorphism and diapause in Gerris: determination and adaptive significance. In. H. Dingle (ed.). Evolution of Insect Migration and Diapause, Springer-Verlag, New York. pp. 218-253.
Vepsalainen, K. and 0. Jarvinen. 1974. Habitat utilization of Gerris argent a tus (Het. Gerridae) . Ent. scand. 5: 189^195.
Vepsalainen, K. 1974a. The life cycles and wing lengths of Finnish Gerris Fabr. Species (Heteroptera, Gerridae). Acta zool. Fenn. 141:1-73.
Vepsalainen, K. 1974b. The wing lengths, reproductive stages and habitats of Hungarian Gerris Fabr. species (Heteroptera, Gerridae). Ann. Acad. Sci. Fenn. A, IV Bioloqica 20 2: 1-18.
Vepsalainen, K. 1974c. Determination of wing length and diapause in water-striders (Gerris Fabr., Heteroptera). Heriditas 77: 163-176. -
Vepsalainen, K. 1973a. Development rates of some Finnish Gerris Fabr. species in laboratory cultures.. Ent. scand. 3: 206-216.
Vepsalainen, K. 1973b. The distribution and habitats of Gerris Fabr. Species (Heteroptera, Gerridae) in Finland. Ann. Zool. Fennici 10: 419-444,
Vepsalainen, K. 1971. The role of gradually changing daylength in determination of wing length, alary polymorphism and diapause in a Gerris odontogaster (Zett.) population (Gerridae, Heteroptera) in South Finland. Ann. Acad. Sci. Fenn. A. IV Bioloqica 183: 1- 25.
Vuilleumier, F. 1972. Bird species diversity in Pataqonia (Temperate South America). Amer. Nat. 106: 266-271-
Werner, E. E.. 1977. Species packing and niche complementarity in three sunfishes. Amer. Nat. 111: 533-578. '
Werner, E. E. 1977. Competition and habitat shift in two sunfishes (Centrachidae). Ecology 58: 869-876.
Werner, E. E. and D. J. Hall. 1976. Niche shifts in sunfishes: experimental evidence and significance. Science 191: 404-406. 299
Whittaker, R. H. and S. A. Levin.. 1977. Role of mosaic phenomena in natural communities. Theor. Pop. Biol. 12: 117-139. I-. •• "
Whittaker, R. and S. A. Levin (eds.). 1975. Niche. Theory and Application. Dowden, Hutchinson and Ross, Inc., Stroudsburg, Pa. 448 pp.
Whittaker, R. H., S. A. Levin and R. B. Root. 1975. On the reasons for distinguishing "niche, habitat and ecotope". Amer. Nat. 109: 479-481.
Whittaker, R. H., S. A. Levin, and R. B. Root. 1973. Niche, habitat and ecotype. Amer. Nat. 107: 321-338.
Wiens, J. A. 1977. Competition in variable environments. Amer. Sci. 65: 590-597.
Wiens, J. A. 1976. Population responses to patchy environments, Ann. Rev. Ecol. Syst. 7: 81-120.
Wilbur, H. 1972. Competition, predation, and the structure of the Ambystoma - Rana sylvatica community. Ecology - 53: 3-21.
Wilcox, R. S. 1 972. Communication by surface waves: mating behaviour of a water strider (Gerridae). J. Comp. Physiol. 80: 255-266.
Zimmerman, J. R. 1960. Seasonal population changes and habitat preferences in the genus Laccophilus (Coleoptera: Dytiscidae) . Ecology 41: 141-152.' APPENDIX I
LAKES STUDIED AND BREEDING GERRID SPECIES 1975-1977 301
A. BECHER'S PRAIRIE STUDY LAKES
LAKE AREA CONDUCTIVITY GERRID SPP.\ NO. LAKE NAME (hectares) RANGE (x_nhos/cm) BREEDING , — 1975-1977 Min. Max. b c p ig L
1 Akhurst Lake 3.89 3472 3683 x X 2 "Akhurst pothole"2 3 0.28 390 X X 3 "Almost Pond" 0.5 1 140 268 X X 4* Barkely Lake "E" 5.81 845 942 X X 5* Barkely Lake "W" 0.51 576 892 X X X X 6* Barnes Lake 17.19 9783 15434 X 7* Blake Lake 3.60 2610 3623 X X 8* "Box 17" 2.65 922 1070 X X X 9* "Box 27" 4.30 26 47 X X 10 "Box 204" 1. 18 208 X X 11 "Box 206" 1.99 1810 X X 12 "Buxton Hill Pond" 2.01 797 874 X X X 13* "Centre Arms Pond" 0.59 230 247 X X X X 14* Clear Lake 2.83 249 304 X X 15 "CMR 1" 0.16 84 103 X X 16 "CMR 2" 0.19 42 140 X X X 17 "CMR 3" 0.94 652 X X X 18 "CMR 4" 0.47 2681 X X X 19 "CMR 5" 3.54 833 X X X 20 "CMR 6" 3.42 1286 X X 21 "CMR 7" 1.06 688 X X X 22 "CMR 8" 1.06 659 748 X X X 23* "Crescent" 0.05 313 376 X X X 24* Drummond Lake 4748 6621 25* East Lake 27.05 952 X X 26* "Gerrid City" 0.60 627 976 X X 27 "Jug Lake" 4.05 1479 X X 28* Lake Greer 15. 17 184 6 3288 X X X 29* Lake Jackson 4.55 4070 4779 X X 30* Lake Lye 46.52 6574 8905 X X 31* Lake Wilkes 8.52 1191 1529 X X 32 "Lonetree Pond" 1.04 1123 X X 33 Long Lake 20.66 6884 8064 X X X 34 Mclntyre Lake 21.12 449 X X
1 Determined by the presence of juvenile stages of the species in question in at least one sample taken during 1975-1977. 2 Lake names in parentheses are unofficial, descriptive names. 3 Underlined lakes are extremely temporary. Each one dried up completely at least once between 1975 and 1977. * Lakes indicated sampled intensively during 1975 (Chapter IV), 302
LAKE AREA CONDUCTIVITY GERRID SPP. 1 NO. LAKE NAME (hectares) RANGE (umhos/cm) BREEDING — — 1975-1977 Min. Max. b c p ig L
35 Moon's Lake 12.87 346 X X 36 "Mud Pond" 1.55 797 843 X X X 37 "Near Akhurst Lake" 2.71 1062 1696 X X 38 "Near Barnes Pond" 0.77 133 359 X X X 39* "Near Blake Pond" 0.64 1025 1180 X X X 40 "Near Box 204" 0.71 540 X X X 41 "Near Box 206" 0.05 X 42 "Near East Pond" 1.17 748 X X 43 "Near Greer Pond" 2.22 2142 X X X 44 "Near Lonetree Pond" 0.65 330 X X 45 "Near Long Pond" 0.60 159 X 46 "Near Lye Pond" 0.51 362 515 X X 47 "Near Mud Pond A" 2.00 1775 2033 X X 48 "Near Mud Pond B" 0.08 910 942 X X 49 "Near Newall Pond" 1 .18 ' 110 159 X X 50* "Near Opposite Crescent" 6.88 1070 1361 X X 51 "Near Pothole Lake" 4. 37 368 7 4947 X X 52* "Near Rock Pond" 1.52 389 775 X X X 53* "Near Round-up Lake" 5.06 1093 1565 X X 54 "Near Sunset Pond" 0.09 1029 1055 X X 55 "Near Wilkes Pond A" 0.65 475 X 56 "Near Wilkes Pond B" 0.13 X X 57 "Near Wilkes Pond C" 1.17 1537 X X X 58* Newall Lake 1.76 132 145 X X 59* "Opposite Crescent Pond" 0.23 118 167 X X 60 "Opposite East Pond" 0.90 1338 X X X 61 "Opposite Pothole Pond" 0.77 730 X X 62* "Opposite Rock Pond A" 0.47 140 233 X X 63 "Opposite Rock Pond B" 0.23 168 X 64 "Opposite Round-up Pond" 0.33 1626 X X X 65 "Pine Pond" 0.39 X X X 66 "Pothole" 0.59 3684 X X X X 67 "Roadside Pond" 0.03 X 68 "Riske Road Pond" 1.07 1367 1520 X X X 69* Rock Lake 34.64 3433 2212 X X 70* Sapper Lake 3.41 1060 1242 X X X 71 "Sunset Pond" 0.47 193 6 2600 X X X X 72 "X28" 0.14 540 X X 73* Round--up Lake 30.84 7762 303
B. SPRINGHOUSE STUDY LAKES
LAKE AREA CONDUCTIVITY GERRID SPP.* NO. LAKE NAME (hectares) RANGE (-^mhos/cm) BREEDING 1975-1977 Min. Max. b c p L
1* Boitano Lake 80.68 5098 6730 X X 2 "Boitano Pond" 611 776 X X X 3* "Boitano West Slough" 0.06 1236 2113 X X 4 Colpitt Lake 17.70 692 755 X X x 5* "Grove Pond" 1 .33 532 866 X X 6* Hayfield Lake 7.55 350 951 X X X 7 "Herrick's Pond" 1.65 932 1259 X X X 8* "Kruger's Pond" 0.03 56 83 X X 9 "Lookout Pond 1" 511 X X 10 "Lookout Pond 2" X X X 11 "Lookout Pond 3" 943 1072 X X X X 12 "Lost Pond" 1.30 877 1039 X 13 "Near Reserve Lake" 1 .06 1257 1511 X X 14 "Near Sp 2 A" 1 .06 1140 1220 X X X 15 "Near Sp 2 B" 0.59 1163 1291 X X 16 "Near Sp 6" 0.47 153 X X 17 "Opposite Grove Pond" 0.83 1023 1262 X X 18* "Pintail Road Slough" 0.02 192 235 X X X 19* Reserve Lake 7.20 1769 3447 X X X 1 20 "Rider s Pond" 1. 18 145 225 X X X 21* Rush Lake 19.60 2967 8311 X X 22* "Sp 1" 2.36 ' 42 56 X X X 23* "Sp 2" 2.71 844 1464 X X X 24* "Sp 3" 0.09 454 1574 X 25* "Sp 4" 0.59 642 1783 X X X 26* "Sp 5" 0.86 407 563 X X 27* "Sp 6" 0.94 109 337 X X X X 28* "Sp 8" 1.06 460 816 X X X X 29 "Sp 9" 1.65 482 614 X X 30 Sorenson Lake 23. 30 1932 2440 X X 31 "Stopover Pond" 0.05 601 755 X X X 32* Warmspring Lake 5.78 580 1098 X X X 33* Westwick Lake 58.30 1598 2448 X X 34 "Westwick 4" 0.59 429 833 X 35 "Westwick 5" 0.47 538 954 X X 36 "Westwick 6" 0.71 943 1322 X X X 37 Willow Pond "N"* 4.72 633 2609 X X X 38 Willow Pond "S"* 1.30 621 1320 X X X 39 "Woodland Pond" 0.04 22 57 X X APPENDIX II
INST AR-SPECIFIC DEVELOPMENT TIMES FOR SIX WATER"STRIDER SPECIES AT VARIOUS LABORATORY TEMPERATURES 305
1. Gerris buenoi ======:— ~ — ~ — ^z ZZ — — — z STAGE TEMPERATURE 15° 18.5° 22° 26° ======:======Egg 30.7±0- 15 11.0±0.00 8.4±0.18 - n=2 0 n=12 n=74 1 8.6±0.22 5.0±0.15 4.7±0.14 3.2±0.07 n=16 n=22 n=49 n=61 2 4.1±0. 14 3.9±0.14 2.8±0.13 - n=20 n=38 n=40 | 3 — 4.9±0.17 4.5±0.21 3.5±0.14 n=20 n=30 n=28 4 — 6.6±0.22 6.5±0.41 4.9±0.23 n=16 n=16 n=10 5 12.0±0.58 7.2±0.48 5.3±0.25 - n=4 n=6 n=4 ======:======:
2. Gerris comatus ======:======zz:~— zzzzzz zz—~~: STAGE TEMPERATURE 15° 18.5° 22° 26° ======Egg 28.8±0.20 12.1±0. 13 10.4±0.15 8,.2±0. 16 n=62 n=84 n=105 n=89 1 7.7±0.12 5.5±0.17 4.6±0.12 3.3±6.10 n=15 n=19 n=54 n=38 2 5.2±0.22 4.5±0. 17 3.9±0.11 2.7±0. 12 n=15 n=15 n=40 n=33 3 7.3±0.19 5.0±0.14 4. 4±0.11 3.0±0.12 n=14 n=15 n = 27 n=25 4 13.3±0.29 7.0±0.29 6. 1±0.27 4.5+0.22 n=7 n=21 n=16 n=22 5 24.6±0.96 11 .9±0.52 8.9±0.35 6. 8±0.22 n=7 n=8 n=10 n=12 ======— ———————~— zz ~: ======: ======—— :=— ======: 306
3. Gerris incoqnitus ======:======TEMPER ATIIRE 18.5° 22° 2 6°
Eqq _ 8.2±0.13 - n=35 1 6.7±0.07 4.5±0.22 3.6±0.13 2.5±0.24 n=15 n=13 n=34 n=13 3.7±0. 12 2 - n=30 - 3:7±0.15 - • 3 n=29 - 5.1±0.25 - 4 n=23 8.3±0.34 - 5 ' - n = 10 ======:
4. Gerris pingreensis ======: STAGE TEMPERATURE 15° 18.5° 22° 2 6°
Eqq 24.0±0.34 10.7±0. 13 8.3±0.08 6.4+0.10 n=30 n=65 n=133 n=43 1 6.5±0.24 4.7±0. 11 4.3±0.09 3.2±0. 10 n=16 n=23 n=32 n=47 2 5.3±0.18 3. 9±0.06 3U±0.13 2.8±0.11 n=15 n=23 n=32 n=36 3 7.6±0.19 4.4±0. 12 3.6±0.12 3.1±0. 13 n=15 n=23 n=32 n=33 . 4 15.6±0.42 6.3±0. 15 5.2±0.13 3.9±0.19 n=7 n=20 n=28 n=19 5 24.0±2.30 12.3±0.54 8. 1±0.63 5.3±0.31 n=5 n=10 n=9 n=8 ——————~ ~ ~ ~— :======:======307
5. Limnoporus dissortis
TAGE TEMPERATURE 15° 18.5° 22° 26° ======Egg - - - — 1 7.5±0.23 5.1±0.14 3.5+0.17 2.9+0.80 n=17 n=6 n=15 n=14 2 - - - — 3 — - - - 4 — - - ' - 5 - - - : ======zz ~™ — — — ZZZ£ ™ — ======
>. Limnoporus notabilis : ======;TAGE TEMPERATURE 15° 18.5° 22° 26° : ======: ======Egg 18.8±0.08 11.0±0.47 n=19 - n=32 - 1 7.6+0.12 5.5±0.55 4.7±0.15 2.7±0.10 n=16 n=15 n=53 n=19 2 6.7±0.19 3.6±0.10 — n=14 - n=35 4 8.0±0.18 - 4.6±0.13 — n=13 n=33 4 13.2±0.64 6.5±0.24 — n=9 - n=28 5 - - 10,. 5±0.65 - APPENDIX III
LINEAR REGRESSION EQUATIONS USED TO ESTIMATE TEMPERATURE THRESHOLDS FOR DEVELOPMENT. Y = the reciprocal of instar duration in days
X = temperature in degrees centigrade ——^ —— — —- — ———-p———_
a. G. buenoi
INSTAR LINEAR REGRESSION EQUATION
Egg Y = - 0.152 + 0.012X F=624.2; df=1,98; p«. 001 , 1 Y = - 0.149 + 0.018X F=448. 7; df=1,162; p«.O01
2 Y = - 0.145 + 0.017X F=147.2; df=1,133; p«.001
3 Y = - 0. 046 + 0.013X F=46.68; df = 1, 82; p«. 001
4 Y = - 0.042 + 0.010X F=23.80; df=1,37; p«. 001
5 Y = - 0.176 + 0.014X F=68.9; df=1,12; p<<.001
b. G. comatus
INSTAR LINEAR REGRESSION EQUATION
Egg Y = - 0.679 + 0.008X F=1307; df=1,279; p«.001
1 Y = - 0. 143 + 0.017X F=253.9; DF=1,124; p«.001
2 Y = - .0102 + 0.018X F=102.8; df=1,101; p«.001
3 Y = - 0.145 + 0.018X F=154.6; df=1,79; p«. 001
4 Y = - 0.089 + 0.012X F=108.7; df=1,63; p«.001
5 Y = - 0.093 + 0.009X F=236. 1 ; df=1,35; p«. 001 310
c. G. incoqnitus
INSTAR LINEAR REGRESSION EQUATION
1 Y = - 0.222 + 0.025X F=156. 4; df=1,55; p«. 001 d. G. pinqreensis
INSTAR LINEAR REGRESSION EQUATION
Egg Y = - 0.095 + 0.010X F=1556. 6; df=1, 245; p«.001
1 Y = - 0.055 + 0.014X F=93.6; df=1,107; p«. 001
2 Y .= - 0.064 + 0.017X F=59.3; df=1,104; p«.0Q1
3 Y = - 0.126 + 0.018X F=120.4; df=1,101; p«.001
4 Y = - 0.146 + 0.016X F=187.2; df=1,72; p«.001
5 Y = - 0.125 + 0.012X F=96.9; df=1,32; p«.001 e. L. dissortis
INSTAR LINEAR REGRESSION EQUATION
1 Y = - 0.219 + 0.023X F = 141.4; df=1,50; p«.001 f• L- notabilis
INSTAR LINEAR REGRESSION EQUATION
1 Y = - 0.162 + 0.018X F = 223.5; df=1,95; p«. 001 311
APPENDIX IV
MAXIMUM WET WEIGHTS OF THE SIX DEVELOPMENTAL STAGES FOR FIVE WATER-STRIDER SPECIES 312
1. Gerris buenoi Kirkl.
INSTAR MEAN MAXIMUM WET S. E. N WEIGHT (mg)
1 0.30 0.006 10 2 0.73 0i031 10 3 1.81 01653 10 4 4.06 0.154 10 5 7.40 0.232 10 Adult Male 7.17 0. 251 10 Adult Female 10.33 0,327 10 ======
2. Gerris comatus D. & H.
INSTAR MEAN MAXIMUM WET S. E. N WEIGHT (mg) ======1 0. 32 0.009 10 2 0. 91 0.041 10 3 2. 46 0. 052 10 4 5.51 0.092 10 5 11.33 0.295 10 Adult Male 11.04 0.298 10 Adult Female 14.09 0.303 10 ======
3. Gerris incognitus D. & H. ======INSTAR MEAN MAXIMUM WET S.E. N WEIGHT (mg) ======— ======1 . 0.35 0.017 10 2 0.91 0.027 10 3 2.01 0.056 10 4 4.88 0.096 10 5 9.76 0.313 10 Adult Male 10.66 0. 172 10 Adult Female 15.48 0.402 10 313
4. Gerris pinqreensis D. & H.
INSTAR MEAN MAXIMUM WET S. E. N WEIGHT (mg) ======: 1 0.35 0.014 10 2 0.86 0.095 10 3 2.24 0.067 10 4 4. 83 0.107 10 5 10.21 0.154 10 Adult Male 11.64 0. 256 10 Adult Female 17.21 0.473 10 ======:zz— zz\ zz\ —— — = —r=~== = =:==~~======:
5. Limnoporus notabilis D. & H.
INSTAR MEAN MAXIMUM WET S.E. N. WEIGHT (mg)
1 0.59 0.020 10 2 1. 86 0.051 10 3 6.71 0.101 10 4 15.37 0.921 4 5 32. 87 2. 165 3 Adult Male 39.92 2. 925 7 Adult Female 48.00 1. 989 10