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Microhabitat selection and regional coexistence in water-striders (Heteropetra: Gerridae) Spence, John Richard 1979

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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 t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January, 1979 (c) John Richard Spence In present ing t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree a t the U n i v e r s i t y of B r i t i s h Co lumbia, I agree tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r re fe rence and s tudy. I f u r t h e r agree tha t permiss ion f o r ex tens ive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s en t a t i v e s . I t i s understood tha t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed wi thout my w r i t t e n pe rm iss i on . Department of Z o o l ° g y  The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P lace Vancouver, Canada V6T 1W5 D a t e January 30, 1979 i i ABSTRACT This study considers the natural history and ecology of water-strider species occurring on the Fraser Plateau of south-central B r i t i s h Columbia. The o v e r a l l aim was to assess the effects of s p a t i a l heterogeneity on factors c o n t r o l l i n g the d i s t r i b u t i o n and r e l a t i v e abundance of gerrid species. The relationships among temperature, population dynamics and habitat use were investigated. From a regional perspective, s p a t i a l 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 f e r t i l i t y are described for Se r r i s comatus and G. pinqreensis in the laboratory and G- incoqnitus in the f i e l d . Egg laying and juvenile growth are shown to be strongly temperature dependent in a l l species studied. Temperature thresholds for development d i f f e r , both among species, and often among stages of a p a r t i c u l a r species. Low thresholds recorded for G. pinqreensis can lead to s i g n i f i c a n t growth advantages for t h i s species during early spring. Instar differences seem to be adapted to seasonal temperature regimes experienced by gerrids. Gerris species and i n s t a r s showed d i s t i n c t optimum temperatures f o r s u r v i v a l . These optima vary with developmental thresholds. I t i s suggested that species may be best adapted for growth under d i f f e r e n t temperature regimes. A method was developed for estimating absolute d e n s i t i e s i n the f i e l d from r e l a t i v e abundance measures using l i n e a r regression techniques. Gerrid s i z e and presence or absence of vegetation markedly a f f e c t capture rates. No ef f e c t of species or type of emergent cover was demonstrated. A v a i l a b i l i t y for capture varies with leg^length i n G. buenoi and G. pinqreensis. This relationship i s used to estimate a v a i l a b i l i t y constants for other water-strider species. F i e l d surveys between 1975 and 1977 established that G. buenoi, G. comatus and G. pinqreensis were the most abundant water-strider species i n - the study area. Each of these was strongly associated with a single type of vegetation in the f i e l d ; G. buenoi with grass/sedge habitats, G. comatus with f l o a t i n g vegetation and G. pinqreensis with bulrush habitat. Limnoporus d i s s o r t i s and L. n o t a b i l i s were commonly encountered on small, temporary ponds, G. incognitus was f i r s t taken during 1976 i n the study area and small populations are confined to brushy, well-shaded habitats. G. buenoi, G. comatus and G. pinqreensis are a l l p o t e n t i a l l y b i v o l t i n e i n the study area; Limnoporus spp. are univoltine. Generation timing varies tremendously among lakes and periods of maximum abundance for each species are not separated i n time. Strong between-lake habitat associations i n the f i e l d result proximately from habitat f i d e l i t y at the time of spring colonization. The tendency of gerrids to overwinter near the iv mother pond and t r i a l and error habitat selection during spring dispersal enforce habitat f i d e l i t y during colonization. Species d i s t r i b u t i o n s within lakes are affected by habitat a v a i l a b i l i t y . Habitat preference experiments demonstrate that G. pinqreensis and G. comatus have active preferences for emergent cover and open habitats respectively. G. buenoi i s a habitat generalist but i t s d i s t r i b u t i o n 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 a l l species. F i f t h i nstar G. buenoi and G. comatus showed greatest weight s p e c i f i c differences i n foraging e f f i c i e n c y among l a t e instars may help produce the habitat associations observed for these two species. F i f t h instar G. pinqreensis showed poor s u r v i v a l when enclosed i n freshwater habitats, suggesting the hypothesis that i t s d i s t r i b u t i o n i s r e s t r i c t e d by the presence of surface-feeding predators other than water-striders. It i s suggested that competition for space, predation, density-independent mortality and colonization dynamics a l l i n t e r a c t on the template of s p a t i a l heterogeneity to produce regional patterns of d i s t r i b u t i o n and abundance. gains when confined in the i r c h a r a t e r i s t i c abitats. Habitat-V TABLE OF CONTENTS ABSTRACT . . . . i i TABLE OF CONTENTS ............. V LIST OF TABLES x i 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 d i s t r i b u t i o n s 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 • —• 3 1 METHODS AND MATERIALS ...... 33 A. Egg production 33 1. E f f e c t s of temperature 33 2. Mating behaviour and fecundity i n the laboratory 35 3. Fecundity i n the f i e l d ....................... 37 B. Growth and development 38 1. Development and temperature 38 v i 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 i n the laboratory . 48 3. Fecundity in the f i e l d 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 I I I . 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 v i i 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. F i e l d temperatures 109 2. Egg production and alary morphism 113 3. Population dynamics 113 4. Habitats ..... . .......... 115 RESULTS . 117 A. F i e l d 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. L i f e 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 i n B r i t i s h Columbia 156 b. Gerrids i n eastern and western North America 158 4. Conclusions .........160 v i i i CHAPTER V. EXPERIHENTAL ANALYSIS OF MICROHABITAT SELECTION IN WATER-STRIDERS 161 INTRODUCTION 161 MATERIALS AND METHODS . . . . . . . . . . . 1 6 3 A. Dispersal . . . Habitat s e l e c t i o n among lakes ... 163 B. Habitat selection within lakes 167 1. F i e l d d i s t r i b u t i o n s ......... — 167 a. Habitat differences among species ........... 167 b. Habitat differences within specie's.. 167 2. Laboratory experiments . . . Responses to a r t i f i c i a l habitat structure ...... 168 a. Laboratory conditions and apparatus ......... 168 b. Species tendencies to enter complex habitats 169 c. Selection of a r t i f i c i a l 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. F i e l d d i s t r i b u t i o n s . . 195 a. Habitat differences among species ........... 195 b. Habitat differences within species .......... 197 2. Laboratory experiments . . .Responses to a r t i f i c i a l 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 s e l e c t i o n 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. F i e l d surveys ................................ 231 2. Analysis .; .... 233 a. Number of species per lake .... 233 b. Population persistence ...................... 233 c. Population success 233 d. Gerrid species d i v e r s i t y vs. plant s t r u c t u r a l d i v e r s i t y 234 B. Habitats, growth and competition ',. •. 234 1. Effects on species growth and survival 234 a. Species growth 234 b. Food-fall 237 2. E f f e c t s 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 d i v e r s i t y vs. Plant s t r u c t u r a l d i v e r s i t y 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, d i v e r s i t y 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 d i s t r i b u t i o n s i n B r i t i s h 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 l a i d by female G. comatus and G. pinqreensis i n the laboratory .52 Table 6. Average c o e f f i c i e n t s of variation for daily batch size in Gerris 58 Table 7. Total (egg to adult) development times i n days at various temperatures 60 Table 8. Percent of t o t a l development time i n each stage at 22°C : 61 Table 9. X 2 values for differences between growth thresholds of d i f f e r e n t l a r v a l stages ,.. 67 Table 10. Comparison of f i r s t i nstar growth thresholds ... 68 Table 11. Intercepts and standard errors for a l l summer in s t a r s and spring adults 88 Table 12. Analysis of variance of regressions of capture e f f i c i e n c y for various developmental stages of Gerris ... 90 Table 13. Proportionality constants obtained for a l l summer instars and spring adults ........................ 91 Table 14. Proportionality constants and t h e i r standard errors for d i f f e r e n t size classes i n two habitats ....... 97 Table 15. Average proportionality constants f o r size classes of G. buenoi and G. p.ingreensis 98 Table 16. A comparison of test estimates with Gerris population estimates from the l i t e r a t u r e ................105 Table 17. C o l l e c t i o n s i t e s 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. P l a n t s p e c i e s and v e g e t a t i o n s t r u c t u r e used to d e f i n e g e r r i d h a b i t a t c l a s s e s 141 Table 20. G e r r i d s p e c i e s abundance i n v a r i o u s h a b i t a t c a t e g o r i e s ....142 Table 21. G e r r i d s p e c i e s abundance i n v a r i o u s c a t e g o r i e s of s u r f a c e water c o n d u c t i v i t y 144 Table 22. G e r r i d s p e c i e s abundance i n v a r i o u s c a t e g o r i e s of l a k e area 146 Table 23. G e r r i d s p e c i e s abundance i n temporary and more permanent h a b i t a t s * . . . 147 Table 24. H a b i t a t p r e f e r e n c e s of g e r r i d s p e c i e s i n B r i t i s h Columbia .. 157 Table 25. Chi-sguare values t o t e s t f o r independence of i n d i v i d u a l p o s i t i o n c h o i c e s on the experimental pools ...174 Table 26. T o t a l numbers of g e r r i d s f l y i n g i n t o e n c l o s u r e s ( s i x 24hr p e r i o d s - May 1977 ., 188 Table 27. Percentage of each s p e c i e s and morph l o s t from experimental e n c l o s u r e s . . ....190 Table 28. Percentage of animals remaining i n e n c l o s u r e s with f u n c t i o n a l i n d i r e c t f l i g h t muscles ...........193 Table 29. Occurrence o f G e r r i s s p e c i e s by h a b i t a t on two l a k e s d u r i n g J u l y , 1976 . . . 7 196 Table 30. Two-way a n a l y s i s of v a r i a n c e f o r the percentage of o b s e r v a t i o n s recorded i n open h a b i t a t s e c t o r s ........205 Table 31. Means and standard e r r o r s of the percentage of t o t a l o b s e r v a t i o n s made on open h a b i t a t s e c t o r s .........207 Table 32. Analyses of v a r i a n c e f o r the e f f e c t s of other s p e c i e s on observed h a b i t a t p r e f e r e n c e s ..........211 Table 33. A r t i f i c i a l h a b i t a t s chosen by th r e e G e r r i s . s p e c i e s i n l a b o r a t o r y experiments 215 Table 34. D i s t r i b u t i o n s of long-winged G. i n c o q n i t u s and G. p i n q r e e n s i s on v a r i o u s h a b i t a t s 225 Table 35. Weights of newly molted f i f t h i n s t a r l a r v a e ....236 Table 36. Number of g e r r i s s p e c i e s recorded per l a k e 1975-r 1 977 .239 Table 37. Percentages of w a t e r - s t r i d e r p o p u l a t i o n s completing one g e n e r a t i o n , 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 sur v i v a l of f i v e gerrids ± standard errors over three day experiments ..........251 xiv LIST OF FIGURES Figure 1. A general l i f e cycle for gerrids of temperate regions. 6 Figure 2. The Becher s P r a i r i e study s i t e . - . 14 Figure 3. An a e r i a l view of the d i s t r i b u t i o n of lakes and the landscape at Becher s P r a i r i e . ....................16 Figure 4. The Springhouse study s i t e . 18 Figure 5. Average monthly temperature maxima recorded at the Williams Lake, B. C. Airport. 22 Figure 6. Average monthly p r e c i p i t a t i o n 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. Di s t r i b u t i o n of surface conductivities ........ 29 Figure 9. Total number of egg batches l a i d by f i v e gerrids during 15 days 41 Figure 10. Average number of larvae hatched per egg batch at four constant temperatures. 43 Figure 11. Mating a c t i v i t y , female s u r v i v a l and fecundity 49 Figure 12- Average size of d a i l y egg batches recorded i n the laboratory 53 Figure 13. Patterns of oviposition and f e r t i l i t y observed for f i e l d populations ................................... 56 Figure 14. Survivorship of a l l l a r v a l i n s t a r s at various , constant temperatures 62 Figure 15. Calculated growth thresholds for a l l 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 a v a i l a b i l t y for capture on mesothoracic leg length ...................... 94 Figure 19. Areas summed in ca l c u l a t i o n of physiological time scales. ...111 X V Figure 20. Physiological time scales for gerrids calculated from f i e l d temperatures - .....118 Figure 21. D i s t r i b u t i o n of reproductive e f f o r t among morphs - ....121 Figure 22. P a r t i a l population curves f o r G. buenoi during 1975 . . . . . . 7 126 Figure 23. P a r t i a l population curves for G. comatus during 1975 i. .- . 128 Figure 24. P a r t i a l population curves for G. pinqreensis during 1975 . . 7.. . 130 Figure 25. P a r t i a l population curves for Limnoporus during 1 975. . . . . 132 Figure 26. Total population curves for a l l water-strider species during 1975. .........................136 Figure 27. Average gerrid biomass per lake during 1975. 138 Figure 28. F i e l d enclosures i n floating/submerged vegetation , .............164 Figure 29. Four types of a r t i f i c i a l habitat i n 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 e f f e c t s of wind ?.-•*• ^8 Figure 32. Unique s p a t i a l configurations of laboratory h abi tats ...... ....... .182 Figure 33. Starting d i s t r i b u t i o n of Drosophila i n laboratory experiments .185 Figure 34. Total number of gerrids disappearing from enclosures i n each habitat. . . . . . . . . I 191 Figure 35. Within lake s p a t i a l d i s t r i b u t i o n observed for various instars .....198 Figure 36. Relationship between depth and density of emergent cover 200 Figure 37. Di s t r i b u t i o n of gerrids on laboratory pools under three surface conditions. .,..,.203 Figure 38. E f f e c t s 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 i n each of four a r t i f i c i a l habitats. •••• ...........217 Figure 41. A plot of gerrid species d i v e r s i t y versus plant s t r u c t u r a l d i v e r s i t y 249 Figure 42. Average weight gains of i n d i v i d u a l surviving gerrids , .253 Figure 43. Average food f a l l per square meter ......256 Fiqure 44. Average daily mortality rates observed i n single-species and three-species experiments i n natural habitats. ..................... 258 Figure 45. The relationship between r e l a t i v e abundance of gerrids and r e l a t i v e abundance of t h e i r c h a r a c t e r i s t i c habitats. .. . . 263 Figure 46. The abundance of a l l predator/competitors oyer a range of conductivity. 270 Figure 47. Areas used by G. b u e n o i G . comatus and G. pinqreensis i n 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 i n the C h i l c o t i n when hip-boots were o f f . Good companionship, various medicinal s p i r i t s , campfires and starry C h i l c o t i n night skies made i t easier to go get "stuck i n the mud" another day. B. Smith helped i n the f i e l d . 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 i d e n t i f i c a t i o n s . Discussions with C..Whitney about assumptions, doubts and philosophy have been most help f u l in pr e c i p i t a t i n g positive ideas. I thank B. Smith and J . van Reenen for constant stimulation and es p e c i a l l y , for understanding the f r u s t r a t i o n s of an entomologist t r y i n g to find a niche i n 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. L i l e y , J.D. McPhail and W.E. N e i l l for reading, t h i s thesis on short notice and offering constructive advice. Dr. N e i l l has been a constant source of encouragement and often helped me to recognize i n t e r e s t i n g segments i n the long st r i n g s of ideas that I have discussed with him. I thank Dr. D. Holm for his generous assistance i n the never-ending task of rearing gerrid food. N e i l G i l b e r t wrote the algorithm used f o r day-degree summation and has always been w i l l i n g to offer and explain x y i i i s t a t i s t i c a l advice. Interactions with Neil have taught me more than I'd ever hoped to know about asking penetrating b i o l o g i c a l questions and using s t a t i s t i c a l methods to help answer them. My supervisor. Dr. G.G.E. Scudder, has made t h i s entire adventure possible. He suggested the gerrid system and shared his r i c h storehouse of information about water-strider natural history and the C h i l c o t i n Study area. His e d i t o r i a l e f f o r t s have much improved the quality of t h i s presentation. I greatly appreciate his support, inte r e s t and encouragement during t h i s project. I am grateful for the generous support, received for t h i s 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 f r u s t r a t i n g 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 u n f a i l i n g patience and understanding, I'd have gone ut t e r l y screamin' crackers! Debbie and I both thank D. Z i t t i n , B. Webb and S. Harrison for help with computer programming and FMT. We are also grateful to J. M i l l e r and C. Whitney for helping to punctuate our weekly e f f o r t s during the "big push". 1 CHAPTER I. INTRODUCTION A. Coexistence Of Species . . . A Plot Questions of species packing and coexistence are f e r t i l e ground for ecologists and evolutionary b i o l o g i s t s because they address the central problem of organic d i v e r s i t y . Ecologists have recently become interested i n describing and comparing community structure and function (Cody, 1974; Orians and Solbrig, 1977). The re s u l t i n g search f o r emergent, community-l e v e l properties has propelled the development of niche theory (MacArthur, 1972a; Pianka, 1976) and concepts of species d i v e r s i t y (Pielou, 1975). E v o l u t i o n i s t s , on the other hand, are interested i n the origins of d i v e r s i t y , and therefore, view extant patterns as clues f o r unravelling the dynamic processes that have produced them ( i . e . Gould, 1978). Obviously, the approaches are complementary and the i r synthesis has led to the new evolutionary ecology (Cody and Diamond, 1975). Hutchinson (1959) f i r s t c r y s t a l l i z e d the s p i r i t of the movement for many b i o l o g i s t s with a simple, but absorbing question - "why are there so many kinds of animals?". This question sprouted from seeds planted when explorer n a t u r a l i s t s from the temperate zone f i r s t ventured into the tropi c s and recorded a staggering variety of l i v i n g things. Relative b i o t i c impoverishment was also observed on islands and is o l a t e d mountains. Numerous hypotheses have been offered to explain the observed gradients in species d i v e r s i t y [summarized by Pianka (1966, 1967) and Otez (1974) J, but r e a l understanding 2 w i l l probably depend upon our a b i l i t y to integrate the e f f e c t s of several processes in consistent models (Menge and Sutherland, 1976). The regional coexistence of c l o s e l y - r e l a t e d species has afforded less mobile b i o l o g i s t s with ample opportunity to study factors responsible for the maintenance of organic d i v e r s i t y (MacArthur, 1965, 1972b). The problem i s usually considered as most acute among congeners because of great morphological and behavioural s i m i l a r i t y . In f a c t , Darwin (1859) predicted that competition " w i l l be generally more severe between them . . . than between species of d i s t i n c t genera'1. Niche theory has developed hand i n hand with studies of the comparative ecology of similar species coexisting i n a small geographical region and laboratory models of population dynamics (Whittaker and Levin, 1975). Factors c o n t r o l l i n g species d i v e r s i t y at a regional l e v e l are usually viewed in the context of t o t a l resource a v a i l a b i l i t y and/or how t i g h t l y species are "packed" into multi-dimensional resource space (MacArthur, 1965, 1972a). The modern approach o f t e n c e n t e r s on the manner in which congeneric species divide up the range of available resources i n order to minimize i n t e r s p e c i f i c competition (Schoener, 1974a) . Most of the data feedback, structuring contemporary niche theory, has come from work with vertebrates, lar g e l y 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 l i m i t i n g 3 s i m i l a r i t y (Cody, 1974; Pulliam, 1975). However, central assumptions of community theory often break down when tested experimentally (Wilbur, 1972; N e i l l , 1974, 1975; Lynch, 1978). Few studies of insect coexistence have conformed to quantitative predictions of community theory (see Price, 1975). This may r e f l e c t May's (1973) observation that niche relationships among insects are extremely complex owing to "intertwining of relevant resource dimensions". It i s also probable i s 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 e q u i l i b r i a because insect l i f e cycles and relevant environmental changes occur on the same time scales. Hutchinson (1975) pointed out that progress in understanding higher-level e c o l o g i c a l relationships depends upon intimate acquaintance with the natural history of the species involved. As b i o l o g i s t s we are challenged, l i k e i t or not, to deal with unique and in d i v i d u a l c h a r a c t e r i s t i c s of the systems that we study (Bronowski, 1973; Elsasser, 1975). Useful theory in ecology must be p l u r a l i s t i c (May, 1973); strong generalizations w i l l result from our a b i l i t y to c l a s s i f y species into groups with respect to process (Southwood, 1977; Whittaker and Levin, 1977). Before any species or population can be so c l a s s i f i e d we must understand the relevant d e t a i l s of i t s 4 natural history. The following investigation was launched from an animal-centered perspective. The ov e r a l l aim was to provide a balanced account of the processes a f f e c t i n g the coexistence of six water-strider species on the Fraser Plateau of south-central B r i t i s h Columbia with special emphasis on responses to s p a t i a l heterogeneity. A f i r s t objective was to establish a detailed baseline of species phenology and natural history in order to construct s p e c i f i c hypotheses about the.processes c o n t r o l l i n g gerrid d i s t r i b u t i o n and abundance. Some of the emergent ideas were. then tested experimentally i n the f i e l d and laboratory. Background information on water-striders and a more s p e c i f i c 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, r i v e r s 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 t h e i r victims. Hunting gerrids are known to respond to both surface vibration (Murphey, 1971a,b; Lawry, 1973) and vi s u a l cues as indicators of potential prey (Jamieson, 1973). Gerrids are the dominant invertebrate predators l i v i n g on the water surface. Their unique s p e c i a l i z a t i o n s and great evolutionary success argue that d i f f u s e competition between gerrids and other members of the aquatic community should be minimal. Gerrids are hemimetabolous insects; they pass through f i v e nymphal stadia p r i o r to dispersal and breeding as adults. A general l i f e cycle for temperate gerrids i s i l l u s t r a t e d in Figure 1. A l l active stages inhabit the water surface. Eggs are attached to f l o a t i n g debris and vegetation or l a i d below the surface on submerged objects (Brinkhurst,.1960; Matthey, 1975). Dispersal to new habitats by f l i g h t occurs mainly i n the spring (Landin and Vepsalainen, 1977). Pre-reproductive f l i g h t s may be extensive i n some species. For example, Leston (1956) noted that Limnoporus rufoscutellatus recolonizes Great B r i t a i n p e r i o d i c a l l y from continental populations. Available evidence suggests that gerrids overwinter in ovarian diapause (Andersen, 1973; Galbraith and Fernando, 1977) under stones,,logs or i n the leaf l i t t e r (Douglas, 1882; Brinkhurst, 1956; Cheng and 6 / Figure 1. A g e n e r a l l i f e c y c l e f o r g e r r i d s of temperate re g i o n s . JUVENILE DEVELOPMENT L A R V A L INSTARS 1-5 ADULTS BREEDING 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 i n females (Andersen, 1973; Vepsalainen, 1974a). Adults not destined to breed during the same season that they reach maturity, begin leaving the water surface f o r overwintering s i t e s as early as mid-July. The only two rigorous studies of gerrid population dynamics suggest that summer population growth i s density dependent i n G. najas (Brinkhurst, 1966) and L . n o t a b i l i s (Maynard, 1969). However the sources of mortality have not been c l e a r l y 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 t h i s sort of population regulation i s strong f o r other invertebrate predators (Fox, 1975a), but i s large l y circumstantial for gerrids. Predation and parasitism do not appear to have s i g n i f i c a n t e f f e cts on gerrid populations. Gerrids harbor a few parasites (Matheson and Crosby, 1912; Lipa, 1968; Fernando and Galbraith, 1970) but heavy i n f e s t a t i o n s 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 t h e i r e f f e c ts are minimal. Among invertebrate predators, notonectids, gyrinid beetles (Jamieson, 1973) and d y t i s c i d larvae (Spence, unpublished) are known to take gerrids, but t h e i r impacts on f i e l d gerrid populations have not been studied. 2. Geographical Distributions Nine species of water-striders have been recorded from B r i t i s h Columbia (Scudder, 1977). The general pattern of species d i s t r i b u t i o n i n the province i s summarized i n Table 1. Detailed d i s t r i b u t i o n a l records are provided by Spence and Scudder (1978), Scudder (1977) and Jamieson (1973). Data at hand suggest that there i s a large degree of range overlap among most species. The ranges of P r a i r i e and Western species meet in the south-central i n t e r i o r of B r i t i s h Columbia. In t h i s thesis I s h a l l follow Andersen (1975) and Calabrese (1977) and consider Gerris s. s t r . Fabricius and Limnpporus Stal as d i s t i n c t genera. The two species of Limnoporus recorded from B r i t i s h Columbia are d i f f i c u l t to separate r e l i a b l y . I have chosen to consider them as a single ecological e n t i t y i n the following discussions because they are comparatively rare members of the gerrid assemblage on the Fraser Plateau and because no s i g n i f i c a n t e c o l o g i c a l differences became apparent during f i e l d surveys. TABLE 1 Gerrid d i s t r i b u t i o n s i n B r i t i s h Columbia SPECIES KNOWN RANGE G. buenoi * Widespread G. comatus * Northcentral I n t e r i o r G. incoqnitus * Southern and Central B.C. Northward along the coast ' G. incurvatus Southern and Central B.C. G. n y c t a l i s Rocky Mountains only G. pingreensis * Fraser Plateau and Northern B.C. G. remigis Widespread in Southern and ~ Central B.C. L. d i s s o r t i s * Northeastern B.C., Fraser Plateau L. n o t a b i l i s * Widespread, most common i n Southern B.C. * species considered i n t h i s study 11 3. The Problem and the Approach Jamieson (1973) attempted to show how cannibalism and i n t e r s p e c i f i c predation might in t e r a c t to produce patterns of coexistence observed among fi v e species of water-striders in southwestern B r i t i s h Columbia. He employed a mathematical model, based on relationships determined i n the laboratory, to simulate population growth and gerrid foraging i n multispecies assemblages. Temperature thresholds for growth, calculated for f i r s t i n s t a r 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 a l l species except the one able to grow f a s t e s t at spring temperatures. These re s u l t s are confusing i n l i g h t of f i e l d data that suggest that multispecies assemblages are the r u l e among gerrids in B r i t i s h Columbia (Scudder, 1971). Brinkhurst (1959b), Vepsalainen (1973b) and Calabrese (1977) have been able to define d i s t i n c t habitat preferences among geographical assemblages of gerrid species. Although Jamieson (1973) noted habitat preferences among gerrids i n the f i e l d , these species c h a r a c t e r i s t i c s were not modeled. Spatial separation, maintained by such preferences, could easily mitigate the effects of i n t e r s p e c i f i c predation predicted by the model and lead to persistent multispecies assemblages observed i n the f i e l d . Because gerrids are opportunistic predator/scavengers. 12 differences i n food use are unlikely as general strategies of resource p a r t i t i o n i n g . Therefore, t h i s study focused on the use of space and the d i s t r i b u t i o n of species i n time. Three aspects of gerrid biology were studied i n some d e t a i l . (1.) Effects of Temperature on Phenology. Laboratory studies were undertaken to calc u l a t e growth thresholds for various species and in s t a r s (Chapter II) . The objective was to c a l i b r a t e physiological time-scales for mating, o v i p o s i t i o n and l a r v a l 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 t i m e - e f f i c i e n t density estimates of water-strider populations (Chapter I I I ) . 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 d i s t r i b u t i o n s were studied among lakes (Chapter V) . Habitat preferences were studied experimentally at both levels (Chapter V) as proximate factors leading to d i s t i n c t habitat associations. I n d i c i e s of population persistence and performance were used to explore how natural s e l e c t i o n might maintain d i s t i n c t species preferences, and assess the importance of s p a t i a l heterogeneity f o r producing patterns of species coexistence observed in the f i e l d (Chapter VI). 13 C. The Study Area . . . The Stage 1• Sites Fieldwork described i n t h i s thesis was concentrated i n the Cariboo-Chilcotin region of B r i t i s h Columbia. The study s i t e s are located between elevations of 950 and 1Q00 m on the Fraser Plateau. Surrounding vegetation i s native grassland interspersed with stands of lodgepole pine, douglas f i r and aspen. Many lakes and ponds are nestled in the r o l l i n g topography of these Cariboo Parklands. Topping and Scudder (1977) and B e i l (1970) have summarized information on the geological history of the area and i t s included lake basins. B e i l (1 970) may also be consulted for an excellent analysis and description of the plant associations of the region. Two s i t e s were chosen for study; one surrounding Springhouse, B. C., and the other at Becher's P r a i r i e near Eiske Creek, B. C. The geographical location of these areas i s depicted by Topping and Scudder (1977)., Figure 2 i l l u s t r a t e s the main features of the Becher's P r a i r i e s i t e . In excess of 75 lakes and ponds occur i n the area which covers approximately 92 km2. Figure 3 i l l u s t r a t e s the d i s t r i b u t i o n and spacing of lakes from an a e r i a l perspective. The Springhouse area i s shown i n Figure 4. The lakes are somewhat less numerous and spread over a wider area (approximately 145 km2) than at Becher's P r a i r i e . Numbered lakes in Figures 2 and 4 were sampled during t h i s study. A key to the lake numbers i s provided in Appendix I. Figure 2. The Becher 7 s P r a i r i e study site.. BECHER'S PRAIRIE STUDY AREA 16 Figure 3. An a e r i a l view of the d i s t r i b u t i o n of lakes and the landscape at Becher's P r a i r i e . 17 F i g u r e 4. The S p r i n g h o u s e s t u d y s i t e . 20 Several points of s i m i l a r i t y can be noted between the two study areas. F i r s t l y , each includes many lakes which span a broad range of siz e and thus provide a diverse array of pot e n t i a l habitats. Secondly, the lakes are not i s o l a t e d so that lake to lake dispersal should be possible f o r f l y i n g insects. Thirdly, most of the lakes and ponds i n both areas were sampled. Ponds were ignored only i f access was d i f f i c u l t or i f two neighboring ponds appeared to provide an i d e n t i c a l range of habitats. Differences between the areas include more intensive a g r i c u l t u r a l use at Springhouse. Also the Springhouse lakes can be divided into several lo c a l groups that are more or l e s s separated by forested areas. Such differences are not considered in t h i s study. 2. Weather Weather patterns in the Chilcotin-Cariboo region show both h i s t o r i c a l and year to year v a r i a b i l i t y . 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 l i f e history parameters of a species may be tuned to the average climate, yearly patterns of d i s t r i b u t i o n and abundance observed i n a short term study are l i k e l y to r e f l e c t the p e c u l i a r i t i e s of each sampling season. Therefore the weather of sample years should be examined i n the l i g h t of average conditions. Figures 5 and 6 present the monthly averages of temperature 21 and p r e c i p a t i o n r e s p e c t i v e l y , as recorded from April-September of 1975-77, at the W i l l i a m ' s Lake, B. C. A i r p o r t . Average valu e s computed from data gathered during 1941-1970 (Atmospheric Environment S e r v i c e , 1975) at the same s t a t i o n are presented f o r comparison. The study years were c o o l e r and wetter than u s u a l . Even so, about 10% of the ponds sampled each year d r i e d up d u r i n g the course o f the season. Wind i s an obvious f a c t o r expected to a f f e c t s u r f a c e -d w e l l i n g i n s e c t s . Two wind i n d i c e s are p l o t t e d i n F i g u r e 7. Both the "gust" index and the average d a i l y windspeed, peak i n s p r i n g and e a r l y summer. T h i s i s l i k e l y to be important f o r semi-aquatic i n s e c t s because emergent v e g e t a t i o n i s not present to serve as wind b a f f l i n g d u r i n g the most c r i t i c a l p e r i o d s . 3. Lakes Some of the l a k e s employed i n t h i s i n v e s t i g a t i o n have been s t u d i e d p r e v i o u s l y . Topping and Scudder (197?) have c l a s s i f i e d a r e p r e s e n t a t i v e s e r i e s of these l a k e s on the b a s i s of p h y s i c a l -c h e m i c a l data. Reynolds and Reynolds (1975) have shown t h a t the d i s t r i b u t i o n s of a q u a t i c v a s c u l a r p l a n t s are a f f e c t e d by lake chemistry and i o n i c p r o f i l e s . Scudder (1969) presented a g e n e r a l d i s c u s s i o n of the i n v e r t e b r a t e fauna i n r e l a t i o n to g r a d i e n t s of s a l i n i t y . Other s t u d i e s (Cannings and Scudder, 1978; Smith, 1977; Topping and Acton, 1976; Scudder, 1975; Reynolds, 1974 and Cannings, 1973) have s t u d i e d the f i e l d -b i o l o g y of v a r i o u s i n s e c t s p e c i e s i n h a b i t i n g these l a k e s . 22 F i g u r e 5. Average monthly temperature maxima recorded at the Williams Lake, B. C. A i r p o r t . 23 \ ©AVERAGE (1941-70) APR. MAY JUNE JULY AUG. SEPT. MONTH 24 Figure 6. Average monthly p r e c i p i t a t i o n recorded at the Williams Lake, B. C. Airport. 25 • A V E R A G E ( 1 9 4 1 - 7 0 ) lOOi A 1975 • 1976 ° 1977 80 z 601 O ,<40| Q_ ^ 20! cr CL APR. MAY JUNE JULY MONTH AUG. SEPT. 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 i n t h i s study, however, have not been studied previously. Appendix I l i s t s a l l of the ponds sampled during 1975-77 and provides some general information about each lake and the water-strider species that were found on i t during t h i s study. Scudder (1969) has found that conductivity i s useful for ordering patterns of species d i s t r i b u t i o n . Figure 8 shows that the d i s t r i b u t i o n of conductivities, as measured from the lakes sampled during summer 1977, i s 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. Dis t r i b u t i o n of surface conductivities recorded from lakes sampled i n July-August, 1977-NUMBER OF LAKES 31 CHAPTER I I . 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 l i k e l y 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 i n order to avoid cannibalism (Maynard, 1969; Vepsalainen and Jarvinen, 1976) and i n t e r s p e c i f i c predation (Jamieson, 1973). This argument i s supported by the work of Jamieson (1973) who has shown that e f f i c i e n c y of prey capture i s d i r e c t l y 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 e f f e c t s that may be compared among species and locations (Gilbert et a l . , 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 s e l e c t i o n (Campbell et a l . , 1974; Trimble and Smith, 1978). This study was undertaken to assess the e f f e c t s of temperature on growth and development for several g e r r i d species occurring together on the Fraser Plateau of southcentral B r i t i s h Columbia. The objective was to quantitatively describe the main events of the gerrid l i f e 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 S e r r i s buenoi, G. comatus, pinqreensis, and Limnoporus spp. were co l l e c t e d from the study lakes during early and mid May 1976. Five.pairs of each species were placed i n rearing containers on May 10 and maintained at 22°C. Twenty a d d i t i o n a l pairs of each Gerris species were established on May 25; f i v e 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 i n i t i a t e d on May 25 were held at 15°C. A l l animals were allowed 48 hours to adjust to laboratory conditions before experiments began. The breeding cultures established are summarized in Table 2. A l l Gerris cultures were kept i n small p l a s t i c containers (7 cm. deep; 9.5 cm. diameter) while Limnoporus were held i n large p l a s t i c 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 a l l other temperatures). Pieces of cork served as Gerris ovipositipn s i t e s and 3/8" plywood blocks (8 cm. x 2 cm.) were provided for Limnoporus. Each pair of gerrids was given 20-25 frozen Drosophila d a i l y as food. 34 TABLE 2 Starting dates of laboratory cultures used to assess the r e l a t i o n -ship between egg laying and temperature TEMPERATURE °C G. buenoi G. comatus G. pingreensis Limnoporus 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. A l l Limnoporus experiments continued u n t i l the females died. Survival of a l l females was converted to day-degrees using thresholds for egg development calculated in subsequent experiments. Cultures were checked d a i l y for the presence of new eqqs. Oviposition blocks with attached egg batches were transferred to separate cumulative hatching containers f o r each culture and the date was recorded. The t o t a l number of f i r s t stage larvae hatching from each culture was divided by the number of dates on which eggs were found i n order to estimate the average batch size f o r each female during the experiment. The t o t a l number of larvae hatched was divided by the female survival i n 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 i n September 1977, were kept through the winter in constant temperature cabinets at 5°C. On A p r i l 22, 1978, twenty pairs of each species were used to establish room-temperature cultures in large p l a s t i c containers (dimensions as above), each containing a single conspecific male . and female. 36 Short sections of birch " s t i r - s t i c k s " (5 cm. x 1 cm. x 0.2 cm.) supported with small pieces of cork were provided as resting and oviposition s i t e s . Cultures were fed to sa t i a t i o n d a i l y with l i v i n g vestigial-winged Drosophila. Laboratory a i r 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 l i s t e d 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 a c t i v i t y . The number of pairs in copula 'plus the number of coupled pairs without genital contact was recorded as a quantitative index of mating a c t i v i t y f o r both species. General q u a l i t a t i v e observations on mating behaviour were also made. The number of eggs l a i d by each female was t a l l i e d every second day. Eggs were counted by scanning oviposition blocks u n t i l successive counts were repeatable. Use of " s t i r - s t i c k s " ensured that a l l eggs were c l e a r l y v i s i b l e . Water was changed, fresh " s t i r ' - s t i c k s " 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 f o r a pa r t i c u l a r culture. Females were dissected at death and the 37 condition of the i n d i r e c t f l i g h t muscles and the number of chorionated eggs remaining i n the reproductive t r a c t were recorded, 3. Fecundity in the F i e l d Groups of 10-12 female Gerris incoqnitus were co l l e c t e d p e r i o d i c a l l y during the spring of 1978 from a small pond adjacent to 16th Avenue on the U.B.C. Endowment Lands, starting with the f i r s t observation of mating pairs on March 27. Subsequent groups were co l l e c t e d at approximate ten day i n t e r v a l s u n t i l the f i r s t teneral adults of the new generation appeared on May 23. Females were brought into the laboratory and placed i n d i v i d u a l l y i n small p l a s t i c containers (dimensions as above) f i l l e d to a depth of two cm. with dechlorinated tap water. " S t i r - s t i c k s " (same dimensions as above) were provided as resting and oviposition s i t e s . Each female was allowed to oviposit for two days i n 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 s i t e of c o l l e c t i o n . Females taken during the preceding i n t e r v a l were ignored in a l l f i e l d c o l l e c t i o n s . The eggs l a i d by each female were counted and held separately u n t i l hatching. The t o t a l number of f i r s t i n s t a r s hatching was recorded for each batch as an index of number of f e r t i l e eggs. 38 B? Growth and Development 1• Development and Temperature a. Eggs Egg batches were isolated d a i l y from cultures of f i e l d -c o l l e c t e d G. buenoi, G. comatus, and G. pinqreensis held at 15°, 18.5°, 22° and 26°C. The number of days required for a l l individ u a l s of these d a i l y cohorts to hatch was recorded at each temperature. The procedure was repeated for Limnoporus at 22° and 15°C. b. Larvae Newly hatched f i r s t i n s t a r larvae of G. buenoi, G. comatus, G- pinqreensis, G. incoqnitus and Limnoporus were reared through the. f i r s t molt at the constant temperatures used for egg hatching. A l l gerrids were held i n d i v i d u a l l y i n 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. n o t a b i l i s females and those from smaller Limnoporus females (presumably d i s s o r t i s ) were kept separately through the f i r s t i n s t a r . At l e a s t 15 f i r s t i n s t a r larvae of each species were reared at each temperature. Larvae were checked for molting and fed an excess of frozen Drosophila d a i l y . The general procedure was repeated for a l l subsequent 39 l a r v a l stages of G. buenoi, G. comatus and G. pinqreensis. A l l stages of G. incognitus were reared at 22°d Limnoporus development times were measured at 15° and 22°C. A l l rearings are summarized in Appendix II. At f i r s t , second and t h i r d stage larvae used were reared from eggs i n the laboratory. Additional fourth and f i f t h stage larvae taken from the f i e l d were used f o r these experiments owing to high mortality during the. l a s t 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 s u f f i c i e n t data were available. The res u l t i n g X-intercepts estimate the temperature at which measurable growth ceases, i . e . the temperature threshold of development for that stage and species (Gilbert et a l . , 1976). Threshold temperatures and t h e i r standard errors were calculated for f i r s t stage larvae of a l l species and a l l other stages of G. buenoi, G. comatus and G. pingreensis. The methods of ca l c u l a t i o n are completely described by Campbell et a l . , (1974). The growth thresholds were compared between stages and between species with weighted analyses of variance to account for differences i n 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 l a i d by gerrid females i s affected by p r e v a i l i n g temperatures. The peaks shown at 22°C for a l l three Gerris species may be accentuated because younger females were used at that temperature (Table 2) . Even i f i n t e r p r e t a t i o n i s r e s t r i c t e d to the four groups started on May 25, the e f f e c t of temperature i s pronounced and d i f f e r s from species to species. Egg production was most severely depressed i n G. buenoi and G. comatus at the two lowest temperatures; G. buenoi females l a i d no eggs at 10°C. G. pinqreensis were least affected by low temperatures. Comparison of the two Limnoporus groups, although started with animals of d i f f e r e n t age, i s legitimate because females of t h i s species do not carry mature eggs u n t i l l a t e May or early June in the study area (Chapter IV) . The average numbers of larvae hatched per egg batch i s compared i n Figure 10. No data are presented for 10°C because no eggs hatched aft e r 65 days. The data i l l u s t r a t e that both G. buenoi and G. comatus lay more eggs per batch with increasing temperature. One-way analysis of variance indicates that the e f f e c t of temperature i s highly s i g n i f i c a n t for G. comatus (F=8.90; df=3,18; p « . 0 5 ) . The number of larvae/batch i s highly variable among G. buenoi females at low temperatures and 41 Figure 9 . Total number of egg batches l a i d by f i v e gerrids during 15 days at various constant temperatures. 8 IOI A G . B U E N O I B G . C O M A T U S ® G . P I N G R E E N S I S • L I M N O P O R U S S P P . 10 15 20 T E M P E R A T U R E 43 Figure 1 0 . Average number of larvae hatched per egg batch at four constant temperatures. in X g CQ •s. O UJ X 6 I UJ < u_ o oc UJ m 20 16 12 8 4 <3. comatus <3. b uenot (3. pingreensis 15 185 22 26 15 18.5 22 26 T E M P E R A T U R E - ° C 15 18.5 22 26 45 the ef f e c t of temperature on batch size i s much less pronounced (F=3.29; df=3,17; p=. 052) . Female G. pinqreensis, l a i d largest egg batches at 18.5°C and, most conspicuously, the number of larvae per batch does not increase with temperature. No s i g n i f i c a n t 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 i s compared i n Table 3. The data show l i t t l e difference between the upper three temperatures, but suggest that absolute rates of egg production f a l l for G. buenoi and G. comatus at 15°G. A two-way analysis of variance was performed over the upper three temperatures. The res u l t s are shown i n Table 4 . There was no s i g n i f i c a n t e f f e c t of temperature over t h i s range. However, the data indicate that there are between species differences in l a r v a l production rate. The means and standard errors presented in Table 3 suggest that o v e r a l l l a r v a l production may peak at 18.5°C i n G. pinqreensis, but larger sample sizes are necessary to es t a b l i s h whether the e f f e c t i s s t a t i s t i c a l l y s i g n i f i c a n t . 46 TABLE 3 Means and standard errors for number of larvae hatched/day-degree/female gerrid at three constant temperatures TEMPERATURE °C G. buenoi SPECIES 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 i n the Laboratory Mating behaviour i n G. comatus and G. pinqreensis does not involve elaborate courtship displays. Males i n 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 r e s i s t s . I n t e r s p e c i f i c coupling has been achieved between several Gerris species i n the laboratory during t h i s study. Coupled animals remain paired for long i n t e r v a l s , 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 i n the laboratory. Individual females and males mate repeatedly throughout l i f e . Mating behaviour i n these species i s concentrated during daylight hours i n both the laboratory and the f i e l d . Oviposition generally occurs l a t e i n the day and overnight. The upper histograms of Figure 11 show that mating continues throughout the period of egg laying. The peak of mating a c t i v i t y precedes the period of maximum oviposition i n both species, although the e f f e c t i s 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 f i r s t two days, suggests that males as well as females require some time 49 F i g u r e 11. Mating a c t i v i t y , female s u r v i v a l and f e c u n d i t y on p h y s i o l o g i c a l time s c a l e s i n the l a b o r a t o r y . (a.) G. comatus, (b.) G. p i n q r e e n s i s . % OF TOTAL EGGS LAID MATING ACTIVITY (OPEN BARS) (OPEN BARS) J> O 0> ro ro o o 8 . 8 8 S % FEMALES LAYING (DOTTED UNE• O ) o g NUMBER SURVIVING (SOLID LINE Q——®) % OF TOTAL EGGS LAID (OPEN BARS) J > 'o a> to MATING ACTIVITY (OPEN BARS) — ro o o 8 8 8 S 8 % FEMALES LAYING (DOTTED LINE© ®) NUMBER SURVIVING (SOLID LINE*——•) 51 to achieve f u l l reproductive ma The lower histograms d i s t r i b u t i o n of egg production time scale. Each day repr degrees at laboratory temperatu occurred between 130 and 16 However, G. comatus survived an laboratory conditions. The aver, species i s pr was s l i g h t l y contained man tendency for f l i g h t muscle down at dea The main caus Senile g e r r i after forced t u r i ty. of Figure 11 i l l u s t r a t e the on the. insect's physiological esented approximately 13.5 day-res. The peak of oviposition 0 day-degrees for both species, d l a i d eggs s l i g h t l y longer under age number of eggs l a i d by individuals of both esented i n Table 5. The data show that G. comatus more fecund than G. pinqreensis. A l l females y chorionated eggs at death, although there was a older females to carry fewer eggs. The i n d i r e c t s of a l l G. comatus females were completely broken th and eggs often f i l l e d the entire body cavity, e of death seemed to be drowning i n both species, ds were unable to stay on the water surface even periods of "drying out" on paper towels. Data presented i n Figure 11 also show that both mating a c t i v i t y 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 i n both species. Thus the decreases shown i n Figure 11 r e s u l t from two factors: (1) fewer females are laying eggs late i n the season; (2) each female lays eggs at a lower d a i l y rate as she ages. 52 TABLE 5 Total number of eggs l a i d by female G. comatus and G. pinqreensis i n 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 5 3 Figure 12. Average size of d a i l y egg batches recorded i n the laboratory for G. comatus and G. pinqreensis. Of I o 0>2O| Cf> U J u_ 161 o !§ 1 2 1 CO U J oc hi 5 8 4 ° CJ. comatus 0 (3. pin Qr Gens is 1 s t a n d a r d e r ro rs 54 108 162 216 270 ACCUMULATED DAY-DEGREES 55 3. Fecundity in the F i e l d Figure 13 shows the pattern of oviposition by G. incognitus taken from the f i e l d . Several points should be noted. F e r t i l i t y i s high, generally greater than 90% throughout the season. However a smaller proportion of females brought i n from the f i e l d lay dai l y egg batches than i n 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 i n the laboratory. Although the mean number of eggs l a i d per female show a pattern s i m i l a r to that seen i n the laboratory, the variances are d i s t r e s s i n g l y large. A one-way analysis of variance i s unable to demonstrate s i g n i f i c a n t differences i n average batch size over the season (F=0.74, df=6,65, p>.10). Coefficients of variation for d a i l y batch size were compared between laboratory and f i e l d populations with one-way analysis of variance. Table 6 l i s t s the means and standard errors of these c o e f f i c i e n t s . , The. analysis indicates that f i e l d populations were s i g n i f i c a n t l y more variable than laboratory populations with respect to dai l y batch size (F=16.68, df=2,26, p<.01). 56 i Figure 13. Patterns of oviposition and f e r t i l i t y observed for f i e l d populations of G. incognitus. AVERAGE NUMBER OF EGGS/DAY/J ( o ro a CD o ro OJ (0 > PRI di r ro ro OJ <£> > oi -<ro ro CO H > z. o > o m 3) o co _ ro ^ <j> oo o o o o o o o V. 9 *S LAYING EGGS OVER 2DAYS0 % FERTILITY (• •) U l T A B L E 6 Average c o e f f i c i e n t s of va r i a t i o n for d a i l y batch s i z e i n Gerris S P E C I E S C . V . ± S . E . G. incoqnitus 0.72±.070 (field) G. pingreensis 0.411 ±. 037 (lab)"" G. comatus 0 T383±.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 s p e c i f i c rates f o r each developmental stage, and p a r t i a l data sets f o r other temperatures, are l i s t e d with standard errors i n Appendix I I . 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 a f f e c t d i f f e r e n t species in d i f f e r e n t ways. In t h i s case G. pinqreensis development i s less retarded by cool temperature than i s that of G. comatus. The percent of t o t a l development spent i n each stage at v 22°C i s recorded i n Table 8. The egg and f i f t h stage were the longest in a l l species while the second stage was generally shortest. However, the f i r s t three l a r v a l stages require approximately the same amount of time. Survivorship of animals hatched i n the laboratory at d i f f e r e n t temperatures i s i l l u s t r a t e d in Figure 14. Although survivorship was generally lower f o r l a t e r i n s t a r s , a comparison of "optimum temperatures" (defined by peak survivorships for the various stages) can be made. Early stages of a l l three species had lower optimum temperatures than l a t e r stages. Peaks in survivorship, recorded for the l a s t three stages suggest that species differences e x i s t . G. pinqreensis had the lowest 60 TABLE 7 Total (egg to adult) development times i n 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 t o t a l development time i n each stage at 22°C STAGE buenoi egg 23.8 1 2 3 4 5 13.4 11.0 12.7 18. 4 20.7 SPECIES AVE Gerris Limnoporus comatus incoqnitus 27.2 25.1 12.0 10. 2 11.4 15. 9 23. 3 11.1 11.3 11.3 15. 7 25.5 25.2 13. 1 10.3 10.9 15. 8 24.7 26. 9 11.5 8.8 11.2 15.9 25.7 25.6 12. 3 10.3 11.5 16.3 24.0 62 Figure 14. Survivorship of a l l l a r v a l i n s t a r s at various constant temperatures i n the laboratory. (a.) G. buenoi, (b.) G. comatus, (c.) G, pingreensis. 15 2 0 2 5 3 0 REARING TEMPERATURE (°C) 64 optimum temperature and G. comatus had the highest. 2. Growth Thresholds Growth thresholds for a l l stadia of G, buenoi, G. comatus and G. pinqreensis are compared i n Figure 15. The regression eguations used f o r each estimation are given in Appendix III. Weighted analyses of variance show that s i g n i f i c a n t differences exis t between thresholds of the f i v e l a r v a l stages for a l l three species. The values of X 2 are given i n Table 9. The general pattern i s that eggs and f i f t h stage larvae have high growth thresholds while intermediate stages have somewhat lower thresholds for development. The ef f e c t i s most pronounced for G. pinqreensis. Signific a n t differences also e x i s t between species as i s i l l u s t r a t e d for f i r s t stage larvae in Table 10. These data show that f i r s t stage larvae of G. pinqreensis have a much lower threshold for growth than other species found i n the study area. The data of Table 10 suggest some evidence for geographical differences. Thresholds tabulated for gerrids on the lower mainland of B r i t i s h Columbia have been taken from Jamieson (1973). Standard errors are not given for the estimates so s t a t i s t i c a l comparisons are not possible. However, G. buenoi seems to have a d i s t i n c t l y lower threshold for growth on the Fraser plateau. 65 Figure 15. Calculated growth thresholds for a l l developmental stages of three gerrid species. m<3. buenoi EGG I 2 3 4 5 DEVELOPMENTAL STAGE ON cn 67 TABLE 9 X 2 values for differences between growth thresholds of different l a r v a l stages SPECIES X 2 df p G. buenoi 16. 24 4 «.01 G. comatus 9.73 4 <.05 G. Pinqreensis 17. 04 4 «-01 TABLE 10 Comparison of f i r s t i n s t a r growth thresholds SPECIES INTERIOR LOWER MAINLAND G. buenoi 8.5±0.66 n=100 •12.9 G. comatus 8.3±0.79 n=126 Does not occur • £ • pinqreensis 3.8±1.62 n=109 Does not occur G. incognitus 8.9±1.04 n=57 9.3 L. n o t a b i l i s 9.0±0.86 n=97 10.3 L. d i s s o r t i s 9.4±1.00 n=52 Does not occur X 2 df P 36.19 5 <<0.01 ; Not calculated 69 DISCUSSION A. Mating Behaviour Wilcox (1972) has presented the only in-depth analysis of mating behaviour i n water-? s t r i d e r s . . He showed that an Australian species of Rhagadotarsus-engages i n elaborate pre-mating displays involving communication by surface waves. Maynard (1969) and Jamieson (1973) have observed s i m i l a r behaviour in L. n o t a b i l i s in southwestern B r i t i s h Columbia and suggested that both male-male and male-female communication i s involved. Comparable mating r i t u a l s have not been observed in Gerris and the only r u l e for males in breeding condition seems to be "catch i t and t r y " . The possible role of pheromones i n orchestrating t h i s behaviour has not yet been investigated. Data presented here show that G. comatus and G. pinqreensis mate repeatedly throughout t h e i r reproductive l i v e s . This seems to be the rule among temperate Gerris. One possible explanation i s that females are unable to store large quantities of sperm. Kaufmann (1971) noted the complete absence of spermathecae in L. rufpscutellatus but t h i s should be substantiated by detailed investigation. Gerrid spermathecae are r e l a t i v e l y small and are very easy to miss when the abdomen i s distended with eggs. A l l species considered in t h i s study have spermathecae s i m i l a r i n structure to that described by Brinkhurst (1960) for G. najas. Frequent mating, then, may serve to keep females f u l l y inseminated and ensure the high f e r t i l i t y observed among G. incoqnitus i n the f i e l d . Gordon 70 and Gordon (1971) have shown that f e r t i l i t y f a l l s rapidly with decreasing sperm leve l s in the milkweed bug, Oncopeltus fasciatus. Concentration of mating behaviour during daylight hours i s consistent with the hypothesis that copulation i n Gerris i s i n i t i a t e d by v i s u a l cues. I t 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 i n copula or the v i s i b i l i t y of mating pairs. B. Fecundity L i t t l e published information exists concerning fecundity i n nearctic Gerris. Labeyrie (1978) stresses that insect fecundity must be considered as a population phenomenon. Therefore r e a l understanding of sel e c t i v e 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 t h i s species lays about twice as many eggs as the species studied here, G. remigis i s about twice as large as G. comatus or G« pingreensjs and continues breeding well into summer. Andersen (1973) reports that the European G. l a c u s t r i s , which i s s i m i l a r in size to G. comatus and G. pinqreensis, l a i d up to 250 eggs under f i e l d conditions. Ovariole number i n temperate 71 Gerris seems to be constant at four per ovary (Brinkhurst, 1960; Kaufman, 1971; Spence, unpubl.) and therefore, ovariole number (Price, 1975) i s not a good comparative index of gerrid egg production. Available data suggest that there . may be some relati o n s h i p between fecundity, body size and female longevity. Fecundity i n Gerris has been investigated by two i n d i r e c t 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 i n the laboratory. His estimates for G. incoqnitus and G. incurvatus are about one half of those determined here for G. comatus and G. pingreensis. I t i s 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 c r i t e r i a (Scudder and Jamieson, 1972; Spence and Scudder, 1978). Two a l t e r n a t i v e explanations involving methods are possible. (1) Jamieson (1973) did not use females overwintered i n the laboratory and i t i s possible that some eggs may have been deposited before animals were co l l e c t e d . (2) Counts of larvae hatched i n the laboratory are misleading because larvae from l a t e r egg batches accumulated i n the laboratory are less l i k e l y to hatch (Spence, unpubl.). Results presented above show that a continuously high proportion of eggs hatch throughout the season i n f i e l d 72 populations of G. incognitus. Therefore neither i n f e r t i l i t y , food quality nor aging are l i k e l y to cause reduced hatching success in natural populations. This information supports Matthey's (1975) contention that the. problem of fecundity in Gerris awaits further i n v e s t i g a t i o n . Matthey (1975) found that drowning was the major source of mortality among ovipositing G. remigis . He attributed t h i s to two factors: (1) loss of water-proofing owing to accummulation of debris i n the surface hair layers r e s u l t i n g from subsurface oviposition and (2) loss of strength to hold themselves o f f the water surface as a re s u l t of muscle h i s t o l y s i s associated with l a t e r stages of ovip o s i t i o n . Observations of t h i s 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 s i g n i f i c a n t post-reproductive period i n Gerris. I t i s established that rates of insect s u r v i v a l and ovipo s i t i o n 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 su r v i v a l of the adult lesser peach tree borer, Synanthedon p i c t i p e s , i s l i n e a r l y related to ambient temperature over the range of l i k e l y f i e l d temperatures. Similarly data from t h i s study show' that 73 l a r v a l production/day-degree i s approximately the same between 18.5° and 26°C for each of three Gerris species. The proximate b i o l o g i c a l mechanism i s that reduced temperatures lead to both fewer da i l y egg batches and less eggs per batch, but increase absolute survival times. S u f f i c i e n t data are not available to estimate exact thresholds of egg maturation by the methods of Greenfield and Karandinos (1976), but Spence et a l . , (1978) have shown that the threshold of egg development estimated i n t h i s paper allows good prediction of time of f i r s t oviposition by G« pinqreensis in the f i e l d . Thermal constants and development thresholds are commonly b u i l t into simulation models of insect population dynamics (Gilbert et a l . , 1976). At l e a s t one obstacle remains before t h i s can be done for Gerris. Although the general pattern of mean fecundity in the f i e l d resembles the patterns seen i n the laboratory, the higher variances of f i e l d data suggest that additional factors must be considered. Two suggestions can be made that r e l a t e to the d i f f i c u l t problem of how insects a c t u a l l y experience temperature i n the f i e l d . (1) Insects on the same pond may experience s i g n i f i c a n t l y different microclimates. (2) Gerrids emerge from overwintering over a period of some weeks and thus several cohorts of breeding animals exist with respect to ph y s i o l o g i c a l age. At present few data can be brought to bear on these matters. 74 C. Growth and Development Vepsalainen (1973a) has reviewed the scant l i t e r a t u r e 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 p o t e n t i a l l y i n t e r e s t i n g information about adaptation. For example Bailer and Bush (1974) have shown that development times vary among geographical and h o s t - s p e c i f i c races of the apple maggot, Rhaqoletis cerasi even though developmental thresholds seem to be approximately constant (Baker and M i l l e r , 1978). These res u l t s imply that the number of day-degrees required f o r pupal development i n Rhaqoletis can be adjusted by selection. Bailer and Bush (1974) show how these adjustments may be viewed as adaptive t a c t i c s . Temperature thresholds also seem to be adaptive. Campbell et a l . , (1974) have pointed out cases of d i s t i n c t geographical variation within aphid species. Tentative comparison of data presented here with those of Jamieson (1973) suggests that s i m i l a r patterns e x i s t i n Gerris. If coexistence i n Gerris i s determined by processes of cannibalism and i n t e r s p e c i f i c predation as argued by Vepsalainen and Jarvinen (1976) and Jamieson (1973), we might expect species to adjust t h e i r growth constants with respect to both l o c a l climate and the growth parameters of potential competitors. G. buenoi and S. pinqreensis often co-occur on the same lakes, es p e c i a l l y during the spring (Chapter V) . I t i s possible that competitive 75 pressure from G. pinqreensis has selected for lower developmental thresholds i n G. buenoi. This idea i s further supported by the fact that neither G. incognitus nor L. n o t a b i l i s have lower growth thresholds on the Fraser Plateau. Neither of these species co-occur frequently with G. pinqreensis. S i g n i f i c a n t differences have been demonstrated among the growth thresholds of di f f e r e n t developmental stages. The consistent pattern observed among three species suggests the following adaptive interpretation. Eggs .develop under water and r e l a t i v e l y high thresholds for egg development ensure that hatching i s delayed u n t i l water temperatures are high enough to buffer low a i r minima l i k e l y during spring. Lower thresholds for early instars lead to rapid development once eggs have hatched. F i f t h stage larvae become abundant i n the study area around the summer s o l s t i c e , consequently low thresholds are unnecessary because by mid-June high d a i l y a i r temperatures are predictable. Data on optimum temperatures f o r laboratory survivorship show that higher thresholds are also associated with increased survivorship at high temperatures. Temperature af f e c t s the development rates of Gerris species to d i f f e r e n t extents. These e f f e c t s can be related to the sequence of appearance of these species i n the f i e l d . G. pinqreensis i s best adapted for cold temperatures because mating, fecundity and juvenile growth and survival rates are not markedly i n h i b i t e d by low temperatures. These same parameters are maximized at high temperatures i n G. comatus and G. buenoi. 76 however, temperature e f f e c t s are les s s t r i k i n g i n G. buenoi. This observed species ranking, with respect to tolerance of cold temperatures, corresponds to the order of t h e i r appearance and speed of development i n the f i e l d (Chapter I V ) . The superior . adaptation of G. pinqreensis to low temperatures should lead to habitat preemption i f gerrid-gerrid predation i s the main factor l i m i t i n g species coexistence among water-striders (Jamieson, 1973). There i s some evidence that t h i s process may help to explain patterns observed i n some habitats (Chapter V I ) , however as a general rule, nature i s not so simple; the fact i s that many temperate gerrids do co-occur regionally and often coexist on the same pond. Other factors involved i n determining the composition of regional water-s t r i d e r assemblages are discussed i n subseguent chapters. 77 CHAPTER I I I . DENSITY ESTIMATES FOR GERRIDS INTRODUCTION A popular approach for studying species interactions i n the f i e l d i s comparison of population dynamics. For most animals the estimation of population s i z e . i s a challenging problem in i t s own r i g h t (Gilbert, 1973). Two types of population measures are employed by ecologists. Absolute, population measures lead to density estimates while r e l a t i v e measures permit only the comparison of r e l a t i v e abundances, given assumptions of egual sampling e f f i c i e n c y over the range of comparison (Southwood, 1966). Absolute estimates are generally desirable but r e l a t i v e methods are more widely employed because they give much better data return per unit e f f o r t . Southwood (1966) discusses the p i t f a l l s of r e l a t i v e methods. Differences between habitats, species, seasons, and weather have been shown to a f f e c t the e f f i c i e n c y of most r e l a t i v e measures applied to t e r r e s t r i a l insects. However guantitative assessment of such e f f e c t s i s rarely available for par t i c u l a r 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 r e l a t i v e i n d i c i e s . Although promising f o r aquatic communities with r e l a t i v e l y uniform habitat structure, the method has not been developed. Recently however, Landin (1976) showed that 78 there are high correlations between absolute and r e l a t i v e population estimates for near-shore hydrophilid beetles. The present study documents a high correlation between r e l a t i v e timed-catch sampling and absolute quadrat-count population measures for s i m i l a r - s i z e d species of Gerris occurring in two s t r u c t u r a l l y d i f f e r e n t habitats. This association can be used as a basis f o r predicting absolute numbers from r e l a t i v e measures. Because a l l l i f e stages operate s i m i l a r l y i n overlapping two dimensional habitats, gerrids are ideal subjects for t h i s approach. 79 METHODS A. Seasons, Species, and Habitats Most ponds and small lakes in the study area have l i t t l e emergent or submergent vegetation i n early spring when f i r s t colonized by overwintering Gerris adults. However, larger lakes with bulrush beds retain a mat of dead growth from the previous year that affords cover f o r colonizing gerrids. Spring adults were sampled between 25 May and 6 June, 1975, on ponds with no development of vegetation. At Becher's P r a i r i e , 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 i n the samples. Summer adult and juvenile populations were measured on Boitano Lake and "Sp 2" at approximate ten day i n t e r v a l s s t a r t i n g with the appearance of new vegetation and concluding i n 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 i n the bulrush habitat of Boitano Lake while both G. buenoi and G. pinqreensis bred successfully on Sp 2. A l l sampling was done between 10:00 am and 5:00 pm on sunny 80 days. The sampling program was designed to y i e l d both r e l a t i v e and absolute population estimates for each developmental stage over a range of natural densities. The r e l a t i v e estimate was made f i r s t 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 s i m i l a r habitat structure. A l l i d e n t i f i c a t i o n s 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 i s well defined by the l i m i t s of aquatic vegetation. Before vegetation appeared i n the spring, l i m i t s were a r b i t r a r i l y defined by the safety margin of my hip waders (approximately 85 cm depth). Samples were collected with a long-handled (1.6 m), c i r c u l a r 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 f l a t nylon mesh bottom (approximate mesh s i z e : 1mm2). When timed-catch samples are to be compared the procedure must be standardized as much as possible. Gerrid d i s t r i b u t i o n s are patchy and diff e r e n t l i f e stages select d i f f e r e n t habitats (Vepsalainen and Jarvinen, 1974; Chapter V) therefore the usual practice of counting standard sweeps was abandoned i n favor of sampling over a standard range of habitat i n a consistent 81 manner. The standard sampling route i s i l l u s t r a t e d 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 i s 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 i s less than some predetermined l i m i t the route becomes rectangular. My sampling r i t u a l required that at least two sides of the route were 15 steps (approximately 10.5 m) i n length. When the vegetation included several structural types, sampling routes were selected that included a l l potential habitats. As the sampling route was transversed, one f u l l 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 i n the net. Netted samples were emptied into a round p l a s t i c tub (35 cm diameter, 18 cm depth), half f u l l of strained pond water, for f i e l d sorting. After gentle mixing to d i s t r i b u t e the accumulated vegetation and debris, gerrids were t a l l i e d as they floated or swam to the surface. Adults and late i n s t a r s were t a l l i e d and released. Young juveniles were preserved i n 70% ethanol and returned to the laboratory f o r positive 82 Figure 16. A schematic diagram of the standard sampling route. 83 SHORELINE 84 i d e n t i f i c a t i o n . The f i n a l 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 p l a s t i c screening. Two foam f l o a t s 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 a l l gerrids captured within the fl o a t i n g 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 f o r each species encountered. The 1 m2 box became impractical with the emergence of vegetation and hatching of early stages for two reasons. F i r s t l y , thick vegetation (especially bulrushes) delayed or prevented the box from s e t t l i n g into the water, surface, thus allowing gerrids to escape. Secondly, high densities of early i n s t a r s were encountered so that counting insects stretched sampling time beyond reasonable l i m i t s . A l l 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 f r a g i l e to throw, samples were taken by slowly wading to a predetermined sample location, 85 s t a n d i n g s t i l l f o r two m i n u t e s , a n d t h e n d r o p p i n g , t h e box a t arms l e n g t h i n t o t h e w a t e r . A d u l t s and l a t e i n s t a r l a r v a e were c o u n t e d and r e l e a s e d . E a r l y i n s t a r s w e r e - c a u g h t w i t h a s m a l l n e t and a s p i r a t o r , p r e s e r v e d i n 7 0 % e t h a n o l and r e t u r n e d t o t h e l a b o r a t o r y f o r p o s i t i v e i d e n t i f i c a t i o n . A t Sp 2 i t was j n e c e s s a r y t o c l i p a n d remove a l l v e g e t a t i o n f r o m t h e e n c l o s e d g u a d r a t b e f o r e c o u n t i n g g e r r i d s . The number o f e m e r g e n t s h o o t s was a l s o t a l l i e d a t Sp 2. O b v i o u s d i f f e r e n c e s were n o t e d i m m e d i a t e l y b e t w e e n a d u l t and e a r l y i n s t a r d i s t r i b u t i o n s . T h e r e f o r e , i n o r d e r t o o b t a i n u n b i a s e d e s t i m a t e s and q u a n t i t a t i v e m e a s u r e s o f h a b i t a t a s s o c i a t i o n ( r e s u l t s p r e s e n t e d i n C h a p t e r V ) , I t o o k q u a d r a t s a m p l e s o v e r a r a n g e o f l a k e d e p t h s . A t Sp 2, 25 m. i s o p l e t h s were m a r k e d by p o s t and s t r i n g a t d e p t h s o f 5, 15, 2 5 , 3 5 , and 45 cm. At B o i t a n o L a k e w h i c h d r o p s o f f much more s t e e p l y , i s o p l e t h s were m a r k e d a t 5, 2 0 , and 35 cm. The i s o p l e t h l i n e s were r e - e s t a b l i s h e d t w i c e d u r i n g t h e summer on Sp 2 o w i n g t o d r o p p i n g w a t e r l e v e l . On e a c h s a m p l e d a t e two g u a d r a t s were c o u n t e d a l o n g e a c h i s o p l e t h a t Sp 2 and f o u r were c o u n t e d a l o n g e a c h i s o p l e t h a t B o i t a n o L a k e . S ample l o c a t i o n was e s t a b l i s h e d by p i c k i n g a t w o - d i g i t random number l e s s t h a n 35. I t h e n waded a l o n g t h e c u r r e n t i s o p l e t h l i n e t h a t number o f s t e p s b e f o r e d r o p p i n g t h e s a m p l e box. C o u n t s o f 10 and 12, 0.5 m 2 q u a d r a t s were a v e r a g e d f r o m Sp 2 and B o i t a n o L a k e , r e s p e c t i v e l y , on e a c h d a t e . E s t i m a t e s o f number p e r s g u a r e m e t e r were c o m p u t e d and r e c o r d e d f o r e a c h s t a g e a n d s p e c i e s e n c o u n t e r e d . To a l l o w c o m p a r i s o n o f s p r i n g 86 and summer re s u l t s , 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 t h i s study, monthly estimates were made for gerrid populations at s i x new lakes i n the study area sta r t i n g i n May 1976. Habitats with grass/sedge, bulrush, f l o a t i n g (Polygonum sp.) and submergent (Myriophyllum and Ceratophyllum) vegetation were sampled. The lakes employed are l i s t e d i n Table 16. Three timed-catch samples were taken at each location at each sample v i s i t and the average value obtained was used to estimate absolute density. These r e s u l t s were compared with the absolute densities recorded with quadrat sampling i n 1975 and with published absolute estimates from other Gerris populations to assess the r e l i a b i l i t y of the method. 87 RESULTS The results of absolute and r e l a t i v e sampling conducted in 1975 were compared with regression analysis. The f i n a l objective was to predict density from timed-catch samples. Therefore i t i s appropriate to treat the absolute measure as the dependent variable for regression. Both variables were transformed to the natural log scale.to s a t i s f y assumptions of the l i n e a r regression model. The untransformed data show that timed-catch samples become r e l a t i v e l y less e f f i c i e n t as absolute numbers increase. Also the r e l i a b i l i t y of either method f a l l s off greatly at low densities f o r a l l stages. However the function log(x+1.0) s a t i s f a c t o r i l y transformed the data to l i n e a r i t y . A. Differences Between Seasons and Size Classes The analysis was f i r s t performed in the usual manner with regression equations that included non-zero intercepts. Data f o r each summer ins t a r ( i . e . s i z e class) and spring adults were considered as separate blocks. The calculated intercepts and t h e i r standard errors are shown i n Table 11. Most intercepts l i e close to the o r i g i n ; f i v e of seven l i e within one standard error. I f the two variables are d i r e c t l y 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 l i n e was constrained to go TABLE 11 Intercepts and standard errors f o r a l l summer inst a r s and spring adults SPRING INTERCEPT STANDARD ERROR N 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 o r i g i n ) . The within-block co r r e l a t i o n between the two measures i s very strong (r=0.903; df=131). Table 12 gives the o v e r a l l analysis of variance obtained. There i s no s i g n i f i c a n t curvature but there i s s i g n i f i c a n t heterogeneity between the slopes observed in the d i f f e r e n t blocks. Table 13 l i s t s the slopes and t h e i r standard errors obtained for each block. The slopes are much greater for the f i r s t four i n s t a r s than for f i f t h stage- larvae and adults. Higher capture rates observed f o r larger gerrids probably result from th e i r greater v i s i b i l i t y during sampling. Slopes are si m i l a r for f i r s t and second i n s t a r s as well as for t h i r d and fourth i n s t a r s suggesting that these data might be pooled i n subsequent analysis. The data of Table 13 also show that real differences exist between size classes and seasons. Regression l i n e s for the size classes suggested by data in Table 13 are shown i n Figure 17. Differences between size classes seem to be r e l a t i v e l y more important than differences between seasons within the adult s i z e class. Data i n Table 13 suggest that the re c i p r o c a l of the slope of the regression l i n e (= a v a i l a b i l i t y for capture) for each species and instar should vary with some. index of body size. A v a i l a b i l i t y i s plotted against length of meso-thoracic leg i n Figure 18. A second order polynomial equation was f i t t e d to the data by the method of l e a s t squares. . The a v a i l a b i l i t y for capture can be adequately predicted ;(r=0.966; df=11) from mesothoracic leg length with the following equation: . a v a i l a b i l i t y = 0.7834 - 0.0910X + 0.0133X2 TABLE 12 Analysis of variance of regressions of capture e f f i c i e n c y f o r various developmental stages of Gerris df M.S. Regression 1 1 8 6 . 6 8 1556 <-QQ1 Regression within blocks 6 4 . 4 6 3 7 . 1 7 <.Q01 Curvature 1 0 . 0 2 Remainder 124 0 .12 TABLE 13 Proportionality constants obtained for a l l summer insta r s and spring adults - regression contrained through the o r i g i n (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 F i g u r e 17. Regressions of a b s o l u t e d e n s i t y on number of g e r r i d s caught per minute. (a.) f i r s t and second i n s t a r s , (b.) t h i r d and f o u r t h i n s t a r s , (c.) f i f t h i n s t a r s , (d.) s p r i n g and summer a d u l t s . 93 94 F i g u r e 18. Polynomial r e g r e s s i o n of a v a i l a b i l t y f o r capture on mesothoracic l e g l e n g t h f o r a l l stages of G. buenoi and G. pingjreensis. 96 B. Differences Between Habitats The four summer groups suggested by Table 13 were each s p l i t into two blocks representing samples from grass/sedge and bulrush habitats respectively. Regression analysis was employed to analyze each group for" s i g n i f i c a n t block differences. The calculated slopes and th e i r S.E. are presented i n Table 14. No s i g n i f i c a n t differences ex i s t between block slopes for any size c l a s s . A l l juvenile stages tend be s l i g h t l y harder to catch by timed-sampling i n bulrushes. However, t h i s relationship i s reversed for adults. Therefore, there i s no need to adjust estimates from these two par t i c u l a r habitats f o r differences i n sampling e f f i c i e n c y when predicting absolute abundance. C. Differences Between Species Gerris buenoi and G. pinqreensis are very s i m i l a r i n size throughout development (Scudder and Jamieson, .1972). Separate regression analyses for each size c l a s s were used to assess possible differences i n capture e f f i c i e n c y that might stem from behavioural differences between species. No s i g n i f i c a n t differences, were found among the four summer s i z e classes analyzed. Means and standard errors are given i n Table 15. TABLE 14 Proportionality constants and t h e i r standard errors for di f f e r e n t s i z e classes i n 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 G. buenoi and G. pinqreensis size classes of 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 t e s t estimates are compared with the range of absolute estimates used to ca l i b r a t e the procedures and available published estimates for other Gerris populations i n Table 16. Data from Vepsalainen (1971) were derived by div i d i n g 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 i s i n s u f f i c i e n t to allow comparison. Density estimates for s p e c i f i c stages cannot be compared because no partitioned estimates have been published. A l l high estimates reported i n Table 16 contained a substantial proportion of early i n s t a r s . The estimates of Jarvinen et a l . (1977) are s i t e s p e c i f i c and therefore, should be higher than estimates derived from t h i s study which are averaged over the whole range of potential habitat. The main point to note i s that absolute densities calculated from timed-catch sampling provide density estimates i n 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 d i f f e r e n t European gerrids on small, uniform habitats. Recently, Jarvinen et a l . (1977) have based estimates of Gerris densities upon the duration of i n d i v i d u a l insect v i s i t s 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. A l l . o f 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 i n t e r v a l . A l l but the method of d i r e c t quadrat counts are r e s t r i c t e d i n practice to adults of multispecies communities. Studies focused at the multispecies l e v e l and comparisons made over a wide range of habitats necessitate the use of r e l a t i v e abundance measures. Brinkhurst (1959) used timed-catch sampling to compare gerrid populations , i n several habitats. Timed-catch sampling has been employed to assess the r e l a t i v e abundance of other freshwater (Taylor, 1968; Zimmerman, 1960) and r i p a r i a n (Andersen, 1969; Spence, 1978) insects over a range of potential habitats. The r e l i a b i l i t y of these methods has been questioned by Andersen (1973) because of suspected differences i n sampling e f f i c i e n c y between habitats and seasons. The present analysis indicates that there i s some hope fo r the continued use of r e l a t i v e methods provided that they are 101 compared with absolute measures to estimate appropriate correction factors. Human beings are not perfect sampling machines. Hairston et a l . (1958) c i t e an unconcious tendency to c o l l e c t i n high density patches or to select the most obvious animals as weaknesses of timed-catch sampling. In t h i s study the f i r s t 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 v i s i b l e animals are collected with greater e f f i c i e n c y . Generally, biases of "human f r a i l t y " should be minimized, and subject to correction, when sampling follows a prescribed r i t u a l with respect to p o t e n t i a l habitat. Landin (1976) found a d i r e c t relationship between size and capture e f f i c i e n c y among aquatic hydrophilid beetles. He pointed out that additional f a c t o r s such as color and behaviour affe c t timed-catch sampling that involves search, pursuit and capture of i n d i v i d u a l animals. For such methods, Landin concluded that only classes of animals with similar capture e f f i c i e n c i e s should be sampled simultaneously. However, a prescribed routine minimizes pursuit of individuals and the foregoing analysis shows that i t i s possible to apply di f f e r e n t correction factors to estimate the absolute numbers of several classes sampled simultaneously. The r e l a t i o n s h i p i l l u s t r a t e d i n 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 i n species behaviour, density estimation i s possible for a l l four Gerris species in the study area. This pragmatic approach, used i n the absence of s p e c i f i c data, w i l l be used to make a l l subsequent density estimates presented. When dealing with animals outside of the size range considered i n Figure 1 8 , however, caution i s necessary. The regression equation may poorly estimate the capture e f f i c i e n c i e s of larger and smaller animals. For example, consider the two species of Limnoporus occurring i n the study area. Adults and f i f t h stage larvae are larger than any stage of co-occurring Gerris species. Conseguently, these two stages should be more v i s i b l e , and hence, more available to sampling. However, t h e i r 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 i n t e r a c t to determine capture rates i s unknown at present. Fortunately, Limnoporus i s much less common than Gerris i n the study area and, at these low densities, errors r e s u l t i n g from application of the Gerris estimator w i l l not seriously d i s t o r t an o v e r a l l view of the water-strider guil d . A surprising r e s u l t of t h i s analysis i s that no s i g n i f i c a n t differences in sampling e f f i c i e n c y were established between grass/sedge and bulrush habitats. However, t h i s does not necessarily mean that habitat differences are unimportant. The 103 seasonal differences observed between spring and summer adults i s probably a r e f l e c t i o n of differences i n amount of vegetation cover present. Vegetation can bring about t h i s e f f e c t in two ways: (1) cover makes large gerrids l e s s v i s i b l e and impossible to pursue with a single net sweep and (2) increasing cover diminishes the actual capture rate of gerrids i n the path of the net by providing refuges and increasing net drag. Assuming that absence of vegetation i s s u f f i c i e n t to account for increasing sampling e f f i c i e n c y in the spring, the presence of vegetation can reduce the proportionality constant by as much as 26% on the study lakes. The upper l i m i t s of error for summer population estimates r e s u l t i n g from variable vegetation densities i s i l l u s t r a t e d by considering the difference between application of spring (no vegetation) and summer constants to the highest adult capture rate recorded i n test-estimates made i n summer 1976. A density of 9.28 G. buenoi adults/square meter was calculated at Gerrid City (Becher's P r a i r i e ) , using the summer constant derived on Sp 2 and Boitano Lake. Application of the spring constant to the same data, lowers t h i s estimate by approximately 50% to 4.62/m2. This i s s t i l l a r e l a t i v e l y high estimate. The best estimate for t h i s sample must l i e 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 i s representative of a t y p i c a l C h i l c o t i n bulrush bed. I suspect that presence of 104 vegetation diminishes sampling e f f i c i e n c y to some extent through interaction of the two e f f e c t s mentioned above. F i e l d experience suggests that the e f f e c t s are greatest i n dense grass/sedge or bulrush habitat. Presently, I have no quantitative basis to compare sampling e f f i c i e n c y across the f u l l range of Gerris habitats encountered. However, the range of estimates presented i n Table 16 show that data at hand are good enough to allow reasonable estimates. Future development of the method should attempt to adjust estimates i n more open grass/sedge habitats by some function of vegetation density. Sampling e f f i c i e n c y in habitats of other structure might be measured and adjusted from the bulrush-grass/sedge baseline by a s i m i l a r function. A si m i l a r approach has been suggested by Caughley et a l . (1976) for c a l i b r a t i n g a e r i a l survey counts of large mammals. The p r i n c i p a l disadvantage of the method i n two-dimensional habitats i s the i n i t i a l work of c a l i b r a t i n g the estimation procedure. Individual sets of correction factors w i l l probably be necessary for each worker to accommodate differences in personal sampling s t y l e . The problem of habitat differences i s d i f f i c u l t and has not been f u l l y overcome by t h i s analysis. Whether the e f f o r t i s worthwhile w i l l be determined only from the c o l l e c t i v e r e s u l t s of studies that attempt i t . The re s u l t s of t h i s study allow summer density estimates for gerrids with the proviso that such estimates are interpreted as setting an upper l i m i t to possible densities. In general, the estimates so obtained compare favorably with the range of 105 TABLE 16 A comparison of tes t estimates with Gerris population estimates from the l i t e r a t u r e ABSOLUTE RANGE OF STAGES/ SOURCE METHOD DENSITY SPECIES PRESENT ESTIMATES Gerris Jarvinen et a l . (19 77) |duration of | i n d i v i d u a l | v i s i t s 0.59-no data arqentatus | adult Jarvinen et a l . (1977) I n II 3.1T15. 1 arqentatus adult Jarvinen et a l . (1977) 1 11 1! 37.3-3 62,7 arqentatus 1-4 Vepsalainen (1971) | mark - | | recapture < 0,61-2.50 odontoqaster adult Sp 2 (this study) | quadrat- | | count 2.6-64.8 buenoi a l l Sp 2 (this study) 1 11 0.8-8.2 pinqreensis a l l Boitano L . ( t h i s study) 1 " 1.8-79.6 I piugreensisj a l l TIMED CATCH TEST ESTIMATES-1976 VEGETATION TYPE I ' ' 1 " ' i Grove Pond | qrass/ | sedge 3.1-176.2 buenoi a l l Gerrid City • II 3. 8-75.2 buenoi a l l Sp 8 Clear Lake |submerged | f l o a t i n g 3.7-25.3 1. 1-67^ 8 comatus 1 comatus a l l a l l Westwick Lake | rush 1.5-79.8 1 pinqreensis a l l Sapper Lake 1 " 3. 4-75.2 pingreensi s a l l 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 l i v i n g in two-dimensional habitats provided that sampling minimizes in d i v i d u a l pursuit. I t may also work for near-shore aquatic invertebrates that do not a c t i v e l y respond to three-dimensional habitat complexity. Three-dimensional complexity qreatly maqnifies the problem of consistent sampling over the f u l l range of potential habitats. Complicated r e l a t i v e sampling techniques would be necessary, thus eliminating the time-efficiency advantaqes of r e l a t i v e 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 B r i t i s h Columbia (Scudder, 1977). Most of these show considerable range overlap i n the province, and several species may inhabit the same small pond. Because gerrids are unspecialized, opportunistic predators (Lumdsen, 1949) , that are l i k e l y to experience periodic resource l i m i t a t i o n (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 B r i t i s h Columbia (U.B.C. Research Forest, Haney, B.C.). He found differences i n l i f e cycle timing that apparently separated the peak abundances of several species over the season, and also noted species differences i n microhabitat preference. The gerrid fauna of the s a l i n e lakes on the Fraser Plateau (Scudder, 1969) o f f e r an in t e r e s t i n g comparison with the fauna studied by Jamieson. Quite d i f f e r e n t from the cool, wet f i r -hemlock forest surrounding Marion Lake, the Fraser Plateau study area i s dry, r o l l i n g , grassland dotted with numerous lakes and sloughs of varying s a l i n i t y (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 l i f e cycles, voltinism and habitats of gerrids i n central B r i t i s h Columbia. These data can be compared with r e s u l t s from other studies and used to formulate more s p e c i f i c hypotheses r e l a t i n g to the coexistence of water-strider species. 109 MAMMALS AND METHODS A. Lakes and Species Studied F i e l d populations of G. buenoi, G. comatus, G. pinqreensis and Limnoporus spp. were studied at two s i t e s (Chapter I) on the Fraser Plateau of south-central B r i t i s h Columbia. Forty-five study lakes were v i s i t e d at approximate ten-day i n t e r v a l s between late May and mid-September. These lakes are l i s t e d and described i n Appendix I. Their exact locations are shown on Figures 2 and 4 (Chapter I ) . 1. Fie Id Temperatures A i r and surface water temperatures were recorded throughout the summer at both study s i t e s with Ryan (Model D) submersible temperature recorders (Ryan Instruments Inc., Seattle, Wa., U.S.A.). Air temperatures were recorded i n the shade, at ground l e v e l , near Westwick Lake at Springhouse and at Opposite Crescent at the Becher's P r a i r i e study s i t e . Surface water temperatures were monitored at small, medium and large lakes at both study s i t e s (Springhouse: Grove Pond, Sp 1 and Westwick Lake; Becher's P r a i r i e : Opposite Crescent, Clear Lake and Lake Lye; — lake surface areas provided in Appendix I.) F i e l d temperatures were used i n conjunction with data on growth thresholds from Chapter II in order to compute physiological time scales for gerrids. I t i s d i f f i c u l t to determine what temperatures should be used in c a l c u l a t i o n s for 110 semi-aquatic animals (i.e. what temperatures are actually experienced by gerrids i n the f i e l d ) . Calabrese (1977) and Matthey (1976) suggest that water temperatures are most r e l i a b l e . However, Jamieson (1973) showed that gerrid body temperatures r i s e where animals are exposed to dir e c t sunlight. In order to correct for the e f f e c t of in s o l a t i o n , Jamieson adjusted day-degree sums by an a r b i t r a r y i n s o l a t i o n c o e f f i c i e n t that decreased symmetrically on either side of the summer s o l s t i c e . I have adopted a somewhat d i f f e r e n t approach in these studies. A l l p h y s i o l o g i c a l timescales presented have been calculated with a modified version of the algorithm for temperature summation which was l i s t e d by Fraser and Gilbert (1976). The procedure assumes that water temperature i s constant at the recorded minimum and that gerrids experience water temperature whenever i t i s greater than a i r 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 e f f e c t s , as measured by recorded a i r temperatures, and allows water temperature to buffer the e f f e c t s of low temperatures when they occur. Spence et a l . (1978) have shown that t h i s summation procedure allows good prediction of egg laying and f i r s t i n s t a r appearance in the f i e l d . 111 Figure 19. Areas summed in c a l c u l a t i o n of physiological time scales. | | AREAS SUMMED AIR TEMPERATURES 113 2. .Egg Production and Alary Mprphism During each sampling i n t e r v a l , females of each species were c o l l e c t e d and preserved i n 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 p a r t i c u l a r species to ensure that species population dynamics were not s i g n i f i c a n t l y altered. The c o l l e c t i o n s i t e s for female gerrids of each species are l i s t e d in Table 17. Limnoporus females were co l l e c t e d at many locations because of t h e i r r e l a t i v e l y low density throughout the study area. At the time of dissection, each female was c l a s s i f i e d with respect to wing morph following the c r i t e r i a of Vepsalainen (1971) and with respect to the presence or absence of chorionated eggs. Data were grouped into bimonthly i n t e r v a l s for analysis. 3. Population Dynamics The entire s e r i e s of study lakes was sampled within f i v e or six days from the beginning of each ten-day sampling i n t e r v a l from late May through mid-September, 1975. Each lake was sampled again during mid-May, 1976, i n order to estimate early spring breeding populations of the following year. At sampling v i s i t s , i n s t a r - s p e c i f i c abundances were estimated f o r each species with timed-catch sweeping over a standard sampling route. Details of the sampling procedure are given i n Chapter III. Animals that could be p o s i t i v e l y i d e n t i f i e d were t a l l i e d and released on s i t e . Early i n s t a r s were preserved i n 70% TABLE 17 C o l l e c t i o n s i t e s for reproductive of female gerrids used dissection SPECIES COLLECTION SITES G. buenoi Grove Pond Gerrid C i t y G. comatus Sp 1 Clear Lake Newall Lake G. pinqreensis Boitano Lake Near Round-up Lake Sapper Lake Limnoporus Grove Pond Sp 5 Gerrid C i t y Opposite Crescent Centre Arms Pond 115 ethanol f o r subsequent i d e n t i f i c a t i o n and counting i n the laboratory. F i e l d 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 I I I , Total gerrid biomass estimates for each lake (expressed as mg wet weight/m2) were obtained by multiplying calculated densities times a biomass c o e f f i c i e n t for each stage and species and then summing a l l values for a p a r t i c u l a r lake and date. Biomass c o e f f i c i e n t s were calculated as the mid-point between the maximum weights of successive in s t a r s of each species. Maximum weights were determined by rearing cohorts of animals fed to sat i a t i o n d a i l y with vestigial-winged Drosophila i n the laboratory. Animals were anesthetized with carbon dioxide and weighed on the day that the f i r s t members of a pa r t i c u l a r cohort molted to a new stage. The average maximum weights recorded for each stage of f i v e species are presented with t h e i r standard errors i n Appendix IV. 4. Habitats The study lakes provide a broad spectrum of potential habitat for aquatic and semi-aquatic insects. Three habitat c h a r a c t e r i s t i c s , that seemed important for water-striders, were measured at each lake sampled regularly i n 1975. (1) Development of aguatic vegetation was noted at each lake, and, 116 by l a t e July, i t was possible respect to the dominant type of vegetation types were presen over the area sampled, the 1 habitat. (2) Surface water monthly inter v a l s at each lake with a Radiometer CD2 conductiv to 25°C. (3) Lake size w permanence i n h i s t o r i c a l time, s e n s i t i v e to v a r i a t i o n on sho considering those lakes that dr 1975 and September, 1977 as tem The t o t a l number of each lake during the 1975 study of p separately, as a measure of re data were used to construct c compare the e f f e c t s of the gerrid d i s t r i b u t i o n s . to c l a s s i f y most lakes with vegetation-present. I f several t i n more or less equal abundance ake was c l a s s i f i e d as "mixed" samples were taken at approximate and conductivity was determined i t y meter; r e s u l t s were corrected as used as an indicator of lake Another c l a s s i f i c a t i o n more rter time scales, was provided by ied out completely between May, porary. gerrid species c o l l e c t e d at each opulation dynamics was t a l l i e d l a t i v e species abundance. These ontingency tables i n order to three habitat c h a r a c t e r i s t i c s on 117 RESULTS A. Fi e l d Temperatures Physiological time scales for gerrids, calculated from f i e l d temperatures recorded i n 1975, are presented i n Figure 20. Figure 20a shows the extent of v a r i a b i l i t y 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 a i r temperatures were used for ca l c u l a t i n g the time scales at each of the two study s i t e s , differences observed among the three lakes can be attributed s o l e l y to va r i a t i o n i n 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 a i r temperatures. Because the ef f e c t i s 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 P r a i r i e s i t e ) . The data indicate that d i f f e r e n t instars develop on vastly different time scales i n the f i e l d owing to the differences in developmental thresholds calculated i n Chapter I I . -118 Figure 20. Physiological time scales for gerrids calculated from f i e l d temperatures at Springhouse and Becher's P r a i r i e . (a.) v a r i a b i l i t y among lakes, (b.) differences between species and ins t a r s . 119 A. PHYSIOLOGICAL TIMESCALES AT SIX L A K E S THRESHOLD TEMPERATURE 3 8.7 °C CO LU LU tr CD LU Q i IOOO 8 0 0 6 0 0 4 0 0 2 0 0 L.LYE CLEAR U S P I W E S T W I C K L G R O V E R O P P O S I T E C R E S C E N T ( D R Y A U G . 7 ) 3 0 5 0 70 9 0 110 1 3 0 B. PHYSIOLOGICAL TIMESCALES AT L A K E LYE USING S E V E R A L THRESHOLDS FOR GERRID DEVELOPMENT 10 3 0 5 0 7 0 9 0 110 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 i n f u l l breeding condition at the st a r t of t h i s study, however, some females of G. comatus and Limnoporus spp. had not yet reached reproductive maturity. Females with chorionated eggs were commonly encountered among a l l four species u n t i l the end of July. Three seasonal patterns of alary morphism were observed among the gerrid species studied. Limnoporus spp. were always f u l l y winged. G. pinqreensis were . most commonly apterous, however some long-winged i n d i v i d u a l s (approximately 13% of a l l specimens collected) were encountered among overwintered and f i r s t generation populations.^ In la t e May, 1976, I also c o l l e c t e d three micropterous G. pinqreensis from Boitano Lake at the Springhouse s i t e . A common pattern prevailed for G. buenoi and G. comatus. Overwintered populations were e n t i r e l y long-winged, but a small proportion of micropterous i n d i v i d u a l s was encountered among the f i r s t summer generation. These patterns are summarized i n Table 18. Vepsalainen (1974) has pointed out that, although a continuous spectrum of wing-length i s encountered, there are only two functionally d i f f e r e n t wing morphs among water--s t r i d e r s : those that can f l y at some point i n the l i f e cycle (long-winged forms: macropters) and those that cannot (short-winged forms: apters, micropters, brachypters). Therefore I 121 Figure 21. Distribution of reproductive e f f o r t among morphs over the summer of 1975. Number at the top of each bar represents the t o t a l number of females dissected during the i n t e r v a l . (a.) G. buenoi, (b.) G. comatus, (c.) G. pinqreensis. A. G. BUENOI g IOO| ID UJ Q U J 8 0 6 0 § 401 o I o 2 0 Q Lul \— . o U J if) 1 0 0 LO 8 0 B. G. COMATUS 6 0 oo < U J L u 4 0 U_ ° 2 0 IO CD CD LO — C. G. PINGREENSIS H LONG-WINGED D. LIMNOPORUS SPP 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* micropterous* long-winged G. comatus long-winged * long-winged micropterous* long-winged G. pinqreensis apterous* long-winged* apterous* micropterous (none found i n 1975) long-winged apterous Limnoporus long-winged* long-winged not^ present * - indicates morphs found carrying chorionated eggs 124 w i l l distinguish only between long-winged and short^winged morphs in a l l subsequent discussion. Figure 21 and Table 18 indicate how the breedinq populations of each species were partitioned among the d i f f e r e n t 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 t o t a l number of females examined for each species. F i r s t generation reproductives of G. buenoi and G. comatus could be e a s i l y distinguished from non-reproductive animals because they are conspicuously marked by pale-white abdominal and, to a lesser extent, thoracic venters. M l non-teneral animals of these two species with reduced,ventral pigmentation c a r r i e d 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 f l i g h t muscles and carried 20 chorionated eggs. Second generation animals with the dark undersides t y p i c a l of overwintered adults, generally c a r r i e d no eggs. The single exception was one female G. buenoi col l e c t e d from Grove Pond on September 9 which carried s i x chorionated eggs and had f u l l y developed i n d i r e c t f l i g h t muscles. The general pattern of wing morphs was reversed in G. .Eiaareensis. Both long and short-winged animals that had overwintered were found i n reproductive condition. F i r s t generation long-winged i n d i v i d u a l s did not breed and l e f t the 125 ponds f o r winter diapause soon a f t e r the adult molt i n la t e 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 i n breeding condition, from the overwintered population or the newly emerged adults destined f o r winter diapause. However, the "fresh" appearance of newly molted animals c l e a r l y indicated that some f i r s t generation G. pingreensis did reproduce i n 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 t r a c t s were always immature. Therefore, I conclude that a l l Limnoporus spp. populations were univoltine during 1975. C. Population Dynamics Figures 22 - 25 i l l u s t r a t e the p a r t i a l population curves for each instar of G. buenoi, G. comatus, G. pingreensis and Limnoporus spp. at the two study s i t e s . Mean densities for each stage are averaged over a l l lakes where the species was recorded during a given sampling i n t e r v a l . The data are plotted as natural logarithms with points for each i n t e r v a l f a l l i n g on the median day of the samplng period. The ten-day sampling i n t e r v a l s were too long for accurate tracking of i n s t a r - s p e c i f i c populations during the early summer, because 126 Figure 22. P a r t i a l population curves for G. buenoi during 1975. (a.) Springhouse, (b.) Becher's P r a i r i e . 128 Figure 23. P a r t i a l population curves for G. comatus during 1975. (a.) Springhouse, (b.) Becher's P r a i r i e . 130 Figure 24. P a r t i a l population curves for G. pingreensis during 1975. (a.) Springhouse, (b.) Becher^s P r a i r i e . 132 Figure 25. P a r t i a l population curves for Limnoporus during 1975. (a.) Springhouse, (b.) Becher's P r a i r i e . LIMNOPORUS SPR DENSITY- LN (NUMBER/SQ. M. +1.0) PER LAKE EC I 134 high temperatures promoted rapid development on the da i l y time-scale. However some general considerations do emerge. Among the p a r t i a l l y b i v o l t i n e Gerris species, v i r t u a l l y a l l stages were present somewhere at each study s i t e throughout the sampling season. Figures 22 - 24 also emphasize that the two breeding generations are not at a l l . d i s t i n c t . There were obvious pulses of f i r s t instar larvae for G, pinqreensis at both study s i t e s but the d i s t i n c t i o n was blurred for both G. buenoi and G. comatus. For a l l b i v o l t i n e species, any i n d i c a t i o n of breeding pulses reflected in f i r s t i n s t a r abundance i s dampened in the curves plotted f o r l a t e r i n s t a r s . The data for Limnoporus spp. (Figure 25) shows that the age d i s t r i b u t i o n of contemporary juveniles was also guite broad throughout the summer. This can be explained, i n part, by the fact that overwintered Limnoporus adults survived and bred u n t i l l a t e in the season. The p a r t i a l population curves suggest that juvenile mortality was high and probably concentrated among the f i r s t four l a r v a l i n s t a r s . This i s most apparent for Limnoporus SPP- (Figure 25) . The open c i r c l e s plotted for the abundance of adult gerrids during mid-May 1976 can be compared with the data from summer 1975 as an ind i c a t i o n of the r e l a t i v e magnitudes of winter mortality. The data suggest that mortality was most severe among overwintering Limnoporus spp. Among the Gerris species, spring adult densities i n 1976 were generally higher than observed at any point during the summer of 135 1975. Increased lake to lake v a r i a b i l i t y i n i n s t a r - s p e c i f i c boundaries i s indicated by the r e l a t i v e magnitudes of the standard error bars of Figures 22 - 25, The data r e f l e c t the divergence i n age d i s t r i b u t i o n s between lakes as the season progressed. The r e l a t i v e l y large standard errors for the Limnoporus data r e s u l t from small sample sizes because in d i v i d u a l s of thi s species were often taken on l e s s than five lakes during a sampling i n t e r v a l . Figure 26 depicts the o v e r a l l population curve for each species at each s i t e as the sum of the respective i n s t a r means for a given sample i n t e r v a l . Populations of a l l species started growing e a r l i e r at Becher's P r a i r i e and remained ahead of those at Springhouse throughout the summer, A spring sequence of species population growth was observed at both study s i t e s . G. pinqreensis populations started increasing f i r s t and were followed by G. buenoi, G. comatus and Limnoporus spp. respectively. However, data i n Figure 26 demonstrates that the timing differences did not persist throughout the summer i n any consistent manner. Separate peaks of abundance for each species did not occur during 1975.. Figure 26 suggests that o v e r a l l g e r r i d 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 a l l water-strider species during 1975. (a.) Springhouse, (b.) Becher's P r a i r i e . 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 P r a i r i e . I 139 A. SPRINGHOUSE i l 4 0 : > 1 2 0 C O «oo 8 0 8 UJ 60S ffi 2 0 S Q_ 6 / 6 6 / 2 6 7/16 8 / 5 8 / 2 5 9/14 ~ B. BECHER'S PRAIRIE j _ 140 U J $ 1 2 0 Q IOOI cr L J J 8 0 O _ j 6 0 ! ^ 4d P . 6 / 6 6 / 2 6 7 1 6 8/5 8 / 2 5 9/14 MONTH/DAY, 1975 140 a l l species present f o r each sampling i n t e r v a l during 1975. Curves obtained for both s i t e s show that gerrid biomass reached a maximum value between mid-July and mid-August. However, the lake to lake v a r i a b i l i t y i n 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 f o r water-s t r i d e r s . These features are summarized i n Table 19. The major factors a f f e c t i n g gerrids seemed to include the density of emergent cover, and seasonal changes i n 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 t o t a l of 16,777 i n d i v i d u a l gerrids were t a l l i e d during 1975. The observed ( d i s t r i b u t i o n of gerrid-species i s c l a s s i f i e d 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 coll e c t e d 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 Beckmannia syzigachne closely-spaced j , much 'emergent (Steud.) Fern. thin,emergent cover by Carex sp. stems i n early July, Juncus balticus Willd. shallow water; no coyer in Lemna sp. f l o a t i n g early spring; P u c c i n e l l i a spp. plants often b a f f l e s both Scolochloa festucacea present also waves and (Willd.)Link. wind Sium suave Walt. Sparganium auqustifolium Michx. b. Floating Vegetation Ceratophyllum f l o a t i n g and/ no emergent demersum L. or submerged1 cover; Myriophyllum vegetation , baffles spicatum L. waves but Polyqonum amphibium L. not wind. Potomoqeton spp. f u l l y U t r i c u l a r i a vulgaris L. developed by early July c., Rush Scirpus validus Vahl widely-spaced much emer-Juncus balticus Willd. r e l a t i v e l y gent cover thick emergent throughout stems season, baffles both wind and waves 142 TABLE 20 Gerrid species abundance i n various habitat categories HABITAT Gerris Limnoporus TOTALS TYPE buenoi comatus pinqreensis spp. Grass/ Sedge N=13 5181 531 717 302 6731 Floating N=10 694 2637 280 8 3619 Rush N=1.1 172 356 3138 3 3669 Open N=3 1 13 1 0 15 Mixed N=7 812 1089 801 41 , 2743 Totals 6860 4626 493? 354 16777 __________ _________ ____________ _____________ ____________ _________ * N=number i n each category 143 G. comatus and G. pinqreensis were strongly associated with f l o a t i n g vegetation and rushes respectively. Data of Table 20 emphasize the r e l a t i v e rareness of Limnoporus on the study lakes. In habitats without a single dominant vegetation type ("mixed" i n Table 20), r e l a t i v e Gerris species abundance was much more evenly d i s t r i b u t e d . However, there was a s l i g h t tendency for G. comatus to predominate. Juvenile gerrids were never collected from the three lakes v i s i t e d without aquatic vegetation (Blake L., Drummond L., Round-up Lake). A l l animals recorded at these open habitats were winged adults c o l l e c t e d at the time of spring dispersal. 2. Surface Conductivity Lakes were c l a s s i f i e d into three groups with respect to maximum recorded surface conductivity (0-1000, 1000-4000, >4000 -Hlmhos/cm at 25°C) . Table 21 presents the relat i o n s h i p 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 i s strongest f o r 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 i n each category 145 3. Lake Permanence Table 22 shows the r e l a t i o n s h i p between species abundance and lake s i z e . G. buenoi and Limnoporus were most common on small ponds while the r e l a t i v e 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 i n Table 23. There i s an obvious difference between species with respect to proportion of the t o t a l population encountered in extremely temporary habitats. Limnoporus spp. invested the highest proportion of i n d i v i d u a l s in habitats l i k e l y 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 i n various categories of lake area AREA (hectares) Gerris Limnoporus TOTALS buenoi comatus pinqreensis < 2.5 N=18 4617 1888 1652 338 8495 2.5-5.0 N = 14 1160 2440 1257 8 4865 < 5.0 N=13 1083 298 2028 8 , 3417 Total 6860 4626 4937 354 16777 * N=number of lakes i n each cateqory 147 TABLE 23 Gerrid species abundance in temporary and more permanent habitats HABITAT CLASSIFI-CATION Gerris Limnoporus TOTALS buenoi comatus pingreensis Temporary N=14 2393 (- 349) 1066 (. 230) 77 5 (.157) 175 (.494) 4409 Permanent N=31 4467 (.651) 3560 (.770) 4162 (.843) 179 (. 506) 12368 Total 6860 4626 4937 354 16777 * N=number of lakes i n each category; number i n parentheses represents proportion of a l l in d i v i d u a l s of that species collected i n each habitat 148 DISCISSION A. L i f e 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 t h i s study i s the f i r s t report of a si m i l a r r e l a t i o n s h i p f o r North American gerrids. When established, t h i s c o r r e l a t i o n greatly s i m p l i f i e s the study of gerrid l i f e cycles because i t allows dir e c t estimation of the size of the summer reproductive generation from f i e l d 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 l i f e cycles among temperate gerrids. Present evidence suggests that nymphs are switched to becoming diapause (-overwintering) adults i f photoperiods begin decreasing before a c r i t i c a l period during the fourth i n s t a r . Thus gerrids that pass through the c r i t i c a l period before the s o l s t i c e w i l l reproduce during the same season. The re s u l t s of my investigations are generally consistent with Vepsalainen*s model. Figures 22 - 24 demonstrate that fourth i n s t a r s of G. buenoi, G. comatus and G. pinqreensis were present at both study s i t e s before the summer s o l s t i c e of 1975. Because fourth i n s t a r s of G. comatus were not common at the 149 s o l s t i c e , the summer generation of reproductives should have been smaller than i n the other two Gerris species. Data in Figure 21 confirm t h i s expectation for the r e l a t i v e abundance of summer generation reproductives between G. buenoi and G. comatus. However, r e l i a b l e comparison with G. pinqreensis i s impossible in the absence of a good morphological marker for summer generation breeders. The p a r t i a l 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 s i t e s and, as predicted by Vepsalainen*s model of diapause c o n t r o l , no fourth instars were c o l l e c t e d before the summer i s o l s t i c e (Figure 25). This was brought about, i n part, because Limnoporus spp. commenced breeding l a t e r than the Gerris species on the Fraser Plateau. Jamieson (1973) found that Limnoporus n o t a b i l i s was also s t r i c t l y univoltine i n southwestern B r i t i s h Columbia, and suggested that breeding i s suppressed u n t i l a c r i t i c a l thermal threshold has passed. Limnoporus n o t a b i l i s occurs as far south as New Mexico (Jamieson, 1973), but no data are a v a i l a b l e concerning the l i f e cycles and voltinism of more southern populations. However, L. canaliculatus i s known to be multivoltine in southeastern United States (Calabrese, 1977). I t therefore seems that L. n o t a b i l i s probably has the pot e n t i a l to produce more than one generation per year. It i s l i k e l y that occasional cool summers at higher 150 l a t i t u d e s select strongly against a second generation among Limnoporus populations. The Limnoporus species are the largest in B r i t i s h Columbia and require the longest time to complete l a r v a l development (40+ days at 22°C; Chapter I I , Appendix I I ) . In the case of la t e summer cold s p e l l s , second generation larvae would have d i f f i c u l t y completing the adult molt and therefore, could not overwinter (Andersen, 1 973). The extended reproductive l i f e observed for Limnoporus i n the f i e l d may compensate for predictable univoltinism i n B r i t i s h Columbia. Jamieson (1973) has presented some evidence that Limnoporus i s also more fecund than co-occurring Gerris species. Although the main features of gerrid l i f e cycles i n B r i t i s h Columbia can be reconciled with Vepsalainen's model, there i s one point that deserves comment. A single long-winged G. buenoi female taken in September, 1975 carried chorionated eggs despite i t s appearance as a t y p i c a l , dark-ventered diapause i n d i v i d u a l . Also, Figure 24 shows a pulse of f i r s t instar G? pingreensis at Springhouse i n September, 1975 owing to the sudden appearance of f i r s t i n s t a r larvae at Westwick Lake. Jamieson (1973) noted a s i m i l a r late season pulse for G. buenoi at Marion Lake i n South-western B r i t i s h Columbia. Vanderlin and Streams (1977) have shown that diapause may be broken by cool autumn temperatures i n Notonecta even though the primary control i s exercised by photoperiod. I t i s possible that analagous e f f e c t s of cold temperatures may explain the l a t e season reproduction observed among gerrids i n B r i t i s h Columbia. Andersen (1973) has reported p a r t i a l l y disintegrated 151 oocytes in G. l a c u s t r i s c o l l e c t e d l a t e i n the season i n 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 i n d i v i d u a l gerrids, the percentage of the population that breeds i s ultimately controlled by spring temperatures. Warm springs w i l l lead to larger second generations because of accelerated rates of juvenile development (Chapter I I ) . Several authors (Andersen, 1973; Jamieson, 1973; Vepsalainen, 1974a) have commented on the pronounced effect of f i e l d microclimates. In f a c t , the lake to lake v a r i a b i l i t y on gerrid p hysiological time scales probably accounts for one of the most puzzling f i e l d observations made during , t h i s study; as the season progressed, lake to lake v a r i a b i l i t y in species age d i s t r i b u t i o n s became s t a r t l i n g . On some -lakes there was no evidence that summer generation reproductives were produced by any species. The gerrid physiological time scales presented i n Figure 20a show that as much as a two-fold difference may exist in accumulated day-degrees by the c r i t i c a l s o l s t i c e period. Differences i n 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 i n t e r a c t i o n , from the perspective of comparative population dynamics, awaits the development of adequate simulation models. These must incorporate the complexity introduced by d i f f e r e n t growth thresholds and l o c a l temperature 152 v a r i a t i o n i f we are to believe the r e s u l t s . B. Comparative Ecology 1. Habitats and Timing This study has demonstrated that there are s i g n i f i c a n t e cological 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 i n the timing of spring appearance and the i n i t i a t i o n of breeding. However, subsequent peaks of abundance overlap greatly i n time among a l l four species studied. In contrast, Jamieson (1973) found differences in l i f e cycle timing that seemed to resu l t i n seasonal separation of g e r r i d species occurring on Marion Lake in South-western B r i t i s h 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 d i s p e r s a l would e n t a i l high r i s k s (Vepsalainen , 1978) and selection operating to segregate species would be most intense with respect to intralake factors. However, the ontogenetic programs governing l i f e 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, i t i s doubtful that timing can be r e l i a b l y adjusted to accommodate for i n t e r l a k e differences i n species composition, e s p e c i a l l y i f there i s moderate year to year variation i n climate. I t i s not surprising, then, that Jamieson (1973) also found s t r i k i n g differences i n microhabitat preference among the species that he studied. In f a c t , 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 l i f e cycle timing, alary dimorphism and habitat with t h e o r e t i c a l models and f i e l d data. In general they have been concerned with the genetic strategies that i n d i v i d u a l species employ to deal with seasonal variation. Results of t h e i r work suggest a strong association between the tendency to occupy temporary habitats and the retention of f l i g h t a b i l i t y . In more permanent habitats, natural selection leads to monomorphism for f l i g h t l e s s n e s s because the cost of l o s t dispersers outweighs the gain from colonizing new habitats. F i e l d 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 i n temporary habitats. Table 23 shows that a l l Gerris species considered here place the bulk of t h e i r reproductive e f f o r t i n habitats that are r e l a t i v e l y permanent over the short run. However, a l l species retain the long-winged phenotype, contrary to t h e o r e t i c a l 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 i n t e r v a l s of "tens to hundreds" of years may be necessary to make ger r i d 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 i n t e r v a l s are s u f f i c i e n t to render gerrid habitats temporary. Two other factors may reduce the effect of natural selection against the long winged morph. F i r s t l y , interlake distances are r e l a t i v e l y small and the intervening grassland does not obscure f l i g h t paths between lakes (Figure 2 and 4, Chapter I ) . Therefore r i s k s involved i n lake to lake dispersal 155 s h o u l d be s m a l l . S e c o n d l y , y e a r l y r a t e s o f p o p u l a t i o n e x t i n c t o n a r e s u r p r i s i n g l y h i g h ( C h a p t e r V I ) and t h u s f r e q u e n t r e c o l o n i z a t i o n o f a b andoned s i t e s i s a d v a n t a q e o u s b e c a u s e h a b i t a t s i n t h e s t u d y a r e a a r e h e t e r o g e n e o u s i n t i m e ( e s p e c i a l l y t h e b a l a n c e b e tween g r a s s / s e d g e and f l o a t i n g v e g e t a t i o n ) a s w e l l a s i n s p a c e . 3. H a b i t a t and R e g i o n a l C o e x i s t e n c e The t h r e e h a b i t a t c l a s s i f i c a t i o n s e m p l o y e d i n T a b l e s 20 23 a r e n o t i n d e p e n d e n t . R e y n o l d s and R e y n o l d s (1976) showed t h a t t h e d i s t r i b u t i o n s o f a q u a t i c a n g i o s p e r m s i n t h e s e l a k e s a r e s t r o n g l y i n f l u e n c e d by c o n d u c t i v i t y . B u l r u s h e s ( S c i r p u s v a l i d u s ) become more common w i t h i n c r e a s i n g s a l i n i t y . The most s a l i n e l a k e s a r e a l s o t h e l a r g e s t a n d most p e r m a n e n t . I n c o n t r a s t , g r a s s e s , s e d g e s and f l o a t i n g a q u a t i c s abound i n t h e s m a l l , f r e s h w a t e r ponds c r e a t e d anew ; e a c h s p r i n q by m e l t i n q snow. Submerged p l a n t s , ( e s p e c i a l l y M y r i o p h y l l u m and C e r a t o p h y l l u m ) p r e d o m i n a t e o v e r t h e m i d - r a n g e o f c o n d u c t i v i t i e s e n c o u n t e r e d among t h e s t u d y l a k e s . T h e s e s t r o n g c o r r e l a t i o n s p l u s c o m p a r i s o n o f t h e s p e c i e s s e p a r a t i o n s o b t a i n e d i n T a b l e s 20 23 a r g u e t h a t s p e c i e s - s p e c i f i c r e s p o n s e s t o h a b i t a t s t r u c t u r e h a ve been a m a j o r theme i n t h e e v o l u t i o n o f g u i l d s t r u c t u r e among g e r r i d s o c c u r r i n g on t h e F r a s e r P l a t e a u . O t h e r r e g i o n a l a s s e m b l a g e s o f w a t e r - s t r i d e r s h a v e been c h a r a c t e r i z e d by d i s t i n c t h a b i t a t a s s o c i a t i o n s ( B r i n k h u r s t , 1959b; V e p s a l a i n e n , 1973; J a m i e s o n , 1973; C a l a b r e s e , 1 9 7 7 ) . S e v e r a l s p e c i e s o c c u r r i n g i n c e n t r a l B r i t i s h C o l u m b i a h a v e been 156 studied elsewhere so some comparisons are possible. a. Gerrids i n B r i t i s h Columbia Table 24 presents data currently available on gerrid habitat preferences i n B r i t i s h Columbia. (Data from the lower mainland are from Jamieson (1973) and from my own records). For completeness I have also included data c o l l e c t e d for _.- incoqnitus on the Fraser Plateau during 1976 and 1977. Species that occur on both the Lower Mainland and the Fraser Plateau show v i r t u a l l y i d e n t i c a l habitat preferences i n both regions. Distributions of G. comatus and G. incurvatus abut, but apparently, do not overlap in B r i t i s h Columbia; to date both species have not been recorded from the same lake (Scudder, 1977). Although G. incognitus i s generally confined to Southern B.C. the known d i s t r i b u t i o n shows no clear relationship to obvious physiographic factors. The two species frequent the same habitats; both are associated with f l o a t i n g vegetation. Thus i t i s possible that interspecific,competition explains the d i s t r i b u t i o n of these two species in the province. The d i s t r i b u t i o n s and habitat preferences of these two species should be examined i n d e t a i l i n south-central B.C. (between Clinton and Kamloops) . The only cases of s i g n i f i c a n t habitat overlap within regional guilds occurs between G. buenoi and the genus Limnoporus - respectively, the "big and l i t t l e " of B r i t i s h TABLE 24 Habitat preferences of gerrid species in B r i t i s h Columbia SPECIES LOWER MAINLAND FRASER PLATEAU STUDY AREA G. buenoi inshore, clean water surfaces; thick emergent vegetation grass/sedge habitat G. comatus does not occur f l o a t i n g vegetation G. incognitus inshore, cluttered water surfaces shaded, often very temporary habitats; under dense willows and alders pinqreensis does not occur rush habitat G. incurvatus offshore, Potomoqeton beds, l i l y pads not present Limnoporus (L. n o t a b i l i s only on LML) inshore, clean water surface, grassy areas grass/sedge habitat, temporary ponds 158 Columbia gerrids. The rati o s of body length between G. buenoi and either Limnoporus n o t a b i l i s or Limnoporus d i s s o r t i s are approximately 2.0 over a l l stages of development. This i s much greater than the values of 1.2-1.3 suggested by Hutchinson (1959) as generally s u f f i c i e n t to permit complete habitat overlap through d i f f e r e n t i a l s e l ection of available food sizes. Schoener (1974b) and Istock (1977) reported s i m i l a r l y high size r a t i o s between co-occurring species of Anolis l i z a r d s and waterboatmen (Corixidae) respectively. Like water-striders, populations of these animals have d i s t i n c t size/age d i s t r i b u t i o n s , Werner (1977), working with sunfish, suggested that elevated size r a t i o s may result when species that p a r t i t i o n food resources are divided into several size classes. In such cases the d i s t r i b u t i o n s of prey size used by each species (niche width) are increased by s i g n i f i c a n t between size class components. It should be possible to extend t h i s analysis to water-striders and determine i f size differences between G. buenoi and Limnoporus are important to the coexistence of these species i n the same habitats. b. Gerrids i n Eastern and Western North America Calabrese (1977) studied the habitat associations of water-s t r i d e r s i n Connecticut. Three species of the central B r i t i s h Columbia gerrid fauna also occur i n Connecticut (G. buenoi, G. comatus, and L. d i s s o r t i s ) . However, straightforward comparisons with Calabrese*s data are d i f f i c u l t to interpret 159 because only q u a l i t a t i v e data based on a d u l t presence or absence are presented. There i s one i n t e r e s t i n g q u a l i t a t i v e c o n t r a s t between the h a b i t a t preferences recorded f o r G. comatus i n B r i t i s h Columbia and Connecticut. C a l a b r e s e ' s (1977), data show a s i g n i f i c a n t tendency f o r G. comatus t o avoid h a b i t a t s with submerged v e g e t a t i o n , but i n c e n t r a l B r i t i s h Columbia f i v e l a k e s dominated by submerged v e g e t a t i o n c o n t r i b u t e d one t h i r d (32.2%) of the i n d i v i d u a l s recorded i n Table 20 from h a b i t a t s with f l o a t i n g v e g e t a t i o n . One hypothesis t o e x p l a i n t h i s c o n t r a s t i n v o l v e s r e g i o n a l d i f f e r e n c e s i n the g e r r i d fauna. Although Calabrese's data do not i n d i c a t e s i g n i f i c a n t p o s i t i v e responses t o submerged v e g e t a t i o n , G. a l a c r i s , G. a r q e n t i c p l l i s , G. marginatus and L. c a n a l i c u l a t u s a l l occurred with f a i r freguency i n areas o f submerged v e g e t a t i o n . In E a s t e r n North America a l l of these s p e c i e s are l a r g e r than G. comatus ( B l a t c h l e y , 1926). Because body length i s d i r e c t l y p r o p o r t i o n a l to l e g length i n g e r r i d s (Matsuda, 1961), e f f e c t i v e s t r i d e l e n g t h a l s o i n c r e a s e s with body l e n g t h . T h e r e f o r e , we can surmise that l a r g e r g e r r i d s should be ab l e t o move f a s t e r and more e f f i c i e n t l y i n h a b i t a t s unencumbered by s u r f a c e v e g e t a t i o n . The l a r g e r s p e c i e s i n C o n n e c t i c u t should t h e r e f o r e have a s i g n i f i c a n t advantage over G. comatus i n h a b i t a t s of submerged v e g e t a t i o n . I t i s i n t e r e s t i n g to note t h a t G. comatus c o l l e c t e d 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 c a l c u l a t i o n s . ) . These data suggest that morphological adaptation to habitat structure i s possible among water-striders. 4. Conclusions The fascinating study of how s p a t i a l heterogeneity contributes to the evolution of pattern i n nature must remain a highly speculative matter u n t i l we .develop s o l i d habitat c l a s s i f i c a t i o n s (Southwood, 1977). These c l a s s i f i c a t i o n s must attempt to r e f l e c t the environment as perceived by the animal (Wiens, 1976; Janzen, 1977). Quantitative data on species d i s t r i b u t i o n s and regional comparisons can help to assess the usefulness of our c l a s s i f i c a t i o n s . In t h i s chapter, I have shown that the d i s t r i b u t i o n and r e l a t i v e abundance of gerrid species on the Fraser Plateau are most sensitive to habitat structure and that the most conspicuous differences i n 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 p a r t i c u l a r habitats often emerge during surveys to assess t h e i r d i s t r i b u t i o n . Habitats are c l a s s i f i e d with respect to environmental variables and the survey data are grouped i n contingency tables for analysis with the chi-sguare s t a t i s t i c (Taylor, 1968; Streams and Newfield, 1972; Fleetwood et a l . , 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 B r i t i s h Columbia suggested that the study lakes were r e l a t i v e l y 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 e s t a b l i s h that species exercise habitat preferences (Klopfer, 1 969). The experimental approach asks i f a species w i l l respond p o s i t i v e l y to a p a r t i c u l a r 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 i n Meadows and Campbell, 162 1972). Some investigators ( i . e . Madsen, 1968; Lock, 1975; Higler, 1975) have demonstrated species preferences that could account for natural d i s t r i b u t i o n s , while i n other cases (i . e . Cummins and Lauf, 1969; Gale, 1971; Dodson, 1975) the observed preferences indicated that other processes were responsible. -In t h i s chapter I discuss the r e s u l t s of f i e l d work and experiments undertaken to assess the p o s s i b i l i t y that active habitat preferences help account for gerrid d i s t r i b u t i o n s observed on the Fraser Plateau. 163 MATERIALS AND METHODS A. Dispersal . . . Habitat Selection Among Lakes Fi e l d enclosures were placed at Sp 6, Sp 8 and Westwick Lake i n early May, 1977. Each of these lakes was characterized by r e l a t i v e l y homogeneous vegetation structure, representing grass/sedge, f l o a t i n g 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 p l a s t i c sheets stretched on a wooden frame, were bolted to cornerposts. Corners were sealed by s t r i p s of foamrubber compressed in corner j o i n t s . Four enclosures were set in approximately 50 cm. of water at each lake and corner posts were pushed into the lake bottom u n t i l 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 P r a i r i e 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 f i e l d temperatures without food. The next morning 20 marked animals of each species (sex r a t i o 1:1) were placed separately i n one of 164 F i g u r e 28. F i e l d e nclosures i n floating/submerged v e g e t a t i o n at Sp 8 i n l a t e J u l y , 1 9 7 7 . 165 166 the enclosures at each of the three lakes. Four apterous _.- pinqreensis which had been s i m i l a r l y marked and handled, were also added to each enclosure as controls. Approximately twenty-four hours l a t e r 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 p o t e n t i a l emigrants were added on May 8, 12 and 19. Lost animals were replaced i n 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 i n d i r e c t f l i g h t muscles. In order to monitor the potential dispersal of summer reproductives, enclosures were checked weekly f o r immigrants between July 12 and August 8, 1977. 167 B. Habitat Selection Within Lakes 1- F i e l d Distributions a. Habitat Differences Among Species Two lakes (Opposite Near Round-up Pond, Becher's P r a i r i e and Sp 1, Springhouse), composed of d i s t i n c t patches of several habitat types, were chosen for more intensive analysis during July 1976. Separate c o l l e c t i o n s , l a s t i n g f i v e minutes, were made within patches of a l l habitats occurring on both lakes. An attempt was made to c o l l e c t every gerrid encountered while moving slowly through each habitat patch. A l l gerrids co l l e c t e d were i d e n t i f i e d on s i t e or preserved i n 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 f u l l y described i n Chapter I I I . Each quadrat sample was assigned to a mean depth thus providing a r e l a t i v e 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 t o t a l numbers of a l l inst a r s c o l l e c t e d of G. buenoi at Sp 2 and _.. pingreensis at Boitano Lake were partitioned according to 168 depth f o r analysis. 2. Laboratory Experiments . . . Responses to A r t i f i c i a l Habitat Structure Three types of experiments were run to investigate the degree to which f i e l d d i s t r i b u t i o n s can be explained by species preferences f o r simple, structural c h a r a c t e r i s t i c s of habitat. The following r e l a t i o n s h i p s were investigated: (1) the e f f e c t 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 a b i l i t y . The f i r s t section below describes the laboratory habitats and sections b, c and d describe the methods used f o r each of the three blocks of experiments l i s t e d above. F i e l d c o l l e c t e d gerrids were used i n a l l experiments with adults. F i r s t i n s t a r larvae were hatched from eggs l a i d i n the laboratory. a. Laboratory Conditions and Apparatus Two p l a s t i c wading pools, 125 cm. i n diameter, were employed i n the following experiments. Pools were f i l l e d to a depth of 1 2 - 1 4 cm. with dechlorinated tap water and a l l experiments were run afte r 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 f i t t e d 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 a r t i f i c i a l habitat are i l l u s t r a t e d in Figure 29. They were constructed as follows: (1.) A r t i f i c i a l rush ("rush"). F i f t y ml. disposible glass pipets were inserted upside-down into holes d r i l l e d at the corners of a 4.0 cm. grid. (2.) A r t i f i c i a l .grass ("grass") . P l a s t i c drinking straws (20cm. i n length, 0.4 cm. in diamter) were painted green and inserted i n 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.) A r t i f i c i a l smartweed ("floating"). Five styrofoam s t r i p s (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 " f l o a t i n g " habitat sectors at the i n t e r s e c t i o n points of a 10 cm. grid. (4.) No vegetation ("open"). These habitat sectors were simply painted grey; no a r t i f i c i a l complexity was added. b. Species Tendencies to Enter Complex Habitats 170 Figure 29. Four types of a r t i f i c i a l habitat in a laboratory pool used for preference experiments. Starting at the bottom left-hand corner, and proceeding clockwise: " f l o a t i n g " , "grass! 1, "rush" and "open" habitat. 171 172 (1.) Basic Procedure The same basic procedure was used i n a l l 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 p a r t i c u l a r configuration of a r t i f i c i a l habitats. These animals were allowed 10 - 15 minutes to d i s t r i b u t e themselves and then the positions of a l l animals were recorded at f i v e successive observation i n t e r v a l s . In order to allow f o r greater adult mobility, observation i n t e r v a l s were set at one minute for adults and two minutes for f i r s t i n s t a r s . The sums of observations recorded in each a r t i f i c i a l habitat over these f i v e observations were used as the basic data f o r analysis. Thus the tendencies to select and to remain i n a pa r t i c u l a r habitat were considered simultaneously. Food was provided ad libitum to a l l experimental animals before use. No food was available on the experimental ponds. Sex r a t i o s i n a l l adult experiments were 1:1. (2,) Experiments (a.) Some Preliminaries I n t r a s p e c i f i c attraction or some other mechanism resulting in clumped d i s t r i b u t i o n s would confound int e r p r e t a t i o n of resu l t s derived from group testing. Therefore i t was necessary to ensure that the position choices of i n d i v i d u a l gerrids were 173 independent. In-order to test the hypothesis of independence for each species, the d i s t r i b u t i o n 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 d i s t r i b u t i o n . The observed d i s t r i b u t i o n s were tested for goodness of f i t by c a l c u l a t i n g chi-square values for each species (Table 25) . These data show that i t i s appropriate to ignore the possible effects of clumping i n the following experiments. The data f o r G. comatus suggest a weak tendency f o r clumping, but i n a l l other species the recorded d i s t r i b u t i o n s were c l e a r l y not contiguous. I f i n d i v i d u a l interactions lead to more uniform spacing, the r e s u l t s of subseguent habitat preference experiments w i l l 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 ( a r t i f i c i a l rush habitats) and open habitat sectors (two contiguous sectors of each) . (b.) The Effect of Surface Conditions A l l 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 i n d i v i d u a l 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- N o 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 e l e c t r i c motor and was thereby a l t e r n a t i v e l y 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 i n the side of the pool and tipped forward u n t i l i t was angled at approximately 60° to the water surface. The fan was centered on the l i n e 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 r e f l e c t i n g surface wind, was generated through the center of the pond, perpendicular to the fan. (c.) The E f f e c t of Stage Ten groups of ten f i r s t i n s t a r 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 F i g u r e 31 . A p p a r a t u s and h a b i t a t c o n f i g u r a t i o n u s e d f o r t e s t i n g t h e e f f e c t s o f w i n d on g e r r i d h a b i t a t p r e f e r e n c e s . G e r r i d s on t h e p o o l a r e L i m n o p o r u s n o t a b i l i s , t h e l a r g e s t s p e c i e s t e s t e d . I333!!!!!'JI!1I!H railttlltt • M l 1 80 respectively with those of adults on calm surfaces as determined in the previous experiments. (d.) The E f f e c t 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 a f f e c t species d i s t r i b u t i o n 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 r e s u l t s of a l l experiments i n t h i s section. For the experiments concerning species interactions, the o r i g i n a l freguencies were weighted by the t o t a l number of observations which varied necessarily with density ( i . e . variances were estimated as c h i -sguares). Analyses of the other experiments with chi-sguare would involve the int e r p r e t a t i o n of three-way contingency tables, with associated problems of non-orthogonality. To avoid t h i s 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 o r i g i n a l freguencies. This i s appropriate, i n t h i s case, because a l l weights are the same (50 observations per run). 181 c. Selection of A r t i f i c i a l Habitat Mimics G. buenoi, G. comatus and G. pinqreensis were allowed to choose between equal areas of a l l four types of a r t i f i c i a l habitat (grass, rush, f l o a t i n g and open sectors) during t h i s experiment. T r i a l s were run using a l l three unique (excluding mirror images) s p a t i a l configurations possible with four d i f f e r e n t 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 i n the middle of a p a r t i c u l a r habitat sector, the positions of a l l animals were recorded after 15 minutes. Groups were started twice i n each type of habitat sector for a given configuration. Therefore 24 groups of ten gerrids were tested for each species. The observations on f i n a l resting place were c o l l e c t e d into a 3 x 4 contingency table (species x habitats) and analyzed for heterogeneity by c a l c u l a t i o n of chi-square, d. Habitat Structure and Foraginq Success The.objective of t h i s experiment was to determine the ef f e c t of habitat structure on the foraging a b i l i t y 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 p l a s t i c extending from the bottom of the pool to a height of 15 cm, above the water 182 Figure 32. Unique s p a t i a l confiqurations of laboratory habitats used i n 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 d i s t r i b u t e d uniformly i n each sector as i l l u s t r a t e d in Figure 3 3 . 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 a l l o t t e d foraging period the six f l i e s were collected from each sector, dried to a constant weight and reweighed. The predicted dry weight was calculated by multiplying the o r i g i n a l wet weight by a c o e f f i c i e n t (0.257±.002) calculated by drying and reweighing 20 groups of Drosophila reared under the same conditions. This c o e f f i c i e n t was adjusted to account for weight l o s t on the water surface by methods described by Jamieson (1973). The difference between t h i s 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 a l l four types of a r t i f i c i a l habitat. 185 Figure 3 3 . Starting d i s t r i b u t i o n of Drosophila i n laboratory experiments to measure foraging e f f i c i e n c y i n various habitats. o STARTING POINTS FOR DROSOPHILA STARTING POINT FOR GERRIDS 187 RESULTS A- Dispersal . . . Habitat Selection Between Lakes 1. Immigration The t o t a l numbers of each species, captured i n the f i e l d enclosures at Sp 6, Sp 8 and Westwick Lake, are recorded in Table 26. The o v e r a l l capture rate was low ( 0 . 1 1 gerrids/sguare meter/day) , however, i t i s apparent that a l l three Gerris species were dispersing during May, 1 9 7 7 . 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 a t t r a c t i v e to f l y i n g i n d i v i d u a l s of that species. The limited data do not permit such comparisons for the other species. A i r temperature had a pronounced e f f e c t on capture rate. Eighty-five percent of the captures occurred during the three days (May 8 , 9 , 23) when maximum a i r temperatures exceeded 14°C at Westwick Lake. Only one G. comatus female was caught during the summer trapping period (Sp 6, July 22, 1 9 7 7 ) . It had a dark venter, t y p i c a l 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 f l y i n g into enclosures (six 24hr periods - May 1977) SPECIES HABITAT Sp. 6 Sp 8 Westwick grass/sedge f l o a t i n g rush TOTALS G. buenoi G. comatus G. pinqreensis Limnoporus Totals 15 2 0 0 17 6 4 1 0 11 3 1 1 0 24 7 2 0 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 a l l species than for G. pingreensis. Therefore, loss from the enclosures can be used as a r e l a t i v e 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 l e a s t from grass/sedge habitats. In contrast the highest disappearance rates for G. pingreensis were observed from the f l o a t i n g habitat. A surprisingly small percentage of potential emigrants actually flew (7.92% over a l l species and experiments), even though gerrid densities i n the enclosures were 2-3 times the ambient population densities at the three lakes. This i s p a r t i a l l y explained by data in Table 28 which show that 20-30% of the Gerris species stocked had non-functional i n d i r e c t f l i g h t muscles by the end of May. This decrease i n the percentage of the population with functional f l i g h t muscles i s brought about by the h i s t o l y s i s of i n d i r e c t f l i g h t muscles that occurs coincidently with egg production i n female gerrids (Andersen, 1973). Comparison of data i n Tables 27 and 28 demonstrates the most a c t i v e l y dispersing gerrids (Limnoporus spp, and TABLE 27 Percentage of each species and morph lo s t from experimental enclosures (six 24hr, periods - May 1977) SPECIES AND ,WING MOEPH TOTAL NUMBER TESTED % LOST G. buenoi macropters 360 5. 8% G. comatus macropters 360 4.4% . G. pinqreensis macropters 360 10. 8% Limnoporus macropters 360 10.. 5% G. pinqreensis apters 288 2.8% 191 Figure 34. Total number of gerrids disappearing from enclosures in each habitat. 281 24 2 0 g or £121 cr U J CD 2 81 g f2 4 1 • GRASS/SEDGE FLOATING BULRUSH G. BUENOI G. COMATUS G PINGREENSIS LIMNOPORUS SPR CONTROLS (APTER0US Q. PIN GREEN 818) TABLE 28 Percentage of animals remaining i n enclosures with functional i n d i r e c t muscles and the sex r a t i o s of emmigrating gerrids SPECIES % OF "NON-EMIGBANTS" | WITH FUNCTIONAL | FLIGHT MUSCLES | SEX-RATIO OF "EMIGRANTS" May 10 May 24 | Male Female G. buenoi 93.1% n=58 70.1% | n=56 | 12 6 G. comatus 96.4% n=56 75.7% | n=58 | 9 5 G. pinqreensis 98.2% n=57 81.5% | n=54 | 20 16 Limnoporus 100% n=51 100% | n=51 | 21 16 194 G. pinqreensis) also showed the highest retention of f l i g h t muscles during the experiment. Observed f l i g h t muscle h i s t o l y s i s was r e s t r i c t e d to females. This probably accounts for the greater proportion of male emigrants among G. buenoi and G. comatus. However i f only data for males are considered. Table 28 confirms the tendency f o r greater d i s p e r s a l among G. pingreensis and Limnoporus spp. The r e l a t i v e species abundances observed among immigrants (Table 26) and emigrants (Table 28) are s t r i k i n g l y d i f f e r e n t . The disproportionate representation of G. buenoi may be explained because i t i s the most common species at the Springhouse site (Chapter IV). The eff e c t of r e l a t i v e abundance should be accentuated because two t h i r d s of the trapping e f f o r t (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 r e f l e c t t h e i r r e l a t i v e l y low abundances in surrounding populations. Less than 15% of a l l G- Pinqreensis c o l l e c t e d at Springhouse between 1975 and 1977 have been long-winged and so the population of potential migrants i s 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 re s u l t from a 195 combination of rareness in natural populations and active avoidance of larger ponds and lakes. B. Habitat Selection Within Lakes 1. F i e l d 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 i s r e s t r i c t e d 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 i n the same habitats as suggested by the lake surveys discussed i n Chapter IV. These data show that gerrid species occurring on the same pond are separated i n space. Comparison of data on percent cover with percent of t o t a l catch f o r 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 t h i s species i s more of a habitat genera l i s t . 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%) X 2 = 27.21; df=6; p<.01 • i 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%) X 2 = 95.07; df=2; p<.001 197 b. Habitat Differences Within Species Figure 35 i l l u s t r a t e s the s p a t i a l d i s t r i b u t i o n observed for various in s t a r s of G. buenoi and G. pinqreensis collected in quadrat samples at Sp 2 and Boitano Lake, respectively. Although a l l 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 l i m i t s of the vegetation zone (i.e. p o t e n t i a l gerrid habitat) during most of the season. At Sp 2 i t was also possible to predict vegetation density as an inverse function of mean depth (Figure 36) . Data presented i n t h i s section show that there was s i g n i f i c a n t s p a t i a l separation among the various size classes of both G. buenoi and G. pinqreensis. Younger in s t a r s occurred more frequently near shore at both lakes studied and in areas of higher vegetation density at Sp 2. 198 Figure 35. Within lake s p a t i a l d i s t r i b u t i o n observed for various instars during 1975. (a.) G, buenoi at Sp 2, (b.) G. pingreensis at Boitano Lake. A.G.BUENOI AT SP2 LO _ 0_ UJ 03 __ LU t LU P UJ Z l o LU h-O LU _J _J o o LU O X o < LU Lu O 50 40 30 20 10 •2 •2 ^ 1 1-2 3 4 5 A . F 3 4 a B. G. PINGREENSIS AT BOITANO LAKE 60 50 40 30 20 10 1-2 1-2 1-2 5 CM 20 CM DEPTH 35 CM 200 Figure 36. Relationship between depth and density of emergent cover at Sp 2 during 1975. 201 5 15 2 5 3 5 4 5 5 5 DEPTH (CM) 202 2. Laboratory Experiments . .. . Responses to A r t i f i c i a l 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. A l l species showed increased preference f o r a r t i f i c i a l rush sectors in response to wind and waves. The e f f e c t of surface wind was more severe than the effect of waves. There were s i g n i f i c a n t differences i n 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 l e s s affected by wind and waves. These differences are c l e a r l y i l l u s t r a t e d by a two-way analysis of variance performed on the percentage data of Figure 37 transformed to arc sins. Table 30 presents the r e s u l t s of t h i s analysis. The interaction between species and surface condition was s i g n i f i c a n t showing that the tolerance to surface 203 Figure 37. D i s t r i b u t i o n of gerrids on laboratory pools under three surface conditions. (a.) G. buenoi y ( b . ) G. comatus, (c.) G. incognitus, (d.) G. pingreensis (e.) Limnoporus. CO cr o LU CO i * CD < LLI CL O CO __: o i cr LLI CO CO O LL. O 70 60 50 40 3 0 20 10 A. G. BUENOI B. G. COMATUS 50 4 0 30 20 10 80 70 60 50 40 30 20 10 C. G. INGOGNITUS E. LIMNOPORUS SPP. D. G. PINGREENSIS CALM WAVES WIND STANDARD ERRORS 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 f o r 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 e f f e c t s absorbed s i g n i f i c a n t l y more of the observed variance than the interaction. Therefore, I conclude that a l l gerrid species tested a c t i v e l y avoid wind and waves by seeking out sheltered habitats but that the extent of the e f f e c t varies from species to species. (2.) The Effect of Stage Table 31 compares the habitat preferences of f i r s t i n s t a r s and adults among the f i v e species tested. The t o t a l range of preferences exhibited by a l l f i r s t i n s t a r s i s smaller than observed for adults. Also, means for f i r s t i n s t a r s of a l l species (except G. comatus) are closer to-50% than for adults. These data suggest that early in s t a r s exhibit l e s s active preference than adults on the scale of t h i s experiment, or that f i r s t i n s t a r s use di f f e r e n t 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 i n Figure 38. The average percentage of each species choosing open water sectors remained roughly constant over both conspecific densities tested. Therefore, i t was appropriate to pool the chi-sguare values computed from species d i s t r i b u t i o n s at both densities with those observed in a l l two-species experiments to calculate a residual variance for TABLE 31 Means and standard errors of the percentage of t o t a l observations made on open habitat sectors under calm surface conditions SPECIES INSTAR F i r s t (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. E f f e c t s of conspecific density and the presence of other species on habitat preferences. (a.) G. buenoi, (b.) G, comatus, (c.) G. pinqreensis. EFFECTS OF CONSPECIFICS A. 6. BUENOI EFFECTS OF OTHER SPECIES 60 40 20 80 60 UJ to 40 CO 9 £ 20 UJ (9 B. G. COMATUS WITH G. C O M A T U S WITH G, PINGREENSIS WITH 6 . B U E N O I 60 UJ a. 40 UJ CD < £ 20 < C. G. PINGREENSIS WITH G, PINGREENSIS 10/POOL 20/POOL DENSITY WITH 6. BUENOI WITH G. COMATUS OTHER SPECIES PRESENT 210 testing the e f f e c t s of heterospecific individuals. Analyses of variance for h e t e r o s p e c i f i c effects are given in Table 32. G. pinqreensis was unaffected by the presence of other species, but both G. buenoi and G. comatus showed s i g n i f i c a n t responses. G. comatus responded to G. pinqreensis and G. buenoi with s l i g h t l y increased preference for open water sectors. Although the observed responses were consistent. Figure 38 demonstrates that the e f f e c t s were too small to have any ecological significance. On the other hand G. buenoi showed a strong tendency to avoid both other species. The e f f e c t i s underscored because G. buenoi d i s t r i b u t i o n s shifted in opposite directions as i f 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 i n d i v i d u a l s affected the average response of G. buenoi by about 10% i n both directions. b. Selection of Habitat Mimics Figure 39 shows the ov e r a l l d i s t r i b u t i o n s of G. buenoi, G. comatus and G. pinqreensis when offered a choice among four types of a r t i f i c i a l habitat structure in the laboratory. The s i g n i f i c a n t 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 e f f e c t s 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. .64 >. . 10 Between species 1 51. 89 51. 89 15. ,72 <. .005 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 <• ,025 Between species 1 0. 79 0. 79 0. .41 • >. , 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. ,60 >. .10 Between species 1 4. 25 4. 25 2. .00 >. .10 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 i n the laboratory. "FLOATING?* "OPEN" "GRASS" "RUSH" G. BUENOI G. COMATUS G. PINGREENSIS 214 Species p r e f e r e n c e s can be d i s s e c t e d by examining more d e t a i l e d data from each experiment as provided i n T a b l e 33. The c h i - s g u a r e values c a l c u l a t e d f o r G. buenoi and G. comatus show t h a t the h a b i t a t s e l e c t e d depended on where the experiment was s t a r t e d . The tendency to remain i n the i n i t i a l h a b i t a t was very s t r o n g f o r G. buenoi and there was no obvious p a t t e r n of h a b i t a t s e l e c t i o n among animals t h a t d i d move. Except when s t a r t e d i n grass h a b i t a t s i n d i v i d u a l s of G. comatus changed h a b i t a t s more f r e q u e n t l y than G. buenoi. The data a l s o suggest that those G. comatus which d i d move p r e f e r r e d h a b i t a t s without emergent cover. G. p i n q r e e n s i s , on the other hand, a c t i v e l y sought out s e c t o r s a f f o r d i n g emergent cover, r e g a r d l e s s o f where the experiment was s t a r t e d . These r e s u l t s c o i n c i d e with the s p e c i e s p r e f e r e n c e s suggested by f i e l d data. G. comatus and G. p i n g r e e n s i s d i s p l a y e d a c t i v e h a b i t a t p r e f e r e n c e s f o r open h a b i t a t s and emergent cover r e s p e c t i v e l y ; G. buenoi showed no obvious h a b i t a t p r e f e r e n c e s . The important p o i n t i s t h a t a r t i f i c i a l mimics of n a t u r a l h a b i t a t s t r u c t u r e are s u f f i c i e n t t o evoke strong responses, even over the r e l a t i v e l y s m a l l s c a l e of these l a b o r a t o r y experiments. TABLE 33 A r t i f i c i a l h a b i t a t s c h o s e n by t h r e e G e r r i s s p e c i e s i n l a b -o r a t o r y e x p e r i m e n t s : a . G. b u e n o i , b. G. c o m a t u s , c - _.. p i n q r e e n s i s a. G. b u e n o i HABITAT | HABITAT AT START | , --| X2 P AT F I N I S H | F l o a t i n g Open G r a s s R u s h I F l o a t i n g | 35 Open | 11 G r a s s | 7 Rush | 7 14 30 5 11 7 9 34 10 7 2 3 48 I I I I 178. 1 df=9 <. 001 b. G, c o m a t u s HABITAT | HABITAT AT START I X2 AT F I N I S H | F l o a t i n g Open G r a s s Rush I I P F l o a t i n g j 24 Open | 20 G r a s s | 6 Rush | 10 17 18 11 14 8 13 34 5 11 18 9 22 I I I I 56.62 df=9 <. 001 c . G. p i n g r e e n s i s HABITAT | - HABITAT AT START I X2 AT F I N I S H | F l o a t i n g Open G r a s s Rush ! P F l o a t i n g | 12 Open | 9 G r a s s | - 1 7 Ru s h | 22 9 5 19 27 10 8 25 17 10 ' 10 13 27 I l I i 9,224 d f =9 0. 42 216 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 a r t i f i c i a l habitats. G. comatus was the only species to show clear s t a t i s t i c a l 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 r e l a t i v e l y better than G. buenoi or G. pingreensis i n both habitats without emergent complexity. These re s u l t s suggest an explanation for the curious tendency of G. comatus to remain i n grass habitat during the previous experiments. Because G. comatus has the longest legs of the species tested i t may actually be trapped by dense, emergent vegetation. The means presented in Figure 40 suggest that £. pinqreensis fared s l i g h t l y better i n emergent vegetation than in open areas. However analysis of variance demonstrates the absence of s t a t i s t i c a l y s i g n i f i c a n t differences among habitats in the data on hand (F=2.07; df=3,36; p<.10). B a r t l e t t ^ test indicates that the variances for G. buenoi were too heterogeneous to allow s t a t i s t i c a l comparison (probability of homogeneity approximately equal to .01). However the average performance of G. buenoi was similar i n a l l habitats except a r t i f i c i a l f l o a t i n g vegetation, where food intake was s l i g h t l y depressed. 217 F i g u r e 40. Amount of food consumed i n each of four a r t i f i c i a l h a b i t a t s . (a.) G. buenoi, (b.) G. comatus, (c.) G. p i n q r e e n s i s . A. 6. BUENOI 300 200 CO _> 100 < Q _ C9 O Q T O § 400 CO LU h- 300 _> O 200 G) 100 __: LU h-< LU INf 300 B. G. C O M A T U S lllilliijjjil!1!!!! Il I i! C. G. PINGREENSIS FLOATING "OPEN" "GRASS" "RUSH" STANDARD ERRORS 219 DISCUSSION A• Dispersal The experiments reported here were designed to investigate the . p o s s i b i l i t y that gerrids a c t i v e l y select breeding habitats by f l i g h t . Therefore, I s h a l l not discuss summer f l i g h t s to overwintering habitats made by diapause i n d i v i d u a l s . The scanty information available on selection of overwintering s i t e s i s summarized by Riley (1919), Brinkhurst (1956) and Landin and Vepsalainen (1977). The seasonal d i s t r i b u t i o n of captured immigrants indicates that the spring i s the major period of interlake d i s p e r s a l for gerrids on the Fraser Plateau. In addition, data presented i n Chapter IV show that summer generation reproductives are generally short-winged and thus incapable of f l i g h t . 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 f i t most of the available data for European gerrid populations. In south-central B r i t i s h Columbia, short summers select against mid-season dispersal because gerrids that develop f l i g h t muscles delay investment i n the reproductive system [ i . e . the oogenesis-flight syndome of Johnson (1969)]. Andersen (1973) 220 showed that f l i g h t muscle development can postpone egg-laying by as much as two weeks in G. l a c u s t r i s . Such delays would seriously shorten the period available for l a r v a l development on the Fraser Plateau. Spring f l i g h t was markedly affected by temperature and reproductive condition i n t h i s study. Most gerrids flew during days when the maximum a i r temperature exceeded 14°C. This i s in good agreement with data of Landin and Vepsalainen (1977) which suggest a threshold of 12-13°C for f l i g h t among spring populations of G. arqentatus i n Sweden, Landin and Vepsalainen also found that female gerrids, taken at r e f l e c t i o n traps, had completely immature reproductive t r a c t s . This observation suggests that the condition of f l i g h t apparatus i s a poor in d i c a t o r of f l i g h t potential and may thus help to account for the r e l a t i v e l y small proportion of animals that flew from the stocked enclosures. These data show that gerrid f l i g h t patterns are sporadic; they w i l l be d i f f i c u l t to study guantitatively i n the f i e l d . Bursts of f l i g h t a c t i v i t y seem to be the r u l e . For example, 1.32 gerrids per sguare meter per day were captured during test runs at Westwick Lake on A p r i l 23 and 24, 1977. I t i s l i k e l y that most dispersal occurs as gerrids leave overwintering s i t e s during the f i r s t few hot days each spring. The subseguent timing and extent of dispersal each year w i l l vary with prevailing weather patterns, Nevertheless major r e d i s t r i b u t i o n s of gerrid species are possible each spring. Because such r e d i s t r i b u t i o n s are not c h a r a c t e r i s t i c of 221 populations on the Fraser Plateau (Chapter VI), colonizing i n d i v i d u a l s must exercise some preference i n choosing habitats. Data on immigration show that G. buenoi flew most frequently into Sp 6. However, i t i s doubtful that t h i s r e f l e c t s an active choice of grass/sedge habitat because there was l i t t l e development of emergent vegetation at Sp 6 u n t i l late May. Temperate gerrids f l y exclusively during daylight i n the spring, and seem to rely upon r e f l e c t i o n from water surfaces as landing cues (Landin and Vepsalainen, 1977). A more l i k e l y explanation of the disproportionate catch of G. buenoi at Sp 6 i s that most gerrids seek overwintering sites near the pond where they developed and therefore have high p r o b a b i l i t y of recolonizing the same habitat on i n i t i a l spring f l i g h t s . The emigration data suggest that species have dif f e r e n t 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 s p e c i f i e d at present, but i t i s clear that cues emanate from the habitat and do not necessarily involve species i n t e r a c t i o n . The observations are consistent with the observed abundance of G. buenoi and G. comatus i n many rush habitats from A p r i l to mid-May (1976 and 1977) and the subseguent f a i l u r e to c o l l e c t larvae of those species from the same locations l a t e r i n the year. 222 The d i s t i n c t preferences observed among emigrating indivi d u a l s suggest that spring colonization proceeds as a t r i a l 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 s i t e s for breeding. B. Habitat Selection Within Lakes 1- Species Differences Data presented in t h i s chapter demonstrate that the gerrid species considered show d i s t i n c t responses to habitat. These responses separate species co-occurring on the same lake and thereby reduce the potential for i n t e r s p e c i f i c competition during periods of food shortage. The responses observed to a r t i f i c i a l habitat mimics i n the laboratory confirm that species choices are iased upon simple structural c h a r a c t e r i s t i c s of habitat, the exact composition of the aquatic vegetation seems to be r e l a t i v e l y unimportant to gerrids. . Similar responses to habitat structure have been recently demonstrated among aquatic invertebrates (Macan and Kitchinq, 1972, 1976) and ly c o s i d 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 r e l a t i v e l y open habitats. G. pingreensis, on the other hand, showed the strongest negative responses to surface disturbance. In the laboratory and i n the f i e l d G. pingreensis seeks out habitats that afford emergent cover. G. pingreensis populations are l a r g e l y 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 e f f e c t s of wind are most severe during A p r i l and May on the Fraser Plateau. Because grass/sedge habitats provide no emergent cover i n the early spring, G. pinqreensis colonizing these habitats may suffer heavy mortality owing to t h e i r poor a b i l i t y to t o l e r a t e surface disturbance. A l l 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 i t appears that G. buenoi may be concentrated i n 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 i f G. buenoi f l i e s more freguently i n the presence of other species. Dodson (1975) has also suggested that avoidance mechanisms explain the d i s t i n c t f i e l d d i s t r i b u t i o n s of two c o r i x i d species that showed the same habitat preferences i n laboratory experiments. 224 incoqnitus preferred emergent coyer to open water in laboratory experiments. This species i s rare in the study area; i t f i r s t appeared as migrant, long^winged i n d i v i d u a l s during the warm spring of 1976. although laboratory preferences of G. incoqnitus and G. pinqreensis are s i m i l a r , l i t t l e habitat overlap occurs on the study lakes. G. incoqnitus i s encountered almost exclusively i n "brushy" habitats beneath alder and willow brushes and on small ponds completely surrounded by forest. A comparison of c o l l e c t i o n s i t e s for lonq-winged individ u a l s of G. incognitus and G. pinqreensis suggests a mechanism to account f o r the observed d i s t r i b u t i o n s (Table 34). G. incoqnitus dispersers appear to land p r e f e r e n t i a l l y i n brushy habitats while G. pinqreensis does not usually colonize them. Because I have commonly collected G. incoqnitus from bulrush and c a t t a i l (Typha sp.) habitats on the lower mainland of B r i t i s h Columbia, i t i s possible that interactions between sympatric populations of G. incognitus and G. pinqreensis have led to habitat r e s t r i c t i o n i n G. incoqnitus on the Fraser Plateau. Further work i s needed to c l a r i f y the exact processes involved. Laboratory preferences recorded for Limnoporus spp. are remarkably d i f f e r e n t from the observed f i e l d d i s t r i b u t i o n s . In the laboratory Limnoporus adults avoid areas of emerqent cover, but in the f i e l d Limnoporus spp. are strongly associated with grass/sedge habitats. The long legs of the l a s t three instars of Limnoporus spp. must i n t e r f e r e with t h e i r a b i l i t y to move among dense vegetation. 225 TABLE 34 Distr i b u t i o n s 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 r e s t r i c t i o n of Limnoporus spp. to small ponds in the . f i e l d (1) Limnoporus spp. start reproducing l a t e r than a l l Gerris species on the Fraser Plateau and therefore s u r v i v a l of early i n s t a r s may depend on the presence of dense vegetation as a refuge against predation by larger gerrids. Spatial refuges have been shown to be important f o r circumventing excessive cannibalism i n 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 s u r v i v a l of the larger stadia of Limnoporus spp. may be maximized by seeking out habitats that s t r i k e a balance between gerrid maneuverability and protection from wind and r a i n . 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 i s further partitioned among the various developmental stages of water-strider populations. Habitat preferences observed i n the laboratory were less marked among larvae than among adults. In the f i e l d , early i n s t a r s occurred most often i n 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 s p a t i a l overlap should be greatest among the early i n s t a r s of co-occurring species. Jamieson (1973) showed that pronounced size differences between predator and prey are required f o r i n t e r s p e c i f i c predation and cannibalism to be e f f e c t i v e among gerrids. Therefore the s p a t i a l separation observed between size classes should minimize gerrid-gerrid predation i n natural habitats. It seems unlikely that gerrid populations on the Fraser Plateau can be regulated i n a s t r i c t density dependent manner by either cannibalism or i n t e r s p e c i f i c predation. 3- Species Morphology and Habitat Structure The experiments to compare adult foraging a b i l i t i e s show that g e r r i d leg length i s associated with foraging success i n certain types of habitat structure. In dense emergent vegetation, r e l a t i v e l y short legs maximize.success at finding food. There must always be a trade-off between maneuverability and locomotory e f f i c i e n c y . Longer legs increase loc.omotory e f f i c i e n c y (Andersen, 1976) and, in the case of G. comatus, seem to allow more e f f e c t i v e foraging i n habitats covered by f l o a t i n g vegetation. Werner and H a l l (1977) argue that morphological differences between the b l u e g i l l and the green sunfish lead to differences i n foraging a b i l i t y that depend upon the habitat background. Feeding morphology and behaviour are often linked with habitat 228 structure and t h i s relationship seems to guide the evolution of community structure i n f i s h (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 r e l a t i o n s h i p between habitat structure and species maneuverability. Because water-striders are unspecialized predator-scavengers (Lumsden, 1949; Matthey, 1974), the main theme i n t h e i r a b i l i t y to coexist i s s p e c i a l i z a t i o n %o habitat structure. The relationships presented i n t h i s chapter suggest that the population performance of g e r r i d species should show marked differences among habitats of d i f f e r e n t structure. In the next chapter I discuss evidence f o r such differences and assess the r e l a t i v e importance of habitat structure i n 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 summarized by Pianka (1976), have organized f i e l d work by allowing quantitative expression of resource p a r t i t i o n i n g thought to explain the coexistence of simialar species (Shoener, 1974a). These niche differences are often assumed to result and p e r s i s t through the action of i n t e r s p e c i f i c competition. The mechanism of competition i s often i n f e r r e d from natural history data, but the action of ccmpetiton i s rarely demonstrated experimentally i n natural communities. Recent papers by Heck (1976) and Weins (1977) argue that we should not be content with patterns developed on t h i s scale of investi g a t i o n . Paine (1966) and Connell (1975, 1978) have offered convincing demonstrations that predation can also allow the coexistence of s i m i l a r species by keeping competitor populations below l e v e l s permitted by ambient resources. The action of predation has been c l e a r l y demonstrated experimentally, but studies to assess the presence of resource partitioning i n these same communities are usually dismissed a p r i o r i as ir r e l e v a n t . In contrast, Connell (1961) has provided one of the few evolutionary ecology. Techniques of niche metrics, recently 2 3 0 convincing demonstrations of competition i n nature with his c l a s s i c 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 i s most important in promoting high species d i v e r s i t y . 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 i n s p e c i f i c communities. We should expect the r e l a t i v e 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, i n complicated questions of community structure, powerful generalization w i l l probably elude us (Whittaker and Levin, 1977). In preceding chapters, I have shown that water-striders on the Fraser Plateau p a r t i t i o n resources along the habitat dimension. In t h i s chapter, I examine pattens of species persistence and population performance and ask i f i n t e r s p e c i f i c competition can account for them. I also assess the potential impact of other ecological processes on the d i s t r i b u t i o n s and r e l a t i v e abundance of these gerrid species. 231 MATERIALS AND METHODS A. Colonization, Persistence and Population Success 1. F i e l d Surveys Yearly surveys were conducted at Springhouse and Becher's P r a i r i e to determine which gerrid species colonized and produced at l e a s t one generation over a representative series of lakes. Data were collected for 60 lakes i n 1975, 74 lakes i n 1976 and 103 lakes in 1977. Sampling methods d i f f e r e d s l i g h t l y from year to year. Species colonization and success was e a s i l y determined from data for the 45 lakes sampled regularly throughout 1975 (Chapter IV). Fifteen additional lakes were sampled once in late May for co l o n i s t s 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 f u l l range of habitats available at the lake. A new generation was recorded as present when a single f i f t h i n s t a r or new adult was colle c t e d . If only fourth or e a r l i e r instars were collected for any species, the lake was r e v i s i t e d two or three weeks l a t e r . Similar spring and summer surveys were ca r r i e d out at a l l lakes sampled in 1976. Quantitative data on species abundances were co l l e c t e d in 1977. The standard sampling method developed i n Chapter III was employed and the resulting data were converted to density 232 estimates. Three samples taken at each lake and were used to calc u l a t e an average density value for spring and summer populations of every gerrid species present. Variables suspected to a f f e c t gerrid population growth were recorded for each lake v i s i t e d 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 t o t a l numbers of backswimmers (Notonecta) , d y t i s c i d ( A e i l i u s , DytiscusV larvae, whi r l i g i g 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 A v a i l a b i l i t y . Potential gerrid habitat was divided into three classes depending upon vegetation structure: grass/sedge, rush and f l o a t i n g vegetation plus open water (cf. Chapter IV) . At the summer sampling v i s i t , 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 a e r i a l photographs. 233 (5) Tree S h e l t e r . Trees and bushes growing along lake margins provide some p r o t e c t i o n a g a i n s t the e f f e c t s of wind. The percentage of the l a k e margin s h e l t e r e d by t r e e s and bushes was determined from a e r i a l photographs. 2. A n a l y s i s a. _ Number of Spe c i e s per Lake The average number of s p e c i e s per l a k e and i t s varia n c e were c a l c u l a t e d f o r s p r i n g and summer data from 1975-1977. Var i a n c e to mean r a t i o s were c a l c u l a t e d t o assess the p a t t e r n of s p e c i e s d i s t r i b u t i o n over the l a k e s surveyed. The method of c a l c u l a t i o n i s summarized by Andrewartha (1961). b. P o p u l a t i o n P e r s i s t e n c e The p r o p o r t i o n of s p r i n g p o p u l a t i o n s t h a t produced subsequent g e n e r a t i o n s was c a l c u l a t e d f o r each s p e c i e s i n 1975, 1976 and 1977. T h i s index of s p e c i e s p e r s i s t e n c e was compared among s p e c i e s . c. P o p u l a t i o n Success A l l d e n s i t y e s t i m a t e s from 1977 were converted to n a t u r a l l o g a r i t h m s . The d i f f e r e n c e between l n ( s p r i n g density+1) and ln(summer density+1) was taken as one index of p o p u l a t i o n success. C o r r e l a t i o n and m u l t i p l e r e g r e s s i o n a n a l y s e s were used to determine which of the lake c h a r a c t e r i s t i c s were most 234 strongly associated with population success i n the f i e l d . Analyses were repeated with biomass estimates calculated as i n Chapter IV. : d« Gerrid Species Diversity vs. plant Structural D i v e r s i t y The Shannon-Wiener index of d i v e r s i t y (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- E f f e c t s on Species Growth and Survival a. Species Growth Twelve small c i r c u l a r enclosures (0.217 sg. meters) were established at Sp6, Sp8 and Westwick Lake i n r e l a t i v e l y homogeneous stands of grass/sedge, f l o a t i n g (Ceratophyllum/Myriophyllum) and bulrush vegetation, respectively (see Table 19, Chapter IV). Enclosures were clea r , 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 i n s t a r s of G. buenoi, G. comatus and G. pinqreensis were.collected from lakes at Springhouse and Becher's P r a i r i e and used to establish mass cultures of each species maintained 235 at f i e l d temperatures and photoperiod. Gerrids were fed l i b e r a l l y with vestigial-winged Drpsophila and insects captured in a l i g h t trap near Westwick Lake. Cultures were checked several times d a i l y for newly molted f i f t h stage larvae which were immediately i s o l a t e d and held up to two days without food in small styrofoam cups. Twenty-five teneral f i f t h i n s t a r s of each species were k i l l e d 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 t h e i r standard errors are given for each species i n Table 35. These weights were used as starting weights for each species in subsequent experiments. F i e l d experiments were started by placinq f i v e unfed f i f t h i n stars of a single species i n an enclosure at each lake. Three days l a t e r , a l l survivors were col l e c t e d , k i l l e d , dried and weighed. Individual weight gains were calculated by subtracting the average i n i t i a l weight (Table 35) from the average f i n a l weight recorded i n each experiment. Ten groups of f i v e 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 f i f t h i n s t a r 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 f a l l i n g onto the water surface at each lake were co l l e c t e d in round p l a s t i c containers (0.123 sq. meters) as a measure of food a v a i l a b i l i t y . Each food-^fall trap contained a d i l u t e solution of formalin. Three traps were run at each lake during each gerrid growth experiment. The coll e c t e d insects were dried and weighed; the res u l t s were converted to mg. per sguare meter per day. 2. E f f e c t s of Competition Between Species The large 4.0 sguare meter, enclosures described i n Chapter V were bisected diagonally with a sheet of heavy gauge p l a s t i c to create 2.0 sg. meter compartments. Fifteen f i e l d - c o l l e c t e d fourth i n s t a r s , 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 t o t a l 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 a l l surviving gerrids were collected and i d e n t i f i e d . 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 f o r 1976 and 1977 r e f l e c t the establishment of G. incognitus populations i n 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 r a t i o s indicate that gerrid species are not randomly d i s t r i b u t e d over the study lakes. In fact the number of gerrid species co l l e c t e d per lake tends to be remarkably uniform during both spring and summer. These data imply that spring dispersal i s egually e f f e c t i v e at mixing species among lakes, and that processes leading to l o c a l species extinction operate with s i m i l a r e f f i c i e n c y across a l l lakes studied. 2. Population Persistence The average percentage of spring populations that l e f t o f f s p r i n g between 1975 and 1977 i s presented for a l l species studied in Table 37. Persistence was .c l e a r l y highest for G. buenoi and lowest f o r 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 I Spp./Lake Ratio |Mean Number j Spp. ./Lake Variance: Mean Ratio 1975 N=60 3.03 .204 j 1.75 .442 1976 N=74 3.31 .248 | 2.28 .445 1977 N=103 3 . 5 0 . 280 j 2.51 .321 240 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 i s 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. F i r s t l y , annual rates of species extinction were r e l a t i v e l y high, even when populations on temporary ponds are omitted from the analysis. However, i t i s 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 r e s u l t s from t h e i r association with small ponds and temporary habitats (Chapter IV) . The very low success rate of G. incognitus may explain why th i s species has not been consistently recorded from the study lakes despite intensive fieldwork i n the area (Scudder, 1971, 1977). A combination of low spring density and p r e f e r e n t i a l colonization of temporary habitats (Chapter V) may exclude _.- incognitus from the study lakes during some years. 3- Population Success Net population change, calculated f o r each lake as in equations 1 and 2, provide an index of species success: net density = ln(summer density+T) - l n (spring density+1) (1) change net biomass = summer density - spring biomass (2) change The pair-wise correlations among population success of a l l water-strider species studied during 1977 are presented for both 2 4 2 density and biomass i n Table 38. With the exception of a weak, positive c o r r e l a t i o n between density changes of G. incognitus and Limnoporus spp., there were no s i g n i f i c a n t r e lationships between p a r t i c u l a r pairs of gerrid species. These data suggest that pair-wise species interactions did not af f e c t species success in a consistent manner from lake to lake. Therefore, i t - i s appropriate to lump the net changes of a l l co-occurring gerrid species i n subsequent analyses of general factors associated with the population success of p a r t i c u l a r species. Table 39 l i s t s the c o r r e l a t i o n c o e f f i c i e n t s 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 s i g n i f i c a n t p o s i t i v e correlations are those of G. buenoi, G. comatus and G. pinqreensis with the r e l a t i v e abundance of th e i r usual habitats (grass/sedge, open/floating and bulrush, respectively; c f . Chapter IV). In addition success in each of these species was negatively correlated with the proportions of the remaining two habitats. Negative correlation c o e f f i c i e n t s were s i g n i f i c a n t for G. comatus and G. pingreensis i n grass/sedge habitat and for G. buenoi i n bulrushes. Success i n G. incognitus and Limnoporus spp. was not related to the abundance of any habitat class. The same pattern held for changes i n biomass. Stepwise multiple regression was used to assess the combined a b i l i t y of six variables to predict population success 243 TABLE 38 Correlations between net changes i n (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 .000 -Limnoporus . 009 . 064 .018 .275* 1.000 b. Biomass G. buenoi 1.000 — — G. comatus -. 060 1.000 - -G. pingreensis .053 .012 1.000 -G. incognitus -. 064 .040 .090 1.000 -Limnoporus -. 122 .066 -.093 -.092 1.000 * s i g n i f i c a n t , p<.05 TABLE 39 Correlations between net increase of gerrid species and proportions of common habitat types SPECIES GRASS/SEDGE RUSH G. buenoi .417* G. comatus -.273* G. ping reensis -.344* G. incognitus -.059 Limnoporus -.085 366* 128 515* 205 101 OPEN S FLOATING -.083 .437* -. 152 .143 -.012 • s i g n i f i c a n t , p < .05 245 for each gerrid species. The s i g n i f i c a n c e l e v e l necessary for in c l u s i o n or exclusion of variables at each step was set at .05. Only lakes with gerrids present at the summer sample were considered i n the analysis; s u f f i c i e n t data were available from a t o t a l of 86 lakes. Table 40 shows the order of inclusion for a l l s i g n i f i c a n t variables for each Gerris species based on data for net density changes. The p a r t i a l correlations of each s i g n i f i c a n t variable with species success at the f i n a l step are given i n parentheses. None of the measured variables allowed s i g n i f i c a n t prediction of success for Limnoporus spp. The best predictors of success for G. buenoi, G. comatus and G. pinqreensis are the respective proportions of t h e i r c h a r a c t e r i s t i c habitat. An inverse relationship with conductivity afforded the best single-variable prediction of success for G. buenoi. However, the qreatest p a r t i a l c o r r e l a t i o n was obtained between G, buenoi success and the proportion of grass/sedge habitat i n the f i n a l eguation. Success in G. buenoi and G. pinqreensis was also p o s i t i v e l y related to the o v e r a l l success of other co-occurrinq waters s t r i d e r s ; no species was s i q n i f i c a n t l y depressed by the success of the other gerrids. These data suggest that i n t e r s p e c i f i c competition alone cannot account for the observed patterns of d i s t r i b u t i o n and abundance i n eco l o g i c a l time. In general, patterns of d i s t r i b u t i o n and abundance were not produced as a dire c t result of d i f f e r e n t i a l species success, but resulted from TABLE 40 Multiple regressions of Gerris population success on habitat variables a. Order of inclusion and p a r t i a l c o r r e l a t i o n c o e f f i -cients in f i n a l eguation Gerris VARIABLE | buenoi comatu s pinqreensis incoqnitus Net Change of | other gerrid | spp. , 1 3 (. 304) n.s. 2 (• 303) n.s. Proportion of | Favoured Habi-| tat Present | (arc sin) | 2 (.401) 1 (.452) 1 (. 570) Summer | Conductivity j 1 (-.319) n.s. n. s. n.s. Abundance of | Predator/ | Competitors | n. s. 2 (.406) n.s. n.s. Lake Area | n. s. n.s. n. s, .. n. s. Proportion of | Margin with | Tree Shelter | (arc sin) | n. s. 3 (-.248) n« s. 1 (-.331) n.s. = not s i g n i f i c a n t (p >. 05) b. E f f i c i e n c y 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 f i d e l i t y at the time of spring colonization. G. comatus exhibited greatest success where other predator/competitors (mostly Notonecta spp.) were abundant. There were no s i g n i f i c a n t negative associations between gerrid species success and the abundance of potential invertebrate predators suggesting that s e l e c t i v e predation cannot provide a general explanation of gerrid species . .success on the Fraser Plateau. Predation does not consistently promote high gerrid species d i v e r s i t y i n the manner described by Paine (1966), because r e a l losses i n gerrid species were recorded during the season at each lake. The only s i g n i f i c a n t variable for G. incognitus was the proportion of the lake margin surrounded by trees. The co r r e l a t i o n i s negative because of the generally poor success of a l l G. incognitus populations during 1977-When the analyses were repeated with biomass data only the variables representing habitat structure were included i n 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 r e l i a b l e predictor of gerrid d i s t r i b u t i o n and abundance on the Fraser Plateau, Although s i g n i f i c a n t regressions were obtained, the f i n a l multiple regression eguations explained only a small amount of 248 the variance observed i n f i e l d population success (Table 40-B). None of the variables measured in t h i s study allow accurate prediction of gerrid population success as defined above. 1« Gerrid Species Diversity vs. Plant Structural Diversity Figure 41 i s a scatter plot of gerrid species d i v e r s i t y versus plant s t r u c t u r a l d i v e r s i t y using data from 90 lakes in 1977- There i s no s i g n i f i c a n t r e l a t i o n s h i p between the two indices (r=. 136; df=90; p>>.05). These data show that water-s t r i d e r species d i v e r s i t y cannot be predicted from the d i v e r s i t y of t h e i r c h a r a c t e r i s t i c habitats. B. Habitats, Growth and Competition 1• Effects of Habitat on Species Growth The average su r v i v a l of f i v e gerrids over the set of three-day growth experiments at each lake i s presented i n Table 41. A l l species survived best i n the bulrush habitat of Westwick Lake. G. pinqreensis showed markedly reduced s u r v i v a l i n grass/sedge and f l o a t i n g vegetation habitats. G. buenoi survived better than G. comatus i n both grass/sedge and f l o a t i n g habitats. The mean survivorship of G. comatus was s l i g h t l y higher in f l o a t i n g vegetation than i n grass, although more data are needed to establish s t a t i s t i c a l s i g n i f i c a n c e . Gerrid remains found f l o a t i n g at the surface indicate that at l e a s t 15-20% of the observed mortality can be attributed to 249 Figure 41. A plot of gerrid species d i v e r s i t y versus plant s t r u c t u r a l d i v e r s i t y during July-August, 1977. 1.5 >-t z C O > C O U J Q _ C O 0 Q t r or UJ 9 • 9 o 9 o9 • 9 9 0 0 9 9 9 0 • 9 9 o © O • © 9 6 0 9 9 • 9 0 0 0 • O e o o o o • 0 • • 9 9 0© 0 9 . 0 o 0 o 9 e ' 9 • 9 9 9 0 • 9 e e 0 • .0.5 1.0 1.5 2.0 PLANT STRUCTURAL DIVERSITY (H1) o TABLE 41 Mean survival of f i v e gerrids ± standard errors over three day experiments i n 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 larg e s t proportion of the remains. Predators with chewing mouthparts (most probably d y t i s c i d beetle larvae) accounted for a smaller proportion of the gerrids eaten. Figure 42 i l l u s t r a t e s the average weight gain recorded per surviving i n d i v i d u a l at each lake. A l l species experienced maximum weight gains i n rush habitat. The performance of G. buenoi was i d e n t i c a l i n rush and grass/sedge habitat but i t did r e l a t i v e l y poorly i n f l o a t i n g vegetation. G. comatus performed equally in grass/sedge and f l o a t i n g habitats. S t a t i s t i c a l l y s i g n i f i c a n t differences could not be detected i n 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 i n Figure 42 indicate that a l l three species showed si m i l a r absolute weight gains i n grass/sedge habitat. However, G. buenoi gained s i g n i f i c a n t l y less weight i n both bulrushes and f l o a t i n g vegetation than did either of the two large species. This suggests that G. buenoi i s able to sustain maximum growth rates in grass/sedge habitat while growth rates for G. pingreensis and G. comatus f a l l . In float i n g vegetation, however, absolute weight gains recorded f o r G. buenoi were smaller than those of the two larg e r species. It i s clear that the best performance observed for a l l species in bulrush habitat was not simply the result of greater absolute food a v a i l a b i l i t y (Figure . 43), Generally, surface 253 Figure 42. Average weight gains of i n d i v i d u a l surviving gerrids over three days i n natural habitats. (a.) G. buenoi, (b.) G. comatus, (c.) G. pingreensis. 254 A. & BUENOI >-cr Q co < s o or i > cr CO < o X (3 LU LU CD < c r LU 1000 800 600 400 200 B. G. COMATUS 1400 I200i 1000 800 600 400 200 BULRUSH HABITAT (WESTWICK L.) GRASS/SEDGE HABITAT (SP 6) FLOATING/OPEN HABITAT (SP 8) STANDARD ERRORS C. G. PINGREENSIS 1200 1000 800 6 0 0 400 200 255 f o o d - f a l l was greatest i n grass/sedge and f l o a t i n g vegetation. The possible e f f e c t s of resource depletion by other surface-feeding invertebrates was not considered in these experiments. Data of Figure 43 also emphasize that f o o d - f a l l i s very patchy in time and space. 2. E f f e c t s of Competition Between Gerrids Figure 44 presents the daily mortality rates calculated for fourth and f i f t h instars of each species i n three-way competition experiments and i n the single-species growth experiments. Comparison of these data suggest that the presence of other species s i g n i f i c a n t l y increased the mortality rates observed f o r a l l species in bulrushes. The e f f e c t i s l e a s t pronounced for G. pingreensis. Only the mortality rate of G. buenoi was s i g n i f i c a n t l y increased by competitive e f f e c t s in grass/sedge habitats. However, both G. pingreensis and G. buenoi suffered s i g n i f i c a n t l y higher mortality i n competition experiments run i n f l o a t i n g vegetation, Although competition and single-species experiments were started at the same i n i t i a l density, precise quantitative i n t e r p r e t a t i o n of these data i s clouded because gerrids molted during the three-species experiments. I t was not possible to rear and handle enough f i f t h instar larvae, under f i e l d conditions, for both sets of experiments. I f molting s i g n i f i c a n t l y increases v u l n e r a b i l i t y i n the competitive s i t u a t i o n , mortality rates may be disproportionately elevated i n the competition experiments, 256 Figure 4 3 . average food f a l l per sguare meter during single-species growth and three-species competition experiments i n the f i e l d . BULRUSH HABITAT, WESTWICK L. GRASS/SEDGE HABITAT, SP 6 FLOATING HABITAT, SP8 STANDARD ERRORS [4i JULY 23 JULY 26 JULY 31 AUGUST3 AUGUST9 STARTING DATE FOR THREE-DAY GROWTH EXPERIMENT AUGUST 19 258 Figure 4 4 . Average daily mortality rates observed i n single-species and three-species experiments i n natural habitats. (a.) G. buenoi, (b.) G. comatus, .(c) G. pingreensis. 259 2 0 | A. G. BUENOI r h LU LO Q U_ O 0.5 UJ CO Z 2.0 UJ <2 > < 1.0 0.5 B. G. COMATUS C. fi PINGREENSIS SINGLE-SPECIES EXPERIMENTS THREE-SPECIES COMPETITION EXPERIMENTS | STANDARD ERRORS r h B U L R U S H WESTWICK L. GRASS/SEDGE FLOATING SP 6 S P 0 260 Fortunately, i t i s the patterns of species mortality among the three habitats that are of r e a l interest, and they are r e l a t i v e l y clear. Presence of other species affects G. buenoi least i n grass/sedge and G. pinqreensis least i n bulrushes. _.- comatus showed s i m i l a r mortality i n competition experiments in a l l three habitats. Because these patterns d i f f e r markedly from those observed i n single species experiments, I conclude that competitive interactions can a f f e c t the success of these three species, d i f f e r e n t i a l l y according to habitat. The data suggest that each species should be favoured i n i t s c h a r a c t e r i s t i c habitat i f forced into a competitive s i t u a t i o n at r e l a t i v e l y high densities. The exact mechanisms remain to be experimentally elucidated. 261 DISCUSSION A. Habitats, Diversity and Population Performance Data presented i n thi s chapter emphasize that water-s t r i d e r s 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, l i k e those of Jarvinen (1976), that address evolutionary guestions about gerrids, s a c r i f i c e considerable realism by assuming uniformity of water-s t r i d e r habitats. The variance of habitat favourablity must be considered. Southwood (1977) has discussed several examples of how variable favour a b i l i t y influences the dynamics of insect populations. Grant (1975) has demonstrated similar differences among habitats occupied by microtine rodents. A combination of habitat e f f e c t s and species i n t e r a c t i o n undoubtedly influence the su r p r i s i n g l y high rates of population extinction observed i n t h i s study, but the precise mechanisms involved are not yet c l e a r . Assessment of species l o s s from discontinuous spring and summer samples may confound genuine extinction with migration from unfavourable habitats. The continuous population data ava i l a b l e from 1975 suggest that both processes are involved. Larvae of a l l colonizing species were usually collected at each lake, but not always in proportions expected from the r e l a t i v e abundances of spring adults. Vepsalainen (1973b) has also noted a consistent loss i n 262 gerrid species per l o c a l i t y i n Finland, I t i s i n t e r e s t i n g that the average number of species reported per lake i n southern Finland i s considerably lower than those reported f o r spring populations on the Fraser Plateau (2.26 vs. 3.28 species per lake), even though Vepsalainen considered d i s t r i b u t i o n records for seven species. This suggests that Finnish l o c a l i t i e s are more e f f e c t i v e l y i s o l a t e d than those studied in c e n t r a l B r i t i s h Columbia, and helps to account f o r the more general occurrence of short-winged forms and dimorphic strategies among Finnish gerrids J. summarized i n Vepsalainen (1978)], Population success rates observed for G. buenoi, 6. comatus and G. pingreensis are generally concordant with the habitat preferences demonstrated experimentally i n Chapter IV. G. buenoi, the habitat generalist, was more persistent i n a l l three years than either G. comatus or G. pingreensis which are habitat s p e c i a l i s t s . At a more detailed l e v e l of population performance, however, habitat structure alone was i n s u f f i c i e n t to accurately predict species success. This i s c l a r i f i e d by reference to Figure 45 where the r e l a t i v e abundance .of each species during summer 1977 i s plotted against the percentage of i t s c h a r a c t e r i s t i c habitat available on each lake (both variables are transformed to arc sin s ) . Patterns of success varied between G. buenoi and the two habitat s p e c i a l i s t s . 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 c o l l e c t e d was highly variable. The 263 Figure H5. The relat i o n s h i p between r e l a t i v e abundance of gerrids and r e l a t i v e abundance of the i r c h a r a c t e r i s t i c habitats. (a.) G. buenoi, (b.) G. comatus, (c.) G. pinqreensis. 88 68 o z UJ CO §48 UJ o < Q CD < CO y j o UJ Q_ CO Q m (T UJ 28 to 1 8 coi88 3 A. G. BUENOI © • • • • • • 8 • J ' " # * ARC SIN (PROPORTION GRASS/SEDGE PRESENT) O o e>l 68 Q48 fe o o 2 8 o_ 2 CO o 8 5 B. G. COMATUS •• • ARC SIN (PROPORTION FLOATING/OPEN PRESENT) 68 48 • 28 ? 8 CO o cr < C. G. PINGREENSIS 8 18 28 38 48 58 68 78 88 ARC SIN (PROPORTION OF BULRUSHES PRESENT) HABITAT A B U N D A N C E 265 pattern i s exactly reversed f o r G. comatus and G. pingreensis; as the proportions of c h a r a c t e r i s t i c habitat increased the r e l a t i v e abundances of both species became more and more indeterminate. Simple explanations do not s u f f i c e ; although resource p a r t i t i o n i n g with respect to .habitat structure i s demonstrably important, other factors must in t e r a c t to produce natural patterns. Strong evidence that gerrids segregate along habitat dimension led me to expect a positive r e l a t i o n s h i p 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 d i v e r s i t y and habitat d i v e r s i t y among a l l sorts of animals. However, the evidence i s often equivocal (Murdoch et a l . , 1972) and, the addition of g u a l i f i c a t i o n s (e.g. Gorman and Karr, 1978; Andersen, 1978) may reduce the generalization to something l i k e "the r e l a t i o n s h i p i s strong except where i t i s n ' t " . In t h i s study no general r e l a t i o n s h i p between gerrid d i v e r s i t y and habitat d i v e r s i t y was found. Tomoff (1974) and Vuilleumier (1972) have found that foliage height d i v e r s i t y i s not always the best predictor of bird species d i v e r s i t y . Allan (1975) noted the absence of a p r e d i c t i v e r e l a t i o n s h i p between habitat complexity and species d i v e r s i t y among stream insects, even though he also presented strong evidence for active habitat preferences. He showed that habitat complexity was f a i r l y constant over the stream gradient and suggested that species 266 d i v e r s i t y was i n s e n s i t i v e to fi n e - s c a l e variation between habitats. ; -A s i m i l a r explanation cannot apply to water-striders on the Fraser Plateau, because the mosaic of habitat structure varies considerably among lakes. The generalization probably f a i l s , in t h i s case, because habitat v a r i a t i o n i s superimposed on variable patterns of resource a v a i l a b i l i t y , gerrid colonization, density independent mortality and i n t e r s p e c i f i c competition. Vepsalainen and Jarvinen (1976) have summarized the arguments f o r resource l i m i t a t i o n i n water-striders. Figure 43 and other data (Spence, unpublished) show that food resources are extremely patchy in time and space. In general, food a v a i l a b i l i t y f a l l s after mid-summer, but gerrid numbers remain high u n t i l l a t e August (Chapter IV). I n t e r s p e c i f i c competition among insects with seasonal l i f e cycles i s often "transient" (Istock, 1967, 1973; McClure and Price, 1975), r e f l e c t i n g such va r i a t i o n i n resource a v a i l a b i l i t y . Transient competition among gerrids could permit elevated species d i v e r s i t y . Gerrid species d i v e r s i t y could be lower than expected on some lakes owing to colonization f a i l u r e and density independent mortality. Only careful experiments and detailed study of natural populations w i l l help explain the scatter observed i n Figure 41. Explaining away the points that do not f i t , a p o s t e r i o r i , i s not l i k e l y to be i n s t r u c t i v e . 267 B. Com petition and Habitat Selection Results of the enclosure experiments indicate that i n t e r s p e c i f i c competition can a f f e c t the d i s t r i b u t i o n and r e l a t i v e abundance of G. buenoi, G. comatus and G. pinqreensis on the Fraser Plateau. However, the natural processes leading to observed d i s t r i b u t i o n s are complex. Bulrushes c l e a r l y afford the optimum habitat for a l l three species, but only G. pingreensis regularly occurs there (Chapter IV), often in v i r t u a l l y monospecific populations. Competitive experiments demonstrated that G. pingreensis has the competitive edge over s i m i l a r - s i z e d G. buenoi and G. comatus i n bulrush habitats. With respect to the habitat axes then, _.• pingreensis occupies a narrower, r e a l i z e d niche that i s "included" ( M i l l e r , 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 i s l i k e l y to evolve innate behavioural dominance", Gerrids are known to prey on other gerrids in laboratory cultures and in the f i e l d (Maynard, 1969; Jamieson, 1973; Vepsalainen and Jarvinen, 1976), and at least part of the mortality observed i n the enclosure experiments resulted from cannibalism and i n t e r s p e c i f i c predation. Jamieson (1973) demonstrated that gerrid species show d i f f e r e n t l e v e l s of aggression. Laboratory observations suggest that G. pingreensis i s , in f a c t , the most aggressive of the three species considered here (Spence, 268 unpublished). I s t o c k (1966, 1967) has found t h a t such d i f f e r e n c e s i n innate a g g r e s s i v e n e s s help t o e x p l a i n n a t u r a l d i s t r i b u t i o n s of w h i r l i g i g b e e t l e s ' (Dineutes) . Jamieson's (1973) l a b o r a t o r y s t u d i e s , i n d i c a t e d t h a t s i z e d i f f e r e n c e was an important component of success i n g e r r i d -g e r r i d p r e d a t i o n . The lower growth t h r e s h o l d s of _.. p i n g r e e n s i s should l e a d to c o n s i d e r a b l e s i z e advantages f o r t h i s s p e c i e s d u r i n g the f i r s t g e n e r a t i o n . However, t h i s does not seem to be a necessary c o n d i t i o n f o r the dominance o f G. p i n g r e e n s i s i n bulrushes because both c a n n i b a l i s m and i n t e r s p e c i f i c p r e d a t i o n o c c u r r e d among s i m i l a r - s i z e d g e r r i d s i n e n c l o s u r e s . Fox (1975b) showed t h a t molting backswimmers (Notonecta) were e s p e c i a l l y v u l n e r a b l e t o c a n n i b a l i s m . Thus, the lower s u r v i v a l r a t e s observed among a l l s p e c i e s may have r e s u l t e d because g e r r i d s molted during c o m p e t i t i o n experiments, R e s u l t s of s i n g l e - s p e c i e s growth experiments demonstrate t h a t c o m p e t i t i o n from other g e r r i d s i s unnecessary t o e x p l a i n the low success of G. p i n g r e e n s i s i n grass/sedge h a b i t a t s . Furthermore, because the e n c l o s u r e s e f f e c t i v e l y e l i m i n a t e d s u r f a c e d i s t u r b a n c e , the p o t e n t i a l biomechanical disadvantage of G. p i n g r e e n s i s , noted i n Chapter V, i s not t o t a l l y r e s p o n s i b l e f o r . the absence o f G. p i n q r e e n s i s from grass/sedge areas. Two e x p l a n a t i o n s can be o f f e r e d i n the f a c e of present data. F i r s t l y , g r e a t e r a g g r e s s i v e n e s s of G. p i n g r e e n s i s may have l e d to higher r a t e s of c a n n i b a l i s m i n s i n g l e s p e c i e s e n c l o s u r e s at Sp 6 because f o r a g i n g became more d i f f i c u l t i n 269 dense emergent vegetation. However, i n one half of the single species experiments i n grass/sedge habitat, no G. pinqreensis f i f t h i n s t a r s survived for the entire three-day period suggesting that mortality factors other than cannibalism were operating, A second, more l i k e l y , hypothesis i s that G. pinqreensis i s 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 i s shown i n Figure 46. Even though there was no s i g n i f i c a n t c o r r e l a t i o n between the percentage of grass/sedge habitat and t o t a l predator/competitor abundance (r= .018; p>>.05), grass/sedge vegetation i s most common on extremely fresh, snowmelt ponds. P i l o t experiments suggest that f i f t h i n s t a r G. pinqreensis are more vulnerable to notonectid predation than are sim i l a r -sized G. buenoi or G. comatus. This hypothesis i s p a r t i c u l a r l y a t t r a c t i v e because i t also helps to explain the strong positive association between population success i n 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 i n s t a r s and adults i s reduced by dense emergent cover (Chapter V), but t h i s factor should not often be important i n the grass/sedge habitats studied on the Fraser Plateau. The r e l a t i v e l y late appearance and reproduction, observed f o r G. comatus in the spring (Chapter 270 Figure 46. The abundance of a l l predator/competitors over a range of conductivity. 271 _ S i 25 _I UJ & LU Eo 15 < b 1 0 Q _ _> o e * o Q o Q _ • • » • • • • 9 • % >2c 5 6 7 8 9 LN (SUMMER CONDUCTIVITY) 10 272 I V ) , w i l l l e a d t o t e m p o r a l c o - o c c u r r e n c e o f i t s e a r l y i n s t a r s w i t h l a r g e r s i z e c l a s s e s o f G. b u e n o i a n d G. p i n q r e e n s i s . P e r h a p s G. c o m a t u s i s r e s t r i c t e d t o f l o a t i n g v e g e t a t i o n h a b i t a t s by a c o m b i n a t i o n o f l a t e s p r i n g a p p e a r a n c e and l o w e r i n n a t e a g g r e s s i v e n e s s . C. E v o l u t i o n and M a i n t e n a n c e o f H a b i t a t P r e f e r e n c e s H a b i t a t s h i f t s ( S h o e n e r , 1974b; Diamond, 1978) h a v e become i n c r e a s i n g l y common e x p l a n a t i o n s f o r c o e x i s t e n c e v i a r e s o u r c e p a r t i t i o n i n g . W e r n e r and H a l l ( 1 9 7 6 , 1977) h a v e shown t h a t t h e m e c h a n i s m i s l a r g e l y b e h a v i o u r a l among c o e x i s t i n g s p e c i e s o f s u n f i s h , a n d t h u s a l l o w s s h o r t - t e r m a d j u s t m e n t s t o s e a s o n a l p a t t e r n s o f r e s o u r c e a v a i l a b i l i t y . D a t a p r e s e n t e d i n C h a p t e r V show t h a t G. b u e n o i may a c t u a l l y a l t e r i t s d i s t r i b u t i o n w i t h i n a pond i n r e s p o n s e t o t h e p r e s e n c e o f o t h e r g e r r i d s . However, s h o r t - t e r m b e h a v i o u r a l p l a s t i c i t y a l o n e c a n n o t a c c o u n t f o r t h e d i s t r i b u t i o n a l p a t t e r n s o b s e r v e d on t h e F r a s e r P l a t e a u . My s t u d i e s s u g g e s t t h a t most s p r i n g a d u l t s r e c o l o n i z e t h e same ponds where t h e y e m e r g e d d u r i n g t h e p r e v i o u s summer. O f t e n t h i s w i l l g u a r a n t e e them a s s o c i a t i o n w i t h a p p r o p r i a t e h a b i t a t s . S e l e c t i o n h a s p r o d u c e d b e h a v i o u r a l p r e f e r e n c e s f o r c e r t a i n h a b i t a t c h a r a c t e r i s t i c s w h i c h e x p l a i n p a t t e r n s o f i n t r a l a k e d i s t r i b u t i o n . P r e f e r e n c e s seem t o be b a s e d upon m a t c h i n g m o r p h o l o g y w i t h h a b i t a t s t r u c t u r e t o a l l o w e f f i c i e n t f o r a g i n g ( C h a p t e r V ) . The p o s s i b l e r e l a t i o n s h i p b etween h a b i t a t p r e f e r e n c e and f o o d d i s t r i b u t i o n ( Werner and H a l l , 1976) i s n o t l i k e l y t o be i m p o r t a n t among g e n e r a l i s t p r e d a t o r s l i k e 273 g e r r i d s . A s m a l l e r p r o p o r t i o n of overwintered a d u l t s make more ex t e n s i v e c o l o n i z a t i o n f l i g h t s and. perhaps, s e t t l e on l a k e s t h a t do not c o n t a i n the a p p r o p r i a t e h a b i t a t f o r the s p e c i e s . Some evidence suggests that such g e r r i d s can f l y again i n a t r i a l and e r r o r attempt to l o c a t e p r e f e r r e d h a b i t a t s (Chapter V). Data presented here i n d i c a t e t h a t g e r r i d s which breed i n u n c h a r a c t e r i s t i c h a b i t a t s pay an e v o l u t i o n a r y p e n a l t y o f reduced r e p r o d u c t i v e success i f r e s o u r c e s become: l i m i t i n g and other s p e c i e s are present. Although h a b i t a t s t r u c t u r e p r o v i d e s only a f a i r p r e d i c t i o n of g u a n t i t a t i v e r e p r o d u c t i v e success, i t i s a p p a r e n t l y the best c l u e a v a i l a b l e to g e r r i d s on the F r a s e r P l a t e a u . D.. Summary I n t e r s p e c i f i c competition from G. p i n g r e e n s i s can exclude G. buenoi and G. comatus from b u l r u s h h a b i t a t s on the F r a s e r P l a t e a u . C o n t r a s t i n g h a b i t a t use by G. buenoi and G. comatus may be e x p l a i n e d as a r e s u l t of d i f f e r e n t i a l p o p u l a t i o n performance i n grass/sedge and f l o a t i n g v e g e t a t i o n . I t seems t h a t G. p i n g r e e n s i s i s r a r e i n freshwater h a b i t a t s because of c o m p e t i t i o n and/or p r e d a t i o n from other s u r f a c e - f e e d i n g i n s e c t s and p o s s i b l y , owing to the absence of s h e l t e r e d m i c r o h a b i t a t s d u r i n g the e a r l y s p r i n g . These . processes have l e d to and enforced the h a b i t a t a s s o c i a t i o n s and p r e f e r e n c e s documented i n e a r l i e r chapters. However, these mechanisms operate on an u n p r e d i c t a b l e background of c o l o n i z a t i o n dynamics and d e n s i t y 274 independent mortality, and therefore, habitat preferences allow only f a i r 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 f o r the observed persistence of G. incoqnitus and Limnoporus on the Fraser Plateau. Therefore competition, predation and environmental heteroqeneity i n space and time are a l l important aspects of qerrid coexistence i n c e n t r a l B r i t i s h Columbia. 275 CHAPTER VII. GENERAL DISCUSSION The preceding chapters lay a foundation for studying the comparative dynamics of gerrid populations in the f i e l d . This study has emphasized how natural patterns of habitat variation can af f e c t the species composition of i n d i v i d u a l 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 i n d i v i d u a l roster of species and t h e i r abundances. The f i r s t challenge for the b i o l o g i s t i s to compress data from i n d i v i d u a l lakes into a framework of more general patterns that imply s i m i l a r operation of natural processes. These c l a s s i f i c a t i o n s w i l l always be more or l e s s imperfect owing to the underlying i n d i v i d u a l i t y of each lake. The next job, only started here, i s to dissect the various: processes affecting population responses experimentally, and thus, explain the natural d i s c o n t i n u i t i e s that allowed us to f i r s t perceive the patterns. The i n i t i a l season of f i e l d work established strong quantitative associations between gerrid species and p a r t i c u l a r 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 f i d e l i t y at the time of spring colonization. 276 H a b i t a t f i d e l i t y seems to be most important f o r producing p a t t e r n s i n the f i e l d ; i t r e s u l t s from the apparent c h o i c e of o v e r w i n t e r i n g s i t e s near the mother pond and, to a l e s s e r extent, a c t i v e h a b i t a t s e l e c t i o n d u r i n g the d i s p e r s a l p e r i o d . For g e r r i d s , however, h a b i t a t c l a s s i f i c a t i o n s based upon v e g e t a t i o n s t r u c t u r e are not s t r i c t l y between-lake, v a r i a b l e s . S e v e r a l s t r u c t u r a l l y d i f f e r e n t h a b i t a t s o f t e n occur on the same lake and g e r r i d s p e c i e s a d j u s t t h e i r d i s t r i b u t i o n s i n response to such v a r i a t i o n (Chapter V). I t was demonstrated t h a t h a b i t a t a s s o c i a t i o n s observed f o r G. p i n q r e e n s i s and G. comatus are p a r a l l e l e d by s t r o n q p r e f e r e n c e s f o r emergent cover and l a c k of i t , r e s p e c t i v e l y . F i e l d o b s e r v a t i o n s and experiments e s t a b l i s h e d t h a t G. buenoi i s a h a b i t a t g e n e r a l i s t , but t h a t i t s d i s t r i b u t i o n w i t h i n a lake can be a f f e c t e d by a tendency to a v o i d other s p e c i e s . Although the a d u l t s and l a t e i n s t a r s of c o - o c c u r r i n g g e r r i d s p e c i e s show d i s t i n c t s e p a r a t i o n i n space, the f i r s t t h r e e i n s t a r s are o f t e n found together i n s h e l t e r e d areas near the lake margin. Thus i t i s a p p r o p r i a t e to c o n s i d e r responses to s t r u c t u r a l c h a r a c t e r i s t i c s of h a b i t a t as p a r t of the intracommunity r o l e or n i c h e of each s p e c i e s and i n s t a r (Whittaker e t a l . , 1975, 1973). MacArthur (1958) used s i m i l a r p o s i t i o n a l c r i t e r i a i n h i s c l a s s i c study of niche d i v i s i o n among warb l e r s c o e x i s t i n g i n c o n i f e r o u s f o r e s t s of the n o r t h e a s t e r n United S t a t e s . Whittaker e t a l . (1975, 1973) argue t h a t the terms "ni c h e " and " h a b i t a t " should be r e s t r i c t e d to s p e c i e s responses to i n t r a - and intercommunity v a r i a b l e s , r e s p e c t i v e l y . T h i s 277 d i s t i n c t i o n i s compelling from a t h e o r e t i c a l viewpoint. I t permits us to separate species packing (alpha diversity) from the e f f e c t s of changing environmental backgrounds (beta d i v e r s i t y ) . However, the argument begs fo r a c l e a r , unambiguous d e f i n i t i o n 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 d i s t i n c t i o n between "niche" and "habitat" i s not often c l e a r in nature. Results of the present study suggest that i t would be u n r e a l i s t i c to separate gerrid niches and habitats in an evolutionary context. Whether or not a particular lake (i.e.."habitat") i s suitable for a given water-s t r i d e r species, seems to depend almost completely upon the presence of i t s c h a r a c t e r i s t i c microhabitat defined by vegetation structure ( i . e . "niche", as argued above). Tight linkage between niche and habitat i s l i k e l y to be common among insect species. Because they have r e l a t i v e l y l i t t l e control over t h e i r environment, insect s have often become extreme habitat s p e c i a l i s t s . The ultimate context of species evolution i s response to the t o t a l array of niche and habitat variables. Whittaker et a l . (1975, 1973) argue that we should consider t h i s combination of variables as "ecotope". I follow this convention i n subseguent discussion. Experimental evidence presented i n 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 t h e i r presence i s a good predictor of habitat permanence f o r water-striders. Also lakes with higher conductivities tend to be more productive (Orians, 1966) which may lead to greater food a v a i l a b i l i t y for gerrids. . Because both G. buenoi and G. comatus exhibited highest s u r v i v a l and growth when confined alone i n bulrush habitat, I conclude that the ecotope of G. pinqreensis i s 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 i n three-species competition experiments i n bulrush habitat. This suggests that G. pinqreensis i s the competitive dominant in bulrushes; the most l i k e l y mechanism i s greater success under conditions leading to i n t e r s p e c i f i c 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 i s the most aggressive of these three Gerris species. I suspect that the more robust morphology of G. pinqreensis leads to greater k i l l and escape e f f i c i e n c y , but these p o s s i b i l i t i e s have not been tested experimentally. Among natural populations, G. pingreensis should gain a d d i t i o n a l advantages because i t commences reproductive a c t i v i t y 279 e a r l i e s t i n the s p r i n g (Chapter IV; Spence et a l . , 1978) and the low temperature t h r e s h o l d s of e a r l y i n s t a r s (Chapter II) promote r a p i d growth. Thus e a r l y s e a s o n a l t i m i n g should l e a d to d i s t i n c t s i z e advantages f o r G. p i n g r e e n s i s . Jamieson (1973) has e s t a b l i s h e d t h a t k i l l e f f i c i e n c y i n c r e a s e s with s i z e d i f f e r e n c e between predator and prey g e r r i d . However, the f a c t t h a t d i f f e r e n t s i z e c l a s s e s . tend to occupy d i f f e r e n t m i c r o h a b i t a t s should d i m i n i s h the o v e r a l l e f f e c t of s i z e d i f f e r e n c e t o some extent i n the f i e l d . Whatever the mechanism, c o m p e t i t i v e s u p e r i o r i t y o f _.- p i n q r e e n s i s i n bulrushes has a p p a r e n t l y s e l e c t e d f o r e c o l o g i c a l c h a r a c t e r i s t i c s to ensure t h a t G. buenoi and G, comatus make minimal p o p u l a t i o n investments i n bu l r u s h h a b i t a t s . However, i t seems that these ecotope boundaries are t e s t e d y e a r l y . As an example, I c i t e the g e r r i d p o p u l a t i o n h i s t o r i e s observed between 1975 and 1977 a t Long Lake and Barnes Lake.at Becher's P r a i r i e . These two l a k e s are at the upper end of the c o n d u c t i v i t y s c a l e (Appendix I) and the onl y g e r r i d h a b i t a t s are smal l (30-50 m2) patches o f b u l r u s h . In 1975 dense, monospecific p o p u l a t i o n s were observed throughout the season a t both l a k e s . P o p u l a t i o n s seemed t o d e c l i n e during 1976, probably as a r e s u l t of the i n t e n s i v e sampling program i n 1975. During May 1977 only a few overwintered G. p i n q r e e n s i s were found at both l a k e s , but a f a i r number of immigrant G. buenoi and G. comatus were a l s o taken a t Long Lake. During the summer survey, no G. p i n q r e e n s i s were c o l l e c t e d a t e i t h e r lake but G. buenoi and G. comatus were common at Long Lake. I t 280 i s tempting to suggest that the reduction of the G. pingreejisis population at Long Lake allowed establishment of the other Gerris populations during 1977. It i s obvious, that G. pinqreensis i s severely disadvantaged outside of bulrush habitats. The marked intolerance of G. pingreensis to surface disturbance (Chapter V) may r e s t r i c t i t to bulrush beds during spring colonization. When t h i s species i s co l l e c t e d elsewhere i n the spring, i t i s almost invariably on lakes that provide some cover among overhanging brush or emergent grass stalks i n flooded meadows. Single-species growth experiments, however, demonstrated that additional factors must be involved. These have not yet been experimentally v e r i f i e d . An inter e s t i n g hypothesis i s 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 i n freshwater habitats leads to diminshed recruitment i n t o the pool of qerrids overwinterinq l o c a l l y . Most G. pinqreensis are apterous and are thus incapable of regular, e f f i c i e n t dispersal. By and large, these gerrids re-colonize the same pond on which they became adults. Therefore, the proximate cause of low Pingreensis abundance i n freshwater habitats i s r i g i d habitat f i d e l i t y at colonization. Data i n Table 34 show that long-r winged G. pingreensis also s e t t l e most freguently i n bulrushes. 281 I t i s clear that selection has acted upon G. pingreensis to enforce minimum population investment i n freshwater habitats. The ecotope boundaries between G. buenoi and G. comatus are less r i g i d than those with G. pinqreensis. Data presented i n Chapter V suggest that habitat use by these two species may be a function of matching foraging e f f i c i e n c y with morphology among late i n s t a r s . A comparison of weight gains recorded i n single-species growth experiments demonstrated that f i f t h instar G. buenoi were s i g n i f i c a n t l y l e s s e f f i c i e n t at foraging in f l o a t i n g vegetation habitats than were f i f t h instar G. comatus (Chapter VI). The extreme patchiness of gerrid food resources in space and time v i r t u a l l y guarantees that populations w i l l be faced with periodic food l i m i t a t i o n each season. I t i s also l i k e l y that high temperature optima (Chapter II) and the exceptional tolerance of surface disturbance recorded for G. comatus i n the laboratory (Chapter V) confer some adaptive advantage i n habitats without s i g n i f i c a n t development of emergent cover. The survey data presented i n Chapter VI show that the best predictors of population performance i n G. buenoi and G. comatus were the r e l a t i v e abundances of grass/sedge and f l o a t i n g vegetation habitat, respectively. Figure 47 summarizes the qu a l i t a t i v e 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 4 7 , but a l l 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 i n 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 i s some suggestion that trophic niches are separated on the basis of prey s i z e . The overriding conclusion of t h i s i n v e s t i g a t i o n i s that the number of gerrid species co^occurring in a small geographical region i s l a r g e l y a function of the number of d i s t i n c t ecotopes avail a b l e . Strong habitat associations are most important on the Fraser Plateau, and seem to be maintained through both i n t e r s p e c i f i c competition and predation by other surface-feeding insects. However, patterns of d i s t r i b u t i o n and abundance are fuzzy because yearly temperature regimes and weather patterns influence food a v a i l a b i l i t y , rates of growth, density-independent mortality and the extent of spring colonization. The i n i t i a l assumption that g e r r i d guild structure was unlike l y to be influenced by "diffuse competition 1' (MacArthur, 1972a) needs to be re-examined because i t i s now c l e a r that other surface-feeding invertebrates may a f f e c t the d i s t r i b u t i o n and r e l a t i v e abundance of water-strider species. Further study of gerrid coexistence must consider the entire assemblage of surface-feeding predators. F i e l d experiments are possible and, i f successfully coupled to s o l i d models of gerrid population growth and habitat occupancy, a fascinating "bird's nest" of eco l o g i c a l relationships i s available for study. 285 The dynamic inter-play of processes that control gerrid d i s t r i b u t i o n and abundance w i l l not be successfully captured by simple, deterministic models. Stochastic.variation i n yearly climate drives gerrid population growth through d i r e c t e f f ects on dispersal, reproduction and development. 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Max. b c p i g 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 H i l l 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 i n at least one sample taken during 1975-1977. 2 Lake names i n parentheses are u n o f f i c i a l , descriptive names. 3 Underlined lakes are extremely temporary. Each one dried up completely at least once between 1975 and 1977. * Lakes indicated sampled i n t e n s i v e l y 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 i g 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 C o l p i t t Lake 17.70 692 755 X X x 5* "Grove Pond" 1 .33 532 866 X X 6* Hayfield Lake "Herrick's Pond" 7.55 350 951 X X X 7 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* " P i n t a i l Road Slough" 0.02 192 235 X X X 19* Reserve Lake 7.20 1769 3447 X X X 20 "Rider 1s 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 ~ := = = = = == = = = === = := = = = = = = = = == —— — :== = = = == = = = : 3. Gerris incoqnitus Eqq 1 6.7±0.07 n=15 2 3 4 5 4 . Gerris pingreensis STAGE 15° Eqq 24.0±0.34 n=30 1 6.5±0.24 n=16 2 5.3±0.18 n=15 3 7.6±0.19 n=15 4 15.6±0.42 n=7 5 24.0±2.30 n=5 306 = == = = = === = : ============== =========== TEMPER ATIIRE 18.5° 22° 2 6° _ 8.2±0.13 -n=35 4.5±0.22 3.6±0.13 2.5±0.24 n=13 n=34 n=13 - 3.7±0. 12 n=30 - 3:7±0.15 - • n=29 - 5.1±0.25 -n=23 ' - 8.3±0.34 -=========== n = 10 == = = = === = = : TEMPERATURE == = = = == = = = :18.5° 22° 2 6° 10.7±0. 13 8.3±0.08 6.4+0.10 n=65 n=133 n=43 4.7±0. 11 4.3±0.09 3.2±0. 10 n=23 n=32 n=47 3. 9±0.06 3U±0.13 2.8±0.11 n=23 n=32 n=36 4.4±0. 12 3.6±0.12 3.1±0. 13 n=23 n=32 n=33 . 6.3±0. 15 5.2±0.13 3.9±0.19 n=20 n=28 n=19 12.3±0.54 8. 1±0.63 5.3±0.31 n=10 n=9 n=8 ——————~ ~ ~ ~— :== = = = = = = ===== = :== = = = = == = = 307 5. Limnoporus d i s s o r t i s T A G E T E M P E R A T U R E 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 n o t a b i l i s : = = = = = = === = ============= ========== ============== ====== ===== ; T A G E T E M P E R A T U R E 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 rec i p r o c a l of instar duration i n 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. d i s s o r t i s INSTAR LINEAR REGRESSION EQUATION 1 Y = - 0.219 + 0.023X F = 141.4; df=1,50; p«.001 f• L- n o t a b i l i s 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 K i r k l . 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 WEIGHT (mg) S. E. N =============== ==================== =========== ===== = = = =: 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 n o t a b i l i s 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 

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