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An experimental test of Dodson’s hypothesis that Ambystoma and Chaoborust have complementary feeding… Giguère, Louis 1974

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AN EXPERIMENTAL TEST OF CCESCN'S HYPOTHESIS THAT AMEYSTCMA ANE CHAGBOBUS HAVE COMPLEMENTARY FEEDING NICHES fcy L c u i s Giguere E. A. , U n i v e r s i t e L a v a l , 1967 B . S c , U n i v e r s i t e L a v a l , 1971 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of ZOOLOGY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA 197 4 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of ^2-^o L. q (rV  The University of British Columbia Vancouver 8, Canada Date dxTs^* 7 JTlj ABSTRACT The removal from an a l p i n e pond of the eggs of a salamander, Ambystoma gracile, had a l a r g e impact on the zooplankton community. The pond was dominated by l a r g e Daphnia while nearby ponds, from which salamander eggs were not removed, were dominated by the s m a l l c l a d o c e r a n Diaphancscma . The s i z e a t m aturity of Diaptomus copepods i n c r e a s e d by 0.3 to 0.4 mm compared to the previous year. Crop content analyses r e v e a l e d t h a t food i n t a k e of I l n d and I l l r d i n s t a r Chaoborus was low. IVth i n s t a r s were s c a r c e and t h e i r dry weight was 50% lower than i n the previous year. When Ambystoma is abundant, i t preys h e a v i l y upon Holopedium gibberum and Daphnia rosea which are not a v a i l a b l e as food to Chaoborus t r i v i t t a t u s . The r e d u c t i o n i n the d e n s i t y of l a r g e z o o p l a n k t e r s permits a competitor, Diaphancscma brachyurum , to f l o u r i s h . I t i s the main food source, when a v a i l a b l e , of I l n d and I l l r d i n s t a r Chaoborus . In t h i s i n d i r e c t way, Chaoborus depends on v e r t e b r a t e p r e d a t i o n to s u c c e s s f u l l y reach i t s IVth i n s t a r . T h i s lends support to Dodson's hypothesis of complementary f e e d i n g niches. TABLE OF CONTENTS ABSTRACT i i TABLE OF CONTENTS . . . . . i i i LIST OF FIGURES v i LIST OF TABLES . v i i i ACKNOWLEDGEMENTS . . . . X I . INTRODUCTION .1 I I . LITERATURE REVIEW ....2 Food S e l e c t i v i t y ....3 i - Food S e l e c t o r s : predators .3 i i - Food C o l l e c t o r s : g r a z e r s .4 S i z e - e f f i c i e n c y Hypothesis ........5 Complementary Feeding Niches ...7 Al p i n e Pond Community Model 8 I n v e r t e b r a t e Predation .......9 I I I . DESIGN OF THE EXPERIMENT .......10 IV. GENERAL METHODS, MATERIALS AND STUDY AREA ....12 V. THE VERTEBRATE PREDATOR:AMBYSTOMA ...................19 A) I n t r o d u c t i o n 19 i - L i f e H i s t o r y ..19 i i - The Problem ..20 B) M a t e r i a l s And Methods ..............20 C) R e s u l t s And I n t e r p r e t a t i o n ..21 i - P o p u l a t i o n S t r u c t u r e .................21 i i - Stomach Contents 21 D) D i s c u s s i o n 23 VI. INVERTEBRATE PREDATOB:CHAOBORUS ..........25 A) I n t r o d u c t i o n . 25 i - L i f e H i s t o r y ..25 i i - The Problem .25 B) M a t e r i a l s And Methods 27 C) R e s u l t s And I n t e r p r e t a t i o n .....28 i - crop Contents 28 1- Q u a l i t a t i v e A n a l y s i s 28 2- Q u a n t i t a t i v e A n a l y s i s ........31 3- Food Density ......34 4- Food P r e f e r e n c e s ..36 i i - P o p u l a t i o n S t r u c t u r e And Density 37 i i i - Dry Weight .40 D) D i s c u s s i o n 41 VI I . ZOOPLANKTON ..44 A) I n t r o d u c t i o n 44 B) M a t e r i a l s And Methods 45 C) R e s u l t s And I n t e r p r e t a t i o n 46 i - Cladocera .......46 i i - S ize At M a t u r i t y For Cladocera 54 i i i - C a l a n o i d Copepods ..56 i v - S i z e At M a t u r i t y For C a l a n o i d Copepods .56 v- S p a t i a l D i s t r i b u t i o n 58 v i - The Shallow Pond 59 v i i - C y c l o p o i d Copepods ..60 D) D i s c u s s i o n 60 i - Cladocera ..61 1- Competition 61 2- Pred a t i o n .61 3- C o n t r o l Pond 62 4- Experimental Pond ................................ 64 5- Daghnia C l u t c h S i z e 64 i i - C a l a n o i d Copepods 65 VIII . GENERAL DISCUSSION ..67 LITERATURE CITED .72 APPENDIX I 76 APPENDIX I I , ..........81 LIST OF FIGURES F i g u r e Page 1. Research area. , 13 2. Morphometry of the ponds. A- C o n t r o l B-Experimental C- Shallow 14 3. D e s c r i p t i o n of the ponds and t r a n s e c t l o c a t i o n s . A- C o n t r o l B- Experimental C- Shallow 15 4. Diet o f Chaoborus. Crop content a n a l y s i s by s p e c i e s and s p e c i e s composition i n the main ponds on August 21st and September 4th 1973. TOP: the r e s u l t s are expressed as a percentage frequency of occurrence i n samples. BOTTOM: the data were transformed from numbers t o bicmasses p r i o r to determining the percentage f r e q u e n c i e s . A- C o n t r o l , August 21st, 1973 B-Experimental, August 2 1 s t , 1973 C- C o n t r o l , September 4th, 1973 D- Experimental, September 4th, 1973 The s p e c i e s code i s as f o l l o w s : a-D. kenai b- Daghnia rosea c- Di_brach_y_urum d-Cy_clo£S e- Others f - R o t i f e r s . Holojgedium i s not i n c l u d e d 30 5. Diagrams of prey d e n s i t y i n the main ponds versus the average number of prey i n a Chaoborus crop i n each pond. The food i s c l a s s i f i e d i n two c a t e g o r i e s : c l a d o c e r a ( f u l l l i n e ) and r o t i f e r s (hatched l i n e ) . Holojsedium i s not i n c l u d e d . The arrow p o i n t s from August 21st toward September 4th, 1973. A l l values f o r c l a d o c e r a were m u l t i p l i e d by a f a c t o r of 10. 35 6. R e l a t i v e p r o p o r t i o n of Chaoborus i n s t a r s i n the main ponds from l a t e August to October 1973. ...... 38 7. Density of Chaoborus l a r v a e : A- i n the c o n t r o l pond f o r I l l r d and IVth i n s t a r s B- i n the experimental pond f o r I l l r d and IVth i n s t a r s C-i n the c o n t r o l and experimental pond f o r a l l i n s t a r s 39 8. Seasonal v a r i a t i o n i n p l a n k t o n i c biomass d e n s i t y i n the main ponds i n 1973. The hatched l i n e s r e present Amb^stoma l a r v a e hatching. C o n t r o l pond: TOP Experimental pond: BOTTOM 47 9. Seasonal v a r i a t i o n i n p l a n k t o n i c biomass d e n s i t y i n the shallow pond i n 1973. The hatched l i n e s r e present Ambyjjtoma l a r v a e hatching. .............. 48 10. Seasonal v a r i a t i o n i n the average biomass per crustacean i n the main ponds i n 1973. ............. 49 11. Seasonal v a r i a t i o n i n cru s t a c e a n and Dia£hancsoja d e n s i t y (TOP) and biomass d e n s i t y (BOTTOM) i n the c o n t r o l pond i n 1973. 50 12. Seasonal v a r i a t i o n i n cru s t a c e a n and Dajjhnia d e n s i t y (TOP) and biomass d e n s i t y (BOTTOM) i n the experimental pond i n 1973 51 13. Holop_edium s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e at maturity 52 14. Dapjmia s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e at ma t u r i t y . 53 15. Dia£tomus kenai s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e a t m a t u r i t y . 57 16. Dia£hanosoma s i z e d i s t r i b u t i o n i n a l l ponds in 1973. The dotted l i n e s r epresent the minimum s i z e a t m a t u r i t y . ................................. 63 17. P r e d a t i o n diagram. From heavy p r e d a t i o n ( s o l i d l i n e ) to l i g h t p r e d a t i o n (dotted l i n e ) 69 18. S i z e d i s t r i b u t i o n of salamanders captured i n the experimental pond i n 1972 83 19. Summary of the estimates of the salamander p o p u l a t i o n i n the experimental pond. .............. 84 LIST OF TABLES TABLE Page I Ponds morphometry. 12 II L i s t of s p e c i e s with g e n e r i c d e s c r i p t i v e terms. ....... 16 III Formulae used to o b t a i n biomass-volume from l e n g t h s , or from d i r e c t e s t i m a t e s . ................ 17 IV Estimates of the number of h a t c h l i n g salamander i n the ponds i n 1973, with t h e i r d e n s i t y (# per c u b i c meter). 21 V Stomach contents a n a l y s i s f o r sub-adult ( l e s s than 55 cm) and adult salamanders (60-75 cm) captured on August 8th 1973. Sub-adults come from the experimental pond and a d u l t s from the c o n t r o l pond. • 22 VI Stomach content a n a l y s i s f o r sub-adult salamanders captured i n the experimental pond on J u l y 5th, 1973. .... 23 VII Chaoborus sample s i z e by i n s t a r s f o r crop contents a n a l y s i s on August 21st and September 4th, 1973. . 29 VIII Average volume per p l a n k t o n i c prey item (food packet s i z e ) on August 21st and September 4th 1973 i n the main ponds. Values are given in c u b i c microns. 33 IX Average volume per p l a n k t o n i c prey item i n crops of d i f f e r e n t Chaoborus i n s t a r s . Values are given i n c u b i c microns x 10^ f o r August 21st and September 4th 1973 i n the main ponds 33 X Average biomass i n crops of d i f f e r e n t Chaoborus i n s t a r s . Values are gi v e n i n c u b i c microns x 10 5 f o r August 21st and September 4th 1973 i n the two main ponds. 33 XI Comparison of the percentage of empty crops f o r the I l n d and I l l r d i n s t a r Chaoborus on August 21st 1973 i n the main ponds. ....37 XII Comparison of the percentage of empty crops f o r the I l n d and I l l r d i n s t a r s Chaoborus on September 4th 1973 i n the main ponds 37 XIII Dry weight i n m i l l i g r a m s of IVth i n s t a r l a r v a e with 95% c o n f i d e n c e l i m i t s . The animals were captured i n the F a l l of 1973 f o r the c o n t r o l pond and, 1972 and 1973 f o r the experimental pond. 40 XIV Summary of Amb^stoma and Chaoborus p r e d a t i o n l e v e l s i n a l l ponds. 43 XV Summary of Ambystoma and Chaoborus p r e d a t i o n by i n s t a r s on d i f f e r e n t prey c a t e g o r i e s with s i z e s 43 XVI A n a l y s i s of v a r i a n c e of minimum s i z e at maturity f o r Daghnia on August 21st and September 4th 1973 i n the main ponds. 55 XVII A n a l y s i s of v a r i a n c e of m u l t i p l e l i n e a r r e g r e s s i o n of c l u t c h s i z e on body s i z e f o r Dap.hnia o n August 21st and September 4th 1973 i n the main ponds. ............................................ 55 XVIII Regression c o e f f i c i e n t s o f c l u t c h s i z e on body s i z e f o r Dapjtinia on August 21st and September 4th 1973 i n the main ponds. 55 XIX C o r r e l a t i o n c o e f f i c i e n t s : s p e c i e s abundance with r e s p e c t t o depth f o r Dajahnia and piaphanosoma i n the main ponds(d.f.=15). ....................... 58 XX C o r r e l a t i o n c o e f f i c i e n t s : s p e c i e s abundance with r e s p e c t t o depth f o r copepods i n the main ponds (d.f . = 15) . 59 XXI Summary of h a t c h l i n g salamander behavior experiments. E x p l a n a t i o n s are i n the t e x t . ........ 78 ACKNOWLEDGEMENTS Th i s work was supported by: 1- Quebec Department of Education bursary # 221-901-488. 2- B.C. P a c k e r s / F i s h e r i e s A s s o c i a t i o n of B.C. award. 3- U n i v e r s i t y of B r i t i s h Columbia t e a c h i n g a s s i s t a n t s h i p . 4- NRC grant # 67-4447 to Dr. C.F. Wehrhahn. I would l i k e t o thank the f o l l o w i n g people who helped with the f i e l d work: K e i t h Reid, Brian MacClean, A r t H i l l i k e r , Derek Roff and L i d i a Jaremovic. I am g r a t e f u l to Con Wehrhahn, Tom Northcote and B i l l N e i l l who read and c r i t i c i z e d manuscripts of t h i s work a t d i f f e r e n t s tages of i t s development. I am p a r t i c u l a r l y indebted t o Steve Stearns who had the patience to work through the o r i g i n a l v e r s i o n s of the t h e s i s , making i n v a l u a b l e comments. It. INTRODUCTION Competitive and predatory r e l a t i o n s h i p s among s p e c i e s are o f t e n c o n s i d e r e d t o be the main determinants of community s t r u c t u r e and d i v e r s i t y . For freshwater l a k e s . Brooks and Dodson (1965) b u i l t a model of community i n t e r a c t i o n s based upon the concepts of s i z e - s e l e c t i v e p r e d a t i o n and d i f f e r e n t i a l e f f i c i e n c y among competitors. Furthermore, Dodson (1970) hypothesized that a h i g h l y s e l e c t i v e predator can a f f e c t the d i v e r s i t y of predator as w e l l as of prey s p e c i e s s i n c e i t excludes i t s p r e f e r r e d (large) food item and thus f a v o u r s the presence of sub-optimal (smaller sized) prey which are the p r e f e r r e d food of the second dependent predator. To t e s t Dodson's hypothesis of complementary f e e d i n g niches s u s t a i n e d by s i z e - s e l e c t i v e p r e d a t i o n , I i n v e s t i g a t e d community i n t e r a c t i o n s through p a r t i a l removal of a predator i n a n a t u r a l s e t t i n g . I removed the eggs of the salamander Amb^stoma f r a g i l e from one of t h r e e a l p i n e ponds near Vancouver, E. C. and recorded the impact of the manipulation on the predatory midge l a r v a , Chaoborus t r i y i t t a t u s , and on the zooplankton community: c l a d o c e r a , copepods, and r o t i f e r s . Throughout the summer, I recorded abundance, age s t r u c t u r e , and s i z e of most s p e c i e s . These s t a t i s t i c s provided i n s i g h t i n t o c o m p e t i t i v e and predatory i n t e r a c t i o n s and u s e f u l i n f o r m a t i o n about the o r g a n i z a t i o n of the community as a whole. H i LITERATURE REVIEW Community s t r u c t u r e and d i v e r s i t y have been s t u d i e d t h e o r e t i c a l l y and e x p e r i m e n t a l l y by many. In the f i e l d , C o n n e l l (1961) has shown that the removal of a predatory s n a i l , T h a i s , a f f e c t s the v e r t i c a l d i s t r i b u t i o n of b a r n a c l e s i n the i n t e r t i d a l . The lower l i m i t of the upper s p e c i e s , Chthamalus, depends on the f a s t e r growing competitor Balanus. Since the s n a i l p r e f e r s l a r g e r prey of the genus Balanus, i t allows the poorer competitor Chthamalus to extend i t s range downward. Paine (1966) and Paine and Vadas (1969) have shown t h a t the removal of a top p r e d a t o r , P i s a s t e r , leads to a decrease i n s p e c i e s d i v e r s i t y . With P i s a s t e r gone, the mussel H j t i l u s e s t a b l i s h e d i t s e l f at the expense of algae and other s p e c i e s such as c h i t o n s , l i m p e t s , b a r n a c l e s and even those t h a t are not eaten by P i s a s t e r such as anemones, and nudibranch-sponge a s s o c i a t i o n s . I f not kept i n check by the predator, M_ytilus w i l l monopolize the a v a i l a b l e space. T h i s o b s e r v a t i o n l e d Paine to suggest t h a t a keystone predator c o u l d s t a b i l i z e the otherwise u n s t a b l e i n t e r a c t i o n s among competing prey s p e c i e s . In freshwater environments, the i n t r o d u c t i o n or e x t e r m i n a t i o n of f i s h i n l a k e s has provided s t r o n g evidence f o r the types of i n t e r a c t i o n s documented by C o n n e l l and Paine f o r the i n t e r t i d a l . Examples have m u l t i p l i e d s i n c e Brooks S Dodson (1965) proposed t h e i r s i z e - e f f i c i e n c y hypothesis based upon l a b o r a t o r y and f i e l d evidence and o u t l i n e d some p r e d i c t i o n s t h a t could be t e s t e d (e. g. Green, 1967; Dodson, 1970; H a l l e t a l . , 1970; S p r u l e s , 1972; Z a r e t , 1972a; Dodson, 1974). I w i l l review the e s s e n t i a l s of t h e i r models, i n c l u d i n g Dodson's a l p i n e pond community model which deals with complementary fee d i n g niches between a salamander and a midge l a r v a . Food S e l e c t i v i t y , i - Food S e l e c t o r s ^ j y r e d a t o r s Predators are o f t e n c o n s i d e r e d to opt i m i z e t h e i r food r e t u r n and s e l e c t prey a c c o r d i n g l y . C e r t a i n l y , the r e l a t i v e abundance of prey of d i f f e r e n t s i z e s and t h e i r d i f f e r e n t c a p a c i t i e s f o r a v o i d i n g capture w i l l a f f e c t t h i s s t r a t e g y . Brooks and Dodson (1965) suggested that the dominant f a c t o r determining the c h o i c e of a l a k e p l a n k t i v o r e i s s i z e , and d i s t i n g u i s h e d between two types of pr e d a t o r s : a) O b l i g a t e p l a n k t i v o r e s which show s u s t a i n e d i n t e r e s t i n p l a n k t o n i c food, e. g. A l o s a , and b) f a c u l t a t i v e p l a n k t i v o r e s which u s u a l l y show i n t e r e s t i n plankton (mostly l a r g e Daphnia ), e. g. Salmo , Perca . I f plankton are not a v a i l a b l e they switch to other food s o u r c e s . Both kinds are s t r o n g l y biased toward the l a r g e s t items a v a i l a b l e w i t h i n an ac c e p t a b l e category. For example, Brooks (1968) found that the s u r v i v a l time of s m a l l c a l a n o i d copepods i n a tank c o n t a i n i n g Alosa was i n v e r s e l y p r o p o r t i o n a l to t h e i r mean body l e n g t h . As a r e s u l t s i z e - s e l e c t i v e p l a n k t i v o r e s produce s h i f t s i n the s p e c i e s composition of the zooplankton community. Most a s t u d i e s (e. g. Brooks and Dodson, 1965; R e i f and Tappa, 1966; G a l b r a i t h , 1967; Wells, 1970) document a decrease i n numbers and even disappearance of some of the l a r g e r s p e c i e s i n l a c u s t r i n e ecosystems. A l t e r n a t i v e l y , a predator might e l i c i t s m a l l e r s i z e at m a t u r i t y of a given s p e c i e s or morphological changes such as the cyclomorphosis of l i m n e t i c c l a d o c e r a (e. g. Green, 19 67, and Brooks, 1965). Other t h e o r i e s have been proposed to account f o r predator s e l e c t i v i t y . Some have to do with b e h a v i o r a l c h a r a c t e r i s t i c s , such as antennal beat frequency(Jacobs, 1967) or locomotion (Lindstrom, 1955). Dodson (1970), Greze (1963), and Z a r e t (1972) cl a i m t h a t n e i t h e r movement or s i z e of prey e x p l a i n i t , and propose t h a t the v i s i b i l i t y of prey to predators a f f e c t s s e l e c t i v i t y . LLz Food C o l l e c t o r s ^ g r a z e r s Brooks and Dodson suggest that g r a z i n g zooplankters a l l have access to food p a r t i c l e s between 1 and 15 microns i n s i z e . Some l a r g e r animals a l s o have access to p a r t i c l e s up to 50 microns. While the c l a d o c e r a can only be s e l e c t i v e by v a r y i n g t h e i r f e e d i n g r a t e s , copepods f o l l o w a p a t t e r n l i k e the r o t i f e r s : they s e l e c t i n d i v i d u a l p a r t i c l e s a c c o r d i n g to chemical and s u r f a c e q u a l i t y c r i t e r i a (Edmondson, 1965). Sympatric s p e c i e s of copepods vary g r e a t l y i n s i z e ( c h a r a c t e r displacement a f t e r Brown and Wilson, ,1956). Hutchinson (1 967) suggested that t h i s i s a s s o c i a t e d with non o v e r l a p p i n g food n i c h e s . For these reasons I b e l i e v e t h a t c l a d o c e r a should be t r e a t e d s e p a r a t e l y from copepods. S i z e - e f f i c i e n c y , H_y£othesis Brooks and Dodson's (1965) model of community i n t e r a c t i o n s can be d i v i d e d i n t o two p a r t s corresponding to the two f o l l o w i n g q u e s t i o n s (Dodson, 1974) : 1- Why aren't l a r g e s p e c i e s found with s m a l l ones? A widely accepted e x p l a n a t i o n to t h a t q u e s t i o n i s t h a t a s e l e c t i v e v e r t e b r a t e predator removing l a r g e animals i s present. 2- Why a r e n ' t small s p e c i e s found with l a r g e ones i n the absence of v e r t e b r a t e predation? The s i z e - e f f i c i e n c y hypothesis o r i g i n a l l y accounted f o r that, I t i s based upon c o m p e t i t i v e i n t e r a c t i o n s whereby l a r g e e f f i c i e n t z o oplankters overcome s m a l l ones i n food resource e x p l o i t a t i o n . A p l a u s i b l e e x p l a n a t i o n f o r the observed c o m p e t i t i v e i n t e r a c t i o n s i s that l a r g e f i l t e r feeders outcompete s m a l l bodied s p e c i e s because they have a lower metabolic r a t e , a slower r a t e of s i n k i n g and are more e f f i c i e n t f e e d e r s . T h i s g r e a t e r f e e d i n g e f f i c i e n c y i s a t t r i b u t a b l e to two main f a c t o r s : (1) they have access to l a r g e p a r t i c l e s which the s m a l l e r animals cannot handle (Edmondson, 1965), and (2) the food c o l l e c t i n g s u r f a c e s are the square of a l i n e a r dimension of the animals f i l t r a t i o n mechanism. E a r l y experiments support that i d e a q u i t e n i c e l y f o r c l a d o c e r a (Sushtchenia 1959, a f t e r Brooks and Dodson 1965): f o r squared body lengths p r o p o r t i o n a l to 1, 2, 9 and 16, the r e s p e c t i v e f i l t r a t i o n r a t e s are p r o p o r t i o n a l to 1, 2, 10 and 18. These predatory and c o m p e t i t i v e r e l a t i o n s h i p s l e d to the f o l l o w i n g e c o l o g i c a l i m p l i c a t i o n s of the so c a l l e d s i z e -e f f i c i e n c y h y p o t h e s i s (cf. Brooks & Dodson, 1965) on which I based my p r e d i c t i o n s at the beginning of t h i s work : 1) P l a n k t o n i c h e r b i v o r e s a l l compete f o r the f i n e p a r t i c u l a t e matter of the open waters 2) Large zooplankters do so more e f f i c i e n t l y and can a l s o take l a r g e r p a r t i c l e s 3) Under low p r e d a t i o n i n t e n s i t y , l a r g e h e r b i v o r e m o r t a l i t y i s low and they e l i m i n a t e s m a l l e r ones through c o m p e t i t i o n 4) Under i n t e n s e p r e d a t i o n , s i z e - s e l e c t i v e p r e d a t o r s w i l l e l i m i n a t e the l a r g e forms a l l o w i n g s m a l l zooplankters ( r o t i f e r s and c l a d o c e r a ) that escape p r e d a t i o n t o become dominant 5) Under moderate p r e d a t i o n i n t e n s i t y , the number of l a r g e r s p e c i e s are c o n t r o l l e d so t h a t s m a l l e r competitors are not e l i m i n a t e d . £°5!£i§£Slii§.II F e e d i N i c h e s Dodson (1970) hypothesized that s i z e s e l e c t i v i t y of p r e d a t o r s a l s o a f f e c t s d i v e r s i t y at the predator l e v e l and t h a t one predator can s u s t a i n another one. He s t u d i e d 50 a l p i n e ponds t h a t are f r e e of f i s h but c o n t a i n salamander p o p u l a t i o n s and observed two b a s i c types of communities: (a) two predators ( Ambjstoma c j r a c i l e and Chaoborus americanus ) and s m a l l herbivorous s p e c i e s , and (b) one predator ( Dia£tomus shoshone ) and l a r g e herbivorous s p e c i e s He estimated e l e c t i v i t y c o e f f i c i e n t s and f e e d i n g r a t e s and was l e d to the c o n c l u s i o n t h a t the salamander maintained a s u i t a b l e f e e d i n g niche f o r the second predator Chaoborus through s i z e - s e l e c t i v e p r e d a t i o n , The salamander i s r e s p o n s i b l e f o r the prey compositon of type (a) communities because i t i s a very d i s c r i m i n a t i n g v i s u a l predator. Analyses of stomach c o n t e n t s r e v e a l e d t h a t the r e l a t i v e abundance of s p e c i f i c food items can be as high as 31 times t h e i r r e s p e c t i v e a v a i l a b i l i t y (with r e s p e c t to other s p e c i e s ) i n the environment. At the same time, however, plankton c o n s t i t u t e s l e s s than 10% of i t s d i e t . CbaoboEMS on the other hand s e l e c t s smaller prey i n a l e s s d i s c r i m i n a t o r y f a s h i o n using a t a c t i l e sensory system . I t s maximum e l e c t i v i t y c o e f f i c i e n t was only 2.8. However, i t feeds h e a v i l y on z o o p l a n k t e r s . I t i s d e s c r i b e d as a dependent predator s i n c e i t i s Ambystoma that accounts f o r the presence of s m a l l h e r b i v o r o u s s p e c i e s . T h i s work l e d Dodson to r e e v a l u a t e the r o l e played by i n v e r t e b r a t e p r e d a t o r s i n a community. He c o n s i d e r s i t an a l t e r n a t i v e to the s i z e - e f f i c i e n c y h y p othesis i n e x p l a i n i n g why smal l h e r b i v o r e s do not c o e x i s t with l a r g e ones and proposes the f o l l o w i n g model of community s t r u c t u r e . A l ^ i n e Pond Community Model Ambyjstoma i s considered a keystone pre d a t o r . I t does not feed h e a v i l y on z o o p l a n k t e r s but i s so s e l e c t i v e that i t has a d r a s t i c e f f e c t on plankton composition. I t s e l e c t i v i t y c o e f f i c i e n t s are complementary with those of the i n v e r t e b r a t e p r e d a t o r s . However the b r i g h t red copepod D. shoshone competes with Chaoborus and i s an easy prey f o r Araby_stoma whereas the c r y p t i c (transparent ) midge l a r v a e can escape p r e d a t i o n and c o e x i s t with s m a l l h e r b i v o r e s . In the absence of v e r t e b r a t e p r e d a t o r s , the copepod e l i m i n a t e s s m a l l h e r b i v o r e s and c o e x i s t s with l a r g e ones. Acco r d i n g to Dodson, Chaoborus i s absent i n type (b) communities, because Dia£tomus shoshone preys upon i t s eggs. l M S I i § 2 i a i § P r e d a t i o n Dodson (1970) does not mention the s i z e - e f f i c i e n c y h y p o t h e s i s . Dodson (1974) conducted experiments to t e s t i t by i n t r o d u c i n g v a r i o u s assemblages of z o o p l a n k t e r s t h a t came from both types of communities i n t o 42 l i t e r cages. He observed very l i t t l e i n t e r a c t i o n between s m a l l and l a r g e s p e c i e s of Eaghnia and suggest t h a t " i f c o m p e t i t i o n f o r food between s i m i l a r daphnid s p e c i e s e x i s t s , i t i s a r a t h e r s u b t l e , long term e f f e c t . Such a weak e f f e c t i s not enough to e x p l a i n the complete absence of s m a l l Da^hnia s p e c i e s from the l a r g e Dap_hnia ponds..." However , i n v e r t e b r a t e p r e d a t i o n by the copepod £ i_shoshone e l i m i n a t e d s m a l l Dapjinia w i t h i n a month. These r e s u l t s along with the d i s t r i b u t i o n of Amby_stoma and D i_shcshcne i n two separate and d i s t i n c t communities l e d Dodson to the c o n c l u s i o n s mentioned i n the review of h i s a l p i n e pond model, i n which predatory r e l a t i o n s h i p s e x p l a i n s p e c i e s d i s t r i b u t i o n s . He suggests t h a t t h i s might a l s o be the case i n systems where Chaoborus , Dia^tomus , Lep_todorus and Cy_clo£S are important and reviews r e c e n t evidence t h a t does not support the s i z e -e f f i c i e n c y h y p o t h e s i s such as l a b o r a t o r y experiments on f e e d i n g e f f i c i e n c y of d i f f e r e n t s i z e c l a d o c e r a (Burns, 1968, 1969; E g l o f f 6 Palmer, 1971) . I H i . DESIGN OF THE EXPERIMENT I s e t out t o t e s t the e x i s t e n c e of complementary f e e d i n g n i c h e s , by p e r t u r b a t i o n of a n a t u r a l system, and to study the i m p l i c a t i o n s of s i z e - s e l e c t i v e p r e d a t i o n and the s i z e - e f f i c i e n c y h y p o t h e s i s . In order t o do t h a t , two main ponds were s e l e c t e d and monitored throughout 1973: 1- A c o n t r o l pond with a s t a b l e water l e v e l c o n t a i n i n g a v e r t e b r a t e predator, Ambyjstoma a r a c i l e (a salamander), and an i n v e r t e b r a t e p redator, Chaoborus t r i y i t t a t u s (midge l a r v a ) . 2- An experimental pond s i m i l a r to the p r e v i o u s one from which approximately 50% of the eggs of Ambystoma were removed i n 1972 and 90% i n 1973. I t i s b e l i e v e d t h a t the h a t c h l i n g s , due to t h e i r great numbers and food h a b i t s , have the g r e a t e s t impact on the zooplankton community (Dodson, 1970; Dodson and Dodson, 1971) . A t h i r d pond was monitored. I t d r i e d up p a r t i a l l y and, t h e r e f o r e , there should be i n c r e a s e d salamander predation p r e s s u r e on zooplanKton during summer. Chaoborus d e n s i t y i s low i n t h i s pond (probably as a consequence of Ambystoma p r e d a t i o n ) . I then t r i e d to o u t l i n e some p r e d i c t i o n s about the p o p u l a t i o n s based upon the s i z e - e f f i c i e n c y h y p o t h e s i s : 1)-Cladocera: In simple systems and i n the absence of v e r t e b r a t e s i z e - s e l e c t i v e p r e d a t i o n , l a r g e c l a d o c e r a should outcompete small-bodied s p e c i e s . I expected t h i s i n the axperimental pond. 2)-Copepods : sympatric s p e c i e s might have a mechanism of r e s o u r c e p a r t i t i o n i n g through food s i z e s e l e c t i o n a c c o r d i n g to body s i z e . I d i d not expect them to f o l l o w any simple model d e r i v e d from the s i z e - e f f i c i e n c y hypothesis. Whereas s i z e -s e l e c t i v e p r e d a t i o n decreases the f i t n e s s of l a r g e body s i z e , the e f f e c t of competition would be to space out the d i f f e r e n t s i z e s of sympatric s p e c i e s . I t i s then d i f f i c u l t to make any p r e d i c t i o n with r e s p e c t to the s i z e - e f f i c i e n c y hypothesis. In the absence of Ambystoma , I expected the l a r g e r copepod s p e c i e s to i n c r e a s e i n s i z e and p o s s i b l y open up a new niche f o r a s m a l l e r s p e c i e s to occupy due to l e s s e n e d c o m p e t i t i v e i n t e r a c t i o n . 3 ) - Chaoborus l a r v a e should not be able to f i n d s u i t a b l y s i z e d prey i n the experimental pond and s t a r v e . Development and weight, as well as abundance and molting' success, should be a f f e c t e d . H i GENERAL METHODSx MATERIALS AND STUDY AREA The f i e l d experiments were performed on H c l l y b u r n Ridge, West Vancouver, i n s m a l l ponds at an a l t i t u d e of 1070 meters (F i g u r e 1) . The ponds were l e s s than one acre i n s i z e and each was from 1 to 3 meters deep (Figures 2 and 3). The c o n t r o l and experimental ponds do not d i f f e r much i n t h e i r morphological c h a r a c t e r i s t i c s . TABLE I Ponds morphometry. i 1 | Area Volume Maximum Volume/ | | depth | I , _ Depth | | mz m 3 m \ I 4 (C o n t r o l 2250 3370 1.9 1.50 | | Experimental 3400 5250 1. 6 1.54 | IShallow 1000 450 0.9 0.45 | A l l samples were preserved i n a n e u t r a l 5% f o r m a l i n s o l u t i o n with 30gm s u c r o s e / l i t r e and b u f f e r e d with borax (Haney & H a l l , 1973). P l a n k t o n i c s p e c i e s were i d e n t i f i e d a c c o r d i n g to Ward 6 Whipple (1959). Table I I l i s t s the s p e c i e s encountered ( a l l s p e c i e s were found i n a l l three ponds) and d e f i n e s the g e n e r i c terms used to d e s c r i b e them throughout t h i s work. An " o c c a s i o n a l s p e c i e s " was d e f i n e d as any s p e c i e s of the s u p e r f a m i l y Chydoroidea which occurs s p o r a d i c a l l y i n the samples and never exceeds 1% of the estimated c r u s t a c e a n p o p u l a t i o n . The " o c c a s i o n a l s p e c i e s " were u s u a l l y c o n f i n e d to the margins of ponds. Table I I I i n d i c a t e s the sources from which formulae were F i g u r e 1. Research area. Hollyburn Mountain Peak (1220m) Mil HMD Hollyburn Ridge (915m) — — — — Water Supply Demarcation l=Rhallow Pond 2=Control Pond 3=Experimental Pond F i g u r e 2. Morphometry of the ponds. A- C o n t r o l B- Experimental C-Shallow A 10 meters 0-40cm Q 40-80cm © 80-120cm '' 6 ^ 120-160cm F i g u r e 3. D e s c r i p t i o n of the ponds and t r a n s e c t l o c a t i o n s , fi-C o n t r o l B- Experimental C- Shallow TABLE I I L i s t of s p e c i e s with g e n e r i c d e s c r i p t i v e terms. PREDATOR. INVERTEBRATE C h a o b o r u s t r i v i t t a t u s ( M i d g e l a r v a ) VERTEBRATE . Ambystoma g r a c i l e ( S a l a m a n d e r ) CRUSTACEAN PLANKTON a) C l a d o c e r a n s D a p h n i a r o s e a D i a p h a n o s o m a b r a c h y u r u m C h y d o r u s s p h a e r i c u s A c r o p e r u s h a r p a e S c a p h o l e b e r i s k i n g i Hoi o p e d i u m g i b b e r u m b) C o p e p o d D i a p t o m u s k e n a i - c a l a n o i d D i a p t o m u s l e p t o p u s - c y c l o p o i d C y c l o p s s p . ROTIFERS K e r a t e l l a s p . INSECTS ( a l s o p r e d a t o r y , w a t e r b o r n e o r a i r b o r n e ) OTHERS S p o n g e s , l e e c h e s , c l a m s , e t c . . . TABLE I I I Formulae used to o b t a i n biomass-volume from lengths, or from d i r e c t e s t i m a t e s . S p e c i e s ( t a x o n ) Fortnul a A u t h o r C o p e p o d W~ 3=0.055L 2 .73 Klekowski. & S h u s k i n a (1S66) (2 ) D a p h n i a W~ 3=0.052L 3 .012 P e c h e n (1965) ( 2 ) D i a p h a n o s o m a W" 3=0.092L 2 .449 II ( 2 ) B o s m i n a ( o t h e r s ) W" 3=0.56L-C .01 S h e r b a k c v (1962) (2) C y c l o p s l x l 0 7 m 3 ( 1 ) R o t i f e r s 5 x l 0 4 / r ' 3 ( 1 ) (.1) f r o m D u s s a r t ( 1 9 6 6 ) ( 2 ) f r o m Edmondson & W i n b e r g (1971) obtained to estimate biomass i n the ponds as w e l l as i n S.^a2^2£!i2 crops. The values were a r r i v e d at most of the time through a s e l e c t i o n of an a p p r o p r i a t e s i z e from the s i z e d i s t r i b u t i o n of a s p e c i e s at a g i v e n time. Biomass wet weight and volume were used i n t e r c h a n g e a b l y s i n c e i t was assumed t h a t plankton s p e c i e s are approximately as dense as water (1gr=1cc). I t i s true that p l a n k t o n i c animals have a c h i t i n o u s carapace and a l s o t h a t l a r g e Daphnia c o n t a i n some water between the valves of t h e i r carapace. However I b e l i e v e t h a t the estimates obtained from the t r a n s f o r m a t i o n s are s u f f i c i e n t l y accurate f o r the purpose of e s t i m a t i n g the average volume of a prey item or the biomass of prey items i n the ponds or i n the stomachs of p r e d a t o r s . Ijl THE VERTEBRATE PREDATOR^AHBYSTOMA A]_ I n t r o d u c t i o n i z L i f e H i s t o r j Metamorphosed a d u l t s gather i n ponds each s p r i n g , as soon as the i c e s t a r t s t o melt. Mating occurs and males pass on spermatheca to females and l e a v e . Eggs are l a i d when the water temperature reaches about 10°C, u s u a l l y during the f i r s t warm week i n June or J u l y on Hollyburn Ridge. They are a l l l a i d on the same day. Each female produces a g e l a t i n o u s clump of about 40 to 90 eggs. The egg masses are attached t o s t i c k s , branches or blades of v e g e t a t i o n a t the edge of ponds. Then, the females leave and the eggs hatch i n approximately 3 t o 5 weeks . A f t e r the f i r s t summer's growth, the l a r v a e go i n t o h i b e r n a t i o n . Towards the end of the second summer (in August), a p r o p o r t i o n of the p o p u l a t i o n metamorphoses and moves onto l a n d . However, an even l a r g e r p r o p o r t i o n " f a i l s " t o metamorphose and stays i n the pond. They reach maturity and breed while s t i l l p o s sessing l a r v a l c h a r a c t e r i s t i c s . T h i s phenomenon i s c a l l e d paedogenesis. According to Neish (1970) , h y b r i d i z a t i o n occurs between the two d i f f e r e n t types o f a d u l t animals. L i t t l e i s known about t h e i r l o n g e v i t y . Metamorphosed a d u l t s of o ther Ambystomidae, e.g. A., t i ^ r i n u m (Blanchard , 1932) and h± l i S i i i a i S I ( p°Pe# 1937) , were known to l i v e as long as 11 and 25 years, r e s p e c t i v e l y , i n c a p t i v i t y . i i - The Problem H a t c h l i n g salamanders were expected to have the g r e a t e s t impact on the zooplankton community because of t h e i r l a r g e numbers and food h a b i t s . Dodson (1970) mentions t h a t plankton i s the major food of salamanders only during the f i r s t few weeks of development a f t e r which i t makes up l e s s than 10% of t h e i r d i e t . I t r i e d to check Dodson's o b s e r v a t i o n through stomach content a n a l y s e s . Laboratory behavior experiments were a l s o performed to i n v e s t i g a t e Amby.stoma h a t c h l i n g ' s food s i z e s e l e c t i v i t y . The r e s u l t s are not d i f f e r e n t from what other authors have found f o r v e r t e b r a t e predators (e. g. Brooks, 1968, B r y n i l d s o n , 1958, and S a l b r a i t h , 1967): within a s p e c i e s or category, l a r g e animals are p r e f e r r e d . In order not to diverge from the mainstream of the f i e l d experiments, I present the d e t a i l s i n Appendix I . 51 M a t e r i a l s And Methods While s k i n d i v i n g I caught salamanders with a s u c t i o n gun made of t r a n s p a r e n t p l a s t i c with a 5 cm opening. I t was a c t i v a t e d by hand. The animals were a n e s t h e t i z e d i n a 0. 1% s o l u t i o n of t r i c a i n e methanesulfonate (MS 222), measured (snout to vent length) to the n e a r e s t m i l l i m e t e r and weighed to the nearest gram. I then pumped and preserved t h e i r stomach c o n t e n t s . Animals were numbered by c l i p p i n g toes a c c o r d i n g to a binary n o t a t i o n and r e l e a s e d . I estimated p o p u l a t i o n s i z e s using Petersen's mark-recapture method as d e s c r i b e d i n B a i l e y (1952). £L E e s n l t s And I n t e r p r e t a t i o n i - P o p u l a t i o n S t r u c t u r e In 1972, I estimated the components of the salamander po p u l a t i o n s t r u c t u r e i n the experimental pond. T h i s i n f o r m a t i o n i s presented i n Appendix I I and s y n t h e t i z e d i n F i g u r e 19. The r e l e v a n t estimates about h a t c h l i n g i n p u t i n t o a l l three ponds are presented here. TABLE IV E s t i m a t e s of the number of h a t c h l i n g salamander i n the ponds i n 1973, with t h e i r d e n s i t y (# per c u b i c meter). | | h a t c h l i n g |#hatchlings with 95$| | | d e n s i t y |confidence l i m i t s | j. 4 + 1 I C o n t r o l | 1.00 | 3370± 375 | lExperimental | 0.78 | 4 0 9 5 ± 4 5 5 | IShallow | 2.62 | 1190 ± 130 | i i - Stomach Contents In 1972 and 1973, I pumped the stomachs of l a r g e animals from Hollyburn Ridge, Lost Lake and the B.C. F i s h and W i l d l i f e Hatchery ponds i n Abbotsford. T h e i r d i e t c o n s i s t e d mainly of i n s e c t s , Chaoborus , l e e c h e s , clams and chironomids. On August 8th 1973, I captured the s m a l l e s t i n d i v i d u a l s I could i n the experimental pond and l a r g e r ones from the c o n t r o l pond. The a n a l y s i s of t h e i r stomach contents i s presented i n TABLE V. The sample from the c o n t r o l pond c l e a r l y demonstrates t h a t l a r g e animals depend more on non p l a n k t o n i c food which occupy most of the volume of t h e i r stomach. These data suggest t h a t Ambystoma i s a f a c u l t a t i v e p l a n k t i v c r e . T h i s i s confirmed by the a n a l y s i s of stomach contents of l a r v a e captured i n the experimental pond on J u l y 5th 1973, before the Spring plankton bloom (TflELE VI). TABLE V Stomach c o n t e n t s a n a l y s i s f o r sub-adult ( l e s s than 55 cm) and a d u l t salamanders (60-75 cm) captured on August 8th 1973. Sub-a d u l t s come from the experimental pond and a d u l t s from the c o n t r o l pond. r t t t | Salamander | #planktonic | t o t h e r s (*) | | s i z e (cm) | cr u s t a c e a n s j | j. x x ^ | SMALL | | 19 51 16 | | 19 12 3 | | 20 50 2 | | 19 117 4 | V  | T o t a l 230 25 | | MEDIUM | | 44 60 2 | | 52 240 8 | | 44 58 1 | | 51 3 6 | | T o t a l 361 17 | j ^ | LABGE | V 4 | 68 9 6 | | 72 0 6 | | 63 2 13 | | 64 44 12 | | T o t a l 92 55 | (*) b i v a l v e s , chironomids, i n s e c t s , Chaoborus , e t c . . . TABLE VI Stomach content a n a l y s i s f o r sub-adult salamanders captured i n the experimental pond on J u l y 5th, 1973. | Salamander | f p l a n k t o n i c | #others | | s i z e (cm) | crustaceans | | ] 50 |~ 2 { 2 0 1 I 20 I 1 - 1 T« I I 17 I 0 I 33 | I 23 | 5 | 21 | | T o t a l | 8 | 85 | I t i s c l e a r t h at when plankton are s c a r c e , s m a l l and medium s i z e salamanders w i l l eat any other food that i s a v a i l a b l e . Otherwise (TABLE V), plankton i s p r e f e r r e d . Thus, we can expect s m a l l and medium s i z e animals t o have most impact on zooplankton. 2.1 Di s c u s s i o n Small ( l e s s than 55cm) l a r v a l salamanders are found i n d e n s i t i e s of about one per c u b i c meter and feed mostly on plankton. Evidence from stomach content analyses of animals caught i n the pond i n d i c a t e s that a prey t h a t i s a v a i l a b l e and easy to catch w i l l be h i g h l y s e l e c t e d . For example, on August 8th, 1973, Holopedium accounted f o r more than 90? of the plankton eaten by numbers (see TABLE V). S f i l o ^ S ^ i i i l l S surrounded by a l a r g e p r o t e c t i v e g e l a t i n o u s sphere and i t cannot escape p r e d a t o r s l i k e other c l a d o c e r a . At the time these o b s e r v a t i o n s were made, Holoeedium was second i n abundance i n the ponds ( a f t e r Daphnia ) and t h i r d i n s i z e ( a f t e r D J L_kenai and Daphnia ). These data suggest that the salamander i s a d i s c r i m i n a t i n g f a c u l t a t i v e predator but t h a t a simple model of s i z e - s e l e c t i v e p r e d a t i o n i s not adequate. Laboratory experiments (APPENDIX I) make i t c l e a r t h a t Amby_stoma i s a t t r a c t e d by l a r g e prey and s e l e c t s l a r g e i n d i v i d u a l s w i t h i n a s p e c i e s , but they a l s o show that d i f f e r e n c e s between s p e c i e s can o v e r r i d e s i z e as a c r i t e r i o n f o r c h o i c e . Prey escape response i s c r i t i c a l . In her study, Fedorenko (1973) found t h a t swimming behavior of zooplankton i s s p e c i e s s p e c i f i c and t h e i r swimming speed i s s i z e and s p e c i e s s p e c i f i c . For these reasons I b e l i e v e the hypothesis of s i z e - s e l e c t i v e p r e d a t i o n , of i t s simple form, does not account f o r predatory performance i n Amby_stoma . I f prey b e h a v i o r a l f e a t u r e s were i n c o r p o r a t e d i n t o the hyp o t h e s i s , i t would have more exp l a n a t o r y power. H i INVERTEBRATE PREDATOR^CHAOBORUS hL I n t r o d u c t i o n i - L i f e H i s t o r y The d i p t e r a n i n s e c t Chaoborus t r i v i t t a t u s pupates i n the s p r i n g . S h o r t - l i v e d a d u l t s emerge and l a y t h e i r eggs. These hatch and go through four d i f f e r e n t l a r v a l i n s t a r s , the l a s t one o v e r w i n t e r i n g under the i c e . The l a r v a e are predaceous and are widespread i n ponds and lak e s throughout Ncrth America. £llJ2bo£iiS forms a s i g n i f i c a n t component of most w e l l s t u d i e d l i m n e t i c ecosystems. I t feeds almost e x c l u s i v e l y on plankton (but a l s o on b e n t h i c i n v e r t e b r a t e s , e. g. clams and chironomids) and can be s a f e l y c o n s i d e r e d to be an o b l i g a t e p l a n k t i v o r e . i i z The Problem According t o Kajak and Ranke-Rybicka (1970) , Chaoborus l a r v a e put on most of t h e i r weight duri n g a very s h o r t time i n t e r v a l corresponding more or l e s s to the p e r i o d c f maximum p l a n k t o n i c standing crop. During t h i s p e r i o d , of about 30 days, they measured a p o s i t i v e c o r r e l a t i o n between the growth of l a r v a e and crustacean plankton p r o d u c t i o n . In Eunice Lake, B.C., Fedorenko (1973) observed that prey d e n s i t y was very low and C i _ t r i v i t t a t u s d i d not feed near c a p a c i t y ( C ^ _ t r i v i t t a t u s has a two year l i f e c y c l e i n Eunice Lake). P o t e n t i a l growth r a t e , measured by f i e l d experiments where e x t r a food was provided to l a r v a e i n s i d e e n c l o s u r e s , was 2 to 6 times higher than observed i n the f i e l d . Food abundance i s c l e a r l y a key f a c t o r f o r the growth of Chaoborus . Even though Ranke-Rybicka found t h a t the l a r v a e feed at approximately a constant rate and t h a t the s i z e of the prey i n g e s t e d determines the biomass i n t a k e , experiments showed that ChaSkSOS f e e d i n g r a t e s i n c r e a s e d with i n c r e a s i n g prey c o n c e n t r a t i o n (Fedorenko, 1973). Other authors found that more than simply s i z e i s i n v o l v e d . S w i f t (1974) found t h a t Chaoborus IVth i n s t a r s t r i k e e f f i c i e n c y was about equal f o r both Dajahnia and Diagtomus but the c o n t a c t e f f i c i e n c y was c o n s i d e r a b l y higher f o r the copepod, at l e a s t f o r prey s i z e above 1mm. Thus, both can be attacked e q u a l l y w e l l , but Da£hnia i s o f t e n l o s t while i t i s being handled. Fedorenko (1973) observed l a r g e daphnids dragging Chaoborus through the water a f t e r an a t t a c k , u n t i l the predator r e l e a s e d t h e i r prey. She a l s o d e s c r i b e d how Chaoborus manages to i n g e s t copepods by breaking o f f the urosome from the cephalothorax. T h i s means th a t they can i n g e s t l a r g e r copepods than Daj>hnia because the l a t t e r has a d i s c o i d a l carapace and a caudal s p i n e . In other words, food q u a l i t y as well as q u a n t i t y i s important to p r e d a t o r s such as Chaoborus . Head p a r t s , mouth appendages and mouth gape are q u i t e constant f o r a s p e c i e s and i n s t a r ( f o r these reasons d i f f e r e n t head measurements are widely used i n t h e i r taxonomy) and put a l i m i t t o the prey s i z e of a p a r t i c u l a r s p e c i e s that an i n d i v i d u a l can handle. I t f o l l o w s t h a t a v e r t e b r a t e predator can d r a s t i c a l l y modify Chaoborus • s f e e d i n g success and growth through i t s impact on zcoplankton. In a l p i n e ponds which are f r o z e n 7 to 8 months of the year, Skaoborus f a c e s the c r i t i c a l problem of f i n d i n g enough food f o r pupating, reproducing and going r a p i d l y through f o u r molts i n a few weeks. In the experimental pond, i f Ambystoma does not remove l a r g e r zooplankton, I expected Chaoborus to be presented with a choice of d i f f e r e n t sub-optimal prey. Feeding success, as w e l l as weight and molting success, should be a f f e c t e d . U l t i m a t e l y t h i s should r e s u l t i n a higher m o r t a l i t y of l a r v a e and a d r a s t i c a l l y reduced Chaoborus p o p u l a t i o n . To i n v e s t i g a t e t h i s and to t e s t the hypothesis of complementary f e e d i n g niches, I analyzed Chaoborus crop c o n t e n t s by i n s t a r and compared them to food a v a i l a b i l i t y i n the two main ponds. The pop u l a t i o n s t r u c t u r e was recorded and I estimated the weight o f the l a r v a e IVth i n s t a r i n the F a l l of 1973. B}_ M a t e r i a l s And Methods £^§2£2£!i§ were sampled using a 30cm square plankton net with 0.65x0.20mm openings. They were immediately a n a e s t h e t i z e d (Straskraba, 1961) to prevent f u r t h e r feeding and preserved. Gut cont e n t s were analyzed l a t e r a c c o r d i n g to S w i f t and Fedorenko's(1973) method and compared to zooplankton a v a i l a b l e i n ponds ( t h i s sampling method i s de c s r i b e d i n S e c t i o n V I ) . To estimate dry weights, r e p l i c a t e s of 11 l a r v a e were d r i e d 24 hours i n an oven at 100°C. C)_ R e s u l t s And I n t e r p r e t a t i o n i - Cro_g Contents TABLE VII g i v e s the sample s i z e of Chaoborus crops that were analyzed, by pond, date and i n s t a r s . I counted a t o t a l of 5989 r o t i f e r s and 321 c l a d o c e r a o r i g i n a t i n g from the gut of 600 l a r v a e . More i n f o r m a t i o n about standing crop can be found i n S e c t i o n VII (see F i g u r e s 8 through 12). B r i e f l y , on August 21st 1973, s t a n d i n g crop was high i n the c o n t r o l pond and at i t s peak i n the experimental pond (the former was dominated by 2ia£hanosoma and the l a t t e r by D a j h n i a ) . On September 4th 1973, i t had d e c l i n e d to low l e v e l s i n the c o n t r o l but Daghnia was s t i l l abundant i n the experimental pond. Copepods were abundant on both dates, more so i n the experimental pond. 1- Q u a l i t a t i v e A n a l y s i s The top p a r t of F i g u r e 4 shows the p r o p o r t i o n a l a v a i l a b i l i t y of crustaceans versus t h e i r r e l a t i v e abundance i n Chaoborus crops the same day. The prey were c l a s s i f i e d i n d e c r e a s i n g order of s i z e . To c o n s t r u c t the bottom p a r t of the F i g u r e , I transformed a l l data to biomasses and i n c l u d e d r o t i f e r s which are the most abundant food item i n Chaoborus cro p s . I n s t a r I does not feed on crustaceans and depends e x c l u s i v e l y on the r o t i f e r K e r a t e l l a . I t i s reasonable to TABLE VII £ii.§oborus sample s i z e by i n s t a r s f o r crop contents a n a l y s i s on August 21st and September <4th, 1973. I n III IV. 1 TOTAL A u g u s t 2 1 s t , 1973 o C o n t r o l 2 35 59 23 119 E x p e r i m e n t a l 46 53 75 •2 176 S e p t e m b e r 4 t h , 1973 C o n t r o l 0 25 104 27 156 E x p e r i m e n t a l 6 108 24 1 139 590 F i g u r e 4. D i e t of Chaoborus. Crop content a n a l y s i s by s p e c i e s and s p e c i e s composition i n the main ponds on August 21st and September 4th 1973. TOP: the r e s u l t s are expressed as a percentage frequency of occurrence i n samples. BOTTOM: the data were transformed from numbers to biomasses p r i o r t o determining the percentage f r e q u e n c i e s . A- C o n t r o l , August 21st, 1973 B- E x p e r i m e n t a l , August 21 s t , 1973 C-C o n t r o l , September 4th, 1973 D- Experimental, September 4th, 1973 The s p e c i e s code i s as f o l l o w s : a- E t _ k e n a i b-Daphnia rosea c- p. brachyurum d- Cyclops e- Others f-R o t i f e r s . Holopedium i s not i n c l u d e d . t COMPOSITION OF PREY o 6 ° § 8 b o cr I F 3 <.:r t~~i CL > 1 ~ , 0 . | Jcr " 1 To- O CT | r> <0 3-o | Z Q. O 3 COMPOSITION OF PREY p o o o . O NJ r- CD CO o o I I I I I -4—f-o o o o o i I I 4—I—I—l—I—I + \ -i- i i- I 0 0 > cr J 3 l cr cr •5L cr n ]<t> cr n 03 Q. > cr 1 Jo Q. fD • n 05" O OJ o assume t h a t i t cannot handle l a r g e r prey. The importance of r o t i f e r s d i m i n i s h e s p r o g r e s s i v e l y i n the d i e t of l a r g e r i n s t a r s i n a b s o l u t e numbers as w e l l as i n r e l a t i v e p r o p o r t i o n to other prey items, even though they are s t i l l taken i n l a r g e numbers For i n s t a r I I , r o t i f e r s are s t i l l an important food item, but l e s s so when s m a l l crustaceans are a v a i l a b l e . I t then r e l i e s upon a few c l a d o c e r a to p r o v i d e most of i t s biomass i n p u t . I n s t a r I I I i s much l e s s dependent on r o t i f e r s and w i l l feed on a d i v e r s i f i e d d i e t of c l a d o c e r a . I t cannot handle l a r g e copepods very w e l l , t a k i n g only an o c c a s i o n a l one. The IVth i n s t a r feeds on l a r g e r items such as copepods as w e l l as on c l a d o c e r a . 2- Q u a n t i t a t i v e A n a l y s i s In order to gain an i n s i g h t i n t o f e e d i n g behavior, I looked at two v a r i a b l e s : the average biomass per prey item (feed packet size) and the average biomass per l a r v a l crop. The data are summarized i n TABLES V I I I and IX. 1) Whereas the food packet s i z e i n the ponds i s greater i n the experimental than i n the c o n t r o l pond, i t appears that the o p p o s i t e trend i s t r u e f o r the food packet s i z e i n crops. Even though the d i f f e r e n c e i n the ponds i s g r e a t e s t on September 4th, i t i s l e s s i n the crops on that day. T h i s i s expected s i n c e very few prey are a v a i l a b l e i n the c o n t r o l pond while the experimental pond has a high d e n s i t y of Da^hnia which i s not a f a v o r i t e food item of Chaoborus. The food packet s i z e of the IInd i n s t a r which feeds mostly on r o t i f e r s i s higher i n the experimental pond s i n c e a few Dajghnia were i n g e s t e d . I t f e d e x c l u s i v e l y on r o t i f e r s i n the c o n t r o l pond. The most important d i f f e r e n c e i n the crops were observed on august 21st when the sta n d i n g crop was high i n both ponds but d i f f e r e n t i n q u a l i t y . We must a t t r i b u t e these d i s c r e p a n c i e s to Chaoborus 's d i f f i c u l t i e s i n h a n d l i n g Dap_hnia i n the experimental pond, producing a r e l a t i v e u n d e r - r e p r e s e n t a t i o n of these l a r g e c l a d o c e r a i n crops. TABLE V I I I Average volume per p l a n k t o n i c prey item (food packet s i z e ) on August 21st and September 4th 1973 i n the main ponds. Values are given i n c u b i c microns. \ J l u g u s t 21st, 1973 ""^September 4th, 1973 ] IControl | 2.61 x 1 0 5 | 0.82 x 10 5 | | Experimental | 3. 24 x 10 5 | 3. 32 x 10 5 | TABLE IX Average volume per p l a n k t o n i c prey item i n crops of d i f f e r e n t Chaoborus i n s t a r s . Values are gi v e n i n c u b i c microns x 10^  f o r August 21st and September 4th 1973 i n the main ponds. ^August 21, 1973 I I I I I I IV ~] I C o n t r o l 26.3 94.6 995.0 ~ j lExperimental 15.0 68.0 | ISeptember 4,1973 IControl l E x p e r i m e n t a l I I 2.7 3.3 I I I 33.9 25. 0 IV 330.0 | TABLE X Average biomass i n crops of d i f f e r e n t Chaoborus i n s t a r s . Values are g i v e n i n c u b i c microns x 1 0 5 f o r August 21st and September 4th 1973 i n the two main ponds. i 1 (August 21, 1973 I I I I I I IV | , , IControl 1.0 207.0 6640.0 4410.0 | |Experimental 1.5 138.0 914.0 | L 1 {September 4,1973 |Con t r o l (Experimental 0.8 II 38. 3 28.7 I I I 637. 0 266.0 IV | 1 1980.0 | I 2) the average biomass per crop presented i n TABLE X shows t h a t Chaoborus performance i n the experimental pond i s q u i t e poor r e l a t i v e to i t s performance i n the c o n t r o l pond. The gap between c o n t r o l and experimental pond i s , i n f a c t , g r e a t e r than we would have suspected from food packet s i z e data alone. I t i s p o s s i b l e t h at food d e n s i t y accounts f o r the d i f f e r e n c e . T h i s i s suggested by a f u r t h e r o b s e r v a t i o n : i n the c o n t r o l pond 2.ia£hanosoma decreased by approximately by a f a c t o r of 10, i n the pond as w e l l as i n the-crops of Chaoborus . 3- Food Density To show these trends c l e a r l y , I p l o t t e d prey d e n s i t y i n the ponds versus t h e i r number per Chaoborus crops, by i n s t a r (Figure 5) . Prey were c l a s s i f i e d as c l a d o c e r a (Bologedium excluded) or r o t i f e r s . Within a prey category there e x i s t s a p o s i t i v e c o r r e l a t i o n between the number of food items i n the lake and i n the crop. T h i s a p p l i e s everywhere except to I l n d , I l l r d and IVth i n s t a r s i n t he experimental pond. F i g u r e 12 f o r the I l n d and I l l r d i n s t a r i n d i c a t e s t h a t o n l y i n the c o n t r o l pond are the crus t a c e a n s u t i l i z e d o p p o r t u n i s t i c a l l y . In the experimental pond, when Daphnia i n c r e a s e s , they are not more e x t e n s i v e l y u t i l i z e d . At the same time, the r o t i f e r s decrease i n d e n s i t y . T h i s f o r c e s the I I nd i n s t a r t o e x p l o i t a d e c r e a s i n g supply of r o t i f e r s more h e a v i l y . They r e p r e s e n t a s i g n i f i c a n t p o r t i o n of i t s d i e t (15% by biomass). F i g u r e 5. Diagrams of prey d e n s i t y i n the main ponds versus the average number of prey i n a Chaoborus crop i n each pond. The food i s c l a s s i f i e d i n two c a t e g o r i e s : c l a d o c e r a ( f u l l l i n e ) and r o t i f e r s (hatched l i n e ) . Holopedium i s not i n c l u d e d . The arrow p o i n t s from August 21st toward September 4th, 1973. A l l values f o r c l a d o c e r a were m u l t i p l i e d by a f a c t o r of 10. PREY DENSITY D O O C - P f 01' 6+ z c 33 O "0 m ~o o1 m ;o o X ml O "0 o .6 6} e o -p-o _£ PL o -p-o p Ul o .6 ui © St PREY • DENSITY _p £ P P-o -p- _£ _ P PL Kt 4- Food P r e f e r e n c e s The evidence has shown so f a r that Dia£hanosoma i s an a p p r o p r i a t e prey f o r Chaoborus and t h a t an i n c r e a s e i n i t s d e n s i t y c o r r e l a t e s with an i n c r e a s e d i n t a k e . I have not shown th a t Chaoborus a c t i v e l y seeks out Diaghanosoma as prey, i g n o r i n g other s p e c i e s . That t h i s occurs i s s u b s t a n t i a t e d by an a n a l y s i s of the percentages of l a r v a e with empty stomachs. On August 21st 1973, more I l n d and I l l r d i n s t a r s had empty stomachs i n the c o n t r o l than i n the experimental pond, d e s p i t e the f a c t t h a t the average biomass per Chaoborus crop was higher i n the c o n t r o l pond. A low percentage of Chaoborus crops have r o t i f e r s only (27%). F i f t y three percent have both types of food, i . e. r o t i f e r s and crustaceans ( Piaghanosgnja almost e x c l u s i v e l y ) . In the experimental pond, 72% of the crops have r o t i f e r s only and 18% have r o t i f e r s and Daphnia . On September 4th, when few c l a d o c e r a except f o r Eaphnia i n the experimental pond were a v a i l a b l e , the percentages of empty stomachs do not d i f f e r markedly. No doubt Chaoborus feeds p r e f e r e n t i a l l y upon Diaghanosgma when i t i s abundant. TABLE XI Comparison of the percentage of empty crops f o r the I l n d and I l l r d i n s t a r Chaoborus on August 21st 1973 i n the main ponds. I ~T I I [ I I I ] |C o n t r o l | 0.200 | 0.120 | (Experimental | 0.037 | 0.066 | TABLE XII Comparison of the percentage of empty crops f o r the I l n d and I l l r d i n s t a r s Chaoborus on September 4th 1973 i n the main ponds, r t i i \ i i i i IControl | 0.080 | 0.076 | |Experimental | 0.063 | 0.083 | iiz £oEolation S t r u c t u r e And Density The r e l a t i v e d i s t r i b u t i o n o f d i f f e r e n t i n s t a r s shows c o n s i d e r a b l e v a r i a b i l i t y from one sampling date to another but i n d i c a t e s t h a t the r a t e of development i s slower i n the experimental pond (Figure 6). Looking at F i g u r e 7a we see that the c o n t r o l pond's l a r v a e reach the I I I i n s t a r , then the IV i n s t a r , l a t e i n the F a l l . In the experimental pond, very few l a r v a e molt s u c c e s s f u l l y from the I l l r d to the IVth i n s t a r (Figure 7b). This i s followed by a c l e a r drop i n po p u l a t i o n numbers (Figure 7 c ) . The winter 1973-74 was unu s u a l l y long and severe f o r Chaoborus l a r v a e . T h i s i s suggested by the f a c t t h a t only IVth i n s t a r l a r v a e s u r v i v e d i n the s p r i n g of 1974. Even IVth i n s t a r l a r v a e m o r t a l i t y was extremely h i g h . The estimated d e n s i t i e s a f t e r 16 plankton tows are 0.72 and 0.12 IVth i n s t a r l a r v a e per F i g u r e 6. R e l a t i v e p r o p o r t i o n of Chaoborus i n s t a r s i n the main ponds from l a t e August to October ^973. % F R E Q U E N C Y e x p e r i m e n t a l c o n t r o l o o •p. o o o • P L - ^ P P P 00 CO 4^ CO CO t o o CO CO CD CO I II < F i g u r e 7. Density of Chaoborus l a r v a e : I l l r d and IVth i n s t a r s B-I l l r d and IVth i n s t a r s C- i n pond f o r a l l i n s t a r s A- i n the c o n t r o l pond f o r i n the experimental pond f o r the c o n t r o l and experimental o-oooecL. 0-00015. O-COOIQ. 0.00005. 0'OOooa] __L —L 1 AUGUST SEFTaeER CCTCBER 2 AUGUST SEPTEMBER CCTCBER AUGUST SEPTEMBER OCTOBER cubic meter i n the c o n t r o l and experimental ponds r e s p e c t i v e l y . This y i e l d s a winter m o r t a l i t y estimate of over 99.5% f o r both ponds. These r e s u l t s s t r e s s tha importance of r e a c h i n g the IVth i n s t a r before winter which i s probably p o s s i b l e only when a l a r g e supply of s u i t a b l e c l a d o c e r a i s a v a i l a b l e to I l n d and I l l r d i n s t a r s . i i i - Dry Weicjht The dry weight of IVth i n s t a r larvae i n the f a l l was determined f o r the e x p e r i m e n t a l l pond i n 1972 (5 r e p l i c a t e s ) and 1973 (8 r e p l i c a t e s ) , and f o r the c o n t r o l pond i n 1973 (8 r e p l i c a t e s ) . The r e s u l t s are found i n TABLE X I I I . The dry weight of animals from the experimental pcnd decreased by about 50? with r e s p e c t to t h e i r weight i n October, 1972, the year preceding Ambystoma egg removal. T h i s i s a l s o 50% l e s s than i n the nearby c o n t r o l pond. TABLE XIII Dry weight i n m i l l i g r a m s of IVth i n s t a r l a r v a e with 9515 confidence l i m i t s . The animals were captured i n the F a l l of 1973 f o r the c o n t r o l pond and, 1972 and 1973 f o r the experimental pond. i r t I | Dry Weight | + + IControl pond, October 30th, 1973 | 0.72 1 0.06 | l E x p e r i m e n t a l pond, October 12th, 19 7 2 i 0.71 1 0.12 | |Experimental pond, October 30th, 1973| 0.36 1 0.05 | L L J P.L D i s c u s s i o n Food consumption increased along with food d e n s i t y . This i s i n agreement with Fedorenko's (1973) f i n d i n g s . However, as suggested by Ranke-Rybicka (1970), food q u a l i t y i s a c r i t i c a l f a c t o r . In the presence of prey such as Daphnia , that i t cannot handle very w e l l , the biomass i n t a k e of some Chaoborus i n s t a r s remains low even though they feed e x t e n s i v e l y on r o t i f e r s . In small ponds, the midge l a r v a probably has access to the whole spectrum of prey s p e c i e s . According to S w i f t (1974), the IVth i n s t a r can a t t a c k most prey e q u a l l y w e l l . Crop content analyses show l i t t l e preference beyond an o v e r r e p r e s e n t a t i o n of l a r g e r s p e c i e s . I t i s an o p p o r t u n i s t i c feeder and t h i s c o r r o b o r a t e s f i n d i n g s of Fedorenko (1973) and of Codson (1970) who found a maximum e l e c t i v i t y c o e f f i c i e n t of 2.8 f o r Chacbcrus i.S§£i.c-§.2.u.'2 • The other i n s t a r s probably hunt i n the same f a s h i o n , except t h a t there i s an upper s i z e l i m i t to t h e i r h a n d l i n g c a p a c i t i e s w i t h i n a s p e c i f i c c a t e g o r y of prey (e.g. any c l a d o c e r a n f o r the 1st i n s t a r , Daphnia f o r the I l n d i n s t a r , l a r g e Caghnia and copepods f o r the I l l r d i n s t a r ) . T h i s r e s u l t s i n a biased r e p r e s e n t a t i o n of food items i n the crops r e l a t i v e to t h e i r a v a i l a b i l i t y i n the ponds. Thus, Chaoborus appears to be " s i z e - s e l e c t i v e " but t h i s i s mostly an e f f e c t of c o n s t r a i n t s . I t i s l i m i t e d by the kinds of prey a v a i l a b l e and t h e i r abundance, e s p e c i a l l y i n the simple communities found i n a l p i n e ponds. However, i t i s not completely at the mercy of the q u a l i t y of i t s food supply s i n c e : a) Each i n s t a r can cover a span of prey s i z e s that allows some leeway b) l i g h t i s an important f a c t o r i n c o n t r o l l i n g i t s development; u l t i m a t e l y i t w i l l reach a s i z e at which i t can u t i l i z e l a r g e prey s p e c i e s such as ccpepcds, provided time and p h y s i o l o g i c a l s t a t e permit i t . My evidence suggests that the most c r i t i c a l stage f o r Chaoborus i n my ponds i s the molt from I l l r d t o IVth i n s t a r . Copepods are a v a i l a b l e to the IVth i n s t a r and allow i t to accumulate the energy necessary to s u r v i v e d u r i n g the winter. Whether or not t h i s stage i s reached s u c c e s s f u l l y depends most l i k e l y on the f e e d i n g success of the I l n d and I l l r d i n s t a r on P-ilEkaJiosoma . F i n a l l y , the f a c t that Chaoborus i s a dependent predator does not r u l e out the p o s s i b i l i t y that i t has an impact on plankton, as suggested by Northcote and C l a r o t t o (in p r e s s ) . I t c o u l d a f f e c t s i z e a t maturity of zooplankton or put a premium on going r a p i d l y through these stages most s u b j e c t to Chaoborus p r e d a t i o n . However, Chaobcrus k i l l s many prey without i n g e s t i n g them. T h e r e f o r e , i t i s d i f f i c u l t to assess i t s impact on zooplankton. In l a b o r a t o r y predation experiments, Fedorenko and Swift found more dead prey at the bottom of a d i s h with £ll52£2£!i§ present than i n a c o n t r o l without l a r v a e . S i n c e Chaoborus can a t t a c k most prey e q u a l l y w e l l , i t i s p o s s i b l e that i t s impact on zooplankton i s not as s e l e c t i v e as crop content a n a l y s i s suggests. TABLE XIV and XV summarize q u a l i t a t i v e l y the i n f o r m a t i o n on p r e d a t i o n i n a l l t h r e e ponds. Mote that there i s p r a c t i c a l l y no food overlap between Ambystoma and Chaoborus . TABL2 XIV Summary of Ambystoma and Chaoborus p r e d a t i o n l e v e l s i n a l l ponds. | POND | v e r t e b r a t e | i n v e r t e b r a t e | | | predator | predator | r + 4 4 jC o n t r o l | normal 1 normal | | Experimental | low | normal | IShallow | high | low | I L I J TABLE XV Summary of Arobystoma and Chaoborus p r e d a t i o n by i n s t a r s on d i f f e r e n t prey c a t e g o r i e s with s i z e s . 1 s t i n s t a r Chaoborus I l n d i n s t a r Chaoborus predator r o t i f e r s c l a d o c e r a I l l r d i n s t a r Chaoborus h IVth i n s t a r Chaoborus Ambystoma prey r o t i f e r s r o t i f e r s c l a d o c e r a r o t i f e r s c l a d o c e r a copepod _ T  | prey s i z e -+ 4 | 0.2-0.3 mm | 0.2-0.3 mm | 0.3-0.6 mm | 0.2-0.3 mm | 0.3-1.3 mm | 0.2-0.3 mm | 0.3-1.0 mm | 0.4-2.2 mm Holop.edium Dap_hnia | 0.5-1.3mm | 1.3-2.3 mm 111:. ZOOPLAUKTON hi. I i l i ^ 2 d u c t i o n Brooks (1968) reviews the e f f e c t s of v e r t e b r a t e s i z e -s e l e c t i v e p r e d a t i o n and c o n s i d e r s two consequences of t h i s behavior: A) F i r s t , a predator can s h i f t the c o m p e t i t i v e balance between the z o o p l a n k t o n i c h e r b i v o r e s . In the presence of a v e r t e b r a t e predator we expect the numbers of l a r g e s p e c i e s to be reduced a l l o w i n g s m a l l poorer competitors to become more numerous. In my experimental pond, I expected a l a r g e h e r b i v o r e to monopolize the supply of p a r t i c u l a t e food, e l i m i n a t i n g s m a l l ones. B) P r e d a t i o n can produce a r e d u c t i o n i n the s i z e of the f i r s t egg bearing i n s t a r , f o r the p r o b a b i l i t y of producing r e p r o d u c t i v e o f f s p r i n g i s s u b s t a n t i a l l y i n c r e a s e d i f some progeny can reproduce while s t i l l s m a l l enough t c a v o i d p r e d a t i o n . " T h i s r e l a t i o n s h i p of predator i n t e r e s t to a b s o l u t e prey s i z e i s of r e l e v a n c e here because the l a r g e morph i n a l l the v a r i a b l e l i m n e t i c Dajshnia i s at the t h r e s h o l d of s u s t a i n e d predator i n t e r e s t " . Many s p e c i e s span a c e r t a i n range of s i z e s at the onset of maturity; f o r zooplankton, the s i z e r e a l i z e d may be a f u n c t i o n of the i n t e n s i t y of predation (Brooks, 1968, and Wells, 1970). In the experimental pond, i n the absence cf p r e d a t i o n , I expected an i n c r e a s e i n the minimum s i z e at maturity f o r the l a r g e s t cladoceran and p o s s i b l y f o r c a l a n o i d copepods as w e l l . F i n a l l y , the r o l e played by i n v e r t e b r a t e p r e d a t o r s should be c o n s i d e r e d s i n c e i t was suggested r e c e n t l y t h a t they might account f o r some of the i n t r a - s p e c i f i c responses e x h i b i t e d by zooplankton (Dodson, i n p r e s s ) . The most abundant i s Chaoborus . Since i t i s found at comparable l e v e l s i n both ponds, i t cannot be co n s i d e r e d as a c a u s a l f a c t o r i n b r i n g i n g about d i f f e r e n t trends between ponds. Cyclops i s known to be predatory. T h e i r r a t e of feeding i s low ( l e s s than one prey per day a c c o r d i n g to B i l l N e i l l , personnal communication) . C o n s i d e r i n g t h e i r d e n s i t i e s , I estimate t h a t t h e i r maximum cropping r a t e i s about 1% t h a t of Chaoborus i n the experimental pond and 10* on the c o n t r o l pond. I t i s u n l i k e l y to have a major e f f e c t i n the community. §.1 M a t e r i a l s And Methods Animals were sampled i n the morning by t r a p p i n g a p o r t i o n of the water column i n s i d e a p l a s t i c pipe 2 inches (5.1 cm) i n diameter along t r a n s e c t s (for more d e t a i l s see Edmondson and Winberg, 1971). A s p i k e 20 cm long kept the pipe out of the mud; a l l l i v i n g m a t e r i a l was c o l l e c t e d on f a b r i c with 0.005 mm square openings. I i d e n t i f i e d and measured a l l crustaceans i n the samples to the nearest 0.036 mm ( n a u p l i i and c y c l o p c i d s excepted). Daphnia *s spine and Diaptomus ca u d a l setae were not i n c l u d e d i n the measurement dona a c c o r d i n g to Ward & Whipple's (1954) standard method. Abundance, s i z e s t r u c t u r e , c l u t c h s i z e and d i s t r i b u t i o n of the s p e c i e s were recorded. D e n s i t i e s were c a l c u l a t e d as unweighted averages f o r a l l samples taken. £1 R e s u l t s And I n t e r p r e t a t i o n i z Cladocera Figures 8 and 9 show the seasonal e s t i m a t e s of p l a n k t o n i c biomass througout 1973. Crustaceans r e p r e s e n t most of the biomass. The average biomass per crustacean i n c r e a s e d s t e a d i l y i n the experimental pond u n t i l a g e n e r a l p o p u l a t i o n d e c l i n e took place i n august whereas i t decreased i n the c o n t r o l pond (Figure 10). The l a t t e r was dominated by the r e l a t i v e l y small c l a d o c e r a n &ia£k&.£2J>oma and the former by the l a r g e Da^hnia which i n c r e a s e d i n s i z e as w e l l as i n numbers throughout most of the summer (Figure 11 and 12) . Other s p e c i e s responded very l i t t l e t o the experimental m a n i p u l a t i o n . Holoj>edium i|ibberura , the next most numerous s p e c i e s , was found at lower d e n s i t i e s i n the experimental pond. The shape of the s i z e d i s t r i b u t i o n (Figure 13) i n d i c a t e t h a t i t s r e c r u i t m e n t slowed i n the experimental pond when Eajghnia became dominant (Figure 14). I t suggests t h a t c o m p e t i t i v e i n t e r a c t i o n i s i n v o l v e d . The o c c a s i o n a l s p e c i e s were not f a v o r e d by the presence of a p r e d a t o r . This might be a t t r i b u t e d to the f a c t that t h e i r d i s t r i b u t i o n i s c o n f i n e d t o the edges c f ponds (75^ were taken i n the f i r s t inshore s t a t i o n of t r a n s e c t s ) . However, F i g u r e 8. Seasonal v a r i a t i o n i n p l a n k t o n i c biomass d e n s i t y i n the main ponds i n 1973. The hatched l i n e s represent Ambystoma l a r v a e h a t c h i n g . C o n t r o l pond: TOP Experimental pond: BOTTOM F i g u r e 9. Seasonal v a r i a t i o n i n p l a n k t o n i c biciaass d e n s i t y i n the shallow pond i n 1973. The hatched l i n e s r e p r e s e n t Am cystoma l a r v a e h a t c h i n g . F i g u r e 10. Seasonal v a r i a t i o n i n the average biomass per crustacean i n the main ponds i n 1973. 12x10 • = co ntrol t = experimental J U L Y A U G U S T S E P T E M B E R F i g u r e 11. Seasonal v a r i a t i o n i n cr u s t a c e a n and piaphanosoma d e n s i t y (TOP) and biomass d e n s i t y (BOTTOM) i n the c o n t r o l pond i n 1973. o-oe • = crustaceans i = Piaphanosoma brachyurum J U L Y A U G U S T S E P T E M B E R 4CO0000 = crustaceans j = Piaphanosoma brachyurum-i °-=J+ F i g u r e 12. Seasonal v a r i a t i o n i n c r u s t a c e a n and Daphnia d e n s i t y (TOP) and biomass d e n s i t y (BOTTOM) i n the experimental pond i n 1 973 . 0-03. i = Daphnia rosea • = crustaceans J U L Y A U G U S T S E P T E M B E R 3000000-= Daphnia rosea F i g u r e 13. §2l22§JiliS s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e at maturity. I 1 7 / 7 / 7 3 > O LU ZD O LU 4 0 „ i 1 I 4 / ^ / 7 3 0 1 control 2 0 1 2 0 experimental S I Z E ( r m m ) H c i — i ' 1 2 s h a l l o w F i g u r e 1U. Daghnia s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e a t m a t u r i t y . 2 0 j 16 -1 1 2 - 1 8 : : u o t-20 M—I—1—I > O LU a LU M l | 20-r ifejiffew |1 L. 20-r i — i 20-r t 'F I I I I I 2 6 / 7 / 7 3 -+M—F—I—I—I 1 - 8 / 8 / 7 3 — I — I I 2 1 / 8 / 7 3 H—I ji^ H imp i j. l , I 4 / 9 / ' 7 3 H—I I 2 1 / 9 / 7 3 »f "I' I—I -I—MM—I—I—I 0 1 2 3 0 1 2 3 0 1 2 3 control experimental shallow S I Z E ( m m ) they showed a response t o s i z e - s e l e c t i v e p r e d a t i o n i n that the l a r g e Acro^erus and Sc a g h a l o b e r i s were the most abundant i n the experimental pond and the much s m a l l e r Chy_dorus elsewhere. In c o n c l u s i o n , s i z e - s e l e c t i v e p r e d a t i o n on a l a r g e z o o p l a n k t e r , £«._E2SSS »  n o ^ open o p p o r t u n i t i e s to s e v e r a l s m a l l e r zooplankters but onl y t o one, Dia£hanosonta . In the absence of a v e r t e b r a t e predator, l a r g e Daphnia were predominantly r e s p o n s i b l e f o r the trends observed. i i - S i z e At M a t u r i t y For Cladocera I randomly picked 20 i n d i v i d u a l s of Daphnia from the two ponds on August 21st and September 4th, 1973. The animals were drawn out with the help of a random d i g i t s t a b l e a f t e r a whole sample had been measured. The 10 s m a l l e s t animals were then used to perform an ANOVA f o r the minimum s i z e at ma t u r i t y . The data (TABLE "XVI) show a s i g n i f i c a n t date X pond i n t e r a c t i o n . No simple t r e n d d e s c r i b e s s i z e at maturity. However, another i n t r a s p e c i f i c response of Daphnia i s e v i d e n t . The s l o p e s (or r e g r e s s i o n c o e f f i c i e n t s ) f o r the c l u t c h s i z e body s i z e l i n e a r r e l a t i o n s h i p vary s i g n i f i c a n t l y (TABLE XVII). An a p o s t e r i o r i t e s t (STP) c l e a r l y i n d i c a t e s that the slope f o r the experimental pond on September 4 t h , 1973 was d i f f e r e n t from the o t h e r three (TABLE XVIII). T h i s i m p l i e s t h a t f o r a given s i z e , a female i n the experimental pond bears fewer eggs at the end of the summer than a female i n the c o n t r o l pond. TABLE XVI A n a l y s i s of v a r i a n c e of minimum s i z e at maturity f o r Dajghnia on August 21st and September 4th 1973 i n the main ponds. | V a r i a t i o n Source d.f. SS MS F - r a t i o | ! ISubgroups 3 0.056 0.019 | | date 1 0.019 0.019 | I lake 1 0.001 0.001 | | Ax B 1 0.036 0.036 6.00* | IWithin Subgroups 36 0.227 0.006 | I T o t a l 39 0.283 | TABLE XVII A n a l y s i s of v a r i a n c e o f m u l t i p l e l i n e a r r e g r e s s i o n of c l u t c h s i z e on body s i z e f o r Daghnia on August 21st and September 4th 1973 i n the main ponds. | Source d.f. S.S. M.S. F | h 4 |Among slopes 3 10.674 3.558 3.6 * | (Weighted average 97 95.844 0,988 | TABLE XVIII Regression c o e f f i c i e n t s of c l u t c h s i z e on body s i z e f o r Daphnia on August 21st and September 4th 1973 i n the main pcnds. i t r 1 | | August 21,1973 | September 4,1973 | h + H -4 IControl | 8.73 (n=18) | 8.04 (n=14) | | Experimental | 8.85(n=44) | 5.10(n=29) | i i i i i i i - C a l a n o i d Copepods The two main ponds show a steady and s l o w l y i n c r e a s i n g n a u p l i i p r o d u c t i o n and are almost completely dominated by the l a r g e D ^ k ^ n a i . I t i s more abundant i n the experimental pond (Figure 15) . 5_._leptopus i s found s p o r a d i c a l l y i n the experimental pond, at very low d e n s i t y . There i s net s u f f i c i e n t evidence to allow me to i n t e r p r e t t h i s as an e f f e c t of the experimental manipulation. Liz S i z e At M a t u r i t y For C a l a n o i d Copepods I c a r r i e d out a one-way A NOV A on s i z e at m a t u r i t y f o r D ^ k e n a i and found t h a t i t d i d not d i f f e r s i g n i f i c a n t l y w i t h i n any of the main ponds during 1973. I then pooled the data from d i f f e r e n t sampling dates. In the experimental pond, the s i z e a t m a t u r i t y showed a s i g n i f i c a n t i n c r e a s e f o r males (t=6.01, d.f.=93) from a mean of 1.87 to 2.14 mm, as w e l l as f o r females (t=13.7, d. f . = 126) from a mean of 2.08 to 2.47 mm. This suggests t h a t the s i z e a t maturity c o i n c i d e s with the t h r e s h o l d of s u s t a i n e d p r e d a t i o n . A decrease i n predator abundance was accompanied by an i n c r e a s e i n s i z e a t m a t u r i t y of copepods. However animals from the c o n t r o l pond are even l a r g e r than males or females i n the experimental pond i n 1972 or 1973. I w i l l comment on t h i s when a l l the necessary evidence i s a v a i l a b l e . F i g u r e 15. PiSEtomus kenai s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e at maturity. 1 0 T > o LU ZD O LU 1 7 / 7 / 7 3 2 6 / 7 J / 7 3 8 / 8 / 7 3 4 H—I—I—I—I-2 1 / 8 / 7 3 H — I — h 0 1 2 c o n t r o l /* / 9 / 7 3 +—I i — I 3 0 1 2 3 0 1 2 3 experimental shallow S I Z E (mm) v- S p a t i a l D i s t r i b u t i o n C o r r e l a t i o n c o e f f i c i e n t s of s p e c i e s abundance with r e s p e c t t o depth were computed. Daphnia has a p o s i t i v e and piaphanosoma a negative c o r r e l a t i o n with depth. TABLE XIX C o r r e l a t i o n c o e f f i c i e n t s : s p e c i e s abundance with r e s p e c t to depth f o r paphnia and piaphanosoma i n the main ponds(d.f.=15), | | C o n t r o l | Experimental | |~ Diaphanosoma | -0.40 (n=2483) | -0.53* (n=61) j I P i i i i i l l I 0.36 (n=151) | 0.77** (n=1464) | Thus the two dominant c l a d o c e r a are s p a t i a l l y separated. However the s e p a r a t i o n i s not c l e a r cut and there i s a gr e a t amount of o v e r l a p . Note that the experimental pond y i e l d s more s i g n i f i c a n t values. T h i s i s probably a s s o c i a t e d with the presence of a medium depth s h e l f with abundant veg e t a t i o n i n the c o n t r o l pond (Figure 2 and 3) which serves as a t r a n s i t i o n zone between the edges of the pond and the deep p a r t . The most i n t e r e s t i n g o b s e r v a t i o n comes from the j o i n t d i s t r i b u t i o n of a d u l t c a l a n o i d copepods. P i _ k e n a i has a n e g a t i v e c o r r e l a t i o n with depth i n the c o n t r o l and a p o s i t i v e one i n the experimental pond. I b e l i e v e t h i s change i n d i s t r i b u t i o n i s a s s o c i a t e d with the change i n v e r t e b r a t e p r e d a t i o n pressure. T h i s evidence suggests that P«._£e..S<ii w i l l make use of the presence of macrophytes f o r cover. As f o r P i _ l e p t o p u s i t s d i s t r i b u t i o n i s i n v e r s e l y r e l a t e d to that of p.. kenai but the s i z e of the samples i n v o l v e d i s too small to allow us to draw any f i r m c o n c l u s i o n . TABLE XX C o r r e l a t i o n c o e f f i c i e n t s : s p e c i e s abundance with r e s p e c t to depth f o r copepods i n the main ponds (d.f. = 15). I I D i . lefitojjus | D_j__kenai | ^ + + ZZZ ^ j c o n t r o l | 0.23 (n=2) | -0.64** (n=216) | lexperimental | -0.46 (n=7) | 0.81** (n=443) | F i n a l l y , t h i s evidence suggests an e x p l a n a t i o n f o r the l a r g e r s i z e (by about 0.4 mm) a t t a i n e d by copepods i n the c o n t r o l pond. Since the c o n t r o l pond i s c h a r a c t e r i z e d by the presence of a r e l a t i v e l y l a r g e and shallow s h e l f with emergent v e g e t a t i o n , t h i s refuge would have the e f f e c t of reducing Amfcy.§toma p r e d a t i o n on copepods. llZ The Shallow Pond U s e f u l i n f o r m a t i o n can be obtained from t h i s pond because i t has the densest p o p u l a t i o n of Albjstoma . As expected i t was l a r g e l y dominated by Oiaphanosoma . The hatching of Ambystoma eggs c o i n c i d e s with two events: A) a HolojDedium gibber um p o p u l a t i o n decrease which can be a s s o c i a t e d with i t s great v u l n e r a b i l i t y to p r e d a t i o n by young salamanders and B) a D_j__le£tO£US p o p u l a t i o n i n c r e a s e which can be a s s o c i a t e d with high v e r t e b r a t e p r e d a t i o n p r e s s u r e and the low Chaoborus abundance. The presence of weed cover p o s s i b l y e x p l a i n s the p e r s i s t e n c e of D ^ k e n a i i n the pond. v i i - Cy_clopoid Copepods Cyclops numbers decreased i n the experimental pond and i n c r e a s e d where v e r t e b r a t e p r e d a t i o n was moderate. The presence of the small D ^ l e g t o p u s i n the shallow pond d i d not a d v e r s e l y a f f e c t i t as i s o f t e n expected f o r sympatric s p e c i e s of copepods. T h e i r abundance i s c o r r e l a t e d with Diaphancsoma . For that reason I suspect t h a t Cyclops a d u l t s might f e e d on young i n s t a r s of c l a d o c e r a . Since Cyclops i s known t o be predatory, i t i s l i k e l y t h a t i t s d i s t r i b u t i o n i s a f f e c t e d by the d e n s i t y of the v e r t e b r a t e predator by v i r t u e of having a complementary f e e d i n g niche. Thus i t responded by an immediate decrease i n p o p u l a t i o n numbers i n the experimental pond, e a r l y i n the summer. Dodson's(1974) a l p i n e community model s t r e s s e s the importance of i n v e r t e b r a t e p r e d a t o r s to account f o r the e x c l u s i o n of s m a l l s p e c i e s by l a r g e ones i n n a t u r a l systems. Let us review the data presented so f a r , i n an-attempt to e v a l u a t e the r o l e played by co m p e t i t i o n and predation. i - C ladocera Daphnia and Diaphanosoma showed the g r e a t e s t v a r i a t i o n i n numbers between ponds. Looking at Figures 8 and 9, one suspects that both c o m p e t i t i o n and p r e d a t i o n c o u l d have combined to produce such d i f f e r e n c e s . 1- Competition S2i2£§dium , the p r e f e r r e d food item of s m a l l Amb^stoma , was not very s u c c e s s f u l i n e s t a b l i s h i n g a l a r g e p o p u l a t i o n i n the experimental pond i n s p i t e of low v e r t e b r a t e p r e d a t i o n p r e s s u r e . Since Chaoborus does not eat Holopedium to any g r e a t extent, competition with Daphnia i s the most l i k e l y e x p l a n a t i o n . Di^phanosoma was almost completely e l i m i n a t e d from the experimental pond. T h i s could not be a t t r i b u t e d e x c l u s i v e l y to i n v e r t e b r a t e p r e d a t i o n s i n c e i n v e r t e b r a t e p r e d a t o r s are found a t almost i d e n t i c a l d e n s i t y i n the c o n t r o l pond without depressing Diaphanosoma numbers. Again, competition with Daphnia must have played an important r o l e . 2- P r e d a t i o n Ambjfstoma can remove most egg-bearing females of Daphnia from a pond causing poor recruitment. Figure 14 d i s p l a y s c l e a r l y the e f f i c i e n c y of Amb^stoma i n removing l a r g e females i n the c o n t r o l and shallow ponds. I t w i l l a l s o have a great impact on the i n t e r m e d i a t e s i z e c l a d o c e r a n , Holopedium , as shown by the stomach content analyses of s m a l l salamander l a r v a e (TABLE V, VI and F i g u r e 13). Diaphanosoma on the other hand can dominate the zooplankton without i n c u r r i n g much p r e d a t i o n by Afi£Istoma (Figure 16). T h i s i m p l i e s that the impact of pr e d a t i o n by Amby_stoma w i l l be f e l t mainly by l a r g e c l a d o c e r a . As f o r the i n v e r t e b r a t e predator, Chaoborus , the crop contents a n a l y s i s i n d i c a t e s t h a t i t ' s main e f f e c t should be on Diaphanosoma . This i s i n agreement with the model of complementary f e e d i n g n i c h e s . However, we have not c o n s i d e r e d the temporal pat t e r n of p r e d a t i o n . Amby_stoma h a t c h l i n g s appear e a r l y i n the summer, while Chaoborus I I and I l l r d i n s t a r s do not appear u n t i l mid-summer. With t h i s i n f o r m a t i o n i n hand, i t i s p o s s i b l e to suggest a mechanism by which c o m p e t i t i v e and predatory r e l a t i o n s h i p s can account f o r the Daphnia - Diaphanosoma d i s t r i b u t i o n i n the ponds. They must be c o n s i d e r e d as a p a i r of competitors able to respond to changes i n p r e d a t i o n l e v e l s (both v e r t e b r a t e and i n v e r t e b r a t e ) . 3- C o n t r o l Pond In the s p r i n g i n the c o n t r o l pond, plankton are f r e e of p r e d a t i o n , except f o r i n s e c t p r e d a t o r s such as c o r i x i d s mostly c o n f i n e d to the edges of the ponds. When Ambjjstoma hatches, l a r g e z o o p l a n k t e r s (mainly Holopedium and Dajohnia ) are a v a i l a b l e to the l a r v a e and are cropped. T h i s favours P-ia£hanosoma and sma l l Dap_hnia . However, i n midsummer, Amb^stoma predation pressure tapers o f f , because the predator i s reduced i n numbers and changes i t s food h a b i t s , while the i n f l u x of Chaoborus I l n d and I l l r d i n s t a r l a r v a e s h i f t s the pressure F i g u r e 16. &ia£haiaosoina s i z e d i s t r i b u t i o n i n a l l ponds i n 1973. The dotted l i n e s r e p r e s e n t the minimum s i z e a t m a t u r i t y . _j h H h H—i-7 / 7 / 7 3 H 1 1 •H 1 1 I 2 6 / 1 7 / 7 3 iUilU ^ — i * H — i 1 8 / 18/73 H 1 I 2 8 / 7 3 H 1 H 1 1 4 / ^ / 7 3 H 1 2 0 1 2 0 1 c o n t r o l experimental s h a l l o w S I Z E (mm) onto young Dajahnia and Diaj:ha.npjsoma . 4- Experimental Pond Zooplankton p o p u l a t i o n growth was not very much a f f e c t e d by Amb_ystoma i n t h i s pond. Da^hnia and piaptomus e s t a b l i s h e d l a r g e p o p u l a t i o n s . Holojaedium and Diaphanoscja were n o t i c e a b l y reduced i n numbers and t h i s suggests that Da^hnia i s the most e f f i c i e n t c ompetitor. As Chaoborus l a r v a e reach i n s t a r I I and I I I , even more pressure i s a p p l i e d on small c l a d o c a r a . Thus, Chaoborus p r e d a t i o n exaggerates the e f f e c t s of c o m p e t i t i o n . In t h i s s i t u a t i o n Da£hnia 's f i t n e s s i n c r e a s e d c o n s t a n t l y while PiSEhanosoma 1 s decreased. 5- Dajjhnia C l u t c h S i z e The c l u t c h s i z e decrease i n the experimental pond could be i n t e r p r e t e d i n the two f o l l o w i n g ways: 1- Producing fewer but l a r g e r young i n c r e a s e s the p r o b a b i l i t y of the progeny s u r v i v i n g i n v e r t e b r a t e p r e d a t i o n . The same r e s u l t c o u l d be achieved through an i n c r e a s e i n the r a t e of growth of young i n d i v i d u a l s . The t a c t i c c o n s i s t s of passing as quxckly as p o s s i b l e through those s i z e c l a s s e s s u b j e c t to i n v e r t e b r a t e p r e d a t i o n . T h i s would be p o s s i b l e i n the experimental pond where the v e r t e b r a t e predator was removed. These females are s l i g h t l y l a r g e r than females with a l a r g e r c l u t c h s i z e . 2- Removal of v e r t e b r a t e predators f a v o r s the presence of l a r g e Daphnia . As the population approaches the c a r r y i n g c a p a c i t y of the environment, i n t e a - s p e c i f i c c o m p e t i t i o n i n c r e a s e s . At t h i s s t a g e , a K - s t r a t e y i s t (MacArthur 8 Wilson, 19b7) i s f a v o r e d , i . e. l a r g a animals with a lower metabolic r a t e , i n c r e a s e d p r o b a b i l i t y of s u r v i v a l , slower development, delayed r e p r o d u c t i o n , lower r e s o u r c e s t h r e s h o l d s , and fewer but l a r g e r young. T h i s would suggest s t r o n g c o m p e t i t i o n f o r food. Both e x p l a n a t i o n s are p o s s i b l e but the c o i n c i d e n c e of t h i s phenomenon with the appearance of I l l r d i n s t a r Chaoborus makes the l a t t e r more p l a u s i b l e . i i ~ C a l a n o i d Copepods The balance among d i f f e r e n t copepod p o p u l a t i o n s was not a f f e c t e d by the experimental manipulation. However evidence from the shallow pond i n d i c a t e s that high v e r t e b r a t e p r e d a t i o n pressure w i l l a f f e c t c a l a n o i d copepod d i s t r i b u t i o n s . What are the e f f e c t s of two d i f f e r e n t predatory processes on the copepod s p e c i e s ? F i r s t , Ambystoma imposes a " c e i l i n g " on the s i z e of a s p e c i e s l i k e Diaptomus . T h i s " c e i l i n g " should be very c l o s e to the s i z e at maturity of the l a r g e r morph. Under high predator d e n s i t y , a s m a l l e r s p e c i e s such as D l e p t o p u s with a s i z e at maturity of 1.9 mm would not be adversely a f f e c t e d and c c u l d dominate, second, Chaoborus IVth i n s t a r ' s p r e f e r r e d food items are copepods. S w i f t ' s (1974) experiments suggest that Chaoborus t£ivittatus has a c r i t i c a l maximum prey s i z e t h r e s h o l d around 2.2mra . Chaoborus could s e l e c t f o r quick growth of copepods and impose a lower " f l o o r " on the s i z e of mature copepods. Thus, there should be an optimal s i z e f o r copepods that f i t s i n s i d e a narrow window whose height i s determined by ve r t e b r a t e and i n v e r t e b r a t e predation pressure. The shallow pond which has a high salamander d e n s i t y and where Chaoborus i s consequently at low d e n s i t i e s puts a low " c e i l i n g " on copepod s i z e but no " f l o o r " . D._ leptopus dominates but kenai i s s t i l l present. In the c c n t r c l pond, copepods encounter both a " c e i l i n g " imposed by the v e r t e b r a t e predator and a f l o o r " by the i n v e r t e b r a t e predator (the l a t t e r i s probably more im p o r t a n t ) . D A kenai dominates completely. In the experimental pond, there i s a " f l o o r " , but the " c e i l i n g " i s removed. P.i iSE^OEUS. i s again at a disadvantage. D A kenai i s abundant and responds with an i n c r e a s e i n the average s i z e at ma t u r i t y from 2.08 to 2.47mm f o r females, and 1.87mm to 2.14mm f o r the males. They have, t h e r e f o r e , a t t a i n e d or surpassed Chaoborus • s maximum prey s i z e t h r e s h o l d of about 2.2mm. T h i s q u a n t i t a t i v e evidence leads me to f a v o r an e x p l a n a t i o n based upon the i n t e r p l a y of v e r t e b r a t e and i n v e r t e b r a t e predation r a t h e r than an e x p l a n a t i o n based upon the s i z e - e f f i c i e n c y h y p othesis. To summarize, there i s evidence that both c o m p e t i t i o n and i n v e r t e b r a t e p r e d a t i o n combined with v e r t e b r a t e p r e d a t i o n to a f f e c t p o p u l a t i o n s of gr a z e r s . Both are necessary to account f o r the r e l a t i v e abundance of c l a d o c e r a i n the ponds whereas pr e d a t i o n alone i s a more p l a u s i b l e e x p l a n a t i o n f o r changes i n s i z e at maturity f o r e i t h e r Diaptomus or Daphnia . llllz. SMIIM DISCUSSION T h i s work i s concerned with a l p i n e pond communities; more s p e c i f i c a l l y with t e s t s of the hypothesis t h a t two major p r e d a t o r s of these communities, Afflb_y.st.oma and Chacborus , have complementary f e e d i n g niches. According to Dodson (1970), although these two predators share food r e s o u r c e s , Amb_ystonia maintains a s u i t a b l e f e e d i n g niche f o r the i n v e r t e b r a t e predator by removing l a r g e z o o p l a n k t e r s , thus f a v o r i n g s m a l l ones. while p r e d a t i o n i s widely accepted as determining the s t r u c t u r e of communities, the r o l e played by c o m p e t i t i o n i s under d i s p u t e . Dodson(1974), f o r example, emphasizes the r o l e played by i n v e r t e b r a t e predation i n s t r u c t u r i n g n a t u r a l communities and even goes so f a r as proposing i t as an a l t e r n a t i v e to the s i z e - e f f i c i e n c y hypothesis. A most remarkable f i e l d study on c o m p e t i t i o n and i t s importance i s G i l l and Hairston's(1972) work on the dynamics of n a t u r a l p o p u l a t i o n s of PafsLESSiM* Although they sampled s m a l l pools i n t e n s i v e l y i n the s p r i n g , they did not f i n d any evidence that the disappearance of one syngen i s due to i n t e r a c t i o n s with another or to c o m p e t i t i o n f o r food. They d i d show that, s i n c e the r e s o u r c e s a v a i l a b l e to Paramecium are coarse grained ( L e v i n s , 1966) , one syngen, an r -s t r a t e g i s t , (MacArthur and Wilson, 1967), can e x p l o i t resources e f f i c i e n t l y while a k - s t r a t e g i s t cannot cope with environmental f l u c t u a t i o n s and i s e l i m i n a t e d . Thus, i t i s q u e s t i o n a b l e whether or not competition p l a y s an important r o l e i n zooplankton communities, e s p e c i a l l y i n a l p i n e ponds where y e a r l y environmental f l u c t u a t i o n s are c o n s i d e r a b l e . I w i l l attempt to draw a s y n t h e t i c p i c t u r e of the communities found on Hollyburn Bidye as r e v e a l e d by experimental manipulation, and t r y to assess the r o l e played by v a r i o u s components of the community. In F i g u r e 17 I present the predatory r e l a t i o n s h i p s of the dominant s p e c i e s found i n the main ponds, r o t i f e r s excepted. Amb£3toma removes l a r g e c l a d o c e r a . Chaoborus i s more s u c c e s s f u l l a t f e e d i n g on a s m a l l cladoceran l i k e £ia£hancscma and on Dia£tomus copepods. The r e a l s t a t u s of Cy_clo£s i s not known. Evidence from the l i t e r a t u r e i n d i c a t e s that i t i s a predator cn r o t i f e r s and s m a l l c l a d c c e r a . Amb^stoma i s the most important pradator because i t hunts v i s u a l l y and feeds very s e l e c t i v e l y cn s p e c i f i c prey items, thus a f f e c t i n g plankton composition. Chaoborus , the most abundant i n v e r t e b r a t e predator, i s not so s e l e c t i v e . I t can a t t a c k mcst prey e q u a l l y well and probably accounts f o r comparable percentages of m o r t a l i t y i n d i f f e r e n t s p e c i e s . On the other hand, the type of prey t h a t i t can capture and handle i s l i m i t e d by mouth appendagas and mouth gape c h a r a c t e r i s t i c s . I t s f e e d i n g success depends on c e r t a i n s p e c i f i c c h a r a c t e r i s t i c s of prey s p e c i e s . I n d i r e c t evidence suggests t h a t Da£hnia , the l a r g e s t c l a d o c e r a n , i s the most e f f i c i e n t g r a z e r . In the absence of v e r t e b r a t e p r e d a t i o n , i t i s very abundant. Hclcpedium i s the next most numerous s p e c i e s . Both of these s p e c i e s encounter l i t t l e i n v e r t e b r a t e p r e d a t i o n . Lliapha nosoma , the s m a l l e s t c l a d o c e r a n , was found at very low d e n s i t i e s i n the experimental pond. We cannot, with c e r t a i n t y , a t t r i b u t e i t s low d e n s i t y to F i g u r e 17. P r e d a t i o n diagram. From heavy p r e d a t i o n ( s o l i d l i n e ) to l i g h t predation (dotted l i n e ) . •Ambystoma .^ Diaphanosoma < Cyclops c o m p e t i t i v e i n t e r a c t i o n s s i n c e Chaoborus i s abundant i n t h a t pond. But i f we compare the experimental p o p u l a t i o n with the 2,i§L£hanosoma p o p u l a t i o n i n the c o n t r o l pond, we see t h a t i t i s dominant there, i n s p i t e of an almost equal d e n s i t y of Chaoborus l a r v a e . T h i s suggest t h a t competition i s an important f a c t o r i n determining community s t r u c t u r e . The r o l e played by Chaoborus would be to enhance the e f f e c t of c o m p e t i t i o n i n depressing P-iSEk^BSSoroa numbers even f u r t h e r . These o b s e r v a t i o n s lead to a model i n which Chaoborus i s a dependent predator as suggested by Dodson(1970). Ambystoma i s the keystone predator and has a l a r g e impact on the assemblage of competing g r a z e r s . In i t s absence, Daphnia and l a r g e copepods dominate. 2iJ52£°£!iS • s feeding success i s subsequently poor. T h i s i s c o r r o b o r a t e d by the r e s u l t s of an a n a l y s i s of crop contents of the l a r v a e i n the experimental pond, and a l s o by a p o p u l a t i o n d e c l i n e i n which very few i n d i v i d u a l s reached the IVth i n s t a r . T h e i r dry weight was only about 50% of t h a t i n the p r e v i o u s year. F i n a l l y , I would l i k e to comment on the importance of i n v e r t e b r a t e p r e d a t o r s i n a q u a t i c communities and Godson's (1974) p r o p o s i t i o n that i n v e r t e b r a t e p r e d a t i o n i s the reason why s m a l l herbivorous s p e c i e s do not c o e x i s t with l a r g e ones. A g e n e r a l o b s e r v a t i o n that a p p l i e s to most predator-prey systems i s that predator numbers are much lower than the numbers of i t s prey. A predator depends on a l a r g e suppply of i n d i v i d u a l prey animals to s u b s i s t . In the s i t u a t i o n that we are i n t e r e s t e d i n , Chaoborus w i l l crop a p r o p o r t i o n of the Diaphanoscma p o p u l a t i o n (and other prey as well) but i t w i l l not depress t h e i r numbers d r a s t i c a l l y or e l i m i n a t e them. Chaoborus goes r a p i d l y through s u c c e s s i v e molts, s h i f t i n g i t s predation pressure to d i f f e r e n t prey s p e c i e s and widening i t s range of p o t e n t i a l feed items. Ihus, Chaoborus, a l a r g e predator, c o e x i s t s with an abundance of a small prey s p e c i e s . T h i s i s p o s s i b l e because Chaoborus i s almost transparent and i t does not i n c u r a very high r i s k of being preyed upon by Ambystoma . In my c o n t r o l pond, the s m a l l prey s p e c i e s i s Diaphanosoma. The abundance of Cyclops a l s o c o r r e l a t e s with Diaphanosoma abundance i n my ponds. It i s a small predator and i t can attack a prey two or three times i t s own s i z e ( B i l l S e i l l , perscnnal communication) while a v o i d i n g v e r t e b r a t e p r e d a t i o n . Both i n v e r t e b r a t e p r e d a t o r s use d i f f e r e n t t a c t i c s but achieve the same g c a l . T h e i r f e e d i n g niche i s s u s t a i n e d by Ambjstomd which accounts f o r most c f the c h a r a c t e r i s t i c s of the community. In the absence of a v e r t e b r a t e predator, a l a r g e and v i s i b l e i n v e r t e b r a t e predator such as D i_shoshone w i l l be found i n high d e n s i t i e s and c c u l d account f o r the e x c l u s i o n of smaller s p e c i e s from l a r g e s p e c i e s a s s o c i a t i o n s . I t i s important to d i s t i n g u i s h between the two types of communities, those with or those without a v e r t e b r a t e predator, when c o n s i d e r i n g Dodson»s p r o p o s i t i o n . In water bodies of l a r g e dimensions, i t i s p o s s i b l e that we w i l l f i n d i n t e r m e d i a t e s i t u a t i o n s because of the added f a c t o r of s p a t i a l environmental h e t e r o g e n e i t y . 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E f f e c t s of a l e w i f e p r e d a t i o n on zooplankton p o p u l a t i o n s i n Lake Michigan. Limnol. Oceancg. 15 ( 4 ) : 556-565. Z a r e t , T. M. 1972a. Predator-prey i n t e r a c t i o n i n a t r o p i c a l l a c u s t r i n e ecosystem. Ecology 53 (2): 248-257. Z a r e t , T. M. 1972b. Predators, i n v i s i b l e prey, and the nature of polymorphism i n the Cladocera (Class C r u s t a c e a ) . Limnol. Oceanog. 17 (2) : 171-184. APPENDIX I Amb^stoma behavior experiments P r e d a t i o n experiments were conducted using l a r v a e hatched i n the l a b o r a t o r y from eggs c o l l e c t e d i n v a r i o u s ponds. Apart from the three p r e v i o u s l y mentioned ponds, animals were c o l l e c t e d i n L o s t Lake on Hollyburn Ridge ( a l t i t u d e 800m), from a pond near Abbotsford at sea l e v e l and from one on Bowen I s l a n d i n Howe Sound ( a l t i t u d e 160m). These animals were fed b r i n e shrimp (Artemia) and were considered "naive" predators u n t i l they fed on plankton. An "experienced" l a r v a had fed once or twice on zooplankton i n a previous run of any experiment. A predator was not fed on the day preceding an experiment . V a r i o u s prey were o f f e r e d (1:1 r a t i o ) a t d e n s i t i e s not v a r y i n g by more than a f a c t o r of two or t h r e e from t h e i r n a t u r a l d e n s i t i e s . Except f o r one experiment that was run i n a p e t r i d i s h and represented abnormally high d e n s i t i e s , I used a white p l a s t i c bleach b o t t l e 15cm i n diameter c u t i n h a l f with 250 or 500 cc of pond water. One or two p r e d a t o r s were used. Experiments were run a c c o r d i n g to one of two methods: a) Each animal taken by a predator was immediately r e p l a c e d , or b) animals were not r e p l a c e d and the experiments were run f o r approximately 15 minutes (minimum 10 and maximum 20) when between 5 and 30% of the t o t a l number of prey had been removed. Any r e s u l t l y i n g o u t s i d e t h a t range was r e j e c t e d "so t h a t chance s e l e c t i o n does not i n f l u e n c e the r e s u l t s , ...[and] the o r i g i n a l r a t i o of prey items i s [ n o t ] a f f e c t e d " (Zaret, 1972a). The data were analysed using the h e t e r o g e n e i t y G-test and c h i - s q u a r e f o r independence (Sokal & Hohlf, 1969) . Data that was not s i g n i f i c a n t l y heterogeneous (with r e s p e c t to the two d i f f e r e n t methods and naive or experienced l a r v a ) was pooled. A l l the r e s u l t s are s y n t h e t i z e d i n TABLE XX. £MSMi3Iii2 # 1 c o n s i s t e d i n o f f e r i n g salamanders l a r g e females and s m a l l e r males of Daphnia pulex . The pooled r e s u l t t h a t 89 /450 females were taken versus 25/450 males c l e a r l y i n d i c a t e s s i z e - s e l e c t i v i t y . I t i s u n l i k e l y t h a t these trends could be a t t r i b u t e d to escape responses of the prey s i n c e females were missed s l i g h t l y more o f t e n than males. EXPERIMENT # 2 o f f e r e d the predator 25 l a r g e and 25 s m a l l females of Daphnia pulex . Data from experiments 2a and 2b (naive predator) were pooled. They were s i g n i f i c a n t l y d i f f e r e n t from the data of Experiment 2c (G=4,00, 0.05>p>0.01) . Both groups y i e l d a s i g n i f i c a n t value f o r c h i - s q u a r e , 57.65 and 93.22 r e s p e c t i v e l y . "Naive" animals scored 66/300 l a r g e and 8/300 small prey, and "experienced" ones 92/225 l a r g e and 3/225 s m a l l prey. Amb^stoma s e l e c t s l a r g e females over s m a l l ones, The heter o g e n e i t y suggest t h a t experience c o u l d p l a y a r o l e i n i n f l u e n c i n g the outcome of a run. EXPERIMENT # 3 compared salamander p r e f e r e n c e f o r 25 Daphnia pulex females with a brcod versus s i m i l a r females without a brood. The presence of eggs or smal l i n s t a r i n s i d e the female carapace c r e a t e s a dotted p a t t e r n and i s a s t r i k i n g f e a t u r e which c o u l d make i t s bearer e a s i e r to see. No s i g n i f i c a n t d i f f e r e n c e s were obtained. TAbLE XXI Summary of h a t c h l i n g salamander behavior experiments. E x p l a n a t i o n s are i n the t e x t . D a t e E x p e r i m e n t # P r e d a t o r C o n t a i n e r Method # r u n s P r e y R e s u l t s 1/5/73 l a 2* - p e t r i a • 6 < •(1) . D. p u l e x 10 f e m a l e s 25 10 m a l e s 1 4/5/73 l b 2* 5 0 0 c c a 6 (2) ii 25 f e m a l e s 25 m a l e s 27 4 6/5/73 l c r* 5 0 0 c c a 19 (7) 25 f e m a l e s 25 m a l e s 37 20 30/4/73 2a i * 5 0 0 c c b 4 ( 1 ) 25 f e m a l e s 25 m a l e s ,31 5 30/4/73 2b i * 5 0 0 c c a 8 (4) 25 f e m a l e s 25 m a l e s 35 4 1/5/73 2c i 5 0 0 c c b 9 25 f e m a l e s 25 m a l e s 92 3 3/5/73 3 i * 5 0 0 c c b 12 (2) D. p u l e x 25 b r o o d f e m a l e s 25 no b r o o d 83 /73 17/7/73 4a i * 2 5 0 c c b 12 15 D. k e n a i 40 15 H o l o p e d i u m 16 20/7/73 4b i * 2 5 0 c c b 5 15 D. k e n a i 23 15 H o l o p e d i u m 14 9/8/73 5a i * 5 0 0 c c b 6 (2) 25 D. k e n a i 6 25 D a p h n i a r o s e a 15 9/8/73 5b i 5 0 0 c c . b 4 (2) 25 D. k e n a i 4 25 D a p h n i a r o s e a 8 9/8/73 5c i 5 0 0 c c b 6 (3) 25 D. k e n a i 6 25 D a D h n i a r o s e a 9 * = " n a i v e " p r e d a t o r (#) = number o f r e j e c t e d r u n s a = w i t h o u t r e p l a c e m e n t b = w i t h r e p l a c e m e n t EXPERIMENT # 4. The c h o i c e g i v e n the salamander was between the copepod D._ kenai and the c l a d o c e r a n Holopedium. I had hoped that the l a r v a e c o u l d not handle Holopedium because of i t s j e l l y mass and t h a t I could have used i t as a predation f r e e z o o p l a n k t e r f o r comparison between ponds with d i f f e r e n t p r e d a t i o n l e v e l s . The value of the G - s t a t i s t i c f o r h e t e r o g e n e i t y i s 33,6 and s i g n i f i c a n t . Some i n d i v i d u a l s l e a r n e d to handle S2l2E®^iSS a n c * o t h e r s d i d not, c o n c e n t r a t i n g on the l a r g e f a s t moving copepod. I saw animals pushing the j e l l y mass with t h e i r snout , t r y i n g t o snap at the enclosed prey only to see i t bounce away. Those t h a t kept pouncing a t i t were f i n a l l y rewarded. C o n s i d e r i n g t h a t D.. k e n a i i s not an easy prey, r e s p e c t i v e s c o r e s of 40/255 or 23/75 f o r D^-Jkenai a n f l 16/255 or 14/75 f o r Hol,2£e^iua| i n d i c a t e t h a t the j e l l y mass does p r o t e c t S2l2£§dium from predator a t t a c k s . As the predator grows and l e a r n s , t h i s advantage must disappear r a p i d l y as suggested by the gut content a n a l y s i s (TABLE V and VI) . EXPERIMENT # 5 1 o f f e r e d 25 D.. kenai and 25 Eaghnia r o s e a to the salamanders . The r e s u l t s of 5a, 5b and 5c were pooled s i n c e t h e r e i s no s i g n i f i c a n t h e t e r o g e n e i t y w i t h i n or between clumps or with r e s p e c t to experience of the predator. A score of 10/225 copepods and 32/225 c l a d o c e r a eaten y i e l d s a s i g n i f i c a n t v a lue of c h i - s g u a r e of 5.85. In t h i s experiment, I f o l l o w e d the p r e d a t o r ' s performance and r e c o r d e d t h e i r u n s u c c e s s f u l attempts as w e l l as s u c c e s s f u l ones . I found t h a t 132 attempts were made to c a t c h copepods and 48 f o r the c l a d o c e r a . For some reason ( s i z e i s a p l a u s i b l e one), more attempts were made to eat the l a r g e copepod; but i t s escape response i s more e f f i c i e n t than Daphnia 's and the c l a d o c e r a n i s the most abundant prey i n ambjrstoma 's stomach. The predator percentage success (capture/attempt) i s 66% f o r Da£hnia and 12% f o r D A kenai . APPENDIX II Population S t r u c t u r e of Ambystoma Throughout t h i s Appendix, I w i l l d e s c r i b e t h r e e components of the Ambjrstoma p o p u l a t i o n : A) Metamorphosed salamanders: on June 26th 1973, eggs were being l a i d i n the experimental pond. Two out of 19 females seen from the shore were metamorphosed females. T h e i r c l u t c h s i z e (75 and 52) d i d not appear to be d i f f e r e n t from that of paedogenetic l a r v a e (60 and 76) when they l a i d t h e i r eggs i n the l a b o r a t o r y . B) Paedogenetic l a r v a e : a Petersen's estimate of 180 i n d i v i d u a l s was obtained f o r the experimental pond i n 1972. Since t h e r e i s no reason to assume t h a t the sex r a t i o i s b i a s e d , there should be approximately n i n e t y females. Seventy nine clumps were l a i d the f o l l o w i n g s p r i n g , 9 of which might be a t t r i b u t e d to metamorphosed salamanders a c c o r d i n g to s i g h t i n g s . A l l o w i n g f o r m o r t a l i t y and assuming t h a t the P e t e r s e n ' s estimate i s not too f a r o f f , i t would appear that most females breed every year. C) Eggs and h a t c h l i n g salamander: i n 197 3 I found 65 egg masses i n the c o n t r o l pond, 79 i n the experimental and 23 i n the shallow pond. M o r t a l i t y before hatching i s q u i t e low. Nine clumps i n s p e c t e d p r i o r to hatching contained 12 dead eggs out of a t o t a l of 300. The h a t c h l i n g i n p u t i n t o our ponds was then estimated. I t was presented p r e v i o u s l y i n TABLE IV. Since the shallow water pond l o s e s h a l f or more of i t s s u r f a c e area and volume by l a t e summer, the predator d e n s i t y there must be 4 or 5 times g r e a t e r than i n the main ponds. D) Immature l a r v a e : Petersen's estimate y i e l d s a value of 1030 (the 95% confidence i n t e r v a l i s 515-1545) f o r 1972. I t was a l s o estimated that from t h i s number, 90 i n d i v i d u a l s (with a 95% confi d e n c e i n t e r v a l of 10-180) metamorphosed at the end of the summer. Fi g u r e 18 r e p r e s e n t s the s i z e d i s t r i b u t i o n of the animals captured i n 1972 i n the experimental pond and Figure 19 s y n t h e t i z e s a l l the i n f o r m a t i o n presented i n t h i s APPENXIX. F i g u r e 18. S i z e d i s t r i b u t i o n of salamanders captured i n the experimental pond i n 1972. F i g u r e 19. Summary of the estimates of the salamander p o p u l a t i o n i n the experimental pond. 

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