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Foraging behaviour of the intertidal beetle Thinopinus pictus (Staphylinidae) Richards, Laura Jean 1982

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FORAGING BEHAVIOUR OF THE INTERTIDAL BEETLE THINOPINUS PICTUS (STAPHYLINIDAE) by LAURA JEAN RICHARDS B.Sc(Hons.), Dalhousie U n i v e r s i t y , 1976 M . S c , The U n i v e r s i t y of B r i t i s h Columbia, 1979 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA May 1982 (c) Laura Jean R i c h a r d s , 1982  In p r e s e n t i n g  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of  requirements f o r an advanced degree a t the  the  University  o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make it  f r e e l y a v a i l a b l e for reference  and  study.  I further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may department or by h i s or her  be  granted by  the head o f  representatives.  my  It i s  understood t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l gain  s h a l l not be allowed without my  permission.  Department of The U n i v e r s i t y of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date  DE-6  (3/81)  written  i i  ABSTRACT Optimal capable  of  foraging  generally  making appropriate  decisions  affect  study  the  of  I  intertidal  burrows  they  live  emerge  amphipods  Orchestoidea  californiana  some d a t a  for  isopods  Alloniscus  important  prey  species.  by  trapping,  pitfall  counts  of  the  searches. beetle  and  However, showed prey 75  In  and  general,  beetles  availability. between  successful for  up  in  to  oviposition I using  and the reaction and the  4-d  intervals  rates  results  food  in  of  distributions of  distance  laboratory and  probability  feeding  that  amphipod  and  at  also  a  so  rate that  during did  was  beach  beetles  a night.  not  affect  direct in  1-h  between patterns.  of  prey,  I  independently  of  with  of  a  were  mean not  Food  always  deprivation  beetle  survival  or  experiments.  amphipod s i z e of  low,  less  patterns  on t h e  sites  on  present  by  spatial activity  foraging  in  prey  Dana,  selection  by  beetles,  a m p h i p o d s m e a s u r e d on t h e  experiments rates.  on  Capture  Beetles  beach,  capture  success,  success  decreased  a n a m p h i p o d was d e t e c t e d  size.  to  distribution  a  Leconte  patterns  active  in  beaches  I  perconvexus  spatial  laboratory  models  these  was a g o o d c o r r e s p o n d e n c e  capture  captures,  obtaining  size  increasing  arrived  sand  (Brandt).  beetles  are  assumptions  night  activity  temporal  Prey  constructed the  there  manipulating the  that  min  of  and t h a t  pictus  on at  predators  measured amphipod a c t i v i t y  beetle  number  amphipod  by  I  these  Thinopinus  beetles  from which  decisions  tested  beetle  Adult  assume t h a t  foraging  fitness.  (Staphlyinidae). temporary  models  observed  increased during  with beach  searches s e l e c t e d l a r g e r s i z e s of amphipods than p r e d i c t e d availability optimal  and v u l n e r a b i l i t y of d i f f e r e n t s i z e s .  foraging  model,  I  estimated  the  from  To apply an  profitability  d i f f e r e n t s i z e s of amphipods from the number of amphipods given  size  required  Profitability small  to  satiate  a  was highest f o r l a r g e  amphipods  and  isopods.  of  of  a  b e e t l e i n the l a b o r a t o r y . amphipods  and  lowest  for  However, amphipod abundance on  the beach was always below the t h r e s h o l d at which s p e c i a l i z a t i o n on l a r g e r s i z e s was p r e d i c t e d to occur. Male b e e t l e s were a c t i v e longer than female b e e t l e s the  during  n i g h t , and fewer male b e e t l e s were observed f e e d i n g .  b e e t l e s tended t o be found higher on the beach more  and  isopods i n t h e i r d i e t than female b e e t l e s .  experiments  I showed that amphipods were h i g h l y  isopods by both sexes of b e e t l e s . approximately items.  the  to  Male include  In l a b o r a t o r y preferred  over  Male and female b e e t l e s were  same s i z e and consumed equal numbers of prey  I conclude that male f o r a g i n g behaviour was a l t e r e d  by  search f o r mates. I  present  an optimal d i e t model f o r two prey types, based  on the expected f o r a g i n g time r e q u i r e d f o r a predator satiation. on  Predictions d i f f e r  maximization  may be minimized then  expands  for  interpretations  intake.  Foraging time  by a predator which begins as a s p e c i a l i s t  of  and  t o i n c l u d e lower v a l u e prey when i t i s  Laboratory experiments  these  reach  i n some cases from a model based  the r a t e of energy  i t s diet  near s a t i a t i o n . support  of  to  predictions, the  but  results.  on Thinopinus give weak I  present I  suggest  alternative that  most  iv  i n v e r t e b r a t e p r e d a t o r s which forage on a c t i v e prey in  their  Future  ability  studies  to  should  assess  variations . in  emphasize  a v a i l a b i l i t y a f f e c t s foraging  how  behaviour.  prey  patchiness  are  limited  abundance. in  prey  V  TABLE OF CONTENTS  ABSTRACT  i i  LIST OF TABLES  viii  LIST OF FIGURES  ix  LIST OF SYMBOLS  x  ACKNOWLEDGEMENTS Chapter  xii  1: General i n t r o d u c t i o n  Background  1  to the problem  1  The study animals  4  The study s i t e  6  Chapter 2: Prey p a t c h i n e s s and f o r a g i n g Introduction  ••••  •  M a t e r i a l s and Methods Predator and Prey A c t i v i t y  7 7  8 Patterns  8  Behaviour at a patch  10  Laboratory feeding experiments  12  S u r v i v a l and o v i p o s i t i o n r a t e s  13  Results Temporal  14 changes i n amphipod a c t i v i t y  14  Beetle a c t i v i t y patterns  18  Behaviour at a patch  27  Laboratory feeding experiments  38  S u r v i v a l and o v i p o s i t i o n r a t e s  38  Discussion When where and how to forage  44 44  vi  Sex d i f f e r e n c e s  48  R e l a t i o n to theory  49  Chapter  3: Prey s e l e c t i o n  51  Introduction  51  Models  53  M a t e r i a l s and Methods  56  F i e l d data  56  Vulnerability  57  Feeding experiments  59  Results  60-  Vulnerability  60  Feeding r a t e s  62  Field results  68  A test  82  f o r prey s e l e c t i o n  M e c h a n i s t i c model  82  Frequency-dependent model  84  Optimal d i e t model  84  Discussion Why  d i d the optimal d i e t model not work?  Isopods versus amphipods Chapter  4: Hunger and Optimal d i e t  89 92 93 95  Introduction  95  The model  96  A t e s t with Thinopinus  105  Discussion  111  Chapter  5: Concluding remarks  Literature cited  115 117  vii  Appendix A  126  Appendix B  128  vi i i  LIST OF TABLES  Table I. Time budgets f o r male and female  beetles  Table I I . R e s u l t s of patch experiments Table I I I . R e s u l t s of the o v i p o s i t i o n Table IV. Capture  25 37  experiment  43 61  success  Table V. Feeding data  65  Table V I . Feeding data f o r b e e t l e s fed to s a t i a t i o n  68  Table V I I . Parameter values f o r exact f i t  85  Table V I I I . Parameters f o r the optimal d i e t model  86  Table IX. Foraging times generated  88  Table X. Values of c  by the model  109  LIST OF FIGURES  Figure  1. Temporal a c t i v i t y of amphipods and b e e t l e s  15  F i g u r e 2. N i g h t l y changes i n amphipod s i z e d i s t r i b u t i o n s  .. 19  F i g u r e 3. b e e t l e and amphipod abundance  23  F i g u r e 4. Frequency d i s t r i b u t i o n of move d u r a t i o n s  29  F i g u r e 5. P r o p o r t i o n of time i n a c t i v e  31  mode  F i g u r e 6. Frequency d i s t r i b u t i o n of attacks/min  35  F i g u r e 7. Mean number of amphipods eaten  39  F i g u r e 8. S u r v i v o r s h i p curves  41  F i g u r e 9. Reaction  63  distances  Figure  10. Mean number of isopods  eaten  Figure  11. Monthly changes i n amphipod s i z e d i s t r i b u t i o n s  Figure  12. Morphological  comparisons  66  of  male  . 70  and female  beetles  74  Figure  13. Mandible spread  76  Figure  14. Scavengers and amphipod s i z e  78  Figure  15. P r e d i c t e d and a v a i l a b l e  Figure  16. Expected t o t a l f o r a g i n g times  Figure  17. Expected t o t a l f o r a g i n g times  amphipod d i s t r i b u t i o n s  case Figure  . 80 100  for  the  general 1 03  18. Mean numbers of amphipods eaten  107  X  LIST OF SYMBOLS  A  probability  that an encountered  c  c o e f f i c i e n t of p r e f e r e n c e (Murdoch 1969)  c(i)  probability  that  prey  of  prey item i s a t t a c k e d  size i  are captured when  detected C  i n the optimal d i e t model, the p r o b a b i l i t y item i s captured, given that  i t i s attacked  d(i)  maximum r e a c t i o n  D  t o t a l food value r e q u i r e d by a predator  e(i)  food value of a prey of s i z e i  f(i)  r e l a t i v e encounter  h(i)  h a n d l i n g time f o r a prey of s i z e i  H  t o t a l time spent i n h a n d l i n g prey  K  proportionality the r e a c t i v e  that a prey  d i s t a n c e f o r a prey of s i z e i  frequency with prey of s i z e i  items  constant which depends on the shape of  f i e l d of the predator  N  t o t a l number of prey items eaten  p(i)  proportion  of  prey  of  size  i  in  the  diet  of  a  n o n - s e l e c t i v e forager p(i)'  p r o p o r t i o n of prey of s i z e  i observed  i n the d i e t of a  i observed  i n the d i e t  n o n - s e l e c t i v e forager r(i)  p r o p o r t i o n of prey of s i z e  R  prey encounter  S  t o t a l time spent i n prey search  v(i)  v u l n e r a b i l i t y of prey of s i z e  i to the predator  w(i)  number  i  of  experiments  rate  prey  of  size  eaten  in  preference  xi  number of  prey  of  size  i  presented  p r e f e r e n c e f o r prey of s i z e  i (Chesson  in  experiments 1978)  preference  xii  ACKNOWLEDGEMENTS First Judy  and  Myers,  foremost  I  f o r making  would  this  l i k e to thank my s u p e r v i s o r ,  study  possible.  Jamie  Smith  deserves s p e c i a l mention  f o r h e l p f u l comments and advice a t most  stages.  to  I  also  wish  thank  the s t a f f of Bamfield Marine  S t a t i o n and the members of my s u p e r v i s o r y committee, Carefoot,  C.J.  McPhail f o r h e l p discussions  Krebs,  C.  in various  Levings, ways.  W.E. I  also  Drs.  Neill  T.H.  and J.D.  benefited  from  w i t h Lee Gass, Sarah Groves, Peter M o r r i s o n , Pamela  Mace, Richard Palmer, and Jens Roland, and from the mathematical e x p e r t i s e of John Parslow. assisted  i n the  field  E l i z a b e t h Boulding in  1979  and  and  Bruce  1980 r e s p e c t i v e l y .  C o a s t a l M i s s i o n s provided the c o f f e e and moral support me awake a t night  Science  Fellowship. J.H.  Myers.  to  The keep  i n 1979, and a l l the people with whom I shared  accommodation l e t me s l e e p i n the morning. NSERC  Till  Scholarship  and  a  H.R.  I was supported by a MacMillan  Family  Research c o s t s were met by NSERC and NAHS g r a n t s to F i n a l l y I wish t o express my a p p r e c i a t i o n  Getman f o r the song "The Moonlight Beach Bug B a l l " .  t o Roy  1  CHAPTER 1: GENERAL INTRODUCTION  Background to the problem Predation communities of  i s one of the  processes  which  structure  (Brooks and Dodson 1965, C o n n e l l 1975). The success  a predator i n f i n d i n g  affect  major  suitable  prey  items  will  not  i t s own f i t n e s s , but w i l l a l s o have consequences  dynamics of the community i n which i t r e s i d e s . One good of  how  this  process  can  experiments,  example  they showed  that seed p r e d a t i o n a f f e c t e d the d i v e r s i t y of annual Sonoran  on the  operate i s a study by Inouye et a l .  (1980). In a s e r i e s of p r e d a t o r removal  the  only  plants  in  D e s e r t . The outcome d i f f e r e d however, depending on  whether the p r e d a t o r s were rodents or ants or both.  for  The o v e r a l l importance  of p r e d a t i o n has lead  general  govern  attempt  rules  which  theory  evolved  reflect the  natural  that  selection.  the outcome of t h i s s e l e c t i o n ,  and  behaviours which maximize i n d i v i d u a l  approach, on  through  the  the i n v e s t i g a t o r must f i r s t animal,  optimization  such  procedure  as is  time then  search  ( f o r reviews  (1971), Pyke et a l . (1977), and Krebs  f o r a g i n g theory i s based on the premise has  a  predator behaviour. One such  i s known as optimal f o r a g i n g  Schoener  to  behaviour  Observed  behaviours  should  approximate  f i t n e s s . To apply t h i s  energy  applied  (1978)). Optimal  foraging  identify  or  see  the  constraints  limitation.  subject  to  The these  constraints. Optimal  foraging  theory  has been s e v e r e l y c r i t i c i z e d f o r  s e v e r a l reasons. One major c r i t i c i s m  i s that the hypothesis that  2  animals optimize i n some way i s not f a l s i f i a b l e 1978).  The  hope,  rather,  is  that  behaviours can l e a d to some i n s i g h t this restriction and  Hanski  approach  Smith  informed m o d e l l i n g of the  into their evolution.  Given  i t i s s u r p r i s i n g , as s t r e s s e d by O l l a s o n (1980)  (1980),  is  (Maynard  that  followed  i n most  without  studies  the o p t i m i z a t i o n  consideration  of  alternate  hypotheses. A  second major c r i t i c i s m of the theory i s the general l a c k  of f i t between q u a n t i t a t i v e p r e d i c t i o n s of the  theory  a c t u a l behaviours observed, although the q u a l i t a t i v e may  be  met.  vertebrates, conditions.  Most  of  these  especially  tests  birds,  have under  been  predictions  conducted  highly  S c h l u t e r (1981) has r e c e n t l y reviewed  and the  controlled the (lack o f )  evidence f o r optimal d i e t s , and concluded that f o r a g i n g of  (mainly)  vertebrates  conducted  p r e d i c t i o n s of the theory. One decisions reason  on  studies  i n the f i e l d do not support  reason  i s that  the  necessary  are o f t e n too complex f o r the animal to make. Another  i s that the energy  investigators  to  content of a prey item, used  by* most  a s s i g n prey v a l u e , i s not always a s u f f i c i e n t  index of i t s value to a p r e d a t o r . I chose predator,  to conduct  a field  study of f o r a g i n g i n  the b e e t l e Thinopinus p i c t u s LeConte  an  insect  (Staphylinidae).  My general aims were (1) t o assess the a b i l i t y of t h i s b e e t l e to make complex f o r a g i n g d e c i s i o n s (2) to assess the importance this  ability  to  individual  optimal d i e t  models  differential  prey  against  fitness an  (3) to t e s t p r e d i c t i o n s of  alternative  vulnerability.  of  model  Thinopinus  based  lives  on  in a  3  structurally  simple environment,  the  sand  beach.  ambush p r e d a t o r s of beach i n v e r t e b r a t e s (Craig amphipod  Orchestoidea  californiana  Adults  are  1970), mainly  (Brandt)  and  the  the  isopod  A l l o n i s c u s perconvexus Dana. I chose t h i s system because of structural  I c o u l d d i r e c t l y observe  simplicity.  b e e t l e s , and  I c o u l d measure the s i z e s of prey  I c o u l d a l s o manipulate of  drift  its  f o r a g i n g of the  items of b e e t l e s .  amphipod a v a i l a b i l i t y by moving  patches  seaweed.  This  t h e s i s i s d i v i d e d i n t o 3 main s e c t i o n s . In Chapter  2,  I d e s c r i b e the behaviour  and a c t i v i t y p a t t e r n s of the b e e t l e and  i t s major prey. I  two  models:  test  of  optimal  (1) that p r e d a t o r s can assess and respond  in prey a v a i l a b i l i t y , and is  assumptions  closely  linked  problem of d i e t received  the  to  (2) that short-term fitness.  In  most  to v a r i a t i o n s  foraging  Chapter  s e l e c t i o n . T h i s aspect of  foraging  3, I consider the  foraging  theory  Chapter  4,  against I  present  behaviours a new  T h i s model i l l u s t r a t e s how alter  optimal  and a simple mechanistic model of prey s e l e c t i o n , and  these models  predictions  has  a t t e n t i o n , probably because i t s p r e d i c t i o n s  are the simplest to t e s t e x p e r i m e n t a l l y . I c o n s t r u c t an diet  success  of  the  observed  the  field.  In  v e r s i o n of an optimal d i e t model.  the a d d i t i o n of model.  simple l a b o r a t o r y experiments.  in  test  The  one  assumption  can  I t e s t these p r e d i c t i o n s i n l a s t chapter c o n t a i n s  g e n e r a l remarks on the f u t u r e of optimal f o r a g i n g theory.  a  few  4  The study animals Thinopinus p i c t u s i n h a b i t s exposed sand beaches on the west coast  of  North  America.  Adult b e e t l e s are a c t i v e on the sand  s u r f a c e only at n i g h t . They spend the day i n on  the  upper  these burrows where  they  drift  part  of  burrows  the beach. A f t e r .dark they emerge from  and move down the beach to forage.  temporary  Beetles  the  high  tide  level  g e n e r a l l y wait w i t h i n a few cm of  seaweed and a t t a c k prey items moving on or o f f the weed by  lunging and g r a s p i n g the prey i n t h e i r mandibles. They injecting  digestive  enzymes  feed  by  and sucking the d i g e s t e d m a t e r i a l  from t h e i r prey. T h i s leaves a c a r c a s s which can  be  identified  and measured. I f r e q u e n t l y observed mating b e e t l e s throughout the summer. When  a  male  and  female  meet,  the male  lunges forward and  attempts t o grasp the female i n i t s mandibles,  as  i t would  a  prey item. The male then mounts the back of the female and l i n k s genitalia.  The  total  time  approximately 2 min. Females  required  for copulation  u s u a l l y r e s i s t the mating  is  attempt  and may continue other a c t i v i t i e s such as burrowing, f e e d i n g , or attacking  prey  items  while  mating.  attempt t o mate with a female,  and  More  males  than often  one male may attempted  to  mount other males. Sexes of Thinopinus can be d i s t i n g u i s h e d by a cleft  i n the l a s t abdominal  segment of the male.  Female Thinopinus l a y t h e i r 1970). weight)  eggs s i n g l y  Eggs  weighed  on  and  hatched  i n approximately  temperatures  (16-20°C).  i n damp sand  average 4.4±0.1 mg (±1SE)  I  was  unable  3 to  wks  at  (Craig  (n=25, l i v e laboratory  rear l a r v a e i n the  5  l a b o r a t o r y to determine Larvae not  the  number  running  across  this  study.  I  the sand surface  occasionally  of  instars.  The  natural  history  C a l i f o r n i a coast has Craig  Orchestoidea  in  soft  sand  spends the day,  drift  on  line  shown by  left it  by  the  feeds  mainly  remain under d r i f t  Alloniscus  on  the  beach  digs  field  burrows  Thinopinus.  on  high  than  tend  drift  seaweed.  amphipods,  It  J u v e n i l e amphipods  seaweed  to  At  tide. It i s  during  the  day.  dusk.  perconvexus, show an a c t i v i t y  s i m i l a r to amphipods. Isopods higher  (1964)  moves down the beach  previous  J u v e n i l e s are a c t i v e mainly at dawn and Isopods,  Bowers  the  the upper p a r t of the beach i n which i t  l a r g e feeding aggregations.  do not burrow, but  on  predominant s p e c i e s at my  c a l i f o r n i a n a emerges and  forms  amphipods  c a l i f o r n i a n a . T h i s amphipod  omnivorous, although sometimes  beach  s i m i l a r to the p a t t e r n  dusk, Orchestoidea the  beach.  sand  The  larvae  or found them  been s t u d i e d e x t e n s i v e l y by  (1971,1973a,b).  s i t e was the  of  so were  observed  i n l a t e afternoon,  in burrows on the upper part of the  to  duration  were not a c t i v e on the sand s u r f a c e at night and  included in  and  or  burrow and  in to  pattern  drier  sand  feed on d r i e d  seaweed. Apart the  from Thinopinus,  families  there are s e v e r a l other  S t a p h y l i n i d a e , Carabidae, and  beetles  Curculionidae  comprise the b e e t l e community on the beach. These other often  join  frequent Dyschirius  Thinopinus  scavengers obesus  are LeC,  in  feeding an  on  a  unidentified  of  which  beetles  prey item. The  most  staphylinid  and  a c a r a b i d , both about 2 mm  in length.  6 Amphipods and isopods o c c a s i o n a l l y  join  in  the  scavenging  as  w e l l . Thinopinus r e a c t s to the presence of scavengers by shaking i t s prey or c a r r y i n g as much as s e v e r a l  i t away from the s i t e of c a p t u r e , sometimes  meters.  The study s i t e Most of  f i e l d data were c o l l e c t e d between A p r i l and September  1979 to 1 9 8 1 . The main study  Beach  (48°53'N  l a t . , 125°7'W  coast of Vancouver sand  beach  about  site  was  located  km  long.  s e m i - d i u r n a l . The two d a i l y  wide  Tides in t h i s  fine-grained  region are mixed  high t i d e s u s u a l l y d i f f e r  and each leave a l i n e of d r i f t  Pachena  long.) near Bamfield, on the west  I s l a n d , Canada. T h i s i s a 1  at  i n height,  seaweed. I d i v i d e d the beach  into  upper and lower s e c t i o n s based on the p o s i t i o n of the d r i f t  line  left from  by the p r e v i o u s higher high t i d e . above  this  of amphipods. The extended  down  The upper beach  extended  l i n e to the backshore and i n c l u d e d a l l burrows lower  beach  included  the  drift  line  and  to the water. For a given night the width of the  upper beach remained c o n s t a n t ,  but  v a r i e d as the t i d e moved i n and out.  that  of  the  lower  beach  7  CHAPTER 2: PREY PATCHINESS AND FORAGING  Introduction The  environment  of  a f o r a g i n g animal i s c h a r a c t e r i z e d by  p a t c h i n e s s i n prey d i s t r i b u t i o n  (e.g. Wiens  1976,  Hassell  and  Southwood 1978). T h i s p a t c h i n e s s extends over a range of s p a t i a l and  temporal  scales.  The  ability  p a t c h i n e s s w i l l determine i n p a r t ,  of a predator to respond to i t s foraging  success.  The  major attempt to i n c o r p o r a t e p a t c h i n e s s i n t o an o p t i m a l f o r a g i n g model  was  made  by Charnov  (1976b). H i s M a r g i n a l Value Theorem  p r e d i c t s that a predator w i l l rate has  in  leave a  when  i t s capture  the patch decreases to the h a b i t a t average  been  moderately  experiments  (Cowie  successful  in  1976,  field  laboratory  1978), but has been  situations  Hanski 1980, Morse and F r i t z  argument of whether  T h i s model  controlled  1977, Cook and C o c k r e l l  found i n a p p r o p r i a t e i n more complex Falls  patch  (Zach  1982). Apart from the  or not an o p t i m i z a t i o n approach i s  correct,  there a r e two simple e x p l a n a t i o n s f o r the f a i l u r e of t h i s First,  there  forager  i s limited  in i t s a b i l i t y  to assess t h i s  variability.  t h e i r environment has been  suggested r e p e a t e d l y to account f o r d e v i a t i o n s between predicted  (Heinrich few  model.  i s always v a r i a b i l i t y among patches. Second, the  The need f o r p r e d a t o r s to sample  and  and  values  in  tests  of  optimal  observed  foraging  1976, Davidson 1978, Krebs et a l . 1978).  Yet  theory only  a  authors (e.g. Pyke 1978) have c o n s i d e r e d how f o r a g e r s might  l e a r n about t h e i r environment. In t h i s chapter I use  Thinopinus  pictus  model that  to  test  the  assumption  inherent  in  the  8  p r e d a t o r s can assess and  respond  r e l a t e short-term predator of  to h a b i t a t  success to f i t n e s s . The  and  spatial  I  then  specific  t h i s chapter, then, are to (1) c o r r e l a t e temporal  b e e t l e and amphipod a c t i v i t y ,  aims  p a t t e r n s of  (2) measure the e f f e c t of  temporal  d i s t r i b u t i o n s of prey on f o r a g i n g success, and  r e l a t e f o r a g i n g success to predator and  variation.  f i t n e s s i n terms of  (3)  survival  oviposition rates.  M a t e r i a l s and Methods  Predator and Prey A c t i v i t y  Patterns  Once each month i n 1980 activity  over  the  set  two  here w r i t t e n as HW, by  the  about For  highest  activity  activity  rows of p i t f a l l  along the d r i f t  monitored  amphipod  chosen as r a i n reduced formed  I  line left and  of  tide  t r a p s spaced  May  sample  with the rows spaced plastic  m  prevented  animals  from  3 m a p a r t . One  of the month, here w r i t t e n as  HHW,  along the d r i f t  would  pass  a  beetle  nights  chosen.  rows of 12 t r a p s 5 m apart  Pitfall  traps  consisted  of  cm deep) f i l l e d to o n e - t h i r d  1973a,b,  escaping  on the  Hayes  1970).  once they f e l l  T h i s method sampled the r e l a t i v e abundance of which  was  left  7.5  of  row  I  line  apart.  1970,  amphipods.  day,  cups (8.5 cm diameter,  with seawater ( C r a i g  amphipod  the  there were two 1  and  by higher high t i d e  10 m towards the backshore from HW the  and  p e r i o d . Dry n i g h t s were  beetles  the second row  high  beetle  sitting  The  i n the t r a p s .  active  motionless  water  amphipods  on the sand.  A l t e r n a t i n g t r a p s were p l a c e d i n p o s i t i o n or removed each  hour,  9  so  that  10  traps  in  each  row were set at any given time. I  counted and r e l e a s e d the numbers of amphipods (estimated as mm),  and  the numbers of b e e t l e s which were caught each hour. I  then counted the number of b e e t l e s found between and  £10  the  water  the  backshore  i n a 10 m wide beach s e c t i o n at each end of the  row of t r a p s . Each 2 h I c o l l e c t e d and p r e s e r v e d amphipods additional  traps  measured from  at  the  the  end  anterior  abdominal segment, a l l o w i n g On  6  July  1980,  pitfall after  began,  I  of  the  head  through  the  f o r body c u r v a t u r e (Bowers  in  pitfall  was  third  1963).  traps.  Before  amphipod  s e l e c t e d f i v e weed patches and p o s i t i o n e d a  t r a p 15 cm from each peak  each row. Amphipod s i z e  I compared the s i z e s of amphipods feeding  on weed patches and caught activity  of  from  amphipod  patch.  activity  had  These  traps  were  placed  o c c u r r e d . One hour l a t e r I  c o l l e c t e d and preserved amphipods from both the t r a p s  and  weed  patches. I  collected  a d d i t i o n a l a c t i v i t y data on b e e t l e s i n 57  searches on 22 n i g h t s between May I wore a h e a d l i g h t and walked  and J u l y  1-h  1981. For each search  systematically  in  a  series  of  t r a n s e c t s . As only a small p o r t i o n of the beach c o u l d be covered in  one  hour,  minimize l o c a l behavior lower  first  were  variation  in  (sitting,  beach)  category  searches  also  for  begun beetle  at  the  same l o c a t i o n to  density.  I  scored  f e e d i n g , mating) and beach p o s i t i o n each  included  sitting  (upper,  beetle  found.  a  b e e t l e s which were moving when  few  The  sex,  behavior  observed. For  each search I recorded day of year,  temperature,  time  10  in  hours  a f t e r sunset,  and amphipod abundance. These v a r i a b l e s  were used i n backwards m u l t i p l e r e g r e s s i o n a n a l y s i s (Draper Smith  1966).  Amphipod abundance was estimated  from the mean of  the number of amphipods caught i n 6 to 10 p i t f a l l the high water  experiments  tested  i f male  b e e t l e s a c t i v e on one night were e q u a l l y l i k e l y the  following  night  and  i n f l u e n c e d emergence on succeeding b e e t l e s were c o l l e c t e d separated  according  group was l e f t with  an  traps  set at  level.  Mark-recapture  burrows  and  in to  two  and  to  i f feeding  female  emerge  on  from  one  night  n i g h t s . On 7 and 19 J u l y , a l l  searches  early  in  the  night,  sex, and d i v i d e d i n t o two groups. One  undisturbed,  and the second  group  was  provided  abundance of amphipods. At the end of the n i g h t , 2-3 h  l a t e r , b e e t l e s were marked according  to  with  enamel  paint  on  the  thorax  treatment group (food, no food) and r e l e a s e d . The  f o l l o w i n g n i g h t , three surveys  were  conducted  to  search  for  marked b e e t l e s .  Behaviour at a patch During  1979  and  1980  I  made o b s e r v a t i o n s  b e e t l e s , g e n e r a l l y f o r 10 min p e r i o d s , observe  beetles  cellophane or  shine  Using all  I  covered  t o reduce l i g h t  of i n d i v i d u a l  f o r a t o t a l of 62  the lamp of the h e a d l i g h t  a stopwatch and coding activities.  I  sheet,  I obtained  e l i m i n a t e d records  were observed f o r periods of l e s s than 5  min,  a  To  with red  i n t e n s i t y . I was c a r e f u l to not  the l i g h t d i r e c t l y on the b e e t l e s d u r i n g  beetle  h.  move  observations. chronology  of  of b e e t l e s which and  of  beetles  11  which  responded  by  movement  towards  my  headlight.  Beetles  s e l e c t e d f o r o b s e r v a t i o n were l o c a t e d near weed patches at In sizes  1980 or  I conducted a s e r i e s of experiments to composition  of  weed  c o n s t r u c t e d patches with the seaweed  patches  test  spaced along HW,  water.  attract  half  Amphipods c o u l d escape  hour  for  6 August,  and 60 cm diameter u s i n g the a  mixture  on  19  of  15 August  garbage  August  mainly  most  from the t r a p s , but b e e t l e s traps  common  patch  type  40 cm  on  the  Egregia  menziesi i  and with N e r e o c y s t i s luetkeana patches on i n diameter. As  a  control,  I c o n s t r u c t e d 40 cm diameter patches from p l a s t i c  bags.  of  seaweed  experiments. Two time.  i n these  Fucus d i s t i c h u s and P h y l l o s p a d i x  I measured the a t t r a c t i v e n e s s species  filled  I t e s t e d patches of 20 cm,  19 August. These patches were 40 cm on  drift  traps were not  s c o u l e r i . I compared F u c u s - P h y l l o s p a d i x with patches  from  I  a t o t a l of f i v e or s i x times i n the f i r s t  p a r t of the n i g h t . On  beach,  beetles.  a l t e r n a t i n g patch t y p e s . A p i t f a l l t r a p was set  c o u l d not. I counted and r e l e a s e d b e e t l e s caught each  which  from the beach. Patches were  beside each patch to c a t c h b e e t l e s . These with  test  characteristics  and P h y l l o s p a d i x c o l l e c t e d  HW.  Similar  p l a c e d along HW.  to  amphipods  s p e c i e s of d r i f t  of  Phyllospadix  in  a  plants  separate were  and  these  series  compared  at  s i z e s of patches of each s p e c i e s were p a i r e d One  f e e d i n g amphipods.  to two hours l a t e r  of a and  I counted the numbers of  12  Laboratory feeding  experiments  Feeding experiments  tested  i f male  and  female  beetles  consumed the same numbers of prey items under s i m i l a r c o n d i t i o n s in  the  laboratory.  individually covered  Twelve  in glass jars  cm  of  each  diameter,  sex were p l a c e d  10  cm  (16-19 mm) amphipods. J a r s  prevent amphipod escape and l e f t  and  amphipods  eaten determined  in  each  were  covered  overnight f o r 20-22 h under  n a t u r a l photoperiod at l a b o r a t o r y temperatures. live  deep)  with a 3 cm l a y e r of damp sand. Each j a r a l s o c o n t a i n e d  2, 4, 6, 8 or 10 l a r g e to  (8  beetles  The  j a r were then counted,  by i n f e r e n c e . There were  two  number  of  and the number  treatments.  group was p r e c o n d i t i o n e d by h o l d i n g f o r three days without  One food.  The other group had been f e d the p r e v i o u s n i g h t . Each b e e t l e was only used once i n each treatment. The rate of search, a, and the time  spent  regression  handling techniques  prey, using  h,  were  Rogers'  estimated (1972)  by n o n - l i n e a r  random  predator  equation ahE-aT E = N[1-e ] where  E  i s the  number of amphipods eaten, N i s the number of  amphipods presented, and T i s t o t a l time and i s s e t to one. Use of t h i s equation enabled comparisons  to be made between male and  female b e e t l e s f o r search r a t e s and h a n d l i n g times. The equation is  similar  for removal  to the d i s c equation  (Holling  of consumed prey from those  1959) but compensates  available.  13  S u r v i v a l and o v i p o s i t i o n  rates  I c o l l e c t e d b e e t l e s f o r these experiments on and  10  May  July  1980  1981 r e s p e c t i v e l y , from T a p a l t o s Beach, about 4 km  from the main f i e l d beetles  4  were  left  site. Prior with  to  beginning  amphipods  the  experiment,  f o r one day to s t a n d a r d i z e  satiation  l e v e l s . I then p l a c e d b e e t l e s i n i n d i v i d u a l j a r s  diameter,  10 cm deep) f i l l e d  to one-half  were kept under n a t u r a l photoperiod  at  (8 cm  with damp sand. B e e t l e s laboratory  temperatures  (16-20°C). I  measured the e f f e c t of temperature and a r e g u l a r feeding  regime on s u r v i v a l r a t e s with f i v e treatments of  each sex per treatment.  (2) food a t 4-d i n t e r v a l s (5)  no  food  p l a c e d three amphipods  at  low  of  eight  These were (1) food at 2-d i n t e r v a l s  (3) food a t 8-d i n t e r v a l s  temperature  removed  and  (4) no  food  (10-12°C). To feed b e e t l e s I  12-15 mm amphipods i n each j a r . The  were  beetles  beetle  following  day  s u r v i v a l was scored. T h i s  procedure was followed f o r 28 days. Beetles deposit their 1970).  I  eggs  singly  i n damp  (Craig  measured the e f f e c t of r e g u l a r feeding on o v i p o s i t i o n  r a t e s with three treatments  of  20  continuous  at  3-d  food  (2)  food  female  beetles  intervals  i n t e r v a l s . Amphipods were r e p l a c e d i n treatment were  sand  each:  (1)  (3) food at 6~d 1, and the  jars  checked f o r eggs on the day f o l l o w i n g feeding i n treatment  2. A f t e r 30  days,  the  remaining  beetles  were  dissected  to  determine the s t a t e of egg development i n the o v a r i e s . Means standard  f o r a l l experimental  e r r o r , except  where  results  indicated.  are  shown with one  Proportions  used  in  14  statistical  t e s t s were f i r s t  All  were  tests  two-tailed  except where i n d i c a t e d . assumptions  transformed by a r c s i n e square r o o t .  of  with a s i g n i f i c a n c e l e v e l of 0.05,  Non-parametric  parametric  tests  were  used  when  t e s t s were not met and i n c l u d e d X , 2  median t e s t , sign t e s t and Spearman rank c o r r e l a t i o n which are d e s c r i b e d i n S i e g e l  (1956). A l l times  coefficient  are  given  as  P a c i f i c D a y l i g h t Time.  Results  Temporal changes i n amphipod a c t i v i t y The peaked  numbers  amphipods  j u s t a f t e r dark on  Abundance on  of  some  then  most  caught  in p i t f a l l  nights  (Fig.  1,  t r a p s at HW solid  line).  d e c l i n e d g r a d u a l l y with a second peak near dawn  nights,  such  as  shown  in  F i g . 1D.  The  peak  was  a s s o c i a t e d with emergence of amphipods from burrows on the upper part of the beach and m i g r a t i o n t o HW, upper  beach  from  HW.  While  at  or with the r e t u r n to the  HW,  amphipods  f e d on d r i f t  seaweed. They sometimes formed l a r g e a g g r e g a t i o n s , e s p e c i a l l y on kelp patches. These feeding amphipods c o u l d not the  pitfall  of  the  night  were  by  moving between patches or r e t u r n i n g to the  line)  were  lower  and  amphipods  caught  in  between the upper beach and  these HW.  at  HHW  (Fig.  l e s s v a r i a b l e than at HW.  amphipods d i d feed on patches of d r i e d most  sampled  t r a p method. Amphipods caught i n t r a p s i n the middle  upper beach. The numbers of amphipods trapped dotted  be  weed  traps  at  were  HHW.  1,  A few  However,  probably moving  15  Figure  1. The mean ±1SE numbers of amphipods caught i n p i t f a l l t r a p s at HW ( s o l i d l i n e ) and HHW (dotted l i n e ) at each hour over the n i g h t , and the number of b e e t l e s observed (open c i r c l e s ) . Beetle data were summed from p i t f a l l t r a p and t r a n s e c t counts. Arrows i n d i c a t e the approximate time of high t i d e . The heavy l i n e gives the p e r i o d of maximum darkness. (A) 6-7 May 1980 (B) 19-20 June 1980 (C) 13-14 J u l y 1980 (D) 1 8 - 1 9 August 1980. Note the s c a l e changes f o r amphipods i n (B) and for b e e t l e s i n (D).  16  18  Amphipod patterns.  activity  The  small  a s s o c i a t e d with Repeated  peaks  observations  in  delayed  the  affected  by  at  i n F i g . 1B  0300  incoming fog and a 1-2°C  night  weather  rise  tide D were  temperature.  occurred  (before about 0100), amphipod a c t i v i t y was  u n t i l a f t e r high t i d e . When the high t i d e occurred  high  tide  intermediate The night  and  ( C r a i g 1973b). When a high t i d e  in the n i g h t , there were a c t i v i t y peaks both the  in  and  i n d i c a t e d that a few amphipods only were  a c t i v e on r a i n y n i g h t s early  was  (Bowers  stage  1964,  in this  Craig  before  later  and  after  1973b). F i g . 1C shows an  transition.  s i z e d i s t r i b u t i o n of a c t i v e  beetles  varied  over  the  and with beach p o s i t i o n ( F i g . 2). J u v e n i l e amphipods were  2 mm i n l e n g t h when r e l e a s e d from the female sexes  could  be  distinguished  over  the p e r i o d  of  pouch,  and  a t 12 mm. J u v e n i l e s were a c t i v e  p r i m a r i l y at dusk and dawn. They were only  brood  trapped  in  low  numbers  the night when b e e t l e s were a c t i v e .  J u v e n i l e s d i d not burrow but remained under weed patches  during  the day ( C r a i g 1971), so that few j u v e n i l e s were trapped  at HHW.  The  juvenile  peak  at  HHW  at  midnight  represented  the brood of a female which was  female f e l l  i n t o the t r a p .  Beetle a c t i v i t y I  ( F i g . 2) released  probably when the  patterns  d i d not  find  active  beetles  each  night u n t i l  amphipod a c t i v i t y had begun ( F i g . 1,  open  the  b e e t l e s and amphipods were  overall  activity  patterns  of  circles).  after  Otherwise  19  F i g u r e 2. The frequency d i s t r i b u t i o n of the s i z e s of amphipods caught i n p i t f a l l t r a p s on 19-20 June 1980 at d i f f e r e n t times of night at HW and HHW. Mean ±1SE s i z e (n) i s given f o r each time.  20  2200 x=4-5±0l(l399)  0000 x = 131 ±0-3 (173)  0200 x= II I + 0 5 (112)  0400 X-II-2+ 0 4 (132)  0545 x=5-2±OI (585)  0  2  4  6  8 10 12 14 16 18 20 22  AMPHIPOD  SIZE  (mm)  21  HHW  02  2200  Jl  OH  S = IO-3±0-8 (60)  0-3 0000  0-21  x = 1074 10 (49)  0 1 l . • i i  0-2 > o UJ  O UJ DC U_  h  0200 x= 1304 0-4 (69)  0|  4=L  02  1  0-  x-135+0 2 (98)  0545  02^ 01  0400  x=IO 44 0 8 (49)  J 0  2  4  6 8  n 1.,.  10 12 14 16 18 20  AMPHIPOD  SIZE (mm)  22  s i m i l a r . Maximum numbers were counted j u s t a f t e r dark,  and  the  number of a c t i v e b e e t l e s decreased g r a d u a l l y over the n i g h t . For the  data  collected  in  1981,  I  found a c o r r e l a t i o n of 0.273  ( F i g . 3, n=57, p<0.05) between the number of b e e t l e s counted the lower p a r t of the beach and  the number of amphipods  Part of the v a r i a t i o n in t h i s r e l a t i o n can for  by  differences  amphipods over the to  total  in  recruitment  summer. I was  patterns  delay  occurred  in  similarly  r e p o r t e d peaks in  a f t e r a high  the  not  middle  of  beetle  accounted  beetles  beetle  when  a  important  high  ( F i g . 1C). C r a i g  activity  both  (1970)  before  from the  1981  female b e e t l e s i n the amount  were counted d u r i n g these in  the  searches  males  on  (Table  the  upper  the  beach and  lower beach. T h i s suggested that female returned  to  during  I).  location  of  50.4±2.0 females There  s i g n i f i c a n t l y more of the  counted on the lower beach. Hence,  longer  and  were  no  mean number of males counted on e i t h e r the  upper or lower beach, but  towards  was  search data, which i n d i c a t e d d i f f e r e n c e s  a c t i v i t y . A mean of 58.8±2.2 males and  differences  and  tide.  between male and surface  For tide  A more d e t a i l e d a n a l y s i s of the b e e t l e a c t i v i t y p a t t e r n obtained  and  counts  amphipod a c t i v i t y .  activity  the night  trapped.  I a l s o found  f i n e s c a l e d i f f e r e n c e s between b e e t l e and did  be  of  unable to s c a l e  s i z e of the b e e t l e p o p u l a t i o n .  example, b e e t l e s  probably  on  sex  females were  ratio  was  skewed  towards females on beetles  fed  and  the then  the upper beach to burrow, while males were a c t i v e the n i g h t . A m u l t i p l e r e g r e s s i o n on the  of males in the upper  beach  counts  included  date  proportion (r=-0.425,  23  Figure  3. The number of b e e t l e s counted on the lower beach in 1981 as a f u n c t i o n of the number of amphipods caught in p i t f a l l traps at the time the b e e t l e count was made (r=0.275, n=57, p<0.05).  24  O O  125  CO LJ  o  HlOOJ  o°  ° o o o o o  LU LU CD  O CD  CD)  5  0  o  o o  o o  6°  o  or  a>  O OP OO O  o  O  25  10  15  20  AMPHIPODS/TRAP  25  30  25  Table I : Time budgets f o r male and female b e e t l e s at d i f f e r e n t beach p o s i t i o n s . Values given are f o r the mean p r o p o r t i o n of time spent s i t t i n g , mating, and f e e d i n g , f o r the mean number of b e e t l e s counted, and f o r the mean p r o p o r t i o n of males in 57 searches. A l l male-female comparisons f o r a given beach p o s i t i o n are s i g n i f i c a n t ( p a i r e d t - t e s t , p < 0 . 0 l ) as are a l l upper-lower beach comparisons on p r o p o r t i o n s f o r males ( p < 0 . 0 5 ) , but not f o r females ( p > 0 . l O ) . For upper-lower beach comparisons on l : o t a l counts, d i f f e r e n c e s are s i g n i f i c a n t f o r females (p<0.0CM), but not  f o r males  (p>0.lO).  upper beach males females  lower beach males females  beach t o t a l males female:  sitting  0.973  0.856  0.934  0.885  0.941  0.865  feeding  0.023  0. 1 33  0.050  0. 103  0.044  0.116  mating  0.004  0.011  0.016  0.012  0.015  0.019  28.6  11.6  30.4  38.7  58.9  50.4  total prop, male  0. 735  n=57,  p=0.00l),  0. 425  temperature  0. 542  (r=0.329, n=57, p=0.0l4) and time  a f t e r sunset (r=0.280, p=0.039) with r =0.230 2  On  the  lower  (r=0.344,  beach,  the r e g r e s s i o n  (n=57,  p=0.003).  included temperature only  n=57, p=0.009). The t o t a l number of b e e t l e s counted on  the lower beach a l s o i n c r e a s e d with temperature (r=0.387,  n=57,  P<0.01). P r o p o r t i o n a t e l y more males were a c t i v e on warmer n i g h t s and  later  i n the n i g h t , and fewer males were a c t i v e r e l a t i v e to  females l a t e r  i n the summer.  Males and females a l s o d i f f e r e d  i n the p r o p o r t i o n of  times  they were observed performing each behaviour. A mean of 11.6% of females  were  observed  feeding  males. Females were observed similar  proportions  at  i n surveys compared  sitting,  feeding  and  to 4.4% of mating  in  e i t h e r beach p o s i t i o n . Males, however,  26  spent s i g n i f i c a n t l y  l e s s time s i t t i n g /  and more time feeding and  mating when on the lower beach. Males on the lower beach fed significantly beach  lower  proportions  than  in  d i d females at the same  position. In an attempt to determine i f a c t i v e b e e t l e s were f o r a g i n g ,  b e e t l e s were p l a c e d i n open amphipods.  Rejection  of  buckets amphipods  on  the  would  beach  suggest that  b e e t l e s were not searching f o r food. There were in  containing  no  active  differences  response of male b e e t l e s from the upper or lower beach, when  b e e t l e s were c o l l e c t e d e a r l y begun.  Forty-five  i n the evening, before feeding  percent (n=20) of males from the upper  and 40% from the lower beach this  experiment was  (n=20) accepted  prey  beach  items.  repeated with males c o l l e c t e d  When  i n the middle  of the n i g h t , a f t e r males d i d have the o p p o r t u n i t y  to  move  the lower beach and feed, 3 3 % (n=12) of males on the upper and  95%  to  beach  (n=20) on the lower beach accepted amphipods (X = 11.3, 2  df=1, p<0.0l). Foraging males were more lower  had  likely  to  be  on  the  beach, although some males on the upper beach d i d feed as  w e l l . I n s u f f i c i e n t numbers of females were found beach on these n i g h t s to perform comparative Mark-recapture  experiments  tested  night,  and  if  upper  if  male to  and  female  emerge  the  feeding a f f e c t e d emergence. In the 7  J u l y experiment, the r e c a p t u r e r a t e was difference  the  tests.  b e e t l e s marked on one night were e q u a l l y l i k e l y following  on  14.4%  (n=208)  with  no  between sexes (X =0.18, df=1, p>0.l0). Twice as many 2  r e c a p t u r e s were b e e t l e s which had not fed but t h i s d i f f e r e n c e was  not s i g n i f i c a n t  the  previous  night,  (X =2.88, df=1, P>0.10). 2  27  For the with  19 J u l y experiment, the recapture  58%  of males and  43% of females- recaptured  p=0.05). Recapture success fed the p r e v i o u s between On  the  night  (X =0.18, 2  overcast  and  rain  13°C  Behaviour at a  and  a  2  had  Differences  be a t t r i b u t e d to weather.  favorable  for  temperature  of  beetle 9°C,  activity  compared to  patch  alternated  of b e e t l e s  between a c t i v e and  at  HW  suggested  than 2 s i n d u r a t i o n . These moves were probably items detected at a d i s t a n c e and (1) forward  that  ambush f o r a g i n g modes. I  c l a s s e d b e e t l e s in ambush mode when they made moves of no  prey  df=1,  on 20 J u l y .  Continuous observations beetles  (n=l95)  (X =3.84,  P>0.10).  df=1,  experiments can probably  impending  50.3%  d i d not depend on whether b e e t l e s  8 J u l y c o n d i t i o n s were l e s s  with  rate was  a  were of three  move - the b e e t l e moved forward  1-3  longer  response  to  types: cm  (2) turn - the b e e t l e changed i t s f a c i n g d i r e c t i o n , u s u a l l y by  180°  (3)  lunge  - the b e e t l e appeared to a t t a c k , although  not observe any  prey  items w i t h i n s t r i k i n g  I did  distance.  Because I c o u l d not d e t e c t whether or not b e e t l e s which remained motionless category  on also  the  sand  included  other a c t i v i t i e s ,  were  in  fact  foraging,  b e e t l e s which may  the  ambush  have been engaged in  such as mate search or d i g e s t i v e pause.  I c l a s s e d b e e t l e s in a c t i v e mode when at l e a s t one move a sequence was than  2  s  in  greater than 2 s in d u r a t i o n and pauses were l e s s  between s u c c e s s i v e moves. Most moves made by  beetles  28  were l e s s than 1 s i n d u r a t i o n but moves ranged up to duration  for  For these  longer moves b e e t l e s g e n e r a l l y maintained  forward  or between weed patches. as  Beetles  moving  between  not always be c o n s i d e r e d Individual  patches,  but  spent  on  they  average  The  rate  active  (Spearman rank c o r r e l a t i o n c o e f f i c i e n t To  also  be  attacked  patches  could  3.2-4.7%  i n t e r v a l , n=362) of the o b s e r v a t i o n  F i g . 5).  constant  as d i s c r e t e u n i t s .  beetles  attack  a  could  Hence,  mode.  P<0.01,  in  or on the sand along  i n a c t i v e mode  amphipods they encountered while moving.  confidence  s  b e e t l e s which I observed i n a c t i v e mode ( F i g . 4 ) .  d i r e c t i o n , e i t h e r i n weed patches,  considered  78  increased  test  in  active  with the p r o p o r t i o n of time  whether  l o c a t e d a good f o r a g i n g s i t e , or  time  (95%  in  =  beetles  0.168,  n=362,  moved u n t i l  response  to  they  unsuccessful  a t t a c k s on amphipods, I compared the p r o p o r t i o n of time spent i n active  mode  by  b e e t l e s before and a t t a c k s on amphipods. There  was a weak tendency f o r b e e t l e s to be more a c t i v e unsuccessful  attack  following  an  (Mann-Whitney U - t e s t , n=248, p=0.08). T h i s  suggested that a c t i v e f o r a g i n g mode was f r e q u e n t l y a response to the d e t e c t i o n of amphipods. The d u r a t i o n of i n d i v i d u a l moves d i d not d i f f e r  before and a f t e r an a t t a c k  Male and female b e e t l e s spent active  (Mann-Whitney  (t=1.l6, df=3350, p>0.10).  similar proportions  of  U - t e s t , n=141,168, P>0.10). However,  27% (n=141) of male b e e t l e s a t t a c k e d amphipods compared (n=168)  of  females  (X =18.6, 2  d i f f e r e n c e between sexes i n resulted  in  capture  time  the  (X =0.06, 2  df=1,  p<0.001).  proportion df=1,  of  p>0.!0).  only  to  52%  There  was no  attacks  which  I  presented  29  F i g u r e 4. The frequency d i s t r i b u t i o n of the d u r a t i o n of i n d i v i d u a l moves made by b e e t l e s observed to a t t a c k amphipods. The mean i s 2.5±0.1 s (n=3389).  30  CD  CVJ CO  oo  < c= r> Q  CD  m  CD  O  csi o  m  — o  g 6  A0N3fl03cJJ  m  o  b  >  O  gure 5. Frequency d i s t r i b u t i o n s of the p r o p o r t i o n of time spent i n a c t i v e mode by b e e t l e s which d i d (white bars, n=149) and d i d not (black bars, n=213) attack amphipods.  0 0  01  0-2  0-3  0-4  0-5  PROPORTION OF TIME  33  amphipods d i r e c t l y to b e e t l e s i n an experiment to differences  in  prey.  differences  in  encounter  rates  Male and female b e e t l e s were c o l l e c t e d e a r l y  i n the  evening before feeding had begun. They were separated to  sex  and  grouped  in  open  buckets  abundance of amphipods. Male and female opportunity  to  attack  prey  on  the  beach with an  beetles  items. However,  according  had  an  2  of  of  the  differences  s i z e s of amphipods  (Fig.  in  activity  patches  1980 the mean s i z e of amphipods was  6.1±0.1  mm  mm  (n=275)  from  12.6±0.2  significant  local  amphipods w i t h i n (X =31.6, 2  (n=388),  the  df=12,  in  pitfall  differences five  for  size  of  10.3±0.8 mm  in  and  the  taken  pitfall  to  There  size  from  patches  weed  a mean of  were  also  d i s t r i b u t i o n s of  from  weed  patches  traps (X =26.8, df=8, 2  amphipods  with  (n=32), compared to 6.5±0.8 mm  b e e t l e s moving over weed  of  i t encounters.  contrast  traps.  samples  p<0.00D  patterns  collected  P<0.001). B e e t l e s observed on sand, captured mean  males  2), the a c t i v i t y and p o s i t i o n  a b e e t l e w i l l determine the s i z e s of amphipods  On 6 J u l y  the  p<0.0l).  (X =15.1, df=1,  Because  equal  63% (n=78) of the  females captured amphipods compared to 30% (n=76) of  different  whether  attack r a t e s between males and females r e s u l t e d  from behaviour d i f f e r e n c e s or with  test  (t=3.23,  df=21,  a  (n=8)  p<0.00l  one-tailed). On  average  amphipods/min attacked.  and  Hence,  beetles captured beetles  attacked 9.1%  (n=440)  0.147±0.019 of  the  (n=362) amphipods  c o u l d expect to wait 6.8 min between  a t t a c k s and 75 min between c a p t u r e s . The frequency  distribution  34  of  attack  from  rates  random  (X =430, percent  observation  df=5,  2  Fifty-nine  expected.  ( F i g . 6) however, was s i g n i f i c a n t l y  of  p<0.00l  beetles  did  to  f i t to  not  attack  Poisson). during  the  p e r i o d , and more b e e t l e s had high attack r a t e s This  suggested  quality  than  that most f o r a g i n g s i t e s were of low  q u a l i t y . E i t h e r b e e t l e s were r e l a t i v e l y high  for  different  unsuccessful  in  finding  s i t e s , or they d i d not d e t e c t , or d i d not respond  amphipods at a s i t e . F u r t h e r , o v e r a l l amphipod abundance d i d  not  i n f l u e n c e f o r a g i n g success.  e i t h e r the number or during  surveys  in  I found no r e l a t i o n s h i p between  proportion 1981  of  beetles  observed  feeding  and mean amphipod abundance (Spearman  rank c o r r e l a t i o n c o e f f i c i e n t  = -0.009 and  -0.087  respectively,  n=57, p>0.10). Beetles  were  not a t t r a c t e d to f o r a g i n g s i t e s on the b a s i s  of .patch s i z e . There were no d i f f e r e n c e s i n the t o t a l number beetles  caught  diameter  (Table I I ) . However, s p e c i e s composition  influenced  the  in  pitfall  number  t r a p s by patches of 20, 40 or 60 cm  of  b e e t l e s trapped.  b e e t l e s were caught i n t r a p s near Egregia near  Fucus-Phyllospadix  a c o n t r o l f o r a patch Beetles  were  significantly patches.  This  caught  result  and  so  were  did  traps  or  not  near  these  near  suggests  that  beetles  N e r e o c y s t i s patches than near  Nereocystis  attract  than  but  more l i k e l y  the  patch  S i g n i f i c a n t l y more  caught  types,  of  than  I used p l a s t i c garbage bags as  which in  fewer were  a t t r a c t e d to a l l patch near,  patches.  type  of  that  amphipods.  patches,  but  Fucus-Phyllospadix  they  t o be trapped  Fucus-Phyllospadix  were  initially  remained  longer  near E g r e g i a and patches,  and  35  F i g u r e 6. The frequency d i s t r i b u t i o n of the number of amphipods attacked/min by b e e t l e s d u r i n g o b s e r v a t i o n s i n 1979 and 1980 (n=362). Dotted l i n e s give p r e d i c t e d values f o r a Poison d i s t r i b u t i o n with the same mean (0.147 amphipods/min).  6 CO  6  I co o  rrO Q-  6 Q-  E o  O LU  <  cr ro  6 6  6  co  6  6  ro O  O  o  A0N3H03dJ  37  Table I I : The numbers of b e e t l e s caught i n p i t f a l l t r a p s l o c a t e d near patches of d i f f e r e n t s i z e or s p e c i e s composition, and sample s i z e s f o r each type (F-P, F u c u s - P h y l l o s p a d i x ; E, E g r e g i a ; GB, garbage bag; N, N e r e o c y s t i s ) . P r o b a b i l i t y l e v e l s are f o r ANOVA. Date  n  No.  patch s i z e  6 Aug.  10  of b e e t l e s counted  comparisons 20 cm  40 cm  60 cm  6.1±1.6  5.6±0.8  4.7±1.1  patch composition  comparisons F-P  15 Aug.  12  E  8.8±0.9 GB  19 Aug.  12  p>0.l0  4.1±0.5  12.2±1.1  F-P  N  6.6±0.8  10.3±1.0  p<0.05  p<0.000l  near F u c u s - P h y l l o s p a d i x patches than near garbage Patches  near  a t t r a c t i v e to amphipods  which  amphipods.  were  counted  bags.  I trapped more b e e t l e s were a l s o more In on  the  amphipod  experiments,  N e r e o c y s t i s than on E g r e g i a (n=24,  P<0.001), on N e r e o c y s t i s than on P h y l l o s p a d i x  (n=22,  on  p<0.05),  Egregia  than  on on  Phyllospadix Fucus  more  Phyllospadix  than  (n=16,  matched-pairs  signed-ranks t e s t ,  (n=14, p<0.005,  one-tailed).  all  p<0.00l), and  on  Wilcoxon  38  Laboratory  feeding experiments  Laboratory  feeding  differences in amphipod  the  experiments  number  densities  for  of  tested  amphipods  if  eaten  there  at  were  different  male and female b e e t l e s which had been  s t a r v e d or f e d . The number of amphipods eaten by s t a r v e d b e e t l e s increased with  the  number  of  amphipods  presented  ( F i g . 7).  B e e t l e s which had fed the night p r i o r to the experiment consumed fewer  amphipods  than those which had been s t a r v e d . There was a  weak tendency f o r fed females to consume more amphipods than fed males, but t h i s trend was not c o n s i s t e n t at a l l d e n s i t i e s . There were no d i f f e r e n c e s between male and female the  estimates  treatment  of  attack  rate  or  beetles  handling  either  time  in  for either  p>0.10).  ( t - t e s t , df=1l7,  S u r v i v a l and o v i p o s i t i o n r a t e s In l a b o r a t o r y experiments b e e t l e s s u r v i v e d at l e a s t 8  days  without food at l a b o r a t o r y temperatures, and at l e a s t 24 days at the lower temperature which more c l o s e l y approximated c o n d i t i o n s on  the beach  2- or 4-d no  ( F i g . 8). Only one of the b e e t l e s which was  i n t e r v a l s d i e d during  differences  i n percent  the experiment, and  and  there  were  weight l o s s of b e e t l e s between these  two treatments (Mann-Whitney U - t e s t , n=16,15, percent  fed at  P>0.10).  Fifty-six  of the b e e t l e s fed at 8-d i n t e r v a l s d i e d between days  10  28. A  total  oviposition  of  nine  experiment.  beetles  died  Oviposition  or rates  were were  lost low  in  the  f o r the  39  F i g u r e 7. The mean ±1SE number of amphipods eaten at each amphipod d e n s i t y f o r s t a r v e d (A) and fed (B) b e e t l e s d u r i n g a 20-22 h p e r i o d . As d i f f e r e n c e s between male and female b e e t l e s were not s i g n i f i c a n t , male and female data were combined f o r curve f i t t i n g . The equations are (A) E = N(1 - exp(0.447E - 1.860)) (B) E = N(1 - exp(0.253E - 1.072))  40  41  F i g u r e 8. S u r v i v o r s h i p curves for b e e t l e s a c c o r d i n g to food treatment: (a) food at 2-d i n t e r v a l s (b) food at 4-d i n t e r v a l s (c) food at 8-d i n t e r v a l s (d) no food (e) no food at low temperature (10-12°C).  43  remaining  beetles  (Table I I I ) . When I d i s s e c t e d b e e t l e s I  found  Table I I I . R e s u l t s of the o v i p o s i t i o n experiment. The t a b l e gives the number of b e e t l e s , the p r o p o r t i o n which l a i d eggs, the p r o p o r t i o n with at l e a s t one mature egg i n t h e i r o v a r i e s , the t o t a l number of eggs l a i d , and the mean egg dry weight f o r each treatment: ( 1 ) continuous food (2) food a t 3-d i n t e r v a l s (3) food at 6-d i n t e r v a l s . Treatment 2 n prop, l a i d  eggs  prop, eggs i n ovar. no.  eggs  that  60%  15  20  0.33  0.35  0.19  0.60  0.60  0.06  12  mean egg dry weight (mg)  of  3  6.2±0.2  16  18  3  5.3±0.2  5.0±0  the b e e t l e s which were f e d c o n t i n u o u s l y  i n t e r v a l s , and one of the b e e t l e s f e d at  6-d  or at 3-d  intervals  had  a  mature egg i n the common o v i d u c t . T h i s suggested that c o n d i t i o n s in  the j a r s were not favourable  for oviposition.  were no d i f f e r e n c e s i n the p r o p o r t i o n of  beetles  However, there which  either  l a i d eggs, or had at l e a s t one egg i n t h e i r o v a r i e s , f o r b e e t l e s fed  continuously  or a t 3-d i n t e r v a l s . I combined these data t o  t e s t f o r the e f f e c t s of feeding at 6-d i n t e r v a l s . There difference  in  the  proportion  of  beetles  which  was  laid  no eggs  (X =0.64, df=1, p>0.10), but fewer b e e t l e s which were fed at 6-d 2  i n t e r v a l s had at l e a s t one mature egg i n t h e i r o v a r i e s df=1,  p<0.00l).  (X =20.4, 2  44  Beetles  which  significantly intervals between  were  greater  dry  (t=3.03, df=28, beetle  egg dry weight  size  continuously  weight  than  p<0.0l).  laid  beetles  There  was  (measured as head capsule  (r=-0.023, n=15,  the number of eggs l a i d mean  fed  of  at  3-d  fed  no  correlation  width) and mean  p>0.10), between b e e t l e s i z e  (r=-0.295,  egg dry weight and  eggs  n=15,  p>0.1u),  the number of eggs l a i d  or  and  between  (r=0.054, n=l5,  p>0.10).  Discussion  When where and  how  to  forage  Although the sand beach amphipod  abundance  habitat  is  structurally  simple,  v a r i e d over a range of temporal and  spatial  s c a l e s . P a t t e r n s of abundance c o u l d l a r g e l y be accounted f o r weather types  and of  occurred there  tide  weed  factors,  patches.  and  In  the d i s t r i b u t i o n of  general,  a  between the timing of b e e t l e and  were  important  discrepancies.  good  assess  or  "know"  prey  h a b i t a t average. Foraging  One  correspondence  to apply an  optimal  to assume that the predator  abundance both at a patch, and b e e t l e s c o u l d encounter very  n i g h t s . T h i s must complicate  but  B e e t l e abundance d i d not  numbers of amphipods at the same time and at consecutive  different  amphipod a c t i v i t y ,  p e r f e c t l y t r a c k amphipod abundance. In order f o r a g i n g model, i t i s necessary  by  any  a d d i t i o n a l feature of t h i s system was  similar assessment  can  f o r the  different sites  on  process.  that the l o c a t i o n and  45  q u a l i t y of patches changed each n i g h t . There was  no b e n e f i t to a  b e e t l e gained by remembering the time or l o c a t i o n of  a  success  on the p r e v i o u s n i g h t . If  b e e t l e s and amphipods use d i f f e r e n t cues to i n i t i a t e or  maintain s u r f a c e a c t i v i t y , b e e t l e s may night-to-night  differences  Alternatively,  in  be unable  to  respond  the times of amphipod  probability  mode may prey  of  eventual  be s u f f i c i e n t l y  capture  activity.  i t i s p o s s i b l e that b e e t l e s need to forage d u r i n g  p e r i o d s of both high and low amphipod a v a i l a b i l i t y , the  outweighs  to  increase  prey c a p t u r e . Foraging i n ambush  inexpensive any  that  energetic  the  probability  by waves. T h i s may have  be an important m o r t a l i t y well-developed  unable to e l i c i t  any response  in  experiments.  preliminary  responses.  from Thinopinus to may  moved  to  at  least  20 m below HW.  (1973a) showed that Thinopinus, Alloniscus  perconvexus  I  seaweed  was odors  be using v i s u a l  such as s i l h o u e t t e s to f i n d patches, as they were patches  away  factor.  chemical  Beetles  Orchestoidea  cues  attracted  In a d d i t i o n ,  and  a l l moved up-slope when exposed to wet simply move up  or down beach a c c o r d i n g to the slope of the beach and degree of  to  Craig  californiana  slopes of 5° i n the l a b o r a t o r y . These animals may  wetness  of  c o s t . However, b e e t l e s  f o r a g i n g before high t i d e were o c c a s i o n a l l y h i t and washed  Insects  to  the sand u n t i l  they encounter  of  some o b j e c t such as a  weed patch. None of these mechanisms f o r f i n d i n g patches provide i n f o r m a t i o n on amphipod abundance. B e e t l e s a r r i v e d at a l l of patches  i n my  experiments,  types  i n c l u d i n g garbage bags, whether or  not these patches were a t t r a c t i v e to amphipods. In g e n e r a l , cues  46  used on  by i n s e c t s to f i n d relative  Southwood  prey  availability  1978).  Selection  patch-to-patch u n t i l a located, 1964)  has  food patches provide l i t t l e i n f o r m a t i o n at  of  site  ambush  with  been suggested  the  more  their a b i l i t y  prey  f o r web-building  availability  is  spiders (Turnbull  1979). also  to amphipods, b e e t l e s were probably l i m i t e d i n  to assess amphipod abundance once at a patch.  frequency  tended  of amphipod  abundance.  arrival  an  at  and  I counted more b e e t l e s at patches which were  attractive  attack  (Hassell  s i t e s by movement from  high  and d a m s e l f l y nymphs (Crowley Although  patch  area  to be too low to a c t as a u s e f u l For  example,  one  attack  soon  The index after  of low amphipod abundance c o u l d lead to a  spurious impression of high amphipod abundance. Scorpions detect prey through Thinopinus  substrate vibrations may  use  However,  are not moving, may available  to  a  and  Farley  1979).  a s i m i l a r mechanism. Short moves and turns-  made by b e e t l e s suggested distance.  (Brownell  responses to amphipods d e t e c t e d  at  a  the number of amphipods on a patch, i f they not  be  a  good  indicator  of  the  number  b e e t l e in ambush mode. Amphipod abundance a l s o  changed over the night independently of b e e t l e a c t i v i t y . I counted more b e e t l e s on the lower beach on warmer n i g h t s . Mark-recapture beetles  emerged  temperatures. temperature probably  experiments from  Polis  suggested  burrows  (1980)  found  on  that t h i s was  because  more  s u c c e s s i v e n i g h t s at higher a  similar  relation  between  and s u r f a c e a c t i v i t y of desert s c o r p i o n s . Thinopinus  required  more food at higher temperatures  during s t a r v a t i o n was  enhanced at low temperatures  as  survival  in laboratory  47  experiments. For example, d a s m s e l f l y mites (Everson at  higher  1980)  larvae  (Thompson 1978a) and  i n c r e a s e t h e i r attack r a t e s on  temperatures.  Temperature probably  prey  items  affects activity  i n d i r e c t l y through i t s a f f e c t on hunger. Hunger has  been shown to l e a d to increased a c t i v i t y  number of d i f f e r e n t Calow  1974).  types of predators  Akre  and  Johnson  suggested that hungry damselfly active  f o r a g i n g modes at low  true f o r Thinopinus,  (Beukema 1968,  (1979)  and  nymphs s h i f t e d  from  as the p r o p o r t i o n of time spent  depleting  abundance  Charnov e_t a_l. (1976) have  of  at  termed  the  in  i n prey  anurans  and  attack.  that one  i s what  depression.  Other  could p o t e n t i a l l y benefit  activity. species  captured  more prey per  unit  a l s o true  B e e t l e s which were a c t i v e on seaweed encountered  proportion  of j u v e n i l e amphipods, and  captured  smaller  on average, than b e e t l e s in ambush mode. One  beetle I  observed by a kelp patch amphipods  disturbed  This  time than species which were ambush f o r a g e r s . T h i s was for Thinopinus.  As  found that species which were a c t i v e  f o r a g e r s took smaller prey items and  amphipods  to  active  (1980) s t u d i e d f o r a g i n g behaviours of s e v e r a l  tropical  a higher  (1979)  ambush  seaweed  patch.  resource  i n ambush mode near the patch  from the r e s u l t i n g increase Toft  1971,  f o r a g i n g mode d i d have a s s o c i a t e d disadvantages.  amphipods,  a  prey d e n s i t i e s . T h i s c o u l d a l s o be  w e l l as the e n e r g e t i c c o s t , a c t i v e b e e t l e s on  beetles  Griim  Crowley  mode tended to increase f o l l o w i n g an u n s u c c e s f u l Active  for  in  a 15 min  l a r g e amphipod  r a p i d l y captured observation (>15  mm)  and  consumed two  p e r i o d . In chapter  is sufficient  to  5  mm  3, I show  satiate  most  48  beetles,  while  up  to  six  juvenile  amphipods  are r e q u i r e d .  Feeding on small amphipods c o u l d lead to a s i g n i f i c a n t  increase  in  amphipod  the  total  time  required  for  abundance or in areas of mostly  Sex  foraging  at  low  juveniles.  differences Sex d i f f e r e n c e s in a c t i v i t y , a t t a c k  experiments  and  mark-recapture  rates,  experiments,  beach  feeding  indicated  either  (1) male b e e t l e s fed l e s s on the beach or (2)  spent  a g r e a t e r p r o p o r t i o n of t h e i r non-foraging time a c t i v e on  the sand s u r f a c e , or (3) both. I d i d feeding  rates  of  male  and  not  female  l a b o r a t o r y experiments to suggest that food  requirement.  However, t h e r e may  find  differences  beetles females  that  that  in  they  in  short-term  had  a  greater  be long-term d i f f e r e n c e s .  Females which were fed c o n t i n u o u s l y l a i d h e a v i e r eggs than those females fed at 3-d egg  weight  conditions  is  intervals  related  (Table I I I ) . For the cinnabar moth,  to  hatching  success  under  adverse  (Richards and Myers 1980). A s i m i l a r r e l a t i o n s h i p i n  Thinopinus may  have  selected  for  more  frequent  feeding  in  females. As w e l l , male b e e t l e s must spend more of t h e i r non-foraging time  active,  and  they p r o b a b l y devote t h i s e x t r a time to mate  search. I observed mating throughout the summer. The problem mate  search  for  male  beetles  is  similar  s e a r c h i n g f o r amphipods (Parker and S t u a r t attempt If  of  to the problem of  1976), and  they  may  to perform both behaviours s i m u l t a n e o u s l y . males  are s e a r c h i n g f o r food and/or mates, the reasons  49  for the s p a t i a l d i f f e r e n c e s between male and  female a c t i v i t y  not obvious,  beetles  abundant  as both prey  on  the  items and  lower  burrows, so I may  beach.  female  Mating  may  were  are more  sometimes occur  have underestimated mating success  of males on  the upper beach. Even i f the p r o b a b i l i t y of o b t a i n i n g a mate the upper beach i s lower, s i t t i n g on the upper beach had advantages.  It  was  energetically  on  several  cheap, compared to a r e t u r n  t r i p of about 50 m to the lower beach. Females emerged from r e t u r n to burrows in the same area. The female  in  (Schlager  a  night  1960,  Parker  R e l a t i o n to  theory  To  casual  a  probably 1970,  in  and  l a s t male to mate with a  fertilizes  eggs l a i d that night  Smith 1979).  observer,  amphipods  might  appear  as  an  u n l i m i t e d food resource. C e r t a i n l y , some b e e t l e s were s u c c e s s f u l i n f i n d i n g patches of high amphipod abundance. I have shown t h i s not  to  be  true  in  general.  l o c a t i o n s of low  amphipod  capture  were  success  were  obviously  foraging  site,  confounded  by  n i g h t s . As one  the  low,  Mean  and  in prey capture  limited and  abundance.  also  frequently unsuccessful  B e e t l e s foraged at times and  beetles  assessment  design  system. Any  a  process  may  of  probably Beetles  have  v a r i a b i l i t y which could e x i s t on  critical  test  were  and  to assess q u a l i t y of a been  successive  assumption of the o p t i m a l i t y approach i s  that f o r a g e r s can assess q u a l i t y of a patch, to  rates  during a n i g h t .  in their a b i l i t y  any  attack  in  an  optimal  i t i s not  possible  patch c h o i c e model in t h i s  d e v i a t i o n between the observed and  predicted  values  50  could  be  accounted  f o r by e i t h e r sampling of the b e e t l e or  lack of f i t of the model. Morse and  Fritz  conclusions  spiders  found  in t h e i r study of crab  spiders  s p i d e r s d i d not were  provided.  flowers  at  good  (1982) reached s i m i l a r on  milkweed.  or stems had  claimed little  that  better  information  a v a i l a b l e to them  new  other  site.  second c r i t i c a l assumption of the o p t i m a l i t y approach i s  not met made  sites  s p i d e r s which moved to  than the number of i n s e c t a r r i v a l s at t h e i r p r e v i o u s A  They  s i t e s more f r e q u e n t l y than random, but  immediately leave poor s i t e s when They  by  with Thinopinus. In most f o r a g i n g  that  foraging  success  s t u d i e s the  does a f f e c t short-term  claim  is  fitness. I  found no d i f f e r e n c e s in s u r v i v a l , weight changes, or o v i p o s i t i o n r a t e s f o r b e e t l e s fed i n the  l a b o r a t o r y at up t o  over a 28-30 d p e r i o d . In g e n e r a l ,  low  foraging  4-d  intervals  success on a  few  nights  appears u n l i k e l y to a f f e c t o v i p o s i t i o n or s u r v i v a l r a t e s  in t h i s  beetle.  Hanski cow  pats  (1980) found that movements of dung b e e t l e s could  be  accounted  for  more  s t o c h a s t i c model than by a model based on energy specific likely  intake.  Mechanistic  systems and to  be  or  closely  by a simple  maximization  stochastic  which i n c o r p o r a t e  between  models,  of based  sensory i n f o r m a t i o n ,  u s e f u l in f u t u r e as t o o l s to p r e d i c t i n g  behaviours. They should  at  least  be  considered  as  mechanistic  on are  foraging alternate  hypotheses. In the next chapter I compare p r e d i c t i o n s of d i e t and  net  models i n prey choice of Thinopinus .  optimal  51  CHAPTER 3: PREY SELECTION  Introduct ion There  i s abundant evidence that animals d i s c r i m i n a t e among  the range of p o t e n t i a l prey types, or s i z e s of a given prey a v a i l a b l e t o them (e.g. see Pyke et a_l. between  the  availability (Eggers  distribution  of  i n the environment  1977).  Selection  c h o i c e by the predator differential  prey  may  result  pictus  of prey types study  Leconte  Any  in  difference  the  diet  prey  and  selection  d i r e c t l y through  (e.g. Zach 1978) or  vulnerability  Thinopinus  types  i s a measure of  In t h i s chapter I d e s c r i b e a f i e l d beetle  1977).  type  active  indirectly  through  (e.g. Pastorok  1981).  of  predation  by  the  ( S t a p h y l i n i d a e ) on d i f f e r e n t  s i z e c l a s s e s of amphipods Orchestoidea c a l i f o r n i a n a  (Brandt).  also  on  present  some  data  A l l o n i s c u s perconvexus certain selection  sizes  or  results  for beetle  Dana. I t e s t  types  of  prey,  predation  I  isopods,  f i r s t whether b e e t l e s s e l e c t and  second  whether  this  from d i f f e r e n t i a l v u l n e r a b i l i t y of d i f f e r e n t  prey types or from a c t i v e c h o i c e . T h e o r e t i c a l attempts two  general c l a s s e s :  to p r e d i c t d i e t  selection  (1) frequency-dependent  fall  into  models (2) optimal  d i e t models. Frequency-dependent models (Murdoch and Oaten 1975, Greenwood and E l t o n 1979) are based on the hypothesis predators  feed  disproportionately  on  the  that  most abundant prey  items. Abundant items w i l l be over-represented i n the d i e t , r a r e items w i l l be under-represented Optimal  diet  models  the  and  r e l a t i v e to a v a i l a b i l i t y .  (Pyke et a l . 1977, Krebs  1978) f i r s t  52  assume that prey items can be ranked a c c o r d i n g to of  their  profitability,  such  The  optimal  diet  in  i n v e s t i g a t o r , such as the net r a t e of energy that  (1)  low  value  p r o f i t a b l e prey are r a r e  prey  will  be  w i l l always  be eaten and  encountered  ( P u l l i a m 1974, Charnov 1976a).  changing  tests  conditions  availability  and  Thinopinus because in  of  rank  chosen  by  the  intake. Predictions eaten only when more  (2) as the abundance of p r o f i t a b l e prey  i n c r e a s e s , predators become more s p e c i a l i z e d  Field  their  c o n s i s t s of the subset of prey types  which r e s u l t s i n o p t i m i z a t i o n of some c r i t e r i o n  are  measure  as the r a t i o of energy v a l u e t o  h a n d l i n g time. Prey types are added t o the d i e t order.  some  unprofitable  these and  diet.  models  chose  prey  never  eaten  are o f t e n d i f f i c u l t  problems I  (3) p r o f i t a b l e prey  i n measurement  when  due to  of  prey  t o measure prey s e l e c t i o n i n  I c o u l d observe t h i s b e e t l e feeding  directly  i t s n a t u r a l h a b i t a t , a sand beach. The aims of t h i s chapter  are (1) t o develop a simple mechanistic model of prey based  on  differential  optimal d i e t Thinopinus  model behaviour  prey  vulnerability  based  on  in  the  active field  choice with  m e c h a n i s t i c , frequency-dependent  and optimal  mechanistic  null  selection  model  provides  i n g e n e r a l , and the  a  (2)  to develop an  (3)  to  compare  predictions  of the  diet  hypothesis  frequency-dependent  d i e t models i n p a r t i c u l a r can be t e s t e d .  selection  models.  The  a g a i n s t which and  optimal  53  Models The  mechanistic  of n s i z e s of a determined active  by  prey an  model p r e d i c t s  the frequency  type  diet  in  observer.  the  of  predator,  as  For a predator which does not  use  c h o i c e , the p r o p o r t i o n of prey items of  in the d i e t  a  distribution  size i  expected  i s given by p(i) =  v ( i ) f (i) Ev(i)f(i) i=1  where  v(i)  i s the v u l n e r a b i l i t y of s i z e i prey and f ( i ) i s the  r e l a t i v e frequency with which s i z e p r e d a t o r . The  term  i prey are encountered  i n the denominator ensures  by the  that  n Ep(i)=1 i=1 The v u l n e r a b i l i t y of s i z e  i prey can be determined  product of c ( i ) , the p r o b a b i l i t y given prey  that  it  is  that  detected, and  a size  i prey i s  the p r o b a b i l i t y  that  from the captured a size i  i s detected. The  probability  proportional  that  a  size i  to the area of r e a c t i o n  prey  is  detected  is  of a p r e d a t o r . I t i s given  by Kd(i)  2  where K i s a constant of p r o p o r t i o n a l i t y shape  of  the  maximum r e a c t i o n in f r o n t  reactive  depends  on  the  f i e l d of the p r e d a t o r , and d ( i ) i s the  distance for a size  of the predator  which  (Holling  v(i) =  i prey,  1966). Hence  Kd(i) c(i) 2  measured  directly  54  Because of the manner i n which I c o l l e c t e d data, I must add another  term  to  the  model.  This  i s the p r o b a b i l i t y that an  observer w i l l  score a feeding event on  proportional  to  h ( i ) , the  a  size i  prey  and i s  h a n d l i n g time, or the time between  prey capture and completion of f e e d i n g . Then p(i)'  =  c(i)d(i) f(i)h(i) 2  Ec(i)d(i) f(i)h(i) i= 1 2  i s the p r o p o r t i o n of prey items of s i z e i I expect to observe i n the d i e t of a predator which does not use a c t i v e c h o i c e . A suitable  index f o r measuring  d e v i a t i o n s from the expected  p r o p o r t i o n s i s the s t a n d a r d i z e d forage r a t i o Let r ( i ) be the observed  in  actual  the  proportion  diet.  of  (c) (Chesson  prey  items  1978).  of  size i  Then p r e f e r e n c e f o r s i z e i prey can be  expressed as o =  r(i)/p(i)'  I[r(i)/p(i)' ] i=1 T h i s index has the advantage i s independent  that  i t v a r i e s between 0 and 1 and  of prey a v a i l a b i l i t y  (Paloheimo  1979).  The optimal d i e t model I develop here i s based on the t o t a l foraging  time  T ( r ) r e q u i r e d f o r a predator to reach  when the d i e t may i n c l u d e prey with s i z e s of rank only.  I  define  j  1  satiation, through  as the rank of a s i z e i prey item, based on  some measure of the value of the prey item to the p r e d a t o r . optimal  diet  is  rationale for this  r  the  set  approach  of  sizes  will  be  which  minimize  presented  The  T(r).A  later.  T(r)  i n c l u d e s both s e a r c h i n g and h a n d l i n g times. I f prey are randomly  55  distributed,  the expected  search time  f o r each prey item can be  d e r i v e d from the mean of the e x p o n e n t i a l d i s t r i b u t i o n with  rate  parameter X given by X=R-A-C Here  R i s the encounter  r a t e , measured i n prey items/time. A i s  the p r o b a b i l i t y that the encountered that  i t has a s i z e of rank  prey i s a t t a c k e d , that i s ,  j to be i n c l u d e d i n the optimal d i e t ,  so  A  r Ed(j) f(j)  =  2  J^J  Ed(j) f(j) j=1 2  C i s the p r o b a b i l i t y that, the encountered given  that  i t has a s i z e of rank  prey item i s captured,  j to be i n c l u d e d i n the d i e t ,  so  C  r Ec(j)d(j) f(j)  =  2  2zl Ed(j) f(j) J =1 2  The is  term  i n the denominator ensures that the maximum value of  1 i f c(i)=1 If  a  C  for a l l i .  mean of N items i s eaten and the t o t a l h a n d l i n g time  i s H, then T(r)  =  N/[R-A-C] + H  56  M a t e r i a l s and  Field  data I collected field  and  Methods  July  1981.  data  b e t w e e n May  and  August  D u r i n g t h e s e months r e g u l a r (see a l s o page 9 ) .  headlight  walked s y s t e m a t i c a l l y i n a s e r i e s of  scored (for  each  time of n i g h t , beach p o s i t i o n , amphipods),  beetle  sex,  and  f o u n d f e e d i n g . Amphipod l e n g t h was the head t h r o u g h the t h i r d curvature  search  prey  beetle  type,  length  I  beetles wore  prey  beetle  a n t e r i o r of for  body  a c h e c k on measurement e r r o r of  lengths  o f p a r t i a l l y consumed a m p h i p o d s e s t i m a t e d  on  I  20-22  laboratory  fed  mm  compared l e n g t h s was  amphipods  to  measured b e f o r e  beetles and  I  estimated  the  high  (8.5  cm  third  water  diameter, full  of  a  were s e t one  feeding.  length,  level. Pitfall 11 cm  deep) set  seawater  10 p i t f a l l  traps  i n t o the  (Craig  s a n d and  ! 9 7 3 a , b ) . The  once they f e l l  sitting  and  There  with  a  motionless  n i g h t each month. In  number  placed  t r a p s c o n s i s t e d of p l a s t i c  i n t o the  on  1981,  t h e p e r i o d when s e a r c h e s were c o n d u c t e d .  the  traps  were  one  prevented  traps. This  sand. In  at cups  filled  water  r e l a t i v e a b u n d a n c e of a c t i v e a m p h i p o d s  beetle  beach,  a m p h i p o d a b u n d a n c e f r o m t h e mean o f t h e  a n i m a l s from escaping sampled the  amphipod  the  the  (n=15).  of a m p h i p o d s c a u g h t i n 1 h i n 6 t o  pass  a f t e r beetle  a weak t e n d e n c y t o u n d e r e s t i m a t e  mean d i f f e r e n c e of -0.8±0.6 mm  i n the  a  length  f o r each  measured from the  May  transects. I  a b d o m i n a l segment, a l l o w i n g  ( B o w e r s 1 9 6 3 ) . As  and  searches for  were c o n d u c t e d and  For  1980  method  which  would  1980,  traps  set  during  57  To  measure  the s i z e s of amphipods a v a i l a b l e to b e e t l e s , I  c o l l e c t e d amphipods from two p i t f a l l t r a p s month.  These  traps  were  left  on  one  night  each  throughout the p e r i o d of b e e t l e  s u r f a c e a c t i v i t y . Samples from the two t r a p s were  combined  and  preserved  i n 5% f o r m a l i n . I grouped the amphipods i n t o 4 mm  size  classes:  (1)  4-7  mm  (2) 8-11 mm  (3) 12-15 mm  (4) 16-19 mm <5)  20-22 mm. J u v e n i l e s were 2 mm i n length when f i r s t the  female brood chamber.  smallest  However,  4  mm  released  amphipods  were  from the  I c o u l d observe when caught by b e e t l e s .  Vulnerability I measured the r e a c t i o n d i s t a n c e of b e e t l e s t o amphipods of different in  s i z e s , and the capture success f o r detected  l a b o r a t o r y experiments. To s t a n d a r d i z e  held  f o r hunger  b e e t l e s at l a b o r a t o r y temperatures without food  three  days  conducted  prior at  to  night  these  in  experiments.  To blocks thread  measure  levels,  f o r two to  Experiments  were  lights  were  covered  with  intensity.  reaction  distance  I formed a g r i d of 0.6 cm  i n the sand. A l i v e amphipod of known s i z e was t i e d on and  dragged  I  buckets or t r a y s c o n t a i n i n g a l a y e r of  damp sand. The overhead f l u o r e s c e n t f i l t e r s to reduce l i g h t  amphipods  in a l i n e perpendicular  2  a  to the head of the  b e e t l e at approximately the walking speed of the amphipod. Three presentations beetle  at  at a given d i s t a n c e were made, moving towards  one block  the  i n t e r v a l s u n t i l a response was obtained. I  d e f i n e d r e a c t i o n d i s t a n c e as the maximum  number  of  blocks  at  58  which  the  beetle  Each b e e t l e was  responded  by movement towards the amphipod.  t e s t e d once only f o r each amphipod  For capture  success  experiments,  one  beetle  amphipods of a given s i z e were p l a c e d i n buckets. beetle  size. and  a  few  I observed  the  and counted the number of a t t a c k s r e q u i r e d to capture  an  amphipod up to a maximum of 10 a t t a c k s . I d e f i n e d an a t t a c k as a forward  lunge  amphipod.  by the b e e t l e which r e s u l t e d in c o n t a c t  This  was  a  conservative  include misdirected,lunges. successful  capture  captures/attack  from  I the  definition  determined product  the  success w i t h i n  only,  successive manually  between  Beetle size capsule  by  each  captures.  head  frequency  of  (b) the frequency  of  size  capsule,  beetle.  removed  B e e t l e s i z e was  estimated  from the  measured  vernier  with  (W)  was  calipers.  to 5.6  mm.  a more c o n s i s t e n t measure of s i z e  Head than by  equations + 0.23L f o r males (p<0.00l, n=40)  W =  + 0.13L  1.57  f o r females (p<0.00l, n=40)  c o n t r o l f o r the use of overhead l i g h t i n g ,  the experiments  with  blinded  beetles,  whose  covered  with enamel p a i n t . B e e t l e s were permitted  a  hours  few  mm  were  W = -0.04  a  16-19  (L) used in beach measurements. They are r e l a t e d  the f o l l o w i n g  As  for  Amphipods  in these experiments ranged from 3.9  width  body length  of  by r e c o r d i n g the number of captures made in 20  attacks  width of the  probability  10 a t t a c k s .  I t e s t e d for the importance of b e e t l e amphipods  the  as i t d i d not  of (a) the  f o r s u c c e s s f u l b e e t l e s , and  with  before  these  eyes  I  repeated had  been  to recover for  t r i a l s began. To determine  capture  59  success, of  I used 12-15 mm amphipods only and recorded  successful  captures  made  the  number  by each b e e t l e i n 20 a t t a c k s . To  determine r e a c t i o n d i s t a n c e , I used 16-19 mm amphipods o n l y , and followed the procedure d e s c r i b e d above.  Feeding  experiments  I measured feeding r a t e s f o r b e e t l e s covered layer  of  damp  sand  and  b e e t l e s t o the nearest about  one  presented  0.1 mg and allowed them  prior  to  experimentation.  with amphipods  or  isopods  amphipods  hour  placed in i n d i v i d u a l  4-22  mm  in  length  and  of  with  to  recover  They  known  size.  consumption weights.  Beetles was  There  were  estimated were  no  immediately from  then  I  used  isopods 8-11 mm i n l e n g t h .  r e j e c t e d as food items. I recorded  feeding.  for  were  and  h a n d l i n g times  nearest minute f o r the time between prey capture and of  thin  j a r s . I weighed  Isopods l a r g e r than t h i s c o u l d not be held by b e e t l e s always  a  the  to the  completion  reweighed  difference  were  and of  gross  the two  a p p r e c i a b l e weight changes i n b e e t l e s  which d i d not feed. To determine the  number  of  amphipods  r e q u i r e d to s a t i a t e a b e e t l e , I presented to  beetles  until  they  of  a  given  size  amphipods c o n t i n u o u s l y  refused f u r t h e r food, at a maximum of 6  amphipods. Beetles were reweighed at the end of the experimental period. I c o u l d not measure satiate  the number  of  isopods  a b e e t l e , as b e e t l e s tended to r e j e c t  required  isopods  to them. A b e e t l e would grasp the isopod between  to  presented  i t s mandibles  60  and  release  i t , although  i t would r e a d i l y accept an amphipod.  Isopod experiments  t e s t e d f o r d i f f e r e n c e s i n preference  male  b e e t l e s f o r isopods and amphipods. The design  and  female  was s i m i l a r to the f e e d i n g experiments (page mm  12).  described  in  Each j a r contained one b e e t l e , 10 isopods  i n l e n g t h ) , and 0, 2, 4, or 6 amphipods. Only  were used and two s i z e s of amphipods  (8-11 mm  between  chapter  2  (under  12  starved b e e t l e s  and 16-19 mm)  were  of f e r e d .  Results  Vulnerability The mean and the v a r i a n c e of the number of a t t a c k s r e q u i r e d to capture an amphipod i n c r e a s e d with i n c r e a s i n g (Table  IV).  For  the  amphipod  three s m a l l e s t s i z e c l a s s e s , a l l b e e t l e s  t e s t e d captured an amphipod i n l e s s than  10 a t t a c k s  IV,  larger  "prop, of  beetles  were  capture/attack  size  beetles"  column).  unsuccessful. decreased  with  For  The  (see  amphipods, some  probability  increasing  Table  of  amphipod  prey  s i z e (see  Table IV, "prob. of c a p t u r e " column). B e e t l e s i z e a l s o a f f e c t e d the frequency amphipods 16-19  were  mm amphipods captured  i n 20 a t t a c k s and  beetle  large  size  Such r e l a t i o n s h i p s between capture  predator and prey s i z e are t y p i c a l  Griffiths  which  captured. The c o r r e l a t i o n between the number of  0.447 (n=41, p<0.0l). and  with  for insects  was  success  (Evans  1976,  1980).  Small amphipods were captured e a s i l y as the e n t i r e body  of  61  Table IV. Capture success as a f u n c t i o n of amphipod s i z e . The t a b l e g i v e s the mean and v a r i a n c e of the number of a t t a c k s r e q u i r e d f o r capture by b e e t l e s which were s u c c e s s f u l w i t h i n 10 a t t a c k s , the number of b e e t l e s t e s t e d , the p r o p o r t i o n of b e e t l e s which were s u c c e s s f u l , and the combined p r o b a b i l i t y of prey c a p t u r e / a t t a c k . size (mm)  size class  mean  var.  n  prop, of beetles  prob. of capture  4-7  1  1 .53  0.81  30  1 .000  0.652  8-1 1  2  1 .88  1 .75  40  1 .000  0.533  12-15  3  3.25  6.58  32  1 .000  0.308  16-19  4  3.84  8.81  48  0.646  0. 168  20 +  5  4.14  8.98  23  0.304  0.073  the amphipod f i t between the mandibles of the b e e t l e . For l a r g e r amphipods, b e e t l e s grasped the amphipod with t h e i r mandibles maintained  a  and  p o s i t i o n on i t s d o r s a l s u r f a c e u n t i l the amphipod  was  subdued. Amphipods responded  of  the  uropods.  Maximum  to capture by repeated  lengths  of  such  escape  flexing responses  observed i n timed bouts were 40 s f o r an 11 mm  amphipod,  for  amphipod. In the  a  14  mm  amphipod, and 96 s f o r an  l a s t example the amphipod escaped. defended  Large  themselves with t h e i r second  t h e i r antennae.  18 mm male  80  amphipods  s  also  (enlarged) gnathopods and  These c o u l d prevent the b e e t l e from g r a s p i n g the  body of the amphipod. The overhead  l i g h t i n g used to conduct  no e f f e c t on capture success. B e e t l e s captured  6.33±0.32  amphipods  (n=l5)  these experiments  which in  had  been  blinded  20 a t t a c k s , or had a  frequency of 0.317  c a p t u r e s / a t t a c k f o r amphipods i n  3.  not s i g n i f i c a n t l y d i f f e r e n t  T h i s value was  had  size  class  from the observed  62  value  of 0.308 f o r b e e t l e s which had  not  been  blinded  (t=0.56,  df=14, p>0.10). Beetles distances blinded  responded  than  small  beetles  b e e t l e s which  to  amphipods  amphipods  were not  had  large  not  ( F i g . 9).  at  much  The  responses  significantly different  been  blinded  (t=1 .48,  greater of  from those of df = 73,  df = 73,  p>0.10).  Feeding  rates  There  were  time or feeding amphipods  or  males and handling  d i f f e r e n c e s i n gross consumption,  r a t e between male and isopods  handling  female b e e t l e s f e e d i n g  ( t - t e s t , p>0.05). I combined r e s u l t s  on from  females f o r the f o l l o w i n g a n a l y s e s . Gross consumption, time and  amphipod largest  no  size  feeding  classes  rates and  increased  over  the  then remained constant  s i z e c l a s s e s (Table V ) . B e e t l e s  first  f o r the  r e q u i r e d more than  two  twice  the amount of time to feed on a l a r g e amphipod as they d i d on small one.  Feeding r a t e s on  than f o r the s m a l l e s t One All  but  isopods  sufficient  on  a  b e e t l e t e s t e d on each of amphipod s i z e c l a s s e s 4 and  sequence  of  the  amphipods on average. Handling greater  greater  to s a t i a t e most b e e t l e s .  5 r e j e c t e d a d d i t i o n a l amphipods (Table V I ) . Beetles fed  a  amphipods.  l a r g e amphipod was one  were only m a r g i n a l l y  3  for  the  smallest  smallest times size  amphipods were  classes,  also with  which consumed  were 3.6  significantly an  overall  c o r r e l a t i o n of -0.429 (n=96, p<0.0l) between amphipod l e n g t h  and  handling  not  time. Handling times f o r feeding to s a t i a t i o n  did  63  F i g u r e 9. The maximum p r e s e n t a t i o n d i s t a n c e at which b e e t l e s responded to amphipods by movement towards the amphipod. Means are given with 95% confidence i n t e r v a l s (n=50). The value f o r b l i n d e d b e e t l e s (n=25) i s given f o r 16-19 mm amphipods ( c l o s e d c i r c l e ) .  64  65  Table V. Feeding data f o r b e e t l e s fed one amphipod or isopod. The t a b l e g i v e s means, standard e r r o r s and sample s i z e s . The s u p e r s c r i p t s d e f i n e l e v e l s which are not s i g n i f i c a n t l y d i f f e r e n t by Duncan's m u l t i p l e range t e s t . size (mm)  size class  handling time (min)  gross consumption (mg)  feeding rate (mg/min)  amphipods 4-7  1  8-1 1  2  26.8±2.3  2  (18)  20. 7±1.5  12-15  3  43.0±2.5  3  (28)  22.812.0  16-19  4  49.6±3.4  3  (25)  28.8±2.4  20 +  5  48.8±3.8  3  (21)  27.1±2.5  8 . 6±1.2  (15)  1  12.0±1.4  (14)  0.70±0.08  2  (17)  1 .38±0.15  2  (28)  2. 17±0.16  3  (25)  2.01±0.20  (21 )  1.93±0.15  (25)  0.74±0.06  1  23  1  2  3  3  3  isopods 13.7±1.2  include  the  search  (25)  time  19.3±1 .4  between  successive  prey items. The  i n c l u s i o n of search time would i n f l a t e the d i f f e r e n c e f o r a g i n g time between small and l a r g e There  were  b e e t l e s fed  to  consumption was P<0.01).  no  between  on  10,  amphipods.  different  r e l a t e d to i n i t i a l  size  b e e t l e weight  classes.  Gross  (r=0.505, n=96,  The c o r r e l a t i o n between feeding rate and b e e t l e weight  the  (r=0.l78, n=96, p>0.05).  experiments  amphipods  and  testing isopods,  isopods than female b e e t l e s when (Fig.  total  d i f f e r e n c e s i n t o t a l gross consumption f o r  satiation  was not s i g n i f i c a n t In  in  open  and  closed  for male no  preference beetles  amphipods  squares,  t=2.l9,  suggesting that males were e i t h e r more s u c c e s s f u l  of  beetles  consumed were  more  available  df=l8, p<0.05), in  capturing  66  Figure  10. The mean ±SE number of isopods eaten at d e n s i t i e s of 10 isopods and 0, 2, 4 or 6 large amphipods (16-19mm, c l o s e d c i r c l e s , n=20) and small amphipods (8-11mm, open c i r c l e s , n=13). Male and female b e e t l e s d i f f e r e d in the number of isopods eaten i n the absence of amphipods only ( c l o s e d or open s q u a r e s ) . For other amphipod d e n s i t i e s , male and female data were combined.  67  2-5  c  2 0  "o  V>  TJ  a is o (A  S  •  males  •  females  •  large  O  small  amphipods amphipods  0  10  -O  E 0-5  0 10 Ratio  2-10 of  4=10  6=10  amphipods 1 isopods  presented  68  Table VI. Feeding data f o r b e e t l e s fed to s a t i a t i o n on one amphipod s i z e c l a s s . The s u p e r s c r i p t s d e f i n e l e v e l s which are not s i g n i f i c a n t l y d i f f e r e n t by Duncan's m u l t i p l e range t e s t . size (mm)  size class  handling gross t ime: consumption (min) (mg)  no. eaten  4-7  1  3 .61±0. 3 4  8-11  2  2 .42±0. 23  12-15  3  1 .30±0. 1 1  16-19  4  1 .04±0. 04  20 +  5  1 .05±0. 05  isopods,  or  feeding rate (mg/min)  (18)  39. 6±4 • 2  (21)  48. 6±3 .6  1  38. 7±3 . 4  (20)  51 . 2±3 .8  1  26. 5±2 .5  3  (25)  50. 6±3 .4  1  30. 4±2 . 8  3  (21 )  49. 5±3 .8  1  27. 4±2 .6  1  2  3  were l e s s l i k e l y  46. 3±4 .4  1  12  3  23  3  (14)  0 .98±0. 1 3  (16)  1 .38±0. 16  (20)  2 .19±0. 19  (25)  1 .99±0. 21  (21 )  1 .93±0. 1 5  isopods  as prey. In  the  presence of l a r g e or small amphipods, only a few  b e e t l e s ate  any  isopods  There  ( F i g . 10,  differences  in  open the  and  number  to r e j e c t  1  closed of  circles).  isopods  r a p i d decrease i n isopod consumption and in  amphipod  consumption  with  suggested strong p r e f e r e n c e feeding them, and  experiments  c o u l d capture  no  eaten by each sex.  The  corresponding  increasing  isopod,  increase  amphipod  f o r amphipods. B e e t l e s  would a t t a c k an  then r e j e c t the  were  abundance  observed  in  isopod dropped i n f r o n t of  even small  isopods  that  they  easily.  Field results In  1980,  c a l i f o r n i a n a . Of amphipod  beetles 423  species  t r a s k i a n a ) present  fed  almost e x c l u s i v e l y on  observations, (Orchestoidea  only  four  were  pugettensis  on the beach in low  Orchestoidea on  and  numbers. One  related Orchestia  beetle  only  69  was  found  pattern  feeding  was  between  different  male  (n=l75)  of  females  the  of  observations  and  pictus. at  These  night  burrows  in in  In  and  beetles,  (n=55)  42% beach were  (X =12.3, 2  relatively  showed  the  The  observations species  length.  feeding  on  three  and  they  had p r o b a b l y  HW b y  (n=114)  df=1, more  the  an  the  lower  the  beach were  time,  compared  but  This on  the  2  summer  amphipods  disappeared after throughout  df=1,  of  June  modal  was  their  observations  on  7 1 % (n=236)  for  d f = 1 , P>0.10).  feeding  on on  isopods the  lower  because  isopods  Female  beetles  beach.  12% (n=97) compared  to  p<0.lO). sizes  (Fig.  juveniles  and J u l y .  size  of  was p r o b a b l y  progressed and  Thinopinus  out  1 6 % (n=114)  different  May,  females,  forced  and  found  upper  For  Mann.  surface  2  difference,  (X =3.12,  abundance  the  LeC.  the  (X =0.76,  to  obesus  tide.  beetles, beach  four  on  feeding  upper  p<0.0l).  been  were  larval  observed  incoming  male  abundant  The  larvae  of  for  on  the  differences.  comprised  fucicola  2 mm i n  only  as  year  of  Emphyastes  the  was weak  population  on  about  observations  changed  the  isopods  Dyschirius  the  the  for  on  were  relative  (n=336)  91.4%  feeding  same t r e n d  (n=236),  Mainly  comprised 7 3 . 1 %  males  isopods  of  to  The  differences  1981, there  sand near  m a l e s on  Amphipods  compared  p<0.0l).  also  in  1981, 67%  However,  6%  1981, the  amphipods female  were  were  addition  both  three  there  perconvexus.  In  two  (Curculionidae), were  males,  diet.  Alloniscus  beetles.  df=1,  of  isopod,  1981 and  diet  the  (Carabidae),  in  female  2  of  an  and  (X =28.8,  remainder  there  on  There  larger  of  1 1 )..  amphipods  The  largest  were  recruited  to  were  also  to  in  1981  year than  in  70  Figure  11. Size-frequency d i s t r i b u t i o n s of amphipods caught in p i t f a l l t r a p s f o r each month i n 1980 (A) and 1981 (B) .  71  1980  6 MAY n=437  0 15 010 005 020  19 JUNE  015  n = 379  >_ 010 2 005 UJ Z>  o  13 JULY  or 020 u_ 0 15  n= 302  010 -l 005 015  18 AUGUST  010  n = 173  005 2 4  6 8  10 12 14 16 18 20 22  AMPHIPOD  SIZE  (mm)  1981  16 MAY  n = 68  0 2  4  6 8  10 12 14 16 18 20 22 24 26  AMPHIPOD SIZE (mm)  73  1980.  The  1981  mean length of captured  (14.0±0.12  p<0.001).  Beetle  than i n 1 9 8 0 length  than in 1 9 8 0  mm)  l e n g t h was  (21.6±0.1  in 1 9 8 1  a l s o greater .t=3.20,  Male  and  (0.237±0.009  females),  n=25  g,  for  (Fig. 1 2 ) .  and  size  variance  males,  As head width  of prey captured.  df=383,  feeding on amphipods feeding females  The  on  isopods  (t=2.50,  was  X2=13.4,  df=4,  likely  unidentified page 6 ) , on  and  larger  i n 7%  P>0.10  were  amphipod n=4l9,  p<0.0l  0.242±0.008  n=27  g,  was  related  to  how  In beach o b s e r v a t i o n s ,  (t=0.8, df=4l7,  for 1 9 8 1 ) .  larger  which  Beetles  attract  staphylinid there was carcasses.  the  the mean females  p>0.l0  However, b e e t l e s  significantly  the far  t h i s could a f f e c t  than  for found  beetles  p<0.05)  and  scavengers  joined  beetles  in  r e l a t e d to the s i z e of amphipod captured  to  for  p<0.05).  with  p<0.0l).  were  i n mean body weight  f o r both males ( t = 2 . 2 9 , d f = 1 4 2 ,  df=304,  frequency  feeding  more  Beetle  of the s i z e of amphipods on which males and  t=0.0,  mm)  1981).  (Fig. 1 3 ) ,  spread  were observed feeding d i d not d i f f e r 1980;  (21.9±0.1  greater head widths than females of  apart mandibles c o u l d be maximum  for  female b e e t l e s d i d not d i f f e r  but males had  same weight  p<0.0l  in  df=802,  p<0.0l).  (r=0.356,  n=385,  of  df=802,  s i g n i f i c a n t l y c o r r e l a t e d in both years r=0.190,  length  also greater t=11.5,  mm,  captured  1980;  the  (12.3±0.2  the  for  and  mm,  amphipods was  of the o b s e r v a t i o n s .  14,  feeding on l a r g e r amphipods were  various  scavengers,  mainly  an  and D y s c h i r i u s obesus, a c a r a b i d  (see  more space f o r attachment In  (Fig.  1980,  Thinopinus was  T h i s f i g u r e was  of  scavengers  j o i n e d in feeding  greater  in 1981  with  74  Figure  12. M o r p h o l o g i c a l comparisons of a sample of male and female b e e t l e s c o l l e c t e d 4 June 1981. Weights were measured 9 h a f t e r c o l l e c t i o n . The slopes, but not the i n t e r c e p t s of the r e g r e s s i o n l i n e s a r e s i g n i f i c a n t l y d i f f e r e n t (p<0.05). Regressions are y = 3.06 + 8.23x f o r males, r =0.853 y = 3.26 + 5.87x f o r females, r =0.751 2  2  75  0.16  0.22  Weight  0.2 8  (g)  0.34  76  Figure  13. Maximum mandible spread f o r male and female b e e t l e s (r=0.937, n=56). Measurements were made on r e l a x e d b e e t l e s using v e r n i e r c a l i p e r s .  77  9.4-J  •  m a l e s  O  f e m a l e s  9.0-1  E E  8.6  0 QCO  O 7.8T  CD , —  CO  O  7.4-  O • • O O OOO OO  O  o o  7.<H O  6.64  O  6.2  1 ^ 3.6  4.0  Head  4.4  4.8  width  •  i  5.2  5.6  (mm)  78  F i g u r e 14. The frequency d i s t r i b u t i o n of the s i z e s of captured amphipods i n 1981 when scavengers were present or absent.  S C A V E N G E R S PRESENT (n=!04) SCAVENGERS ABSENT (n=28l) LU  3 0 - 41 Cf LU QC  ^ 0 Z \  ^1  4-7  8-11  12-15  16-19  AMPHIPOD SIZE (mm)  20 +  80  Figure  15. The observed frequency d i s t r i b u t i o n of amphipod s i z e c l a s s e s i n the p o p u l a t i o n obtained by weighted averages of p i t f a l l t r a p catches from each month (white b a r s ) , the p r e d i c t e d d i s t r i b u t i o n of amphipods captured a c c o r d i n g t o the mechanistic model ( s t r i p e d b a r s ) , the observed d i s t r i b u t i o n of captures (black b a r s ) , and the s t a n d a r d i z e d forage r a t i o s ( i n s e t , see page 54) f o r both years of data. The dotted l i n e g i v e s the expected forage r a t i o s i n the absence of p r e f e r e n c e .  82  scavengers  present  P<0.001). Only  in  27%  of  the o b s e r v a t i o n s (X =57, df=1, 2  5% of the b e e t l e s f e e d i n g on isopods i n 1981 were  j o i n e d by scavengers.  A test I  f o r prey  selection  obtained  amphipods  average  available  frequency feeding  an  to  distribution  beetles  f o r the  made  on  of  i n each year by weighting the  of each amphipod s i z e i n each month by the observations  sizes  number  of  b e e t l e s i n that month ( F i g . 15,  white b a r s ) . The observed d i s t r i b u t i o n s of the s i z e s of captured amphipods  (Fig.  15,  black  a v a i l a b l e f o r both years P<0.001).  Beetles  bars)  differed  from  the  sizes  (X =376, df=4 f o r 1980, X =128 f o r 1981 2  2  d i d not  capture  sizes  of  amphipods  in  p r o p o r t i o n to the r e l a t i v e abundance of those s i z e s . I used  the  model  from  described  above  to  test  whether  this  resulted  d i f f e r e n c e s i n the r e l a t i v e a v a i l a b i l i t y of d i f f e r e n t  sizes  to  b e e t l e s , or by a c t i v e s e l e c t i o n by the b e e t l e s .  M e c h a n i s t i c model The mechanistic model was based amphipods  of  size i ,  I  expected  on p ( i ) ' , to observe  b e e t l e i f the b e e t l e d i d not use a c t i v e p(i)', 15,  i n the d i e t of a  selection.  To  predict  I combined data on the s i z e s of amphipods a v a i l a b l e ( F i g .  white  bars)  handling times p(i)'  the p r o p o r t i o n of  differed  with  l a b o r a t o r y values f o r v u l n e r a b i l i t y and  ( F i g . 9, Table IV, Table V ) . P r e d i c t e d values between  the two years  of  ( F i g . 15, s t r i p e d b a r s ) .  83  The  observed d i s t r i b u t i o n s  (Fig.  of the s i z e s  of  captured  amphipods  15, black bars) a l s o d i f f e r e d between years (X =34.8 df=4, 2  P<0.001),  but  showed s i m i l a r  fromp(i)'  (X =540, f o r 1980; X =85 f o r 2  2  The  p a t t e r n of food s e l e c t i o n  the  s t a n d a r d i z e d forage r a t i o s  size  trends i n both years and d i f f e r e d  classes,  a  size  1981,  df=4,  p<0.00l).  i s seen most c l e a r l y by examining ( F i g . 15, i n s e t ) . With f i v e  class  which  i s neither  p r e f e r e n t i a l l y nor avoided has a r a t i o of 0.20.  In  selected  both  b e e t l e s appeared i n d i f f e r e n t to the s m a l l e s t s i z e c l a s s , at  least  one  of  the  middle  p r e f e r e n c e f o r the l a r g e s t The  size  size classes,  amphipods c o u l d r e s u l t i f  overestimate  amphipods  the  captured  underestimated insufficient distributions  in  probability. largest  by t h i s c a l c u l a t i o n ,  to account f o r the of  this  two  As size  only.  the numbers of classes  measurement e r r o r  preference  This  were  was s t i l l  observed.  The  new  p ( i ) ' d i d however conform more c l o s e l y t o the  observed d i s t r i b u t i o n s for  true the  test  p ( i ) ' u s i n g data on the p r o b a b i l i t y  of c a p t u r e / a t t a c k from b e e t l e s which were s u c c e s s f u l would  avoided  and showed strong  I underestimated capture success f o r these s i z e s . To I recalculated  years  class.  apparent p r e f e r e n c e f o r l a r g e  possibility,  prey  (X =239 p<0.00l, f o r 1980; X =14, 2  2  p<0.0l  1981). It  i s possible  to  choose  parameter  agreement between p ( i ) ' and the observed which  result  values which  distributions.  force Values  i n an exact f i t f o r a change i n one parameter are  given i n Table V I I .  For example, very h i g h values  h(i)  5, r e l a t i v e t o the other s i z e c l a s s e s ,  for size class  f o r d ( i ) or could  84  improve listed from  the f i t of the model. None of the values f o r parameters i n Table VII were w i t h i n the  experiments.  range  However, i t i s s t i l l  d i f f e r e n t . parameters could combine to significant  difference  between  of  values  obtained  p o s s i b l e that e r r o r s i n  produce p(i)'  a  spurious  and  the  but  observed  distributions.  Frequency-dependent  model  The frequency-dependent model c o u l d a l s o comparison  of  p(i)'  in  the most abundant  the  by  a  to  the  predictions  of  this  s i z e c l a s s e s should be o v e r - r e p r e s e n t e d  d i e t 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 , and c o n v e r s e l y f o r  the l e a s t abundant trends.  tested  and the observed d i s t r i b u t i o n of the s i z e s  of captured amphipods. A c c o r d i n g model,  be  s i z e c l a s s e s . The data  showed  Preference appeared to be s t r o n g e s t  l e a s t abundant  size  the  opposite  f o r the l a r g e s t  and  class.  Optimal d i e t model For most optimal d i e t models, prey  type  is  defined  as  the  the ratio  profitability  of  each  of net energy intake to  h a n d l i n g time. I approximated t h i s r a t i o by the feeding r a t e f o r each s i z e c l a s s and could not d i s t i n g u i s h between the l a r g e s t size  classes.  This  simple  form  of  i n a p p r o p r i a t e , as I measured d i f f e r e n c e s size  the in  model  was  clearly  selection  between  c l a s s e s 3 and 4. An a l t e r n a t i v e method I chose was  prey items based on the number of  amphipods  3  of  a  to rank  given  size  85  Table V I I . Parameter values f o r f ( i ) , h ( i ) , d ( i ) and c ( i ) which r e s u l t i n an exact f i t of the observed d i s t r i b u t i o n of the s i z e s of captured amphipods to p ( i ) ' . These values were c a l c u l a t e d f o r a change i n the l i s t e d parameter o n l y . Other parameters used i n c a l c u l a t i o n of p ( i ) ' were those obtained from the experiments. Values f o r h ( i ) , d ( i ) and c ( i ) were s t a n d a r d i z e d to a maximum value of 1. Values are shown f o r both 1980 and 1981 d a t a . i  f (i)  1980  h(i)  d(i)  c (i)  data  1  0.3995  0.1646  0.0333  1 .0000  2  0.1038  0. 1147  0.0387  0. 3304  3  0. 1764  0.0429  0.0245  0. 0648  4  0.1921  0.2323  0.1741  0. 1510  5  0.1281  1.0000  1 .0000  0. 3009  1 981 data 1  0.1663  0.1715  0.0347  1.0000  2  0.0870  0.1259  0.0425  0. 3478  3  0.1960  0.2786  0.1581  0. 4009  4  0.3597  0.4965  0.3722  0. 3095  5  0.1910  1.0000  1.0000  0. 2888  c l a s s r e q u i r e d to s a t i a t e a b e e t l e . T h i s was because an increase in  the  number  time r e q u i r e d .  of I  items eaten would i n c r e a s e the t o t a l  could  still  not  distinguish  search  between  size  c l a s s e s 4 and 5 however, and these s i z e c l a s s e s shared a rank of one. I c a l c u l a t e d the expected feeding  to  satiation  on  d e s c r i b e d on page 54. When  f o r a g i n g times, T ( r ) , f o r b e e t l e s  different beetles  sets f e d on  of  s i z e c l a s s e s as  mixtures  of  size  c l a s s e s I used the data i n Table VI to estimate the mean numbers  86  of  items  eaten and the h a n d l i n g times  (Table V I I I ) . I combined  Table V I I I . Parameter v a l u e s f o r the optimal d i e t model. The t a b l e g i v e s a l l p o s s i b l e combinations of s i z e c l a s s e s of prey items i n the d i e t , and the mean h a n d l i n g time h ( i ) , and the mean number of items eaten (N) f o r each combination. Combinations a r e l i s t e d as the number of the s i z e c l a s s i n the order of capture for a maximum of four-items eaten. order of capture 2 3  4  handling t ime(min)  no. eaten  1  1  1-2  42.2  3.6  1  1  3-5  39.4  3.6  1  1  2  42.4  3.0  1  1  3-5  36.6  3.0  1  2  1 -2  42.2  3.0  1  2  3-5  38.0  3.0  1  3-5  32.5  2.0  2  1  1-2  42.2  3.0  2  1  3-5  38.0  3.0  2  2  1-2  42.2  2.4  2  2  2-5  39.4  2.4  2  3-5  33.8  2.0  3  1-2  32.5  1 .3  3  3-5  28.3  1 .3  28.3  1.0  1  4-5  data on h a n d l i n g times f o r s i z e c l a s s e s classes  1 and  2  and  for  size  3, 4 and 5 where d i f f e r e n c e s were not s i g n i f i c a n t . When  f e e d i n g occurred on  mixtures  of  size  classes  with  distinct  87  handling  times, I used an average feeding  proportion  of the t o t a l  food  intake  time, weighted by the  from  the  different  size  classes. One  prediction  of  optimal  d i e t models, i s that only the  subset of s i z e s which minimize f o r a g i n g time are i n c l u d e d optimal when  d i e t . Using 1980 data,  a l l size  T(r)  was  c l a s s e s were i n c l u d e d  r a t e s l e s s than 60 amphipods/h, and than  68  amphipods/h  f o r encounter  f o r 1981 data.  At higher  time was minimized by e x c l u d i n g  classes  from  diet.  The  long  search  IX)  rates  less  encounter r a t e s ,  the s m a l l e s t  two  size  foraging times f o r a b e e t l e  which f e d on the two l a r g e s t s i z e c l a s s e s only, these s i z e c l a s s e s were l e s s  (Table  i n the d i e t f o r encounter  foraging  the  minimized  i n the  abundant  and  r e s u l t e d because  required  a  longer  time.  To  t e s t p r e d i c t i o n s from t h i s model with the f i e l d  used the numbers of amphipods caught i n  pitfall  traps  data, I on the  beach as an estimate of the amphipod encounter r a t e . The s i z e of the For  t r a p opening approximated the area 1981 data,  the  maximum  amphipods/h. According threshold important optimal  at  encounter  rate  measured  First,  I  could  not  distinguish  d i e t p r e d i c t i o n from the n u l l hypothesis.  of  according  encounter  30  occur. T h i s r e s u l t had two the  A good f i t t o  the model c o u l d r e s u l t i f b e e t l e s were foraging e i t h e r or simply  was  to Table IX, t h i s rate was w e l l below the  which s e l e c t i o n should  implications.  of r e a c t i o n of a b e e t l e .  optimally  to prey a v a i l a b i l i t y . Second, f o r the  range  rates observed, f o r a g i n g time d i f f e r e n c e s between  most s t r a t e g i e s were w i t h i n one standard  error  of  experimental  88  Table IX. Foraging times (min) generated by the optimal d i e t model f o r b e e t l e s feeding to s a t i a t i o n on a l l combinations of s i z e c l a s s e s . (A) the g e n e r a l i s t (B) s i z e c l a s s e s 2-5 only (C) s i z e c l a s s e s 3-5 only (D) s i z e c l a s s e s 4-5 o n l y . Encounter r a t e s are i n amphipods/h. C a l c u l a t i o n s are shown based on amphipod s i z e d i s t r i b u t i o n s f o r 1980 and 1981. * i n d i c a t e s the lowest v a l u e . encounter rate  A  B  C  D  1  1980 data 258.4*  308.7  358.2  2187.7  10  52.8*  57.4  61 .2  244.2  20  41.4*  43.4  44.8  1 36.2  30  37.6*  38.8  39.3  100.2  40  35.7*  36.4  36.5  82.2  60  33.8  34. 1  33.8*  64.3  1 00  32.3  32.2  31.6*  49.9  1  1981 data 291.1 *  311.7  431 .4  754.6  10  56.4*  58.3  68.6  100.9  20  43.4*  44.2  48.4  64.6  30  39.0*  39.5  41 .7  52.5  40  36.9*  37.2  38.3  46.4  60  34.7*  34.9  35.0  40.4  32.9  33.0  32.3*  35.5  100  values Table and  f o r mean handling  time. The only t e s t a b l e p r e d i c t i o n from  IX was that b e e t l e s would not s p e c i a l i z e on s i z e c l a s s e s 4 5.  In  fact,  the p r o p o r t i o n of captured  amphipods i n s i z e  c l a s s e s 4 and 5 i n c r e a s e d with amphipod encounter rate  for  and  = 0.410,  June  1981  (Spearman rank c o r r e l a t i o n c o e f f i c i e n t  n=23, p<0.05). This was not true f o r J u l y  1981  (Spearman  May  rank  89  correlation coefficient  = 0.038, n=25, p>0.l0), probably because  small  relatively  amphipods  were  more abundant i n J u l y . I t i s  p o s s i b l e that I underestimated encounter r a t e s . especially  true  if  observed c o n t i n u o u s l y  prey  were  clumped.  This  However,  would  be  beetles  I  i n 1979 and 1980 had a mean encounter rate  of 8.8 amphipods/h (page 33). The feeding rate on isopods on the s m a l l e s t s i z e  class  of  c r i t e r i a d e f i n e d above, isopods the  same  rank  and  isopods  was s i m i l a r t o the feeding amphipods  (Table  V).  By  and s i z e c l a s s 1 amphipods should  be  eaten  rate the share  as encountered  whenever the s m a l l e s t s i z e c l a s s of amphipod i s i n c l u d e d i n optimal  diet.  For  amphipod  the  abundances l e s s than 10 amphipods  trapped/h i n 1981, 18% (n=135) of the feeding o b s e r v a t i o n s on  isopods. Above t h i s l e v e l only 6% of the f e e d i n g  were on isopods  were  observations  (X =10.7, df=1, p<0.0l). 2  Di s c u s s i o n I have shown that the d i s t r i b u t i o n of s i z e s of amphipods i n the  diet  hypothesis  of  Thinopinus  frequency-dependent  both  evaluated. measured field.  complete  A  vulnerability.  a null However,  and  or optimal d i e t models. In order t o model, I must assume that  that  In p a r t i c u l a r in  from  s i z e s e l e c t i o n c o u l d not be accounted f o r by  r e j e c t the n u l l hypothesis is  significantly  model based on d i f f e r e n t i a l  the p a t t e r n of prey either  differed  I  the  must  parameters  assume  that  the  were the  model  correctly parameters  the l a b o r a t o r y a c c u r a t e l y r e f l e c t behaviour i n the  combination  of  estimation  errors  in  any  of  the  90  parameters  could  result  in  an  apparent  preference  or  i n d i f f e r e n c e f o r some s i z e c l a s s e s . One p o t e n t i a l source sizes  of  amphipods  of e r r o r i s i n the assessment  not perform as p r e d i c t e d . In chapter  did  and  were important  not monitor these  the  actual  to the success  beetles continuously  densities  or  average,  beetles  2, I argued that  s p a t i a l and temporal v a r i a t i o n s i n the s i z e s occur  of  did  not  sample  local  amphipods  did  of f o r a g i n g b e e t l e s . I and I  did  not  know  s i z e s of amphipods encounted by each  b e e t l e . In p a r t i c u l a r , the s i z e s of amphipods caught i n traps  the  a v a i l a b l e . T h i s i s because the p r e d i c t i o n s  were based on means. I can only s t a t e that on did  of  pitfall  the s i z e s of amphipods encountered by a  b e e t l e i n a c t i v e mode (see page 33). I d e s c r i b e a more p r e c i s e t e s t of p r e f e r e n c e classes  2  and  only  f o r s i z e c l a s s 4 by s t a r v e d  b e e t l e s , and I could not measure any preference not  been  predominantly the  did  starved.  mechanistic  not  This  opportunistic,  biologically,  model  were  suggested  of b e e t l e s which  that  beetles  In prey,  statistically,  but  only  weakly  s i g n i f i c a n t . B e e t l e s which I observed on the beach  always  attack  amphipods near them. T h i s was true f o r beetles  to  and then r e j e c t amphipods of any s i z e . order  f o r a b e e t l e t o a c t i v e l y s e l e c t c e r t a i n s i z e s of  i t must have the a b i l i t y  ability  were  and that measured d e v i a t i o n s from  l a r g e as w e l l as small amphipods. I d i d not observe capture  size  4 i n l a b o r a t o r y experiments i n Chapter 4. I was  able to measure weak p r e f e r e n c e  had  between  to  distinguish  prey  to  distinguish  prey  size.  The  s i z e has been w e l l documented i n  91  vertebrates 1981;  (salamanders, Jaeger and  b i r d s , Zach 1978,  Brown  1981).  There  (crabs, E l n e r and and  Farley  Paruroctonus vibrations.  Bernard  Goss-Custard 1977; is  also  some  Hughes 1978;  (1979)  ants,  detects  Davidson  Thinopinus  may  that  prey  Such on  prey  effectively  if  an  amphipods landed size  jumped.  was  observed  high.  of  vibrations  size.  This  Brownell  desert  scorpion  substrate-borne  blind could  mechanism  rapidly  sighted  also  provide  would  attacked  Mistakes  in  as  tests  an  in  judging  explanation  of optimal  work  most  Frequently  amphipods  which  Jaeger and  Barnard  d i d not account for sizes  of  in  a  short  capture,  measuring  the  of  and  small amphipods which I of  different  d i e t model. Small amphipods  s e v e r a l could be captured  in  one  area  time i n t e r v a l . Large amphipods were more c o s t l y to  A  between  the  the  amphipod  b e e t l e feeding on a l a r g e item a l s o a t t r a c t e d a  v a r i e t y of scavengers. T h i s l e d to s t r u g g l e s and portion  are  discrepancies  profitability  as a s t r u g g l e f r e q u e n t l y ensued  beetle.  for  profitability  1981).  amphipods for the optimal  tended to be clumped and  prey  prey  d i e t p r e d i c t i o n s (e.g. E l n e r  There were advantages to feeding on  costs.  and  amphipod walked toward a b e e t l e . Beetles  cited  Hughes 1979,  and  invertebrates  near them. Hence, the p o t e n t i a l f o r e r r o r i n judging  frequently  in  and  use a s i m i l a r mechanism. There were  sand-borne  information  the  Barnard  1978).  through  no d i f f e r e n c e s in the r e a c t i o n d i s t a n c e beetles.  f i s h , Gardner  shrews,  evidence for  demonstrated  mesaensis  1981;  prey  item,  a  loss  with a s s o c i a t e d time and  of  a  energetic  92  Why  d i d the optimal d i e t model not work? T h i s i s a case  and  did  i n which I measured a l l the necessary  everything  predictions  were  "correctly".  identical  to  Yet, the  the  values  optimal  n u l l hypothesis.  diet  Pastorok  (1981) reported r e s u l t s s i m i l a r to those I have presented He  constructed  different  an  s i z e s of  here.  optimal d i e t model f o r Chaoborus feeding on Daphnia  which  was  based  on  differential  v u l n e r a b i l i t y . The model p r e d i c t e d that Chaoborus should i n c l u d e all  s i z e s of Daphnia i n i t s d i e t  measured i n the f i e l d . T h i s was, In  g e n e r a l , attempts  invertebrate  predators  f o r the range of abundances he in f a c t , the p a t t e r n  observed.  to measure s i z e s e l e c t i o n of in  (Thompson 1978b, G r i f f i t h s  the  have  by  been u n s u c c e s s f u l range  of  on l i m i t s set by  the  mechanics of prey capture, and on d i f f e r e n t i a l v u l n e r a b i l i t y  of  sizes  eaten  different  appears  sizes.  1980,  field  prey  largely  Griffiths  Murtaugh 1981). to  depend  The  (1981) showed that hungry a n t - l i o n s  would a t t a c k small prey f o r which the e n e r g e t i c c o s t s of capture exceeded Chaoborus  the and  benefits.  For  Thinopinus,  predators  which encounter  prey s i z e and abundance, a g e n e r a l i s t the  best r u l e . Morse and F r i t z  more important  such  as  ant-lions,  large v a r i a t i o n s in  foraging strategy  (1982) have suggested  f o r c r a b s p i d e r s to l o c a t e  good  than t o s p e c i a l i z e on c e r t a i n prey while at a  may  that i t i s  foraging  site.  be  sites  93  Isopods versus amphipods Both  frequency-dependent  q u a l i t a t i v e l y predicted rate  on  isopods was  amphipods. amphipod  Beetles  the  types of  lower than the included  abundance  only  experiments  and  preferred  to  isopods.  distinguishing different  I  prey  diet  models  The  feeding  the  smallest  eaten.  r a t e on a l l but  the  in  their  diet  at  low  r e l a t i o n s h i p with abundance  as p r e d i c t e d  showed  Beetles  between  optimal  isopods  appeared to follow a t h r e s h o l d laboratory  and  that may  by  amphipods  be  amphipods  and  s i z e s of amphipods. B e e t l e s  the  more  In  were h i g h l y  successful  isopods  would  model.  pick  than up  in  between and  drop  isopods they c o u l d e a s i l y c a p t u r e . There  were a l s o d i f f e r e n c e s between sexes i n the  with which isopods were i n c l u d e d beetles for  were  mates and  such  making some kind of searching  (Jaeger et a l . 1981) and  and  and  predator  territorial  birds  Presumably,  " t r a d e - o f f " between  f o r food. E f f e c t s  as between f o r a g i n g  foraging  in the d i e t .  of  other  male  searching trade-offs  defence in salamanders  (Kacelnik  avoidance  frequency  in f i s h  et  al.  1981),  ( M i l i n s k i and  1978), have a l s o been shown to reduce attack  r a t e s and  or  Heller  decrease  diet specialization. Selander foraging that  (1966)  evolved to reduce  morphological  between males and have  larger  capturing  proposed  that  sexual  intersexual  differences  in  food  the  differences competition,  food-getting  females r e f l e c t e d t h i s competition.  mandibles  and  may  be  better  than  isopods. However, f i e l d data i n d i c a t e d that  in and  apparatus Males  do  females  in  i t was  the  94  smaller b e e t l e s of each sex which were feeding on small  isopods  isopods.  The  used i n l a b o r a t o r y experiments c o u l d e a s i l y have  been captured by a l l b e e t l e s . Morphological resulted  from  differences  the  in  mandible  mating  probably  use of mandibles by male b e e t l e s to capture  female b e e t l e s as w e l l as prey items. Females any  size  attempt.  tended  to  resist  More than one male sometimes attempted to  mate with a female at a given time, l e a d i n g to male-male Larger males may shown fly  for  be more s u c c e s s f u l  the milkweed  (Borgia  (McCauley  has  a  1982) and f o r the dung  in prey  behavioral  selection  response  on  between the  sexes  must  for  by  Male b e e t l e s spent more time on the upper beach where  isopods were more abundant isopods  be  p a r t of the b e e t l e s  rather than a m o r p h o l o g i c a l one, and could be accounted learning.  been  1980).  The d i f f e r e n c e primarily  beetle  i n such f i g h t s as  fights.  more  frequently.  than d i d females, and would Female  beetles  may  encounter  not  recognize  isopods as prey items. I  have  shown  that b e e t l e s are s e l e c t i v e i n the s i z e s and  types of prey i n c l u d e d i n t h e i r d i e t . both  from  selection  of  foraging  of  larger  should  place  amphipod more  sizes.  emphasis  i n v e r t e b r a t e p r e d a t o r s d i s t i n g u i s h prey from non-prey how  resulted  d i f f e r e n t i a l v u l n e r a b i l i t y and from a c t i v e c h o i c e of  amphipods over isopods and studies  This  l e a r n i n g and memory a f f e c t  t h i s process.  Future on  how  items, and  95  CHAPTER 4: HUNGER AND OPTIMAL DIET  Introduct ion Models of optimal d i e t  (reviewed by  Pyke  et  a_l. (1977))  assume that predators behave so as to maximize t h e i r rate of net energy  i n t a k e . By knowing the energy  values e ( i ) , handling times  h(i),  r e l a t i v e f r e q u e n c i e s f ( i ) and o v e r a l l encounter  i p o t e n t i a l prey types, i t i s p o s s i b l e into  the  predict  be  Charnov  1976a). These prey should always be eaten, and  never eaten when encountered  D e v i a t i o n s from the always or never  optimal d i e t  which  should  prey  incorporated  to  rate R of  ( P u l l i a m 1974,  by  the  failure  of  excluded  - the "always or never"  the  have  model to account  (Milinski  and  1978, H e i n r i c h 1976),  Heller  1978),  Hughes 1978), n u t r i e n t balance  avoidance  prey r e c o g n i t i o n time  (Elner and  (Pulliam  1975, Westoby 1978), and (Pulliam  1974).  The optimal d i e t model does not allow f o r changes  i n the  of predator hunger. I n c r e a s i n g hunger i s known to a f f e c t  the p r e d a t i o n process by i n c r e a s i n g feeding r a t e Beukema 1968, McCleery predator  activity  and S i s o j e v i c eaten  (Krebs  predator  the random nature of encountering prey  degree  been  f o r other  c o n s t r a i n t s on the f o r a g e r s such as the need f o r sampling et a l . 1978, Davidson  rule.  r u l e t y p i c a l l y occur however  in experimental t e s t s of the model p r e d i c t i o n s . They explained  prey  ( E r n s t i n g 1977,  1977), r e a c t i o n d i s t a n c e ( H o l l i n g  1966),  (Beukema 1968, Calow 1974), prey use (Haynes  1966, Johnson et a l . 1975), and s i z e range of prey  (Heatwole  and Heatwole 1968, K i s l a l i o g l u and Gibson  1976).  Some of these r e s u l t s may be i n t e r p r e t e d i n terms of the optimal  96  diet  model  i f r e c o g n i t i o n or assessment  predators  is  approach  satiation,  and  modified  by  they  Here  opposite  result.  I  describe  diet  prey.  A  using  predator  t e r m e d an e x p a n d i n g whether diet  this  Consider attempts obtain the  food  diet  on  predation process,  the  Maximizing period  the evidence  equivalent  determine which prey For  prey  types  types  changes  1971), a p r e d a t o r (T)  I make i d e n t i c a l  that e(a)>e(b),  three  reasonable  this  above  f o r the e f f e c t  assumption  i n t a k e over  minimizing  T.  i s not the The  and e ( a ) / h ( a )  >  only  food  o f hunger  unreasonable.  entire  foraging  problem  t o i n c l u d e i n the optimal  A prey  to  assumptions to  t o make up i t s  i s to  diet. prey,  e ( b ) / h ( b ) . Then t h e r e a r e  alternatives:  f e e d on t y p e  which  necessary  A a n d B, l e t A be t h e more p r o f i t a b l e  such  (1)  pictus,  p r e d i c t e d by t h e m o d e l .  f o r a g i n g time  cited  to  (1980) h a s  one a d d i t i o n , t h a t t h e p r e d a t o r c a n  t h e r a t e of e n e r g y  is  value  d e s c r i b e an e x p e r i m e n t t o  t h e f o o d v a l u e D, r e q u i r e d f r o m p r e y Given  Charnov  lower  i s what H e l l e r  (Schoener  requirement.  deficit.  1974,  model  minimizer  model w i t h  were a b u n d a n t  to include  i n the d i r e c t i o n  t o minimize the t o t a l  optimal  assess  a time  a fixed  rule  predators  conditions, predators  the b e e t l e Thinopinus  The  as  by  model w h i c h p r e d i c t s t h e  satiation  s p e c i a l i s t . I then  satiation  Then  1971, P u l l i a m  two p r e y  near  one p r e d a t o r ,  near  a  availiability  behave a s i f p r e y  I show t h a t under c e r t a i n  expand t h e i r  test  levels.  (Schoener  should  its  should  become more s e l e c t i v e  1976a).  hunger  of f o o d  (specialist)  97  (2)  feed on type A and B prey as encountered  (3)  feed on type A prey i f D i s l e s s than  otherwise  feed  on  type  (expanding  specialist).  A  and  B  (generalist)  a  prey  given as  encountered  The model I present here p r e d i c t s the mean and v a r i a n c e total  foraging  time  value,  of  the  f o r a predator f o l l o w i n g one of the above  rules. First search  c o n s i d e r the case where  for  and  predator  will n+1  i s an i n t e g e r such that 0<z<e(b)  D = ne(b) + z, the  The  feed on at most one type A prey, or at most  type B prey. Here n>0  For  D<e(a).  specialist,  expected  h a n d l i n g time E(H  ) i s the S  food requirement  d i v i d e d by the intake rate while f e e d i n g on the  item. I assume that  intake r a t e , and hence  handling  zero v a r i a n c e . Expected search time E(S ) and  time,  i t s v a r i a n c e V(S )  S can  has  S  be d e r i v e d from the e x p o n e n t i a l d i s t r i b u t i o n with parameter  R f ( a ) . Here R i s the r a t e at which prey (assumed  constant),  encountered  and  f(a)  is  the  item i s a type A prey. Hence  f(a) + f(b) = 1 Then expected f o r a g i n g time i s E(T ) = E(H ) + E(S ) S S S = Dh(a) + 1  iTaT  with v a r i a n c e  Rf (a)  items  are  probability  encountered that  the  98  V(T For on  the  Expected E(T ) G  S  )  =  l/[Rf(a)V  the g e n e r a l i s t , the number of items eaten order  i n which  type  A  and  will  depend  B prey are encountered.  f o r a g i n g time i s  =  E(H ) + E(S ) G G  =  Dh(a) + [ h(b)-h(a) ] [ D - n e ( b ) ] f ( b ) elaT e l b l iTaT  n+1  + [ h(b)-h(a) eTBT eTaT  ]e(b)[l-f(b) ]f(b) ITaT n  n+1 + [1~f(b) ] Rf (a) D e r i v a t i o n s of E(H ) and E(S ) are given i n Appendix A and G G B r e s p e c t i v e l y . The f i r s t specialist.  for The  f o r the  on  incurred  f o r f e e d i n g on an amount z from a type B prey.  term i s the e x t r a h a n d l i n g i n c u r r e d by the  feeding  generalist  type B prey before encountering a type A prey.  f o u r t h term i s the search time. Expected  greater  time  The second term i s the e x t r a h a n d l i n g time  by the g e n e r a l i s t The t h i r d  term i s the h a n d l i n g  f o r the g e n e r a l i s t ,  while  handling  expected  search  g r e a t e r f o r the s p e c i a l i s t . The g e n e r a l i s t w i l l have f o r a g i n g time when E(T ) < E(T ) G S or when [ h(b)-h(a)]{ z + e ( b ) [ l - f ( b ) eTBT eTaT  n  -n ]f(b) } fTaT  <  1 Rf (a)  time  is  time i s the  lower  99  The  extra handling  the s p e c i a l i s t Foraging V(T where  G  )  V(H  search  )  V(H  ) + V(S  G  and  V(S  r e s p e c t i v e l y . Search  than  time.  time v a r i a n c e f o r the g e n e r a l i s t i s given  =  G  i n c u r r e d by the g e n e r a l i s t must be l e s s  S  )  G  by  )  are  derived  time v a r i a n c e  in  is  Appendix  always  A  greater  and for  B the  s p e c i a l i s t . Hence the g e n e r a l i s t w i l l have a lower f o r a g i n g time variance i f V(H  G  )  Fig.  <  V(S  Total  ) - V(S  G  16 shows expected  requirement. generalist  S  For  certain  reaches  ) f o r a g i n g times as a f u n c t i o n of ranges  of  parameter  s a t i a t i o n more q u i c k l y than  foraging  time  increases  with  values,  the  D.  food the  specialist.  Increases  are  d i s c o n t i n u o u s at the p o i n t where D i s an exact m u l t i p l e of e ( b ) , that  is,  when  discontinuity  an  additional  represents  s u c c e s s i v e prey  the  prey time  items when f o r a g i n g  item spent time  is  required.  i n searching between increases,  intake does not. An  increase in e ( b ) , or a decrease  or  the  f(a),  extends  g e n e r a l i s t has variance  is  the lower independent  range  of  total  values  foraging  of time.  D  but  in  Next  the  time  increases where  an  take  any  item i s r e q u i r e d .  consider  gamma  h(b)/e(b)  Foraging  of D f o r the s p e c i a l i s t and  the  general  case  v a l u e . The mean and v a r i a n c e of f o r a g i n g from  food  f o r which the  with D f o r the g e n e r a l i s t , with small d i s c o n t i n u i t i e s a d d i t i o n a l prey  The  distribution  as  in which D can time  can  be  derived  f o r the r e s t r i c t e d case.  The  100  Figure  16. Expected t o t a l f o r a g i n g time f o r the g e n e r a l i s t ( s o l i d l i n e ) , and s p e c i a l i s t (dotted l i n e ) as a f u n c t i o n of the food requirement, D. E r r o r bars represent one standard d e v i a t i o n . Parameter values are R=0.65, e(a)=1 and h(a)/e(a)=20. Other values are given i n the f igure.  5  TOT  TOTAL 8  8  P  FORAGING  5  TIME 8  8  $  102  specialist  search f o r and feed o n at most m+1  will  type A  prey.  i s a n integer such that  Here m>0  D  =  me(a)  + y,  0<y^e(a)  Then E(T  S  )  =  )  =  m+1  RfTa)  +  Dh(a)  elaT  and V(T  (m+1) TRfTaTT  S  Hence  foraging  function of The shown  time  2  variance  for  specialist  i s a step  D.  formula f o r the g e n e r a l i s t  by  the  simulation  that  for  i s complex, but  certain  parameter  it  can  be  values the  g e n e r a l i s t w i l l have a lower f o r a g i n g time than the  specialist.  The  specialist,  interesting  case however, i s f o r the expanding  the predator which switches from s p e c i a l i s t it  has consumed m type A prey. That  is required for s a t i a t i o n . foraging the sum  time f o r the expanding of  specialist on Fig.  the  mean  and  mean  i s , at most one type A prey and  variance  specialist  variance  after  of  for  total  c a n be obtained from  foraging  time  for  a  f e e d i n g on m type A prey and f o r a g e n e r a l i s t f e e d i n g  D-me(a) 17 which  reaches  The  to g e n e r a l i s t  energy u n i t s o f is  analogous  type A and B prey. T h i s i s shown i n to  F i g . 16b.  If  the  generalist  s a t i a t i o n more q u i c k l y than the s p e c i a l i s t when D<e(a),  then the expanding  s p e c i a l i s t must have a  for the same range o f  v a l u e s when D>me(a).  lower  foraging  time  103  Figure  17. Expected t o t a l f o r a g i n g time f o r the g e n e r a l i s t or expanding s p e c i a l i s t ( s o l i d l i n e s ) and s p e c i a l i s t (dashed l i n e ) as a f u n c t i o n of the food requirement, D. E r r o r bars represent one standard d e v i a t i o n . Parameter values are R=0.65, e(a)=1, h(a)/e(a)=20, h(b)/e(b)=40, e(a)=1, and e(b)=0.25.  105  A t e s t with Thinopinus F i e l d data on the predator Thinopinus  preferred  large  Thinopinus p i c t u s suggested that  s i z e s of the amphipod Orchestoidea  c a l i f o r n i a n a as. prey items. I designed a to  test  whether  this  preference  was  Short-term l a b o r a t o r y experiments had small  (8-11  required  mm)  to  mg/min and  amphipods or satiate  1.99  a  mg/min on  affected  (16-19 mm)  Mean  small and  experiment by  hunger.  shown that on average  large  beetle.  (Table V I I ) . These values used in F i g .  1.0  laboratory  amphipods were  feeding  r a t e s were  approximately  corresponded  to  those  16a.  prefer  satiation  large  over  small  show no p r e f e r e n c e . 2, 4,  densities  of  10  small  design  chosen so that I c o u l d a l s o t e s t the p r e d i c t i o n  s e l e c t i o n that the d e n s i t y  and  10 l a r g e amphipods. T h i s p a r t i c u l a r  all  (3) p r e f e r e n c e  optimal  diet  models  of  for l a r g e amphipods i n c r e a s e s  from prey with  of large amphipods.  To o b t a i n food  8 or  used  (2) b e e t l e s near  and  frequency-dependent  6,  I  amphipods  starved  amphipods was  1.38  l a r g e amphipods r e s p e c t i v e l y  Q u a l i t a t i v e p r e d i c t i o n s based on the model were (1) beetles  2.4  two  hunger l e v e l s I e i t h e r h e l d  beetles  without  f o r 3 d ( s t a r v a t i o n treatment) or fed them the night p r i o r  to the experiment male and  ( s a t i a t i o n treatment).  For each treatment,  12 female b e e t l e s were each placed  diameter,  10  cm deep) c o n t a i n i n g  l a r g e and  small  amphipods.  a 3 cm  Because  of  in a g l a s s j a r (8  l a y e r of damp sand this  variable  number of amphipods were a c t i v e on the  any  time, with the  given  sand  12 cm and  layer,  a  sand s u r f a c e  at  remainder of the amphipods in  burrows.  106  Beetles  also  amphipods escape  and  were  of  eaten  recovered  left  live  feeding,  Jars  were  by  in  inference.  a l l large  for  temperatures  amphipods  so they  covered  overnight  at laboratory  number  number  after  continuously.  photoperiod the  burrowed  each  to  d i d not  encounter  prevent  amphipod  20-22  h  (16-19°C). jar  In c o n t r o l  and jars  a m p h i p o d s a n d a mean o f  under  natural  then  counted  I  determined  without  9.3±0.3  the  beetles  (n=8)  I  small  amphipods. I the  separated  number  each  the  of amphipods of  amphipod s i z e  male  effects  beetles  treatment  t o f e e d on more  variances  were  differences  small  df=1,1l0,  homogeneous.  i n t h e number large  of  df=4,110,  P>0.10, F i g . 1 8 , w h i t e  number  of  large  amphipods  was  sex  large  more  amphipods  significant amphipods  presented  pronounced  df=4,110,  p<0.000l)  df = 4 , 1 1 0 ,  p=0.05),  (Bartlett's the  test,  interaction  df=4,110,  in  p>0.10).  with  at  and p=0.0l not  different  the  (F-tests, interaction  P>0.10).  The  number  of  the  This  treatment  were  and  significant  1 8 , shaded b a r s ) .  variances  for  p>0.l0)  treatment  df=4,110,  for  male-female  no  n o r were  on  starvation  other  amphipods eaten  the s a t i a t i o n  sex were  the  were  either  ANOVA  tendency  df=1,1l0,  starvation  in  in  but  increased  p<0.00l  terms with  amphipods  bars),  (Fig.  although  w a s a weak  (F-tests,  the  than  df=4,  in  eaten  i n a 2-way  (F-tests,  small  of  with  There  There  densities  terms  eaten  p=0.06),  not s i g n i f i c a n t  were  sex and amphipod abundance  each s i z e  and t r e a t m e n t .  (F=3.57,  comparisons  of  (F=6.22,  treatment not  effect  (F=2.42,  homogeneous  respectively).  significant  Again  (F-tests,  1 07  Figure  18. The mean ±1SE number (n=24) of l a r g e amphipods (shaded bars) and small amphipods (white bars) eaten at d e n s i t i e s of 10 small and 2, 4, 6, 8, or 10 l a r g e amphipods. (A) s t a r v a t i o n treatment and (B) s a t i a t i o n treatment.  108  109  I combined r e s u l t s from male and female b e e t l e s to t e s t f o r differences Beetles than  between hunger treatments at each amphipod  from the s t a r v a t i o n treatment ate more  beetles  were present  small  density. amphipods  from the s a t i a t i o n treatment when l a r g e amphipods at d e n s i t i e s of 2  and  10  only  (t-tests,  p<0.05), and ate more l a r g e amphipods at a l l d e n s i t i e s  df=46,  (t-tests,  df=46, p<0.0!).  Table X. Values of c at d i f f e r e n t d e n s i t i e s of l a r g e amphipods for each hunger treatment. Values given are c o r r e c t e d ( c o r r ) or not c o r r e c t e d (ncor) f o r l o s s of small amphipods. stavation corr ncor  satiation corr ncor  2  1.99  2.78  1.15  0.86  4  2.26  1.68  1.25  0.89  6  1.43  1.02  0.77  0.59  8  1.72  1.25  0.79  0.55  10  0.94  0.75  0.84  0.54  density  To  test  for  preference  I  first  computed the measure o  suggested by Chesson (1978) where preference  f o r l a r g e amphipods  i s given by oi  where w, and w  2  eaten,  and  x,  =  (w,/x,)[(w /x )+(w /x )]1  1  are the numbers of and  x  2  are  the  amphipods presented. Preference a2  =  2  large numbers  1  2  and of  small  amphipods  l a r g e and small  f o r small amphipods i s given 1~oi  by  110  This  m e t h o d was c h o s e n  obtained  when  beetles  on one  compare  and  test  found  preference  p=0.05)  and 8  10  (n=23,  of  the  (n=24,  p=0.09).  also  2,  4,  compared  satiation decrease  at  differences  at  To c  c, f o r  test  (Murdoch  the  for  used  of  small  c either large both  with  amphipods  satiation  test.  6,  4 and in  amphipods  22,  and  and  8  10 w e r e  in  19  by  for  predicted.  a  near  significant beetles  (n=24,  with  none  beetles  was  the  density  24 a n d  for  I  (n=22,  treatment, 24,  amphipods  2  at  P>0.10) a s  There  preference  for  =  near  p<0.05),  same  and  direction.  density,  I  computed  w,  w , x /w x , 2  and w  lost  no t r e n d  2  2  from F i g .  a correction jars.  or  with  decreased This  frequency-dependent  and  was  For  the  repeated mean  both  to  diet  the  number  treatments,  increasing  opposite  optimal  18 a n d  for  from c o n t r o l  (Table X).  selection.  (n=24,  then  beetles  densities  the  large  and w i t h o u t  amphipods showed  at  I  be  where  mean v a l u e s  calculations  amphipods  beetles  could  2  starved  in  2,  c I  For  2  small  median  changes  1969)  o .  o  amphipod o n l y .  10 r e s p e c t i v e l y ,  densities  for  of  for  starved  densities  c, a n d  size  significant  8 and  for  and  beetles  preference  satiation  large  p=0.06),  were  6,  using in  for  For  comparisons  densities I  to  a value  fed  used a s i g n weak  because  density  predictions models  of  of of  prey  111  Discussion The  optimal  diet  model  (Pulliam  1974,  Charnov  1976a)  p r e d i c t s that predators should s p e c i a l i z e i f 1  < e(a)h(b) - h(a)  eTBT  Rf(a) This  condition  Clearly, prey,  i s met  f o r the  parameter values i n F i g . 16.  i f predators can assess the food  the  net  rate  of  food  value  required  intake does not p r e d i c t when to  s p e c i a l i z e . The optimal d i e t can change i n composition items of lower value are i n c l u d e d satiation.  Essentially  this  as  the  i f i t accepts the f i r s t  search f o r a high value  predator  approaches  can  be  satiated  item i t encounters,  intake  prey.  requirements  s i z e s . For Thinopinus  more  r a t h e r than  A range of D values can a l s o be i n t e r p r e t e d as a food  such that  means that i f only a p o r t i o n of a  high value item i s r e q u i r e d , a predator quickly  from  to  satiate  predators  range  of  of d i f f e r e n t  p i c t u s f o r example, I found a  correlation  of 0.505 between the weight of food r e q u i r e d to s a t i a t e a b e e t l e and  beetle  are expected rate  with  weight  (page 65). Hence, d i f f e r e n c e s i n preference  f o r b e e t l e s of d i f f e r e n t the  high  value prey  The  If  is sufficiently  p r e d i c t s switches between s p e c i a l i s t f o r a g i n g r u l e s with a continuous  sizes.  and  the  encounter  low, the model  expanding  i n c r e a s e i n predator  specialist size.  g e n e r a l i s t or expanding s p e c i a l i s t may have a  (1) lower mean and v a r i a n c e of f o r a g i n g time (2) higher mean but lower (3)  higher  v a r i a n c e of f o r a g i n g time  mean and v a r i a n c e of f o r a g i n g time r e l a t i v e to  1 12  the  specialist  Whether a specialist  or  predator  specialist,  responds to v a r i a t i o n outcomes  (1)  forages  and  as  will  a  generalist,  depend  in  on  in encounter r a t e s of the prey  specialist  for  D=0.20 mean d i f f e r e n c e s i n f o r a g i n g times  how  it  types.  (3), the g e n e r a l i s t or expanding  and  specialist  part,  expanding^  For  specialist,  s t r a t e g i e s r e s p e c t i v e l y are favored. For example,  and  specialist  will  be  for the  statistically  expanding significant  (p<0.05) a f t e r about 20 f o r a g i n g bouts ( o n e - t a i l e d z t e s t ) using values  bouts  using  values from F i g . 16c. For p r e d a t o r s which feed many times  during  their  from F i g . 16b,  or a f t e r  about  6  foraging  l i f e t i m e , even small time savings may For  strategy  outcome  (2),  may  favored  be  risk-aversive  the  become  g e n e r a l i s t or expanding by  manner (Caraco  a  predator  1980,  of  constant  over  Conversely, foraging  (1981) has  variable  a forager may  on average to achieve  food  favor a s p e c i a l i s t  are  in  a  and  a  lower  satiation.  wasps  prefer  expectations.  strategy i f  it  is  r e q u i r e a s h o r t e r time  i n c u r s the r i s k of t a k i n g a  time.  Heller  (1980)  randomly  distributed  developed  a model from which he showed  that an expanding s p e c i a l i s t may prey  have  rewards with equal  s a t i a t i o n , but  Rarely w i l l prey be assumes.  also  shown that bees  in a r i s k - p r o n e manner. I t w i l l  much longer  foraging  t a k i n g a much longer time to achieve  For example, Real  specialist  Caraco et a l . 1980). I t w i l l  have a higher mean f o r a g i n g time, but w i l l probability  important.  clumped.  Hunger was  also  have  an  not c o n s i d e r e d  as  this  advantage  model  when  in h i s model, but  113  p r o f i t a b l e prey c o u l d be  rapidly  depleted  from  a  patch  and  i n t e r p a t c h t r a v e l times were long. It  is  commonly  variety  of  prey  (e.g. Schoener work of I v l e v selective Johnson  h e l d that hungry p r e d a t o r s a c c e p t a wider  types  than  do  predators  near  satiation  1971). The best evidence f o r t h i s comes from the (1961) who  as  they  showed that  carp  became  increasingly  approached  s a t i a t i o n . In c o n t r a s t , Akre and  (1979) found a decrease  in p r e f e r e n c e near s a t i a t i o n f o r  d a m s e l f l y naiads feeding on two prey types. C o n t r a r y evidence i s given by the experiments Thinopinus preference  pictus  with  r e p o r t e d here. showed  satiation  a  significant  decrease  in  f o r l a r g e amphipods at most amphipod  d e n s i t i e s when s t a r v e d b e e t l e s and b e e t l e s near  satiation  were  compared. Although t h i s r e s u l t was i n q u a l i t a t i v e agreement with the  model presented here, the q u a n t i t a t i v e p r e d i c t i o n s were not  met. B e e t l e s i n both treatments fed on small densities. density  as  deviations  amphipods  Preference f o r l a r g e amphipods d i d not i n c r e a s e with predicted. from  the  The  model  "always  could  account  specialist  to  generalist  s a t i a t i o n d u r i n g an experimental interpretations  of  damselfly  naiads  satiation  and  these  may  alter  if  beetles  f o r a g i n g r u l e s as they run.  results,  change  for  these  or never r u l e " i f v a r i a t i o n among  b e e t l e s was r e l a t e d to s i z e d i f f e r e n c e s , or from  at a l l  their  There  however. search  the r e l a t i v e encounter  were  switch approach  alternative  Both b e e t l e s and behaviour  near  r a t e s of d i f f e r e n t  prey t y p e s . The model presented here i s most  applicable  to  predators  11 4  near  satiation,  as  other  factors  likely  affect  foraging  d e c i s i o n s of a predator which has been s t a r v e d f o r a s u b s t a n t i a l p e r i o d of time p r i o r to experimentation. Future t e s t s must  also  allow  for  satiation.  Prey  size  should  be  sufficiently  to  food  requirement,  so  that  changes  in  predator behaviour large  changes  in  near  relative preference  are measurable.  S u i t a b l e p r e d a t o r s f o r f u t u r e t e s t s are a n t h o c o r i d 1976)  or  relatively  salticid few prey  spiders  (Givens  1978),  items for s a t i a t i o n .  the  which  bugs  (Evans  require  a  11 5  CHAPTER 5: CONCLUDING REMARKS The  s t a p h y l i n i d -.beetle  structurally  simple  Thinopinus  environment.  pictus  Yet,  the  food  Thinopinus v a r i e s over a range of s p a t i a l and Thinopinus  is  obviously  this  Most  of  result the  invertebrates  spatial variation.  i s a general  support  supply  to assess  I  have  for invertebrate  optimal  foraging  with  a c t i v e f o r a g e r s which search  slow-moving or sedentary  prey  (e.g. crabs  and  Pianka  (Davidson  (1981)  1978), and  compare  foraging  modes  based  Species  which ambush g e n e r a l l y eat  lower metabolic c o s t s , and  on  have  and  for  Hughes  bumblebees (Pyke 1978)). Huey  various  ambush  and  predators.  from  ants  of  argued  theory  (Elner  a  scales.  come  1978),  has  for  one  in  temporal  limited^ in i t s a b i l i t y  respond to t h i s temporal and that  lives  correlates studies  of  of  active  desert  and  lizards.  fewer, more a c t i v e prey, have a  limited  learning  ability,  r e l a t i v e to species which are a c t i v e f o r a g e r s . These r e s u l t s can probably  be  applied  to  i n v e r t e b r a t e s as w e l l . One  which emerges i s that there are not govern  foraging  behaviours  simple general  range of  to  c a p a c i t y of the  the  "correct  forager,  and  the  behaviours  which  the  rules  available  forager  to make  decisions".  A second c o n c l u s i o n respond  on  rules  of a l l animals. Instead,  depend on prey behaviour, on the  conclusion  to  Morse and predictions  prey Fritz from  i s that the c a p a c i t i e s of  foragers  to  v a r i a t i o n depend on the s c a l e of measurement. (1982) optimal  suggest  that  for  f o r a g i n g theory  ambush  predators,  based on patch  are more s u c c e s s f u l than p r e d i c t i o n s based on d i e t c h o i c e .  choice This  11 6  was  not  true  for  Thinopinus,  p r e f e r e n c e f o r the prey type highest  feeding  rate.  as  Thinopinus  (amphipod)  However,  because  a  wider  which  Thinopinus  p r e f e r e n c e s among s i z e s of amphipods. results  on  variety  The of  showed it  showed  had  are  d i s t i n g u i s h among prey types than among s i z e s of  the  only weak  difference  cues  strong  probably  a v a i l a b l e to a  given  prey  type. The  most  behaviours behaviour  successful  have  been  (Hanski  based  1980),  c a p a c i t y of the animal  (Pyke  in  a v a r i e t y of experiments  al.  (1981) and Real  depend  recent  (1981)  on  or  on  1978,  attempts simple  to p r e d i c t f o r a g i n g  stochastic  assumptions  of  O l l a s o n 1980).  rules the  For  of  memory example,  Caraco et a_l. (1980), Waddington e_t have  shown  that  food  preferences  not only on the expected outcome of a f o r a g i n g bout,  but  a l s o on the v a r i a n c e of that outcome. Hence, as i s so o f t e n true in e c o l o g i c a l s t u d i e s , the d i s t r i b u t i o n the  mean.  variation  Problems  of how  i s more  f o r a g e r s l e a r n about  i n prey a v a i l a b i l i t y  important and  w i l l continue to be one  and necessary d i r e c t i o n f o r f u t u r e r e s e a r c h .  than  respond  to  exciting  1 17  LITERATURE CITED  Akre, B.G. and D.M. Johnson. 1979. 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Behavior 67:134-148.  1 26  APPENDIX A Expected  handling  time  f o r the  generalist  can be  expressed as E(H  )  =  G  n i n+1 E[iP+W]f(a)f(b) + X f ( b ) i =0  where P  =  h(b)-e(b)h(a)/e(a)  W  =  Dh(a)/e(a) and X  =  Dh(b)/e(b)  The term i n square brackets i s the time to feed on i type prey,  and then to complete  f e e d i n g on a type A prey.  term i s m u l t i p l i e d by the p r o b a b i l i t y that such of  events  will  reach s a t i a t i o n  occur.  a  B  This  sequence  The second term g i v e s the time to  f o r feeding on type B prey o n l y ,  times  the  p r o b a b i l i t y that no type A prey are encountered. The  mean  generating M (t) H  and  v a r i a n c e can be d e r i v e d from the moment  function n [ip+w]t i f ( a ) Ee f(b) i =0  =  +  Xt n+1 e f(b)  Wt (n+l)Pt n+1 Pt -1 + e f(a)[l-e f ( b ) ] [1-e f(b) ]  =  Xt n+1 + e f(b) Then E(H  G  )  =  =  M' (0) H n+1 n P[nf(b) -(n+1)f(.b) + l ] f ( b ) / f ( a )  n+1 n+1 + W[l-f(b) ] + Xf(b) and  127  V(H  G  )  =  M"(0)  =  [X-W]  - [E(H ) ] G  H 2  2  n+1 n+1 [1-f(b) ] f ( b )  n+1 n n+2 - 2P[X-W][nf (b) - ( n + D f ( b ) + l ] f ( b ) / f ( a ) 2 2 n+1 - P [n d + f ( b )  n+1 )+2n]f(b)  2 2n+2 2n+1 n+1 + P [2nf(b) -(2n+1)f(b) +f(b) n 2 - f(b) +l]f(b)/f(a)  128  APPENDIX The  moment g e n e r a t i n g  generalist  M (q) S  is  given  of  search  time  for  the  by  n+1 T q t - R t n+1 n -1 f (b) J e R t [n! ] dt 0  +  integration  function  of  waiting  times  multiplied are  function  .co . . n 1 r q t - R t i+1 I -1 Ef ( a ) f ( b ) J e R t [ i ! ] dt i =0 0  =  The  B  the  by  performing  gamma to  the  eaten,  and the  term  describes  the  distribution,  obtain  exactly  probability summed  that  over  integrations  and  moment  the i+1  distribution  prey  exactly  all  generating  items.  i+1  values  of  summations t h i s  of  It  prey  items  i.  After  becomes  n+1 M  (q)  =  S  Rf(a) (l-[Rf(b)/(R-q)] Rf(a)-q +  The E(S  )  expected =  G = with  n+1 f(b) {  R/(R-q)  value  of  }  n+1 }  search  time  is  given  by  M'(0) S n+1 -1 [1-f(b) ][Rf(a)]  variance  V(S  )  =  G =  M"(0) S  -  [E(S  )]  is  2  G  n+1 n+2 2n+2 -2 [ 1 - 2 (n+1 ) f ( b ) +2(n+Df(b) -f(b) ][Rf(a)]  

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