Ecology, 1994, v.75, n.2, pp.532-547
ISSN: 0012-9658
DOI: 10.2307/1939556
http://www.esajournals.org/loi/ecol?cookieSet=1
http://www.esajournals.org/doi/abs/10.2307/1939556
©1994 by the Ecological Society of America
F . < ~ / r i q \ .75(2). 1994. pp 532-547
i
1994 h) thc Ecologtcal Sorlrty ol Arncrlca
ASSESSING HOW FISH PREDATION AND INTERSPECIFIC PREY COMPETITION INFLUENCE A CRAYFISH ASSEMBLAGE1 . l I ~ s f t . u ~ ~In
t . northern Wisconsin lakes. the introduced crayfish 0rc.otlrc.tcs r~lsticusis
replacing 0. propiilqir~c.r.a previous invader. and 0. ~.irili.s,a native craqfish. Herein. \ve
cxplore how fish predation and competition interact to drive this change in crayfish species
composition. In outside pools. we conducted selective predation experiments exposing
craqlish to largemouth bass. .21ic,t~optc,russultlro~dc.r,to quantify patterns of crayfish vulnerabilitq. T o deter-mine how interactions among crayfish influence susceptibility. we quantilied shelter usc and behavioral interactions among craqfish in aquaria and outside pools.
At equal s ~ / e 0.
. ~.rrilrsw as more susceptible to fish predation than either o f t h e invaders.
0. ru.\tic,~(.sa nd 0. ~ ~ r o p t t i y ~ rthe
u i : two inbadcrs mere equally susceptible to predation.
Howc\er. s i x s of these craqfish in the field are 0. 1.tr111.r> 0. ~ I I S I I C L I S> 0. prol1it1q1l1l.r.
s
Because fish predators prefer sniall crayfish. at unequal size. small 0. p r o p r t q ~ ~ u\Irere
can replace 0.
morc vulnerable to predation than large 0. rlc.rfirus. Thus. 0. rli.\t~c~ls
Although 0.~Yrilisgrowslarger than the invaders.
~ ~ r c ~ ~ ~ ~ t ~toynatural
u ~ r s dsile
u e difftrences.
~t was more susceptible e\,en when 3 nini larger. We hqpothesized that 0. l,irili.s, although
large. part~cipatcdin beha\ iors that increased its risk to predation.
When provided with unlimited shelters. all three species increased refuge use under
was excluded from
predatory risk. When shelters were limiting and fish present. 0. ~>irilis
shelters bq in\ ader-s. 0. ~.trtltsa lso participated in risk! behaviors. such as increased activity
and swimming. Both agonistic interactions with congeners and approaches by largemouth
bass Increased risk! behaviors in 0. 1.iri1t.s.In addition. 0. 1.1rili.rwas innately less aggressive
than in\.aders. G i \ e n these bcha\.iors. 0. 1.rt.111swas consumed at high rates and \vould be
e\entuallq replaced in lakes.
In northern Wisconsin lakes. lish predation and cra!fishxrayfish con~petitioninteract
to Influence crayfish replacements. Based on our results. largemouth bass predation modifies
the outcome of ~nterfercncecompetition among the three crayfishes and. in turn. competitive Interactions among the crab fishes influence susceptibility to fish predation. We predict
should suffer high mortalit) to fish predation in the presence. rather than in
that 0. 1~rt.111.s
the absence. of the two in\.ading species. O u r results support the hypothesis that. in areas
of sqmpatrq whcrc predators are \electi\e and pre! species compete. predation and competition interact to d e t c r m ~ n ecommunity structure.
j
o r . ugyrc,.\.\rot7:
o~oidiitrc~~
hc~h~vror:
c ~ t ~ ~ ~ ~ c tcruj./l.~h;
i i r o n ; iruhitut use; Iur~(1tno~th
huss;
Micropterus salmoides: t~lorpirolo,g~~:
north tetnpcrutc luke~;Orconectes propinquus: Orconectes mst~cus:Orconectes v~rilis:prcw'utron; rc'fic,gc use,; S I Z ~ :.sp(,ci~s~ nt~usron.
Ecolog~stsh a\.e long sought to understand how biotic
forces shape c o m m u n ~ t i e rIn
. a vast number of studies.
both predation (see Sih et al. 1985. GliwicI and Pijanowska 1989 for re\ iews) and competition (Hutchinson 1959. MacArthur and Levins 1967. Price 1978
Smith and ('ooper 1987. " c o r ~ n1983. Johnson et al.
1985, see Schoener 1983 for- re\ iew) h a l e been found
to dramat~call! influence communitq structure. .41though these two forccs can work indcpendentlq. the!
also can act together In c o n ~ p l e xwaqs to shape many
c o m m u n i t ~ c s( e.g.. Hairston et al. 1960).
Rclat~onshipsbetween predation and competition
are Laricd. Selective predators can rcnio\,c competi- ti\.eI? dominant prey and ther-eb) increase prey species I Manuscr~prreccl\ed I8 No\eniber 1992: revised 6 Ma?
1993: accepted 2 1 Ma? 1993.
divcrsitq (Paine 1966 l n o u l e ct al. 1980. Morin 198 1 .
1986. Steneck et al. 199 1). Predation and conipetition
can \.ary in importance along environmental gradients
(Mcnge and Sutherland 1976. Lubchenco 1978. Southerland 1986)and amongdifkrent seasons (Cubit 1984).
Bq reducing pre) densit). predation can release sur\.iving PI-e) from conipetition for limited resources
( W ~ l h u er t al. 1983). Clearlq. predation and competition are interrelated n ~ e c h a n i s n ~that
s influence cornnlunlt) structure.
Predation and competition can interact such that one
b ~ o t i cprocess modifies the other (Kotlerand Holt 1989).
Although p r c d a t ~ o ncan directlq modifq competition
bq removal ofcompetitivel! dominant preq it also can
indircctl! influence pre! behaviors and. hence. pre)
resource use (Mittclhach 198 1 . Werner et al. 1983).
Both nonconipetiti\.c and competitive interactions
anlong preq can alter the outcome of predation. T o our
March 190.1
SELECTIVE P R E D A T I O N A N D P R E Y C O M P E T I T I O N
533
knowledge. such relationships have on11 been denl- rimultaneouslq evaluating predation and competition.
onstrated for noncompeting prey. Noncompetitive In- we assess how these two ecological processes could
~nllucnceongoing species replacements in northern
t e r a c t ~ o n sbetween salanianders and isopods modif)
predator) risk of both species (Huang and Sih 1990. Wisconsin lakes.
199 1 ). Similarlq. crab fish reduce predator) rrsk of sculpins b) thcir shared predator (McNeely et al. 1990).
.Although. for these systems. preq only share a common
We conducted experiments in 200-L laboratory
predator and not common resources. evidence exists aquaria (photocycle: 16 h light : 8 11 dark) and in cirthat competition for limited resources among sqmpat- cular. outdoor pools (1.8 m diameter. I n~ depth) at
ric c r a ~ f i s hspecies does result in modified predation
Universit! of Wisconsin. Center for Limnology. Trout
b) fish (Butler and Stein 1985. Soderhack 1990).
Lake Station. Vilas County. Wisconsin. during J u n e
Herein. we explore how fish p r e d a t ~ o nand interspe- through August. 1990 and 199 1 . T o maintain water
cilic competition interact to inllucnce a crayfish species qualit!. all pools and aquaria were supplred with conreplacement. In northern Wisconsin lakes. the crayfish tinuous flow-through water from the epilimnion of
0. rlr\t~c,lc,,the most recent invader. I S replacing 0. Trout Lake. Temperatures in pools and aquaria ranged
~ ~ r o j ~ i t ~ cal ~
previous
i ~ r s , invader. and 0. ~~irilis,
a native from 16" to 73°C.
crab fish (Capelli 1982. Lodge et al. 1986. Olsen ct al.
199 1 ). 0.[ I ~ O ~ I I ~ I ~ L Ican
I ~ . \ replace 0. ~,irilis
(Capelli 1982.
Largemouth bass. . ~ I ~ ~ ~ I . ~ I.suIt~~oic/c~s
I ~ ( , I . L I (s250-275
Lodgc ct al. 1986. Olsen et al. 199 1). Replacement
m e c h a n i s n ~ slikelq include li-equencq of human intro- n1n1 total length [TL]). were collected from C'arrol Lake.
d u c t ~ o n .predation. and competitive exclusion (Lodge Oneida County. Wisconsin with electrofishing gear.
Largemouth bass, housed in outdoor pools. were fed
ct al. 1986. Lodge 1993). Replacenlent rates arc relativelq rapid and monotonic (Olsen et a!. 199 1 ) . Such hapharardly chosen craq fi sh (all three species). mincharacteristics provide a n ideal $)stern in whrch t o ex- nows. and nightcrawlers.
Craqlish were hand collected by snorkeling in lake
plore how predation and con~petition contribute to
littoral zones. 0. ~ L I S ~ ~ and
C I ~ 0.
S j ~ r ( ~ p ~ t ~were
y u u scolchanges In species asscnihlagcs.
Fish predation. crayfish interactions. and crayfish lected from Trout Lake. \vhereas 0. ~.it.ili.rwas collected
from White Sand Lake. Vilas Count). Wisconsin. Upon
behavior likely contribute to the replacement. Crayfish
are important to fish diets(Saik1 andZiebell 1976. Stein capture. craylish carapace length (CL) was measured
1977) and. when tethered in the field. crayfish are se- with vernier calipers to the nearest 0.5 nim. Carapace
lectively consumed by fish (Kershner 1992. Mather and length was defined as the distance between the rostra1
Stein 1993, DiDonato and Lodgc, it1 prcss). Thus. se- tip and posteroniedian edge of the carapace. Unless
e
used in experiments
lective predation by fish likely influences crayfish pop- otherwise stated. all C L s i ~ classes
ulatlons. Crayfish interactions and behavior also can were ? I m m . Similar-sized craylish were housed in
aerated 10-L aquaria before experiments and fed fish
influence the replacement by modifying susceptibilit)
to fish predators. Although 0. r'li.stlc.us. 0.p r ~ q i t ~ q u u s , flakes. tr-out chow. n~acrophytes(mostly Elodc'a spp.
spp.). and leaf detritus daily. Beand 0. viri1r.r are morphologically similar (Capelli and and C'c~r'cztoph~~llu,,~
Capelli 1980). they differ in body (Corey 1988. Olsen cause most crayfish arc nonovigerous o r Form I1 (nonet al. 199 1 ) and chela sire (Garvey and Stein 1993). reproductive) during summer. only these life stages
These crayfishes also differ in aggressive (Capelli and \yere used during experin~ents.
Munjal 1982) and nonaggressive behaviors and, in the
absence of predator) risk. difTerentially compete for
(;c~t~c~ral
tt1c~rhod.5.
- In outside pools and laboratorq
shelter (Capelli and Munjal 1982). Because size, beaquaria. we quantified relative susceptibilities ofequalhavior, and refuge use determine vulnerabilitq to predation in craqfish (Stein and Magnuson 1976. Stein and unequal-si~edcrab fish to largemouth bass predators starved for 24 h. Lcss than 5% of the craqfish.
1977). congeneric differences in these characteristics
marked with a uropod clip t o monitor difi-rential relikel! influence species-specilic susceptibilit).
sponses. were used in more than one trial. Experimerlts
Although many mechanisms likelq drive the replacement. we focussed on the roles ofprcdation and com- ran for 4-8 d and were replicated 3-1 0 tlnies. each with
pctrtion. T o determine how selective predation is me- a difftrent largeniouth bass. During these experinients.
dlated through competitive interactions among these era> fish fed o n decaqing leaves and other detrital matthrce crayfishes and through their difftrential rnorpho- ter In the pools. T o retrieve cralfish dur-ing cxpcrinients, we used a hand net and identified crayfish to
logical and behavioral character-rstics, we pursued two
specles and sex. Consumed cra!lish her-e not replaced.
objectives. First. we quantified sclective fish predation
among 0. r ~ c , ~ i c ~0.
~ r s~.~ r o l ~ ~ t l qand
u u s 0.
, 1.1r11rs.We Upon craylish return to pools. largeniouth bass mere
then explored how crayfish n ~ o r p h o l o g l .interference restrarned with a net until craqlish settled to the botconipetrtion. nonaggressive behaviors. and aggression tom.
We compared numbers of crab fish remaining at the
~nfluencedcralfish \.ulnerabilit> to fish predators. By
534
JAMES E. GARVEY ET AL.
Ecolog?. Vol. 75. No. 2
end of each selective predation experiment using rep- 0. rlr.c[~c,~ic,
two 0. I I ~ . O ~ J ~ ~ I C Itwo
~ I L I 0.
S , ~.it.ili.\( 17 crab licated ( i test\ (Sokal and Rohlf 198 1). expecting equal fish rn' densit). I : 1 sex ratio. 30 nini CL) and a largeconsumption of each species. All cells were .\ + 1 mouth bass in a 300-L aquarium with a sand bottom.
transfbrrncd such that pooled totals were 2 5 (Sokal .After 4 h. we recorded the number and species ofcraband Rohlf 198 1 ). As a conscrvati\ c test for selection. lish consunled. F i \ e replicates were performed.
these anallscs d ~ not
d account for continued selection
I't7c~llrul-ti:ccl c,ru~.fr.\h.-Although 0. r~l.sric-~l,.
0.
in the face of declining numbers of the preferred spe- pt~opit~qlrlrs,
and 0. \.1r111sa re niorphologicall~similar.
cics.
body sires in the lield arc ordered 0. 1.iri1i.r 0. r1i.st<qlrtrl-\~:c~l
c ~ r ~ z ~ ~-At
l i \ h . equal \ites. 0. rirs[rc,uc. 0. fic,~l_\ 0. p r o p i t ~ q ~ c throughout
~r.~
life (Corey 1988.
~ t . o ~ ~ i t l qand
~ r ~0o ., \.lt.ilic share siniilar niorphological Olscn ct al. 199 1). Because lish predators prefer small
charactcristics (Capelli and C'apelli 1980). Howe\.cr. crabflsh (Stein 1977). this site difkrential maq influ0 t.~r\rtc,~rc
and 0.~~rol~itlillc~c.\
males ha\,e larger chelae ence susceptibilit?. Bq reducing 0.pt.ol~itlq~c~c.s
site relthan 0. \~r.lll\m ales or females (Ciarveq and S t c ~ n1993). ative to 0. t.~r.sric~lr.s
sire. we could evaluate if 0. pt'oT o determine 11'these three species were differentiallb 11111illr~trb e c a m e m o r e susceptible. In t u r n . bq
susceptible to fish predation. we exposed equal-sired
manipulating site difkrentials hctween 0 . 1.ir111cand
~ n diduals
~ \ to largcmouth bass p r c d a t ~ o non sand. coh- the two in\.aders. 0. ( ~ t ~ o l ~ i t ~ qand
l r l r .0.
~ t~lr.sric~~i.s,
we
blc. and rnacrophgte substrates. We then e\.aluated could determine if increasing 0. vrr111.cbod? si/c rewhether chela site diffcrenccs contribute to differential duces 0. ~,irili.,s usceptibilit? to predation.
\ ulnerahil~tqanlong the three congeners.
O u r approach with unequal-sited craylisli in outside
In most cxpcrimcnts. 10 0. rlr\trc.lo. 10 0. pt.oplt1pool experinients was siniilar to that for equal-sired
qlrltr. and I0 0 . ~.o.illc( I : I sex ratio) were introduced ones. Exper~nientswcre conducted on sand. hegun at
at 0900 into outside pools ( 1.8 n~diameter. I nl depth). 0900 and checked at 0900 and 1700 each dab for 4 d .
In experiment\ using 35 rnm C L craylisli onlb. 5 male A11 <'L t i l e classes werc i 0 . 5 m m . In our first exper0. t.lc\[rc,lr\. 0 ~ ~ r o o i t ~ q i rand
~ r . ~0, . \,~t,ili.swerc used. imental set. we exposed 10 each of 18 nini 0. propltlTotal crab fish d e n s ~ t i c swere I 3 indi\ iduals m'. well qlrlrc and 70 nlnl 0. rlrs[ic,lis ( 1 : l sex ratio). We inw ~ t h i nnatural densities (Capelli 1975. Lorman 1980). creased CL of 0. t.lrsr~c.lr.sbb I -mni increments until
On sand. we tested differential selcctivit? at 15. 30. 0. 11rol11t7q~clcc
hccamc more susccptihle to predation
and 75 nlnl C'L. For 15 and 20 m m CL. crayfish were (at equal s i ~ e s .0. I I I . O I I I I I ~ L I I I \ and 0. t.~t.srrc~~i.s
were
checked at 1700 and 0900 for 4 d. Because 35 m m chosen equallb in our experiments. see Rc~s~r1r.s).
Our
craqlish wcre consumed slowlb. 25 m m cxpcrimcnts second suite ofcxperiments exposed 10 each of 18 mni
ran for 8 d and were checked dailq.
~ r snlnl
. C L 0. propitlqlrlrs, and 3 1 mrn
C'L 0 . r ~ r s r ~ c ~18
Craqlish are abundant on macrophyte and cobblc ('L 0. \.it.il~.\ (1: I sex ratio) to largcmouth bass prcsubstrates (Stein 1977. Saiki and Tash 1979. Kershncr dation. We increased C L of 0. ~ , i t ~ l by
i s I-mm incre1993). Therefore. we tested the relati\ e susceptibilities ments until all three craqfish species were selected
ol' crab fish on both of these habitats. We quantified equally (at equal sites. 0. \Yt.ili.s was selected o\.er 0.
difkrential susceptibilities of 20 m m ('L c r a l f ~ s hon t'~rcl/c'~rs
and 0.. ~ t . o p ~ t z q ~in
r ~our
r \ experiments. see Kcniacroph! te-covered substrates cons~stingof' I50 shoots .All/[.\).
of ( ' c ~ t . t i r o ~ h ~ ~ lspp.
l ~ r t plus
? i 50 shoots of f ' o ~ u t ? ~ o , q ( ~ ~ o t ~
spp. (total shoot densit). 78.5 s h o o t s m ' . similar to
Pre) interactions and behavior often determine susd c n s ~ t i e sin T-rout Lake). Collected from Carrol Lake.
shoots werc weighted with rocks. which werc then bur- c c p t ~ b ~ l i (Sih
1987). Thus. we examined how preq
tl
~ c din sand. S ~ m i l a rexperiments tested susceptibilit? ~nteractionsand heha\ ior inllucnced vulnerahilit? in
on cobble (80-100 m m diameter) placed on sand at selection experiments bb quantifying. for each species.
c ~ t h e rh ~ g h(300 pieces pool) or low (50 pieces~pool) ( 1 ) how predators influenced diff'erential refuge use In
outside pools. ( 2 ) individual responses to presence of
dcnsities. ('obblc dcn5ities in the lield gcnerallq Larq
between these two extreme densities. All macrophgtc congeners and predators. (3) efkcts of congeners on
and cobble experiments lasted 4 d . When experiments susceptibility to predation. and ( 3 ) individual aggreswerc chccked on da! 3. all cobblc and macrophbte sion.
shoots wcre renlo\ed. then replaced. We also deter. S l ~ ( ~ / f (c ~~t o. t ) ~ p ~ (~.\-OC~IIIIC~II.S.
~ i f ~ o t ~ - T o u nderstand
n l ~ n e dwhether chela si/c difkrenccs influenced difkr- how rcfugc a~ailabilitqand predator? risk afftct crabc n t ~ a l\-ulncrabilitb to p r c d a t ~ o nbq removing chelae fish shelter use. we quantilicd differences in shelter use
of I5 m m ('L crallish and exposing them to large- among these three craqfishes at two shelter densities
in outside pools. Ten crabfish. 70 m m CL. of each
mouth bass predation on sand for 4 d . Chela removal
itself did not cause mortalit? in cra)fish. In tlicsc cx- s p c c ~ c swith a I : I sex ratio wcre introduced at 0900 to
pcriments. cra!lish were checked at 0900 and 1700 pools containing either I0 o r 35 shelters. Shelters were
each d a ? .
lengthwise h a l ~ c sof polqvinyl chloridc (PVC) plastic
T o cxplorc the congruence in selectivit? hctween
pipe cut into 5 x 10 cm segments and embedded in
o~~ts~
d c and laboratory aquaria. we placed two sand-covered bottoms. Experiments lasted 2 d . For 3
pools
--
--
March 1994
SELECTIVE PREDATION A N D PREY COMPETITION
535
d before the experiment. crayfish were allowed to ac- min observation was arcsine-<x transformed and anclimate to pools with no predators. At 1700 o n day 2 alyzed with a two-way ANOVA for each species to test
of the acclimation period. a satiated largeniouth bass for effects of other crayfishes and fish predators (Wilwas introduced in five replicate sets of experimental kinson 1990). We also compared behavioral differences
treatments for both 10 shelters and 35 shelters. There- among species using individual one-way ANOVAs with
after. any crayfish consumed was replaced with a new post-hoc Tukey's multiple-comparisons tests. Largecrayfish marked with a clipped uropod. Controls with- mouth bass orientation was analyzed with a replicated
out largemouth bass were replicated 4 times each. When G test (Sokal and Rohlf 198 1).
Congeners and susccptibilit)~to predation. -To asexperiments were checked daily at 1000. shelters were
retrieved and species/sex of the inhabitant was record- sess whether interactions among crayfishes in pools
ed. After all shelters were removed. remaining crayfish influenced their susceptibility, we prevented crayfish
were captured. After largemouth bass addition. we from interacting by tethering them in outside pools
conipared proportion shelter use (pooled for the two with sand (as per Kershner 1992, Mather and Stein
1993, DiDonato and Lodge. in press). We used three
experiniental days) among species for the two shelter
densities using a two-factor MANOVA with shelter male and three female 20 mni CL crayfish of each
densit? and fish predator presence as the two factors species. T o tether crayfish. we tied 12 cm of monofiland shelter use for each species as dependent variables. ament line to swivels and then glued swivels to the
Shelter use of individual species was analyzed using cephalothorax using cyanoacrylate. Tethers were atunivariate ANOVAs. We also compared species sur- tached to clay tiles (15.2 x 15.2 cm) buried in sand.
vival in shelter experiments. again pooling data for the Crayfish were arranged by grouping species, alternating
2 d , using a C; test, in which we expected equal con- males and females. along the pool perimeter. Species
were grouped to minimize the chance of interspecific
sumption of all the three species.
l
with nearby. tethItzdividiral crayfish behavior. -Crayfish modify their interactions caused by ~ i s u acontact
behavior under predatory risk (Stein and Magnuson ered crayfish. Tethers allowed at least 10 c m between
1976). T o understand underlying reasons for differ- individuals. A largemouth bass predator. starved 24 h,
ential vulnerability. we quantified how other crayfish was introduced at 0900 and experiments were checked
and largemouth bass influence crayfish behavior. Due at 1700 and 0900 daily for 4 d. Consumed crayfish
to logistical constraints, only male crayfish beha\,iors were replaced. Crayfish consumed over the 4 d were
were recorded. although females occurred in some conipared among species using a replicated G test (Sotreatments. N o crayfish (20 m m CL) were reused. kal and Rohlf 198 l).
Largemouth bass were starved 24 h; if a crayfish was
Crayfish a,qgressi,.e intrracfions.-Aggressive enconsumed during the experiment. we ended the trial. counters between crayfish can influence reproduction
Four treatments. replicated at least 5 times, were as (Capelli and Munjal 1982. Berrill and Arsenault 1984),
follows: (1) i n d i ~ i d u a l :one crayfish, no largemouth acquisition of shelters (Capelli and Munjal 1982), combass. plus no other crayfish: (2) individual plus con- petition for food (Hill and Lodge 1993), and suscepgener: one crayfish. no largemouth bass. plus one fe- tibility to predation (Stein 1977, Mather and Stein 1993.
male conspecific. plus one male and one female of each T o evaluate the importance of aggression among the
of the other species ( A r = 6 crayfish): (3) individual plus three congeners, we quantified aggression among equal
largemouth bass: one crayfish, one largemouth bass, sizes and unequal sizes of 0. rusticus, 0. propinquus,
plus no other crayfish; (4) individual plus largemouth and 0. virilis. Aggression was defined a s "tension conbass and congeners: one crayfish. no largemouth bass. tacts" (Bovbjerg 1953) where one crayfish (the winner)
plus one female conspecific, plus one male and one caused another crayfish (the loser) to change direction
female of each of the other species (1'1' = 6 crayfish).
during a confrontation. Experiments were done in 200-L
Crayfish were individually marked with typing cor- aquaria on sand, lasted 1 h. and were replicated at least
rection fluid for easy identification, then introduced to 5 times. One male and one female of each species were
the tanks for each 30-min trial. During each trial, male used for each trial. Because males tend to be more
behavior was recorded every I 0 s using a behavioral aggressive than females (Berrill and Arsenault 1984)
recorder (Datamyte 1000. Electro General Corpora- and intermale conflicts within species might influence
tion) and a voice-activated tape recorder. Beliaviors susceptibility. we also quantified intermale behavior
recorded were ( I ) chelae displays-crayfish posturing between two males of each species in another set of
with chelae extended and spread. (2) activity levelfive trials. We did not observe interfemale conflicts.
crayfish moving o r stationary, and (3) swimmingDuring each trial, total number of fights and outcomes
crayfish swimming in water column. For swimming of each fight were recorded. For equal sizes, 20 m m
observations, which were infrequent and easy to re- CL crayfish were used. For unequal-sized crayfish, inicord. we quantified both male and female behaviors. tially 18-mm C L 0. rusticus and 18-mm CL 0. proLargemouth bass orientation to crayfish also was quan- pinquus were placed with 21-mm C L 0. virilis, the
tified. Proportion behaviors (number of each behavior same size differential as in size-selection experiments.
recorded/frequency of all behaviors) during each 30- We then increased 0. virilis C L by 1 m m until 0. virilis
JAMES E. GARVEY ET AL.
Ecolog). Vol. 75. No. 2
portion tights won during each bout was the dependent
variable, and body and chela sire differences between
0. ~ . i n l i sa nd the invaders were the independent variables.
V Op male
Within species, male and Semale susceptibility did
not difler ((; test. P > .O5): thus, we pooled male and
Semale data. Because repl~cateswere homogeneous ((;
test for heterogeneity. P > .I()). they were pooled. T h e
small number oS reused crayfish d ~ ndot difyer in suscept~bilityfrom naive crayfish (CI' test. P > .I()). .4pparently, crayfish did not learn to avoid fish predators
w ~ t hincreased experience.
Fy1icll-.s1zc,r/ ( ~ ~ l ~ . f i . ~Largemouth
/i.
bass consistent1) chose 0. 1.1t~1i.so ver equal-sired 0. rzl,srr~,lr.sand 0.
O I . O ~ I ~ I ~ I for
I I I SI 5-. 20-. and 25-mm C L sire classes on
r Op female
A
- C
MACROPHYTE
.
25mm
DAY 8 *
r
I
-
i
=*6
~
'N=3
I
I
I
I
I
DAYS
FIG.I. Number of equal-si/ed Olz~otrc,~~rc.\
r . i r c r r c ~ l r c (Or).
0. proprrzyu~rt(Op). and 0 1.iri1ic( O \ ) s u n I\ ing largemouth
bass predation for three crayfish carapace length classes (CL
-t 1 m m ) on sand in 1.8 ni diameter outside pools through
tlme (note differences in .u axis scale among panels). Means
are gi\en -t 1 SE. Statistics are results of replicated (; tests
comparing number of each specles remaining on final day of
each experiment (see lower left of each panel). NS I'
.05:
*f'
.05: ** P -- .0 1: *** I' . .OO 1.
#-
I
I
I
LOW DENSITY
COBBLE
N=5'
-
won more lights than the other two species (at equal
sires. 0. r1rili.s was less aggressive. Capelli and Munjal
1982. our stud)). Number of lights won for each sex
and species was a n a l q ~ e dusing replicated G tests (Sokal
and Kohlf 198 1).
U'lth \ erniercalipcrs. we measured chela length (palm
length) ofeach crayfish to determine whcther chela sire
~nfluencedoutcomc of lights. For 20-mm CL craqlish.
chela length d i f i r e n c c s between the winner and loser
of each tenslon contact were regressed against proportion of fights won (number of lights won 'total number
oflights during that 1-11bout). These relationships wcrc
anal) red using linear regression (Wilkinson 1990). We
also determined whether increasing body o r chela s i x
difirences influenced 0. ~,!t.~li\
aggression using multiple-regression analqses (Wilkinson 1990) where pro-
1
6
I
C
-
HIGH
DENSITY
COBBLE
- N= 5 NS
1
v
~p male
Ov male
v
~p female
Ov female
DAYS
(Or). 0,
proFIG.2. Number of equal-siled 0. rirrr~c~ir,
pit~yiiici(Op). and 0. virill\ (0\) surviving after 4-d exposure
to fish predation with high density ofcobble (700 pieces pool).
low density ofcobble (50 pieces pool). and macroph)tes (78.5
shoots m = )a ~ a i l a b l eas bottom structure In 1.8 ni diameter
outside pools through time. All crayfish were 70-nim CL (carapace length). Means are shown i- I s ~Statistics
.
are results
of replicated (; tests comparing number of each species re.05: * P 5 .05.
maining on da) 4. NS P
'-
SELECTIVE PREDATION AND PREY COMPETITION
-
:A:\-
b \ 6
Or 20 rnrn CL
Op 18 mm CL
-
N=5
-
-
NS
-
o r 21 rnrn CL
Opl8rnrnCL
***
N =5
-
-
Or 18 rnrn CL
Op 18 rnrn CL
Ov 21 mrn CL
-
N=l0
I
I
I
I
I
I
I
I
p,
Or 18 mm CL
Op18rnrnCL
Ov 22 mm CL
N=5
I
I
I
NS
I
I
Ov
0
I
I
I
I
I
DAYS
FIG. 3 . Number of unequal-sired cralfish surviv~ngafter 4 d of largemouth bass predation on sand in 1.8 m diameter
outside pools. Top panels (A. B) re\-eal effects of reducing 0 pro.r~irzquus(Op) slze on suscept~bility:bottom panels (C. D)
re\eal effects of increasing O viri1c.s (Ov) C L on d~ffcrentlals uscept~b~l~ty
of 0 rlc.$tic.us(Or) and 0. propirzyuus. Means are
shown i I SF. Rcpl~cated(; tests uere performed on number of crallish remaining on da) 4. N S P > .05: * P 5 .05: *** P <
.oo 1 .
~~~1
. smni
t h asniallt
sand ((; test. P ;.05. Fig. I). Number of 0. r l i ~ f i c ~ i s ones (Stein 1977).0.~ ~ r o p ~ t ~ q ~ were
.
and 0.~ ~ r o ~ ~ i r / yremaining
////.s
by the end ofexperiments cr than 0. r.lrsric~u.\ were chosen equally (Gtest. I' ;
did not differ ((; test. P > . l o . Fig. 1). Patterns in 100-L . 10. Fig. 3.A).but 0.propirlq/i/o that were 3 nini smaller
aquaria w ~ t hsand substrates matched those in outside than 0. rlrsric~lrsu e r e consumed more ofien bq daq 4
pools. During five replicate trials. more 0. 1~iri1i.s( I . 4 ((; test. P ;.O5. Fig. 3R).
t 0.23 individuals. mean i 1 SE) were consumed than
Although sire-selective predation contributes to the
either 0. rll.rrlc,/r.s or 0 prol~lnq/r/rc( 0 consumed) after
differential susceptibilitg of 0 0ro111rzq1110, for 0. 1.13 h with a star\ed largemouth bass.
r./li.\, which grows larger than either invader. factors
When substrates wcre macroph1tes o r low densit1 other than bod) size d~fferenceswere invol\-ed. For
cobble. 0. 1.lrili.5 also was selected b) largemouth bass example. 3 m m larger 0. ~ ' i r ' i l still
l ~ was selected over
((; test. P
.05. Fig. 1A. R). Apparentl). these sub0. r~rsr1c~~r.s
and 0. l~rol~irrql/~r.\
((; test. P ;.O5. Fig.
strates did not provide adequate cover tbr craqfishes.
3C). When 0. r1rcli.c u e r e 3 m m larger. all three era)especiallq 0. 1.iri1i.s. At high cobble densit). 0. ~,/rill.s fishes s u r v i ~ e dfish predation equally ( G test. I' > .O5.
Fig. 3D). We caution. however. that in these experiwas no longer selected ((; test. I' > . l o . Fig. 2C) simp11
nients sample sites differed between the 3-mni (.V =
because craylish wcre eaten quite ~nfrequently.Most
10)and 4 - m m (,V = 5 ) difference experiments (Fig. 3C.
likclq. largeniouth bass became less selective as prey
capture rates declined (MacArthur and Pianka 1966). D). Potcntiallq. we detected susccptibilit~patterns In
l r s have the 3-nim diff'erential experiment because sample sires
Recall that 0.r1r.ti1c~l.tand 0.~ ~ r o p i r ~ y l rmales
were greater. Perhaps if we increased sample si/es in
longer and wider chelae than 0 ~./rili.s(Garveq and
Stein 1993).Yet. afierwe removed chelae from all three 4-rnm d ~ f k r e n c eesperlments. we again would have
speclcs. 15-nirn C'L 0. 1.iri1i.c was still highly selected
found \.ulnerabilit) diffkrences. Therefore. we can on11
over s a m e - s i ~ e d0. r~/.cric,lrsa nd 0. propitrylcrrs ((; test. cautiously conclude that increasing sire decreases sus.I' = 3. P .; .05). Thus. mechanisms other than chela
ceptibilitq of 0. ~ , / r . ~ l l r .
si/c difkrences contribute to difftrential vulnerabilitq.
~ ~ t ~ c ~ y ~ r a l - . ~ i z c-7'0
~ r / / further
~ ~ r u ~ understand
~ / ~ s / ~ . horn,
predation influences the abilit) of both 0. r ~ ~ ~ r i ( and
,~l.\
-We conipared
.Y/~elrcr. c,or?r1wrir/orz c~.\-/~(~rirr/(~r/r,s.
0. 11ro111rzq//~l.(
to replace 0. ~ , / r ~ l i \natural
.
difkrences among-species difkrences in shelter use after largein era) lish body si/es ordered 0. \,ir.lli.\ > 0. r./r,r/c.~ts mouth bass addition (pooled for 2 d). Both increasing
> 0. [ I ~ O / I / / Z ~ L ~within
~/.\
each age class (C'orey 1988. shelter availabilit? and introducing largemouth bass
Olsen et al. 1991) also must be considered. U'ithin a caused all three specics to increase shelter use (twospcc~es.lish predators choose small cra) lish over large waq MANOVA. F ~ s hand Shelter effects. P .r .001.
\
JAMES E. GARVEY ET AL. 10 shelters available
Shelter "*
Fish *"'
Shelter X Fish "
Ecology. Vol. 75. No. 2
35 shelters available
A
/
NO FlSH
FISH
FIG. 4. Proportion of 10 0.rztsricin (Or). 10 0. proplnqlrus (Op), and 10 0. vrrilis (Ov) in shelters for 2 d (pooled
percentages, mean and I SE)after largemouth bass were added to 1.8 m diameter outside pools. Shelter was either limited
(panel A: 10 shelters) or unlimited (panel B: 35 shelters). Dashed lines are expected shelter use if all three species used shelters
equally. Statistics result from two-way MANOVA examining species (Shelter) and largemouth bass (Fish) effects for the two
shelter treatments. Only significant (P 5 .05) effects within a treatment are plotted. See Table 1 for MANOVA and univariate
ANOVA results. ** P i.01; *** P < ,001: **** P .c ,000 1 .
Fig. 4A, B, Table I). A significant Shelter x Fish interaction from both the conservative MANOVA analysis which includes all dependent variables (P = ,004.
Table I ) and for the corresponding ANOVA decomposition for 0. viriiis (ANOVA. P = .0002. Table 1)
indicated that 0. viri1i.s was unable to acquire limited
shelters when fish predators were present, potentially
due to exclusion by other crayfish (Fig. 4A). Apparently, with largemouth bass plus unlimited shelters, all
three species increased shelter use to avoid predation.
However, when shelters were limited, 0. iYrilis was
excluded from shelters by the two invading species.
Susceptibility patterns in shelter experiments were
similar t o those in other outside pool selectivity ex0.56 indiperiments. Slightly more 0. virilis (2.2
viduals [mean I 1 SE]) than 0.propinqlius (1.2 i 0.2 1 )
and 0. rlrsticus (0.6
0.17) were consumed during all
*
*
TABLE
1. Results oftwo-way MANOVA and univariate ANOVAs for proportion of shelter use bq 10 Orconrctrs rusl~cus(Or), 10 0.propinquus (Op), and 10 0. ririlis (Ov) in
outside pools with 10 shelters (limited) or 35 shelters (unlimited) available for 30 crayfish. Results are pooled across
7 d. See Fig. 4 for related data.
Effect
Shelter
Vanablc
(qroportlon in ANOVA
shelter)
P
Or
OP
ov
Fish
Shelter
x
Fish
Or
OP
Ov
Or
OP
Ov
Wilks'
X
MANOVA
P
shelter experiments ( G test. df = 2, G = 6.98. P < .05).
More crayfish tended t o be consumed with unlimited
shelters (2.2 i 0.37) than with limited shelters (G test,
d f = I . (; = 3.95. P < .05). Due to low c o n s u n ~ p t i o n
rates, we were unable to anal!~c differential species
susceptibilities for each individual shelter density.
Indrvlduul oaj.fish l~ehai~ior.
-In treatments without
largemouth bass. using one-way ANOVA comparisons
across species. we found that 0. virilis was less active
than 0. propitlqlrl/s (Tukey's multiple-comparisons. P
= ,001. Fig. 5A) whereas 0. ~ L I S / I C I I S a c t i ~ i t ydid not
differ from either 0. pt.opit?yurrs or 0. vinlrs (Tukey's
multiple-comparisons, P > .O5, Fig. 5A). Largemouth
bass reduced activit) l e ~ e l sof all three species (twoway .ANOVA. Fish effect. P < .000 1, Fig. 5A. Table
2). However, in the individual plus largemouth bass
treatment. 0. i~irillsdecreased activity more than the
two invaders (Tukey's multiple-comparisons. P = ,002.
Fig. 5A). .4lthough addition of congeners t o tanks containing individual crayfish did not influence crayfish
activity, congeners plus largemouth bass increased activit) of 0. 1,irili.s relative t o its activity with largemouth bass only (two-way ANOVA, Fish x Congener
effect. P < .05. Fig. 5A, Table 2). Interestingly, in the
individual plus congeners and largemouth bass treatment. 0. virtlis activity n o longer differed from that of
the t n o i n ~ a d e r (one-way
s
ANOVA, P > .05. Fig. 5A).
Indeed, 0. virilis, the most vulnerable species, did reduce activit) more in the face o f predation, possibly
to avoid detection by the predator. However, interactions with other crayfish increased 0. vtril~sa ctivity
levels, thereby confounding this predator avoidance
behavior.
With n o congeners present, largemouth bass orientations t o crayfish increased chelae displays of all three
species (two-way ANOVA, P < .05, Fig. SB, Table 2).
M a ~ i h1004
SELEC TI\ E PKEI)A rION AND PKEY
< O'vlPETI [ION
5 10
M'ithout largemouth hass present. introduction o f c o n Activity levels
Chelae displays
geners t o tanks M ith i n d i t idual craqfish d i d not influ_ B
ence cra)lish chelae dlsplays (Fig. 5B. T a b l e 2 ) . M'ith
Or
FlSH
a largemouth bass present. h o a c ~ e r .increasing t h e
n u m b c r o f craq fish in a t r e a t m e n t could reduce t h e
0.6
t?cquency o f i n d i idual
~
chelae d l s p l a ~ simply
s
because
fish \%ere orienting o n six i n d i \ iduals I-athcr than o n c .
z 0.3
Indeed. o t h e r crabtishcs reduccd 0. ,11ro,1)1r7yiiii.cc helae 5
displays ( t ~ o - m a ?A N O V A . P = .O2I , Fig. 5l3. T'ablc 0
(1
i
0.0
2) r e l a t l ~ et o t r e a t m e n t s without o t h e r craqlishes. LJsOP
OP
ing onc-waq 4 N O v . 4 ~t o c o m p a r c chelae d i s p l a l s
FlSH
FlSH
a m o n g cra)tishes for cach t r e a t m e n t . we found that 0.
FlSH x CON '
~.iri/i\ d i s p l a ) e d chelac lc\s li-equentlq than 0. 11ro,11iilyiili.\ (Tukeq's multiple-comparisons. 1'- . 0 10. Fig. 513)
a n d . possibl). 0. t . l r s / i c ~ ~ (i \T u k e l ' s multiple-comparisons. I' = , 0 9 4 . Fig. 5B). 0. i.lurii,iit a n d 0.p r o i p i t ~ y i i i l s
d i s p l a ~ c dchelae sirnilarl) (Tukcy's niultiplc-comparOv
isons. I' ., .05. Fig. 5U). In cxpcrlnientr. 0 . r~r\/ic,lc.s
a n d 0 pi.opctryllrr\ \\,ere q u i t e aggressi\,c toward largeFlSH "" m o u t h hass. often walking t o ~ . a r dlish predators with
FlSHxCON ' chelae raised. C o n \ c r s c l ) . 0 . ~.iri/i.\, rathcr than pos- turing mith chelae spread in responsc t o a prcdatorq threat. sirnpl? reduced its actl\ it!. 0. I , / ~ . I / I \ also s w a m m o r e Srequcntlq (.\. = 79 stvim
Ilights: d u r a t i o n 7.5 t 0.8 s [ m e a n ? I SE]. range 7IND CON IND CON
IND CON IND CON
18 S ) a n d h r longer t h a n the i n ~ a d e r s .which rarel)
s w a m . T h e s e s w i m flights ranged 20-30 cnl a b o ~ teh e
FIG. 5. Propol-t~onofot>ser\ations ofcra!iish act1\
(A
hottorn. 0.,~)i.ol)lilylrlrsa n d 0. i.ll\irc.il.t s w a m in O.juir) column) and chelae d~splays( B column) o f the three species
o f trials. with n o swim f i g h t exceeding 3 5. B e c a ~ ~ s e during 30-rn~nobserbations In 200-L aquaria as all'ected b!
.
ofother cra~lishe,(Ind). and
the n u m b e r o f 0. ~,ri.rlrtm a l e a n d female s u i n i flights largcmouth bass ( F ~ s h )absence
one consprc~ficplus four crab fishes of the othcr specles (C'on).
per treatment d i d not d i l k r (tno-\\a! 4 N O V 4 . P
Data arc means and I SE. Onl! signlficant (I' . 4 5 ) treatment
.5). we pooled sexes in a n a l l s e s . Both congcncrs a n d efict, of tuo-aaq ANOV4 are presented. Scc l'able 2 lor
largemouth bass lncreased t h e frequencq o f 0 . ~.irrlrc two-uav ANOV4 rcsults. * I' .O5: **** I' . 0001
""
\
""
""
&
-
-
TABLE
2. TWO-ma! ANOVA rcsults Ihr pr-oportlon chelac di5pla)s. proportlon actlvlt! le\els. and number o f s m ~ mH~ghts
) congoicrs (Con) d u r ~ n g3 0 - m ~ nobservat~on,.0rc.oiic~cictrii.ci~c.ir.\(Or) and 0.
~nflucnccd b! largemouth b a s ( F ~ s l iand
prop~iicliiii\ (Op) never part~c~patcd
In s n ~ mH~glitsIn the presence of largemouth ba,s.
Ezpc~-~rne~it
Tckt
locat~on
Sncc~cs
Source of
\ ariat~on
Fish
Con
F ~ s hx
F~sli
C on
F ~ s hx
F~sli
Con
F19h x
F~rli
( on
F ~ s hx
F~sli
( on
F ~ s hx
F~sh
C on
F15h x
F~sh
Con
Flsh x
Con
Con
Con
(
on
Con
(
on
(
on
df
MS
F.
I'
540
JAMES E. GARVEY ET AL. Z
2.5
Z
-
-
1NO FISH
T
FISH
g
IND CON
1
IND CON
s wlm flights i 2 - s duration
FIG. 6. Number of 0. 1.1r11rc
as alTected b) largeniouth bass (Fish). absence of other trap
fishes (Ind). and one conspec~licplus four invaders (Con).
(I' 5 .05) treatment
Data are means and I SE. Only ,~gn~ficant
cfects of two-wa) 4 N O V 4 are presented. See Table 2 for
4NOVA results. * P 5 .05.
swim flights (two-way ANOVA. P ;. O j . Fig. 6. Table
1).but nelther congeners nor largeniouth bass influenced swim flight duration (two-wal ANOVA. P i .5).
S w ~ m m i n gcrallish. unable to use chelae Ibr defense.
are easilq consumed (Stein 1977, Mather and Stein
1993). Indeed. o n four occasions, 0. viri11.s swim flights
resulted in consumption by largemouth bass. Therefore. 0. virilis clearly was a t a disadvantage w hen par-
Ecolog). Vol. 75. No. 2
ticipating in this beha\ior. When 0. r~rsfic.lr,.0. propitiyirlrs, and 0. viri1i.s occurred together. largeniouth
bass o r ~ e n t e don all sexes and species equally (G test.
P > .5). Therefore. 0. ~ ~ r i lbehaviors
ts
were not caused
b? preferential largemouth bass orientations. Both the
presence of in\ aders and predators initiated potentially
lethal swim flights for 0. rirills.
C'otl,yc2nc2,sund .su.,cc,l~rif~ili~y
10 prcdutlon. - lnteractions among crayfishes appear to be important in
determining susceptibilit? to fish predation. However.
when crayfish in outside pools were prevented from
interacting via tethering, on14 0. \,tt.tlts was consumed
by largemouth bass during Sour trials (mean I1 SE =
1.5 2 0.29 individuals): neither tethered 0. rli.sric~1l.s
nor 0. r ~ r o l ~ i t ~ ywere
l l ~ i ~eaten during these 4-d treatments. Apparently. indi\ idual activities. such as infrequent chelae displays and swimming. were sufficient
t o increase the susceptibility o f 0 . vlri1t.s to largemouth
bass.
<'riq.f2.\/1ugi:yt.c,\.\~~.c,
itif~ruct1or1\.
-Although individual behavior alone appeared to place 0. \~rili.sat great
predatory risk. agonistic interactions with other crayfishes influenced swim flights. refuge use. and activity.
In experinients where we observed agonistic tension
contacts. all I -h duration replicates were homogeneous
((; test. I
' i. l o ) and. therelbre. were pooled Ibr anallsis. Similar to Capelli and Munjal (1983). we found
that. in male-female experiments. 0. 1~trt11.s
males and
k m a l e s lost most lights with equal-sued 0. rllslic,lls
and 0.~t.opitly~l~l.s
males and feniales ((; test. P .: . O j .
TAB umr of tension contacts won and re,ult, of replicated
(
;
tests (pooled result,) between cralfish 1 ( C f l ) and
species were u,ed for each
crayfish 2 (C12) during I-h observations In 200-L aquaria. T w o each of the three Or~,otrc~crcs
(Ov) same 5i7e as congeners. O v 3-mm CL (carapace
r-epl~catein both intra- and interspec~fict r ~ a l sTreatments
.
were 0 ,i,rr~/tr
0 v 3 m m larger
Cra? tish equal-si/ed
C'onte5tants
Interaction
CfI vs. C11
Cfl
CP,
Intraspecilic
Or6 vs. Or9
Op6 vs. OpP
-
64
85
26
27
44
30
Ov6 vs. OvP
Interspecific
Or6 v5. Op8
Or8 v\. 0 v 6
0176 vs. Ov8
-
(;
Or9 v\. OpP 0v9 Op9 vs. Ov9
Or? vs.
-
-
-
('fl vs. CfZ
Cfl
NS
Or8 vs. O m
Op6 vs. OpP
0 v 6 vs. OvP
14
3
13
NS
Or6 v,. Op6
-
16
14
5
o m vs. OPP
o m vs.
9
3
16
33
69
70
90
14
21
-
* P -..O5. ** I' .OI, *** P ,001, **** 1' ,0001.
t Male-onl? fights: o t h e n v ~ s esex ratio was I:I.
48
14
68
15
67
56
P
12.12
17.99 0
***
0.76
43.32
37.22
****
****
****
25.24
Or6 v\. OpP
Or6 v\. OvP
Op6 vs. Or9
0 p 6 vs. OvP
Ov8 vs. OrP
Ov6 vs. OpP
Contestants
Numbcr
of fight,
won
Number o f
fights won
**t*
2.9 I
17.99
7.63
NS
0.14
29.67
0.02
46.47
28.02
12.38
NS
****
**
*t**
NS
****
****
**t
Or6 vs. Oc6
Op6 vs. Ov8
OpP vs OvP
Or6 vs. OpP
Or6 v,. OvP
Op6 vs. 01%
Op6 vs. OvP
Ova vs. Or9
Ov6 vs. OpP
9
II
3
6
19
3
hlarch 10'14
SELECTIVE PRED4TION 4ND PREY C'OMPETITION
54 1
Table 3). In male-onl! cxperlmcnts. 0. r~rsrrc~ir.~
won
.O5. Table
~,it.rlr,d o m ~ n a t c dthe in\.ader-s ((; tcst. I'
on14 a feu more lights with 0. ~Yrrli,((; tcst. . I 0 > P 3). For hoth CL sire differcnccs. large 0. ~,ri.i/e\a ppar. O j . Tablc 3 ) and e\.en fewer fights w ~ t h0. (~rol~etl- entl! reduced the total number of agonistic bouts durIng each l -h trial (Tablc 3). Although 0. ~ , r r i l i ,a ppears
((; test. I'
. O j . Table 3). As in male-female
yi~i~.c
niales won most lights with
experiments. 0./~ro[~rnyi(ir.c
Innately less aggressive. i.c.. lcss able to wln tcnsion
0 ~,crrle.cmales ((; tcst. I' .: .05. Table 3). We cannot contacts than the in\ading species. a fair11 large bod!
explain \\hy 0. i-~rc~cc~iic
males were less successful in si/c differential reverses the outcome of equal-si/ed
male-on11 experiments. Perhaps the presence of con- interactions.
specific males influenced light outcomes. However. 0.
Chela length increases with body sl/e ( C a n e ! and
1.1t.elrcwas consistcntlq less successful in a g g r e s s i ~ cenStein 1993). Large chelae p o s i t i ~cly influenced the
0. rir\/cc,~1.co r 0. ~~t.oj~r/lyirirs.
counters ~ i t h
number of interspecific tights won among all c r a ~ f i s h c s
At eclual si/cs. 0.~,o.clicmales and females fought except 0. rirt1i.s females. in that. as chela si/e differeach other less fi-eiluently than male and female in- ences betv.een the winner and loser increased. the like\.adet-s ((; tcst. df = 2. (; = 29.62. 1' .: .O5. Table 3). lihood of winning increased (I.' test. I'
.O5. Fig. 7).
.Also. in male-onl! obser\ ations. more interspccific (.I' When exploring the relative contribution of increasing
= 3-12), rather than intrarpecitic. fights (.I' = 71) ocbod! and chela si/e dilkrenccs t o 0 ~,irrlrrd o n ~ i n a n c e
curred among males of the three specles ((; test. P .:
o~c0
r . ri(.,ric.ii, and 0. prol)i/lyirii.~,h oth increasing
. 0 5 ) . In male-kmale cxpcriments within specie>. 0. bod! and chela sl/e difkrences were important for 0.
t.i(sri(,iic and 0 . /~r.o/~c/rc/ilirc
males \\on more agon~stic ~ ~ c i . e l c m
\ ales ( n ~ u l t i p l eregression. .\' = 85. K' = 0.58:
bouts than females ((; tcst. P --- .O5. Tablc 3). whereas
1i)r bod! si/e. 7- = 5.58. I' ;,0001; for chela si/e. 7.
0 ~ . i t . c l i c niales and females Mere ecluall! aggre>s~\.e = 1.95. P -. ,055). H o ~ e \ , e rfor
. female 0. ~~wtlcc.
onl!
(Table 3). Thus. for 0. ~,crr/cc.i nterspecilic fights most
tncrcasing bod! site dilkrenccs influenced the number
likel! occurred more frecluentl! than ~ntraspccificones of fights \ o n o\.er O t . i t s ~ r c ~ i ( and
c
0.~~t~o[)rriyirirc
(mulin select~\,ep r e d a t ~ o ncxpcrlnients.
tiple regrcsslon. \' = 85. K' = 0.50: for bod! si/e. ? ' =
With incrcas~ngbod) si/e. 0. ~,rrelrsn o n more lights
6.08. P . ,0001 ).
Intercrttngl!. \\hen
n i t h 0 t.itvfrc.ir\and 0./~~.o/)etryirir\.
0.~.rrilrc\\ere 3 nlm larger than 0. r i i r ~ c ~ and
i ~ c 0.
/~rol)r/iyrri(,,n iales \\on an c c l ~ ~ number
al
of lights (TaM'e propose that t\\o rncchan~smscontribute to the
ble 3). HoneLer. 0.~~o.eler
males and li.males n o n more
I-cplacemcnt 01'0./~t'o[~et~i/iri(c
and 0. ~ , / r / I r ch! 0. t.ir.ctension contacts than 0.t.i(tri~~ir.r
females ((; tcst. I'
rrc.ic\. Large 0.t.i(.\~rc,i(c
replaces its small congener 0.
.O5. Table 3). M'hcn 0 ~,o.c/ccMere 4 nlm larger. 0. j~t.ol~ctiyiri~s,
in part due to s ~ / e - s e l c c t ~ vpredation
e
b!
--
-
-
Icngth) larger. and 0 v 4-mnl CL largcr. Contestants not shar-ing n common under-line \\on s~gn~ficantlb
(P
numbers of fights.
-
.O5) d~ferent
0 v 3 mni larger
-
Ov 4 nini larger
Number
01' ligh~s
('ontestants
M 011
('12
(;
I'
('(1
\ s . Cl2
Number of
fights won
Cfl
c'17-
1
0
0
23.09
NS
1.0 1
10.73
NS
'7.
I
28
7
20
15
26
0
0
9.64
20.89
(;
P
ops \ s. OpP
0 \ 6 \s. OvP
Or6 \ s. Op6
Or6 \ s. Ovs
Op6 vs. Ovs
10
12
8
0.03
0.03
NS
NS
1.20
9
0.36
5.94
5.0 1
NS
NS
5
14
t
t
OrP vs. OpP
OrP vs.
OpP \s. O\.k
Or6 vs. OpP
Ops vs.
Op6 vs. OrD
opS vS. O\.P
Ovd vs. OrP
OvS vs. OpP
-
3
****
**
**
****
March 1994
SELECTIVE PREDATION A N D PREY COMPETITION
Lodge, in press). size-selective fish predation likely will
reduce 0. propinquus populations more quickly than
sympatric 0. rusticzts populations.
Large crayfish are dominant over small crayfish. Large
0. ru.rticus, likely via aggressi~ebehavior. outcompete
small 0. propinquus for limited refuge. In turn, when
crayfish densities become high (as they can in northern
Wisconsin lakes reaching 60 individuals/m2; Capelli
1975. Lorman 1980), relative to available refuges, their
shelters can become limiting. In such situations, intense size-mediated shelter competition can lead to the
exclusion of 0.propinq~llisby competitively dominant
0. rusticus.
We argue that both size-selective fish predation and
perhaps interspecific competition underlie the replacement of 0. propinquus by 0. rusticus. Si~e-mediated
fish predation and availability of shelter limits recruitment ofjuvenile lobsters to their adult life stage (Smith
and Herrnkind 1992. Wahle and Steneck 1992). Similarly. when 0. rusticus is present, small size and availability of shelter likely reduce recruitment of 0. propirlqiius to subsequent age classes. In areas of sympatry,
0. rusticus likely replaces 0. propinquus.
Sek~ctiveprc~datiotz.- 0. r.irilis was more susceptible
to predation than similar-sized invaders on sand, nlacrophyte, and low density cobble substrates. These results agree with those of DiDonato and Lodge (irzpress).
who tethered three size classes (1 5-1 8, 23-25, 33-35
mn1 CL) of all three species at six sand sites within
Trout Lake. Like us, they found that 0. virilis was
selected over 0. rllstic~isa nd 0.propinquus. Their field
verification of our pool and aquarium results confirms
that 0. virilis is at a distinct disadvantage when confronting predators.
O n dense cobble substrates, differential susceptibility did not occur and predation rates were low. Not
surprisingly, orconectid crayfish are abundant in such
habitats (Stein 1977, Kershner 1992). However, in surveys of three macrophyte and three cobble sites in
Trout Lake, 0. ~'iriliso ccurred exclusively in macrophyte habitats (J. Rettig, prrsonal cotninunication).
Similarly, invaders displace similar-sized 0. virilis from
cobble substrates into macrophytes and sand (Hill and
Lodge 1993). Therefore. whereas 0. r'irilis may not be
selected by fish predators in abundant cobble, invaders
may force it into nonpreferred, high predatory-risk
habitats. Such forced shifts in habitat-use of congeners
by invaders have been observed in the Great Lakes
(Crowder et al. 198 1) and streams (Karp and Tyus
1990, DeWald a n d Wilzbach 1992), often to the exclusion of native species. In northern Wisconsin lakes,
the native species 0. virilis, excluded from dense cobble
habitats by the two invaders, likely suffers greater predatory risk. Without access to a refuge from predators,
0. virilis populations should be dramatically reduced.
543
As with differential habitat use. morphological differences among organisms influence susceptibility to
predation (see Zaret 1980 for review). With crayfish,
large chela size reduces vulnerability to fish by increasing capture and handling times (Stein 1977). Because
the invaders possess larger chelae than 0. virilis (Garvey and Stein 1993), we expected removal of chelae to
eliminate differences in susceptibility. Surprisingly,
however. chela size did not determine susceptibility;
0. virilis was still more susceptible. We also predicted
that differences in body size would influence differential selection. Although 0. ririlis grows larger than
the invaders, potentially, it must exceed body size o f
the invaders by 4 m m just to attain equal susceptibility.
Thus, although 0. ilirilis is larger than the invaders, it
is still selected by fish predators and eventually replaced. Below. we address characteristics unique to 0.
iirilrs that render it more vulnerable to fish predators
than the invaders.
Interactiorzs and behavior. -Factors such as differential coloration and palatability can influence vulnerability. Although such morphological characteristics are important, behavioral responses to predators
also can contribute to abundance and distribution of
a wide variety of organisms, including armored catfishes (Power 1984), shrimp (Main 1987), odonates (see
Johnson 199 1 for review), and crayfish (Stein and Magnuson 1976, Collins et al. 1983, Mather and Stein 1993).
T o understand crayfish behaviors that potentially influence susceptibility to predation. we quantified shelter competition, activity, predatory defense, swimming, and agonistic interactions. We then assessed
whether 0. virilis behaviors differed from those of its
congeners, specifically exploring how these behaviors
influenced vulnerability.
Risk of fish predation increases crayfish shelter use
(Stein and Magnuson 1976, Stein 1977, Mather and
Stein 1993). Without predatory risk, 0. rusticus outcompetes both 0.propinquus and 0.virilis for shelters;
in turn 0. propinquus can displace 0. virilis from shelters (Capelli a n d Munjal 1982). When unlimited shelters were combined with a fish predator, all three species used shelters, presumably to avoid predators.
However, when limited shelters were combined with
fish predators, 0. virilis were inferior competitors for
refuge. In the field where cobble is sparse, crayfish densities are high. and fish predators are present, 0. virilis
loses the battle for limited shelter and is replaced by
the invaders.
Interestingly, 0. virilis was selected over the invaders
in limited and unlimited shelter experiments. In fact,
more crayfish were consumed in unlimited shelter
treatments. Conceivably, we did not detect a relationship between shelter availability and predatory susceptibility because we only quantified refuge use by
rather crude, daily "snapshots" of shelter occupation.
Likely, finer scale, unobserved interactions, such as
fights over occupied shelters in limited o r unlimited
544
JAMES E. GARVEY ET AL.
Ecolog). Vol. 7 5 . No. 2
threat. increased the frequent) of risk! behaviors in
treatments. were important in determining vulnera0. \,irili,. In our view. 0. \,irili.s is adapted to s)stems
bilit! t o predators (Sih et al. 1988). Indeed. in both
pilot laborator) experiments (J. E. Garve). ~rtlp~thli,shrd where interactions with other craLfish and frequent enr l i ~ t u a) nd other shelter competition experiments (Ca- counters with fish predators are rare. Frequent enpelli and Munjal 1982). 0. r ~ r s / r c ~and
~ i s 0.p r o ~ ~ i t l y ~ tcounters
~ ~ . ~ with invaders and with fish predators (given
were observed to evict 0. \.irili.s from shelters. Because shelter eviction bq invaders) increase 0. \,irili\ lethal
lish predators can earil! capture an evicted swimming behaviors which. in turn, contribute t o its replacement.
Morphological differences such as coloration in fisho r exposed crayfish (Stein 1977. Mather and Stein 1993:
es (Stacey and Chiszar 1975), size in pinnipeds (BarJ . E. Garvey. persotlul oh.sovution). 0. virilis is at a
tholomew 1970). and plumage color in birds (Rhijn
competitive disadvantage when seeking either limited
1973) influence aggression. For crab fish, increased bod!
o r unlimited shelters and therefore is vulnerable to
and chela si/e reduce predator) susceptibilitq in two
predation. This maq explain why. in low densitycobble
mra)s: ( 1 ) directlq. bq rendering fish consumption difpools where shelters were still abundant for crayfish
licult and (2) indirectl). b) increasing aggressive d o m ( z 5 0 rocks for 30 crayfish), 0. virilis was still selected.
inance. thus reducing riskq behavior and ensuring shelAn inabilit) to successfully occupy shelters in the prester acquisition. l n d ~ r e c teffects of increasing bod) and
ence of invaders and fish predators, even when shelters
chela s i x on aggression translated to susceptibilitq difare apparently unlimited. could clearly lead t o the exferences in 0. \.irili.s. For a n individual 0. \~tri/tsto
clusion of the least aggressive species. 0. virilis. In our
overcome its morphological and behavioral disadvanview. predator) risk likely enhances aggressive. intertages, it must be larger than its invaders. Indeed. 3 - m m
ference competition for refuge among crayfish congelarger 0 \,irilis, n o longer forced Into risk? behavior
ners. thus increasing vulnerability t o predation of the
and larger than the invaders (and therefore more difnative species.
ficult to handle), became equally vulnerable to largeIn a d d ~ t i o nto interactions at shelters. behavioral
differences in response to congeners or predators influ- mouth bass predation. .4lthough slightlq larger In the
enced susceptibility. Often. susceptible prey ma! re- field (Olsen et al. I99 I ) . O ~,irrli.smust maintain such
spond more to predatory risk than relat~vely~ n v u l - a si/c difrerential to avoid selection b) predators. Innerable pre) ( S t e ~ n 1979. Sih 1983). For example. teresting]?. mean 0. ~Yri1i.cC L increases in lakes as O
juvenile porcupines (Sweitjer and Berger 1992). sun- t.ir.\iri~ir.\ abundance increases (Olsen et al. 199 1 ) reinlishes (Werner et al. 1983). lobsters ( S m ~ t hand Herrn- lhrcing our interpretation, 0. \.rrrlis. owing to its smallkind 1992. Wahle and Steneck 1992).and craq fish (Stein cr chelae (Garvey and S t a n 1993) and innate. low
and Magnuson 1976) modil) their b e h a v ~ o rin re- aggression (Capclli and hlunjal 1982. this stud!). could
sponse to predators more than their adult counterparts. not avoid selective consumption b) predators. c \ e n
Sirnilarlq. under predator? risk. indibidual 0. \ , I ~ I I I \ with a slight si/e advantage. Therefore. in areas o f s h m c ~ Lr ,,I .~ I I I \ is
reduced activit) more than invaders. possibl? to avoid patr) wit11 0. rlc.\/rc,lc.\ and 0. ~ ~ r o i ~ r n y ~0.
predator detection. t + o \ \ e \ e r . cra)fish congeners in- selected b) lish predators and eventuallq replaced.
creased 0. \.irilis activit! \\ hen fish \\ere prcscnt. like11
through aggressixe interactions. Because interspecific.
rather than intraspecific. interactions occur more freT h e ability of a n introduced organism to establish
clilcntl! for 0. \,irili.,, contact with inxadcrs. rather than and increase in numbers is difficult t o predict (see Ehrconspecifics. more likel! compromised p r e d a t o r a v o ~ d - lich 19x6. Drake et al. 1989. Lodge 1993 Ihr reviews).
ancc behavior b) 0. \,rrrlr\ and increased its \.ulnera- Both predation (Simberloff 198 1 . Robinson and Wellbilitq. Although chelae d i s p l a ~ sd e t e r p r e d a t o r s born 1988) and competition (Crowell 1973. Le\.ins and
(Bo\'bjcrg 1956. Stein and Magnuson 1976). 0. riri/i.c Hcatwole 1973. Cole 1983. McLachlan 1985. Moulton
d i s p l a ~ e dchelae less than the inxadcrs. 0. \,rrrlr\ also and Pimm 1986) inflilence in\.asions. B) assessing ho\\
participated in risk). lethal s h i m flights generated b) an in\ ading species replaces its congeners. we can begin
interactions nit11 predators or congeners. ,411these be- to understand ecological mechanisms ilnderl) ing comhaviors rendered 0. ~,irilismore susceptible to lish milnit) structure. In this stild). \\e considered how
predation and competition interact to modif! craqlish
predators.
\l e conclude that 0. r~r\rrc,lrcand 0. p t ~ q ) t n y ~ r ~rcr . \ species assemblages cornposcd of Invading and nat1L.e
duccd their acti\.it! \\hen threatened. !ct continued to species. For the 0. rlc.\rrc.~i.\-O. pt~opolyiril.\ sbstem. preact aggrcssivcl) via chelae displahs toward predators. dation interacts directlq with prc) si/c to detcrmine
For thc ~ n x a d e r s neithcr
.
interactions with other craq- pre) susceptibilitq. i.e.. the smallcst species. 0. prolirhes nor with lish predators caused riskq beha\ iors. /)111q1e1c.\,silffers differential predation. Con\ crsel), difand 0. ric.c~rc,~rc
When alone. 0 ~,/t.rIr\r educed actix 11) when confront- ferences in si/c between 0. /~ropttly~rri\
ing a largcmouth bass and appeared to respond less ~nfluencccompetition. \\ hich Ilkel) influences \ illneraggressi\.cl> than the ~ n \ . a d c r sto it. In pool-tethering abilit). For the 0. \,otlt.\-0. ~'~rs/ti'~r.\-O.
propo~y~rlr.,
cxperlments, i n d i ~d u a l beha\ lor alone was sufficient sqstem. p r c d a t ~ o ninteracts directl? witll pre! b e h a ~ i o r
to render 0. ~.trili.\more susceptible to predation. How- to determine predator choice. Con\.ersel?, prel Interactions, modilied b) pre) aggression, predator) rec \ c r . interspecific interactions. as well as predator)
March 1994 SELECTIVE PREDATION A N D PREY COMPETITION
545
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Assessing How Fish Predation and Interspecific Prey Competition Influence a Crayfish
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