Competition, predation and microhabitat selection of Zygoptera larvae in a lowland river

2005·2.1 MB·English

by  Hofmann, T A

Odonatologica34(1):27-36 March 1,2005
Competition, predationandmicrohabitat
selectionofZygopteralarvaeinalowlandriver
T.A.Hofmann*andC.F.Mason
DepartmentofBiologicalSciences,UniversityofEssex,Wivenhoe Park,
Colchester,Essex,CO4 3SQ,UnitedKingdom
ReceivedOctober20,2003/RevisedandAcceptedMay 12,2004
Themicrodistribution of4loticspp.wasinvestigatedinthefield. Microhabitatselection
ofCalopteryxsplendensandErythrommanajaswasfurtherexaminedin thelaboratory,in-
dividuallyatdifferentlarval densitiesandinthepresenceoftheotherspeciesandapredator.
— E.najas,IschnuraelegansandPlatycnemispennipesshowedsignificantpreferencesfor
particularaquaticmacrophytescomparedtoothersinthefield,whereasC.splendensdidnot
discriminatebetweentheinvestigatedplantspecies.Onlylimitedspatialseparationwasap-
parentbetweenthelarvaeofdifferentspecies,aspreferencesforthesamemacrophytespe-
cies werefound.—Whenkeptseparateandatlowdensities,larvaeofC.splendensandE.
najasinhabited significantlydifferentmicrohabitatsinthe laboratory.Athighintraspecific
abundances,spatialoverlapbetween thetwospeciesbecame apparentasboth increasingly
occupiedless preferredsubstrata,which isin concurrencewiththe ideal free distribution
model ofhabitat selection.E.najasshowednochangeinperch selectioninthepresence of
C. splendensathighdensities. - Inthis instance,intraspecificcompetitiontherefore ap-
pearedtobemoreimportantthaninterspecificcompetitionwith otherZygopterain deter-
miningthe microdistribution ofE.najas. In the field,theniches ofthetwospeciesmay
bemoreadequatelyseparatedonthebasisofprey selectionorhuntingbehaviour. E.najas
alsoactivelyreactedtothepresence ofapredator,indicatingsomeflexibilityofresponse
regardingperchselection.
INTRODUCTION
ThelarvaeofOdonatafamiliesandespecially thoseoftheAnisoptera vary inphysi-
calappearance, adapting themto specific microhabitats.Incontrast,mostZygoptera
larvaeareessentially weed-dwelling anddifferonly slightly inbodyshape, withdiffer-
encesmainly inbodysizeandactivity levels.Insuitablesectionsofrivers anumberof
speciesofZygopteralarvae oftenoccurtogether instandsofaquatic macrophytes, but
*Authortowhomallcorrespondenceshouldbeaddressed: telephonenumber:01206873298
email:[email protected]
28 T.A.Hofmann&C.F.Mason
distinctionsonamicrohabitatlevelhaveseldombeeninvestigated. Anumberofques-
tions thereforeremain. Doesspatial separation, e.g onthe basis of differentialperch
selection, play animportantrole incompetitiveavoidance?Towhat degree areinnate
preferences ofmicrohabitatselectionmodifiedby intra-andinterspecificcompetition
forfoodandspace,predation andprey distribution?
JOHANNSSON (1978)foundspatialdifferencestoberelativelymoreimportantthan
temporal separation basedonlifehistory traits, inseparatingthenichesofCoenagrion
pulchellum, Ischnuraelgans, Enallagma cyathigerum andErythromma najas inalen-
tic system. Each species occupied amore or less distinctmicrohabitat,i.e. lily-beds,
reed-bedsoropenmud,andareasfavouredby larvaedidnot necessarily correlatewith
adultoviposition sitepreferencesandvariedbetweendifferentinstarsofthesamespe-
cies.Directcompetitiveinteractions, asopposed tocompetitiveavoidanceduetoinnate
preferences, havebeendemonstratedtoaffectlarvalcommunity structureanddistribu-
tion(BENKEetal., 1982;MERRILL& JOHNSON, 1984).Influencesofinterference
competitiononsurvivalrateswereshowninEnallagmaaspersum(PIERCE etal., 1985)
andCelithemiselisa(JOHNSONetal., 1985),whereasashiftinmicrohabitatselection
ofOnychogomphus uncatus occurred as a directresultofasymmetric interspecific in-
terference,with increasing densitiesfoundinnormallyless preferred substrata(SUH-
LING, 1996).Interspecific competition canevenresultincompleteexclusionofaspe-
ciesfrompotentially suitablehabitats(CASTELLA,1987).
Intraspecific interference,predominantly athighpopulation densities,affects surviv-
al andgrowthrates (GRIBBIN &THOMPSON, 1990),as well as larval distribution,
andevencannibalismhas beensuggested tooccur insomeodonates(CROWLEY et
al., 1987).
Thevulnerability oflarvaetopredation iscorrelatedwithmicrohabitatuseofvegetat-
edandunvegetated patches, whichisoften species- andhabitatspecific. Microhabitat
selectionmay shiftwiththeappearanceofapredator, so thatlow-risk vegetatedareas
arepreferentially occupied (PIERCE, 1988).Aquatic plantsare utilisedinahighlynon-
-randomfashion.Leaf-axialareas are preferred(WELLBORN&ROBINSON, 1987),
as well as spatially heterogeneous macrophytes with complex leafstructures (LOM-
BARDO, 1997),wheresusceptibility topredation is diminished.LOMBARDO (1997)
suggested thatthepresenceofcomplex leafedplants mayberesponsible forthecoex-
istenceofotherwisemutually exclusivespecies, duetolargeravailability ofmicrohab-
itats asrefuges. Hiding behaviourhasbeenshowntobeconditionalupon thetypeof
vegetation providinghabitatstructure.Active ‘hiding’as aformofantipredator behav-
iouris morecommoninmacrophytehabitatsofsimple leafstructure(DIONNEetal.,
1990).Interspecific differenceswithrespect toactivity patternsandability tocapture
prey undervarying levelsofpredation canbeevoked toexplain differencesintheoc-
currenceofOdonataspecies,e.g.betweenfish-containing andfishless ponds (PIERCE
etal., 1985;JEFFRIES, 1990).
STERNBERG(1994)concludedthattheobservedmicrohabitatdistributionofOdo-
natalarvaeatanypointintimeistheresultofinnatespecies-specific habitatpreferences,
Zygopteralarvaeinalowlandriver 29
modifiedby competitiveinteractionsthatmaychange locally andseasonally.
Theaimsofthisinvestigation were(a) toexaminewhetherZygoptera larvaearespa-
tiallyseparated ataloticsite,(b) to confirminnatemicrohabitatpreferences incontrol-
ledlaboratory experiments forsomespecies and (c)toassess theeffectsofintra-and
interspecific competitionandpredation onmicrohabitatpreferencesofthesespecies.
METHODS
FIELDSURVEY. —Four 1-minutesweepsamplesweretakenfromeachofthe followingpuremacro-
phyte stands:Phragmitesaustralis,Rorippanosturtium-aquaticumandGlyceriamaximawithina20msec-
tionoftheRiverStouratNaylandinSEBritain(51°56’83”Nand 1°17’88”E)inMarch2000.Theriverin
this sectionconsistsoftwochannels,anartificial channel,which wasdugforflood preventionandtakesthe
mainwaterflowawayfrom thesettlementatNayland,andtheoriginalriver channelwhereflowiscontrolled
byaweir. Inthiscutoffsectionflow velocitieswereextremelylow(<0,05ms'),andwithinthemacrophyte
stands, wherethe samplesweretaken velocities werebelow thedetectable range ofthe flow meter.Each
samplewastakenfrom anareaof0.5m2ofvegetation.Samplesweresortedforpenultimateandfinalinstar
Zygopteralarvae immediatelyuponreturntothelaboratory,identifiedandcounted.
CHOICECHAMBER EXPERIMENTS. — Choice chamber experimentswerecarriedouttocompli-
mentfield observations onmicrohabitat preferencesoftwo ofthespecies foundin theriver, Calopteryx
splendensandErythrommanajas.Thesewerechosenastheyareweed-dwelling,occurredinrelativelyhigh
numbersintheRiverStour and co-existedatthe sampledsite.Their comparativelylargesizealsoallowed
easierobservation.
Threesix-sidedchambers(height=29cm,basearea=412cm2)weredividedintofourequalsections,each
containingonlyonespecies ofaquaticvegetation.Phragmitesaustralis, Glyceriamaxima,Rorippanastur-
rium-aquaticumandFontinalis sp. wereselectedastheyarefrequentlyencountered alonglowland rivers.
Threeofthese speciesalsooccurredtogetherinthesampledriver sectionandarephysiognomicallyverydif-
ferent fromeachother.Plantspecieswereassignedtochoice chambersections usingrandom numbersand
individual specimenwerearrangedsoastocoversimilarareas.There werenophysicalbarriersseparating
thesectionsandthefourplantspecieswereequallyaccessibletolarvaefromthecentreofthechamber.The
bottomwascoveredwithgraveland neitherprey speciesnorexternaloxygen wasintroduced. Thechamber
wasfilledtoapproximately75%ofitsvolume withtapwater.
E.najasandC.splendensinparticulararegenerallyknowntopreferflowingwater.However,bothC.splend-
ensandE.najaslarvaewerefoundto-
getherinstagnant waterwithinmac- TableI
rophytestandsatthisandanumberof Species,numberofindividuals andreplicatesusedineachexperi-
otherfieldsitesalongtheRiverStour. mentaltreatment
Itwastherefore considered adequate
toutilise this experimentalsetupfor TTrreeaattmmeenntt EErryytthhrroommmmaa CCaalloopptteerryyxx GGaasstteerroosstteeuuss NNoo..ooff
theresultstoreflectthefieldconditions NNoo.. nnaajjaass sspplleennddeennss aaccuulleeaattuuss rreepplliiccaatteess
encounteredatthesestagnantriverine
siteswithinplantstands.Aflowcham- 11 - 11-2_ 020
ber,whichisgenerallyconsideredtobe 22 -- 1155 - 55
moresuitabletomimicloticconditions 33 -- 3300 - 55
intheriverchannel,wasalsolesssuit- 44 11 -- -- 2200
abletoalloweasyobservation oflar- 55 1122 - - 55
valsettlementTheresultsaretherefore 66 2244 - -- 55
limitedtothedescribedfieldconditions 77 1122 1122 -- 55
andcannotbeusedtomakeinferences 88 1122 2244 -- 55
aboutfasterflowingriver sites. 99 1122 -- 11 55
30 T.A.Hofmann&C.F.Mason
LarvaeofC. splendensandE. najaswerekeptinseparatetanks ongravelonly andfedonDaphniaand
chironomid larvae.Atotalof243 C.splendensand176E.najas,penultimateandfinalinstar larvaeofcom-
parablesizes,were available fortheexperiments.All had beentaken fromtheRiverStour.
Allexperimentswerecarriedoutintheopen,atambienttemperatureand light.Thepositionofthechambers
waschangedaftereachtreatmenttoaltertheincidenceoflight,whichmayaffectlarval settlementchoices.
Nine differentcompetitivetreatments werecarriedout(Tab.I).Within eachtreatmentnewindividuals were
usedforeachreplicate.Duetoinsufficientnumbers oflarvae,particularlyofE. najas,individuals werere-
usedin differenttreatments.
Larvaewereintroduced intothechamberbyplacingthemontothegravelinthemiddlesection.Theposi-
tion ofeachindividual wasnotedoneitherofthefourplantspeciesorgravelafteraperiodof1hourintreat-
ments 1 and4,and24hoursinallothertreatments.Thelongertimeperiodallowedcompetitiveinteractions
todevelopandaffectlarvaldistributionsbeforetheirpositionwasrecorded.Thefinaltreatmentincludedthe
presence ofapredator,athree-spinedstickleback, Gasterosteusaculeatus.
DATAANALYSIS. The microdistribution oflarvae,asestablishedbythefield survey,was compared
usingax 2contingencytable,aswell asKruskal-Wallis singlefactoranalysisofvariancebyrank,appropri-
ateforsmallsamplesizes(ZAR, 1999).
Within-treatmentchoicechamber resultswereanalysedusingx2andFriedman’sanalysisofvarianceby
ranks. Corrections werecomputedfortiedranks (ZAR, 1999).Differenttreatmentswere comparedusing x2
contingencytables.Replicateswerepooledwhereappropriatefollowinganalysis ofheterogeneity.
RESULTS
FIELDSURVEY
ThelarvaeofC.splendens, I.elegansandE. najashadsignificantly differentmicrodis-
tributionswithrespecttothethreeaquatic macrophytes sampled(x2=36.43,d.f.=4,p<
0.0u I.elegans wasfoundingreatestnumbersonG. maxima,withfewonP.australis
(Fig. 1).Comparingrankeddensities,thedifferenceissignificant (Kruskal-Wallistest,H
=8.171,d.f.=3,p< 0.05).E. najas wasfoundinequal abundanceonP.australisand
G. maxima,butnumbers
were significantly lower
onR. nasturtium-aquati-
cum (H =7.765,d.f.=3,
p < 0.05). Platycnemis
pennipes occurred main-
ly onG. maximaandwas
notfoundonR. nasturti-
um-aquaticumorP.aus-
tralis (H = 8.518, d.f. =
3, p < 0.05). No prefer-
ence couldbe detected
Fig.1.Mediannumber(nr2)ofCalopteryxsplendens,Ischnuraelegans, in the distributionofC.
Erythrommanajas andPlatycnemispennipeslarvae on three aquatic
splendens.
macrophytes(n=5).
Zygopteralarvaeinalowlandriver 31
CHOICECHAMBEREXPERIMENTS
Inexperiments withspecies testedindividually, thedistributionofC. splendens and
E. najas differedsignificantly betweenthefivesubstratumtypes atalllarvaldensities,
withtheexceptionofonetreatment(E. najas at24individuals)(Fig. 2).
C. splendens showedapreferenceforFontinalisinallexperimentaltreatments. Fifty
percentofindividualswereonaveragefoundonthemoss atlowlarvaldensities(Fig.
2a).This figure decreasedto 31%athigh densities,whilstproportions on R.nasturti-
um-aquaticum andP.australisincreasedfrom7%to21%andfrom1%to 13%respec-
tively. Larval densityhadasignificant effectonthe microdistributionofC. splendens
individuals(x2 = 18.04,d.f.=4,p <0.01).
In contrast, thehighest mean percentageofE. najas was foundonP. australis at
all abundance levels (40%
at low density) (Fig. 2b).
With increasing density, a
greater proportion of larvae
was foundonR. nasturtium-
-aquaticum (increase from
8% to 11%) and particular-
ly G. maxima(increasefrom
2%to22%).Again theeffect
was significant (x2 = 13.76,
d.f. = 4, p < 0.01). In both
species arise in density led
toamoreevenspreadofindi-
vidualsoverthefivesubstra-
tum types.At low densities
the distributionsof the two
species differedsignificantly
fromeachother(x2=43.70,
d.f.=4,p< 0.01).
Neitherofthetwocompeti-
tivelevels(5or12individual
ofC.splendens) hadaneffect
onthedistributionofE.najas
compared tothecontrol(Fig.
3).Highestmeanproportions
werefoundonP.australisin Fig.2.Proportionatemicrodistribution(%)of(a)Calopteryxsplend-
allthreetreatments. ensand(b)Erythrommanajaslarvaeonfivesubstratumtypesatthree
Inthe presence ofG. ac- eoxnpeeinridmiveidnutaallde(nns=iti2e0s),,cmhoeiacennauftmerboenre(±hSouDr)foafrteexrp2e4rhimouernstsfroarnblwoicthk
uleatus at the same larval experiments(n=5).Significancewastestedwithx2forfrequencies
density (12 individuals) the and Friedmantestforblockexperiments (d.f.=4).
32 T.A.Hofmann&C.F. Mason
mean proportion of E. najas occupying G. maximaand R. nasturtium-aquaticum in-
creasedfrom2%to5%andfrom8%to23%respectively, whilstonly 5%oflarvaere-
mainedongravel. DifferencesinthedistributionofE. najaswithand withoutpredator
(control) weresignificant (x2= 11-63, d.f.=4,p<0.05).
DISCUSSION
MICRODISTRIBUTIONINTHEFIELD
Theresultsofthefieldinvestigation indicatedthatthefourZygopteraspecies differed
significantly fromeachotherindistribution,withrespecttothethreeaquaticplants sam-
pled. Withtheexception ofC. splendens, each species occurredinsignificantly higher
proportionsonsomeofthemacrophytespecies comparedtotheothers.I.elegansseemed
toavoidP.australis,butwas foundonbothG. maximaandR. nasturtium-aquaticum.
In contrast,E. najas wasrarely presentonR. nasturtium-aquaticum, occupying mainly
P. australisandG. maxima.P.pennipes was absentfromR. nasturtium-aquaticum.
Whilstthesespecies-specific differencesensuredacertaindegreeofspatialseparation,
someoverlap wasapparentanddifferentzygopteransessentially coexistedinthesame
microhabitat,e.g.allspecies werefoundonGlyceria maxima.Theobservedmicrodis-
tributionmaybeexplained onthebasisofinnatepreferences forparticularplantspecies,
modifiedbyintra-andinterspecific interactions.Theseverityandoutcomeofcompetition
differsbetweenplants (SUHLING,1996),asperchavailabilitychanges inlinewithvar-
yingleafandstructuralcomplexity, andalsobetweenOdonataspecies, dependentupon
thesizeofthecompetitors. Competitiveinteractionsarereducedwhereperchavailabil-
ityishigher(GRIBBIN
&THOMPSON, 1990)
and in more heteroge-
neous microhabitats
(complex-leaf versus
simple leaf structures)
(LOMBARDO, 1997).
Innate species-specific
preferences maybere-
latedtosizedifferences
betweenlarvalOdonata
ofthesameinstar, mak-
ing particular aquatic
plants more suitable
Fig.3.Proportionatedistribution {%,±SD) ofErythrommanajas(12indi- as perches thanothers.
viduals, n=5)persubstratum type,whenkeptseparately(control),atlow
Differentialselectionof
levels (12individuals)andhighlevels(24individuals)ofcompetitionfor
percheswithCalopteryxsplendensandinpresence ofapredator,Gasteros- perchesofdifferentdi-
teusaculeatus (1 individual). ameter on the basis of
Zygopteralarvaeinalowlandriver 33
certainbody dimensions(eye spacing) was demonstratedby ASKEW(1982)inadult
Odonata.Ifasimilarprinciple applies tolarvae, itcouldexplain why thetwo small-
estspecies (I. elegans andP.pennipes)wererare onP. australisstems, whichhave a
comparatively greaterdiameter.
Wherespecies coexist, e.g. onG. maxima,size differencesbetweenlarvaemay al-
lownicheseparation onthebasis ofprey selection, afactoralsoevokedtobepartial-
lyresponsible fortheseparation ofdifferentinstarsof thesame anddifferentspecies
(CROWLEY& JOHNSON, 1982).
Onacumulativebasis, mostZygopteralarvaewerefoundonG. maxima,leastonP.
australis. Differencesinstructuralcomplexitybetweenplant species mayberesponsi-
bleforlarvalpreferences. Phragmites hasarelatively simplestructure,offering limited
perchspaceperplantinonly onedimension(verticalstems).Concealmentfrompreda-
torsislimitedcompared tomacrophytes withmorecomplex leafstructures.G. maxima
hasverticalandhorizontalstructuralcomponentsforlarvaetooccupy. Leavesarefairly
simple(linear),butmayformdensefloating mats(PRESTON&CROFT, 1997),which
increasesperchavailabilityperplantandeffectivenessasrefuge.R. nasturtium-aquati-
cum is structurally themost complex of thethreemacrophytes. However, leavesare
oval-shaped andmaybeless suitableforoccupancy by somelarvae,apossibility that
requiresfurtherinvestigation. Selectionofmicrohabitatonthebasisofpredator avoid-
ance hasbeenrepeatedly suggested (THOMPSON, 1987;CONVEY, 1988;PIERCE,
1988;JOHNSON,1991).Preferenceforcomplex leafhabitatsandparticularpartsofa
plant hasbeenobserved(WELLBORN&ROBINSON, 1987;DIONNE etal., 1990,
LOMBARDO, 1997)andgreaterpreyavailability inadditiontoconcealmentfrompre-
dationhasbeenproposed aspossiblereason.
CHOICE CHAMBEREXPERIMENTS
In thefield, manyenvironmentalfactors may influencethe perch selectionby Zy-
gopteralarvae, including watertemperature,competition andpredation. Theobserved
microdistributionatany point intimeistheresultofthecombinedeffectofallofthese
onlarvalcommunities(STERNBERG, 1994).Controlledlaboratory experiments can
help toidentify ‘true’larval choicesand illustratehowthese maychange when other
influencesareadded.
Inexperiments carriedoutwithindividualspecies, itbecameapparentthatC.splend-
ens larvaefavoured Fontinalismoss overanyofthemacrophytes andthatthisprefer-
encepersisted as larvaldensitiesincreased. Unfortunately this couldnotbecompared
tofieldresults, asFontinaliswas foundattoo lowaquantity tobesampled intheriver
section.However,basedonfieldresultsfromanotherriver,ZAHNER(1959)proposed
thatC.splendensavoidedFontinalisduetoitsdensegrowth.Thesedifferencesinobser-
vationmaybeanindicationofhowperchchoiceismodifiedinthefield.Increasedflow
velocity at someriverinesites mayrenderFontinalisunsuitableforlarvaloccupancy,
as themosshas nosupporttissuetowithstandflow.Wherethereisnoflowas was the
34 T.A.Hofmann&C.F. Mason
case atthesitesampled inthisstudy, Fontinaliscanofferastructurally heterogeneous
habitat, enabling concealmentfrompredators andconspecifics. Aninnatepreference
forthe mostcomplex perch substratum, offeringeffectiveconcealment,mayexist ir-
respective ofthepresenceofapredator.
Increasedlarvaldensitiesledtomorelarvaestartingto coloniseless preferredsubstrata,
inordertolimitintraspecific interference,conformingwiththe‘idealfreedistribution’
modelofhabitatselection,whichstatesthatovercrowding reducesthesuitabilityofthe
mostpreferred habitat,increasing thatofintermediatehabitatswheredensitiesrise ac-
cordingly (FRETWELL, 1972).SimilarobservationsweremadebySUHLING (1996)
regarding Onychogomphus uncatus larvae.Aggressive interactionsandperch defence
behaviourareknowntooccurinZygopteralarvae(JOHNSON, 1991).Thismayimply
thatcompetitively inferiorindividualsareforcedto occupy microhabitatsthatareless
suitable(e.g. gravel)andmayconferfurtherdisadvantages (e.g. greatervulnerabilityto
predation, limitedprey availability) (BEGON etal., 1996). Itappears thatlarvaldensi-
ties inconjunction with intraspecific interferencecompetition affectthe microhabitat
selectionofC. splendens larvae.
ThemicrodistributionofE. najas differedsignificantly fromthatofC. splendens at
lowlarvaldensities.E.najas larvaepreferentially occupiedPhragmites stems,butwere
alsofoundathigh proportionsonFontinalisandgravel. Incontrasttofieldobservations,
E. najas larvaeavoidedG. maximainthechoicechamber.
Thedecisioncriteria withrespect toperch selectionmay vary between species, de-
pendent on ecological differences, e.g. differentiallevel ofintraspecific interference
(includingcannibalism).Also,subjecttolife style (e.g. active versuspassive hunting),
microhabitatselection maybeevolutionary fixedorflexible(PIERCE, 1988).
SimilartoC. splendens, microdistributionofE. najas changed as larvaldensitiesin-
creased, with larvaemore evenly spread over allsubstratum types.Atthe sametime
the degreeofspatial overlapbetweenthetwo species increased, sothatthedifferences
becamenon-significant. Thismaybe amorerealistic representationofthefieldsitua-
tion, wheredensitiesare higher.
ThemicrodistributionofE.najaschanged significantly inthepresenceofapredator.
Themainobservationswereadecreaseinproportional occupancyofgravelandPhrag-
mitesandanincreaseinrelativeabundanceoflarvaeonFontinalisandRorippa. Complex
macrophytes thatoffersuitablerefuges werethereforeincreasingly selected,assuggested
byLOMBARDO (1997).High-riskhabitats,suchasgravel,werelargelyavoided.Perch
selectioninE. najas was thereforeflexibletosuitachanging environment.
Itmaybesurprising, inthefirst instance, thattheintroductionofC.splendens larvae
didnot haveasignificant effectonE. najas microdistributionin thecompetition ex-
periments.Theresults discussedabovesuggestthatthetwospecies avoidintensecom-
petition by limiting spatial nicheoverlapatlowintraspecific larvaldensities.Whenin-
traspecificcompetitionislow,innateperchselection preferences,e.g.based onphysical
characteristicsofthelarvae, may bemoreeasily realised.
Inconclusion, intraspecificcompetitionappearstobemoreimportantthaninterspecific
Zygopteralarvaeinalowlandriver 35
competitionwithotherZygopteraindetermining themicrodistributionofC. splendens
andE. najas inmarginalmacrophytestandswithdiminishedflow.Thespecies showed
differentpreferences withrespect toperch substratum.However, dependenton larval
densities,spatial separation was limited, as intraspecific interferencecausedindividu-
als toincreasingly occupy less preferredsubstrata.The nichesofthetwo species may
bemoreadequately separated onthebasis ofprey selectionor hunting behaviour.C.
splendens individualsofthesameinstarare onaveragelargerthanE. najas larvaeand
may consumelargerprey.E. najasalsouse amoreactive huntingmethod,whereasC.
splendens areessentially sit-and-waitpredatorsandare moreactiveat night(BROOKS,
1999). E. najasalsoactively reactedtothepresenceofapredator, indicating flexibility
ofresponseregarding perch selection.
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