Variation inherbivorywithinandamongplantsof Daphnelaureola(Thymelaeaceae):correlationwithplant sizeandarchitecture
CONCHITAALONSOand CARLOSM.HERRERA
EstaciónBiolôgicadeDouiana,CSIC,Apartado1056,E-41080Sevilla,Spain
Summary
1Herbivory bynoctuid mothlarvae(Lepidoptera,Noctuidae) onplantsofDaphne laureola L.(Thymelaeaceae)wasstudiedundernatural conditions inasouth-eastern Spanishmontane habitat. Themainobjectiveofthestudywastodeterminehowsize andarchitectural featurescorrelated withherbivorylevel(meanpercentageleafarea removedbytheendofthelarvalgrowthseason)andherbivoreload(meannumber ofcaterpillars recordedperday)bothamongandwithinplants.
2Asignificantcorrelation wasfound betweenherbivoreloadandherbivorylevelof individualplants.Herbivorylevelsdifferedwidely(range 0.1—12.8%leafarea)and wereconsiderablysmallerthanthoseoftenusedinartificialdefoliationexperiments.
3Variation among plantsintheincidenceofnoctuid larvaewasdirectlyrelatedto thenumberofleafwhorls,andinverselytothemeanbasaldiameter ofstems.These responsestosizeandarchitectural traitsmaybe explainedbydiscrimination by ovipositingfemales.
4Within plants,larvaepreferentiallyselectedleafwhorlshavingshorter supporting
stemsandlowerbranchingorders. Movementcostsmaybereducedbylarvaeusing plantarchitecturaltraitsascuesforwithin-plant foodselection.
5Thereasonsforand potential implications ofthedifferentfeatures usedbyadult
noctuidsdiscriminating betweenplantsand bytheirlarvaeselectingleafwhorlsare discussed.
Keywords: foodselection,foragingbehaviour,insectherbivory,Noctuidae, plant architecture
Introduction
The evolutionaryimplicationsof plant—herbivore relationships cannot be understood without elu cidating(1)whyallindividualsinaplantpopulation arenotequallydamaged, andwhetherdifferencesin herbivorylevelsarerandom, relatetoenvironmental conditionsordependonintrinsic(i.e.potentiallyheri table)plantfeaturesand(2)whetherdifferentialher bivoryhasfitness-relatedconsequencesforeitherthe plant, theherbivores, or both (reviewedinKarban
1992;and Marquis 1992).Somerecentstudies have focused on characteristics extrinsic to individual plants, suchasmicrohabitatconditions (Lincoln & Mooney 1984;Bowers et al. 1992;Dudt Shure
1994),recentherbivoryhistory(McCreaAbra hamson 1987)and interaction withnatural enemies (Moran1984;Hare1992;OhsakiSato1994).Other analyses haveconsidered relationships betweenher-
bivory levelsand plant genotypes (Marquis 1990; Fritz 1990; Bowers et al. 1992)and, more rarely, associatedphenotypes(DirzoHarper1982;Nuñez Farfán Dirzo 1994).Athird lineofinvestigation concentrateson individual variation in phenotypic traits,suchassecondarychemistry(DirzoHarper
1982; Zangerl & Berenbaum 1993), phenology
(Moran & Whitham1990),physicalcharacteristics of foliage(Howard 1988)or plant gender(Elmqvist& Gardfjell1988;Boecklenetal.1990).
Most investigations on correlates and conse
quencesofvariationinherbivoryhave adopted an experimental approach wherebyherbivory levelsare simulated artificially and/or plants are cultivated undercontrolledconditions.Comparativelyfewstud ieshaveexamined thephenotypic correlates ofindi vidualplant variation inherbivore loadand foliage lossundernaturalfieldconditions. Wefocusedonthe basisfortheinteraction betweenDaphnelaureolaand
Table1Effectsofsizeand architectural plant traits on individual variation in herbivore load (caterpillars perday)and herbivorylevel (%leafarea removed),asassessedbymultipleregressionanalyses.Onlyvariablescontributingsignificantly (ormarginallyso)areshown.Significancelevelstestedusingrandomizationmethods.b=standardizedregressioncoefficient
Caterpillars/dayLeafarearemoved
R2=0.613,P=0.02R2=0.214,P=0.05
Variable / b / b / PNumberofleafwhorls / 0.8285 / 0.0002
Meanstemdiameter / —0.3749 / 0.0684 / —0.4788 / 0.0174
itsmajor herbivores(lepidopteran larvae)byinvest igatinghowvariation insizeandarchitecture among shrubsrelatestodifferencesinherbivoryinthefield.
Variationinplantarchitecturecouldinfluenceher bivoryratesviaitseffectsontheforagingcostsand behaviour ofcaterpillars. Plantsizeandarchitecture havebeenconsideredtoexplaindifferences inher bivory between species (Lawton 1983), but more rarelyamongconspecifics.Outofatotalof43studies reviewedbyMarquis (1992:Table 13.3)that exam inedtheeffectofplantcharactersonintraspecificvari ationin herbivoredamage,onlyfourconsidered architecturaltraits.
Arealisticassessment ofherbivory levelsexperi
encedbyindividuals ofD.laureolawasobtained by evaluatingtheoveralldamagecausedbyinvertebrate herbivores(MaddoxRoot1987;Marquis1990).In sometreestheconsequencesofherbivorydependon location effects(Heichel Turner 1984; Marquis
1988),butintracrown variation inherbivoryhasnot
been documentedpreviouslyforshrubs.Within-plant variation in herbivory was therefore studied in addition toamong-plantdifferences.Acomparison ofwithin-andamong-plant analysesmayillustrate possible conflictsbetweenpreferencesoflepidopteran larvaeandhostselectionby ovipositingfemales (Thompson 1988).
Thespecificquestions addressed inthisstudyare
thus:(1)Do D.laureolaplantsdiffersignificantlyin sizeandarchitectural traits,andinherbivory levels? (2)areindividualdifferencesinherbivory among D. laureolaplantsrelatedtovariation insizeandarchi tectural traits,(3)doherbivoresexhibitwithin-plant selectionand(4)arewithin-andamong-plantchoices consistent?
STUDY SPECIES
(Obeso1985;Hulme 1992).Plants shedtheiroldest leavesinearlysummer.
D.laureolaplantsconsistofavariablenumberof erectstemsthatriseatground level fromacommon trunk(Fig.1).Leavesarefoundonlyatthedistalend ofeachbranch,formingasingle,well-defined leaf whorl.Individualstems mayremainundividedormay ramifyinto two,threeorfour branches, whichalso may branch again or remain undivided. Variation amongplantsinthenumber,lengthanddegree of branchingof stemsgeneratesabroadspectrumof architectural types inthisspecies.Fromtheviewpoint ofslow-moving,flightlessinvertebrate herbivoresthat havetocrawlalong barestemstoreachleafwhorls (e.g. lepidopteran larvae),foodresourceswithinD. laureolaplantsoccuraswidelyspacedpackages.Each oftheseprovidesfoodin excessofindividualrequire ments during asingleforaging bout, but the need forregularsearchpresumablyimpliescertaincostsin termsoftimeandenergy.
Foliage-feedinglepidopteran larvaewerethemost frequent herbivoresonD.laureola. Mammalian her bivores donotbrowseonD.laureolaplants,probably becauseofthepresenceofavarietyoftoxicandrepel-
DaphnelaureolaL.(Thymelaeaceae) isalong-lived evergreenshrubthat,in theMediterraneanregion, occupiespreferentiallytheundergrowth of shady mountain forests.Inthestudyareafloweringoccurs inFebruary—March,shortlybeforenewleaves are produced. Fruits(single-seededdrupes)ripenin early June, earlier than most fleshy-fruited plants in the area, and are heavilyconsumed bymiceand birds
Fig.1Characteristicarchitecture ofaDaphnelaureolaplant. Numerals denote thedifferentbranching orders associated withindividualleafwhorls.
lentsubstancesin thefoliage(Hegnauer1973).Grass hoppers wereoccasionally observedon studyplants buttheirdamage(which couldbeeasilydistinguished from that caused bycaterpillars) wasnot recorded. Somegeometridlarvaewere alsoobservedspor adicallyonD.laureolaplantsbut,astheywerenever seen feeding on the foliage, were not considered further.Alllarvaerecordedfeedingwerefromnoctuid moths(Noctuidae).Themostabundantspecieswere TrigonophoraflammeaEsper.(44.1%oftotalrecords) andNoctuajantheBkh.(38.4%). Other species observedwerePseudenargia ulicisStaud.(10.2%)and Noctua‘fimbriata’(=N.fimbriataSchreb.+ N.tir renicaBieb.,Spdl.Hngk.) (7.3%) (fortaxonomic identityseeYela1992).Inthestudyregionthesefloe tuidspeciesareallpolyphagous onavarietyofher baceousandwoodyplants(J.L.Yela,personalcom munication; T. flammea,for instance, isknown to feedonsuchdisparate plantsasCrataegus,Quercus, Arbutus,Phillyrea,NarcissusandPaeonia,inaddition toDaphne). Weonlyevaluated theimportance of caterpillarsas consumersof foliage(largelyyoung leaves), although they also consume flowers and unripefruits.
Noctuajanthe larvaeweremostfrequentonD. laureolaearlyintheseason(Apriltomid-May). Lar vaeremainedinactive duringdaytime,hiddeninthe litterbeneath theplants,andclimbedatnighttothe leafwhorls to feed.They did not usually feedon thesameleaf whorlduringconsecutivenights.This speciesbelongstothe‘shortpupalduration’groupof Mediterranean Noctuidae(Yela& Herrera 1993)and adultsemergeearlyinthe summer,shortlyafter pupation.LarvaeofTrigonophoraflatnmeaweremost frequentonD.laureolafromearlyMaytomid-June, when pupation occurred. Their foraging behaviour wasfoundtovarywithage.From thefirsttofourth instars,larvaearegreen-coloured,remainhiddendur ingdaytime among theleavesofD.laureolaplants, and feedon thesameleafwhorl forseveralnights. Afterenteringthe fifthinstar,theybecomebrown- coloured and hide in the litter beneath the plants duringthedaytime,climbingtotheleafwhorlsevery nighttofeed.Atthisstage,theydidnotfeedonthe sameleaf whorlduringconsecutivenights.Tn gonophoraflammeapassesthesummeraspupa(‘long pupalduration’ groupofYelaHerrera 1993),and adultsemergebyearlyOctober.
Methods
ThestudywascarriedoutduringApril—June1994on apopulationofmarked Daphnelaureola individuals (N=28plants)locatedintheSierradeCazorla(Jaén province, south-eastern Spain), on a north-facing slopeat1235ma.s.l.(CAsiteinYelaHerrera1993). Themarkedshrubswerepartofotherinvestigations (YelaHerrera 1993;andJ.L.Yela,unpublished), andrepresentedarandomsubsample ofthelocalD.
laureola population.Nearest neighbourdistances betweenmarkedplantsrangedbetween0.2—30m,and thetotalstudyareawasabout 1500m2. Atthestudy site, D.launeolagrows asan understorey shrub in adense mixedforest, with Quercusrotundjfolia,Q. faginea,Pinusnigra,PhillyreafatjfoliaandJuniperus oxycedrusasdominant treespecies,and Rubusu/mi folius,CrataegusmonogynaandRosacaninaasdomi nantshrubs.
PLANT CHARACTERISTICS
Plant structure wascharacterized according tosep arate size andarchitectural measures,although relationshipsmayexistbetweenthetwotypesoffea tures.
Sizetraitsaresinglemeasurementsthatprovidean
overallcharacterizationforanindividual plant. For eachmarkedplantweusedthenumberofleafwhorls, theheight ofthetalleststem(‘plantheight’hereafter) andthegeometricmeanofthemajordiametersofthe plant’sverticalprojection(assimilatingittoan ellipse) asestimatesofplantsize.
Measurements used to characterize the archi tectureofmarkedplantsweremadeonindividualleaf whorls,and thus provided aseriesofreplicates per plant that reflectedintraplantvariation instructure andcharacteristicsof stemsandleafwhorls.Measure mentsweremadeinlateJune, whenallcaterpillars had either beencollected for other studies or had alreadylefttheplantstopupate.Wecharacterizedall theleaf whorlswherecaterpillars hadbeenpreviously recorded,whichhadbeen markedduringobservation sessions (‘markedwhorls’ hereafter)anduptoa maximumof10leafwhorls(dependingonplantsize) chosenhaphazardly amongunmarked ones(‘control whorls’hereafter).Since herbivoreshadnotbeen chemicallyorphysically excludedfromplants,the limitations of exciosure-based approaches(Strauss
1988)did not apply, but the control whorls could have beenfed upon bycaterpillars on dates when noobservations were made.However,only40%of control whorlshadatleastoneleafwith 26%area removed,incomparison with82%ofmarkedwhorls
(x2 =74.6,P<0.0001).Thisindicates that con
trolwhorlsactuallyrepresentareliablesampleofleaf whorlsthatwere lessoftenselectedbycaterpillars.
Forbothmarkedandcontrolwhorls,wemeasured thelength(fromground leveltotheinsertion ofthe firstleaf)and basaldiameter(atground level)ofthe supporting stem,counted thenumber ofleavesand determinedthebranchingorder.Branchingorderwas takenasthenumberofforksoccurringfromthebase ofthestem(whichwasconsidered asthefirstfork) uptotheleafwhorlunderconsideration,i.e.thenum ber offorksa climbinglarvahadtopassovertoreach theleafwhorl(Fig.I).Averagebranchingorderthus reflectstheinternalcomplexityoftheplantfromthe viewpointofforaginglarvae.Bycomputing weighted
plantmeans,architectural datacanbeusedtomake amongplantsaswellaswithinplantcomparisons.
INCIDENCE OF HERBIVORES
TheincidenceofherbivoresonmarkedD. laureola plantswas assessedbothasthefrequencyofoccur renceofcaterpillars during nocturnal censuses(‘her bivoreload’)andastheproportion ofleaftissue removed(‘herbivorylevel’).
Herbivoreload
Inorder todetermine thenatural herbivoreloadon markedplantswemadeeffortstodisturbcaterpillars aslittleaspossible.Observations oflarvaewerecon ducted at night (from 2l.O0hours to 24.O0hours GMT), whentheywereactiveandmostconspicuous. Observations were conducted on two consecutive nights each weekover the whole study period, to enable short-term variation in the distribution of caterpillars tobeassessed.Plants werecheckedina differentorder oneachnighttoavoidpossiblearte factsduetolarvalactivityrhythms.Oneachoccasion, thefoliageofallmarked plantswascarefullyexam inedatcloserangeusingaheadlamp todetectactive caterpillars. Werecorded thetaxonomic identity of everycaterpillar observedandmarkedoneleafofthe whorlonwhichitwasfeeding.Differentcolours of plastictapewereusedformarkingondifferentdates.
Herbivorylevel
Foreverycurrent yearleafinallmarkedandcontrol whorlstheproportionofleafarearemovedbytheend ofthelarvalgrowthseasonwasestimated.Individual leaveswereclassifiedintooneof6herbivoryclasses according tothepercentageleafarearemoved:0,no signsofherbivory;1,1—5%arearemoved;2,6—25%;
3,26—50%; 4,51—75%;and 5,>75%. An overall
estimate ofherbivory levelforeach leafwhorl was obtained as:
wheren =numberofleavesinherbivoryclassi,and
H,=midpointofherbivoryclassi,asdefinedabove.
1?representsaweightedaverageoftheproportionof arearemovedperleafineachwhorl. Reduction ofa continuousmeasurementscale(percentageleaf area removed)toanordinalone(theclassscoreusedhere) mayleadtobiasedestimatesofmeanherbivory(Wil liamsAbbott 1991), butthisanalytical sim plification allowsfor more extensivesampling and thuslargersamplesizes.Furthermore, inourstudy systemtherearenoreasons toexpectthat potential biaseswouldhavea differentialeffectontheestimates
obtainedfor differentplantsorplantparts,hence estimatesmaybeproperlyusedforcomparative pur poses (see,e.g.Dirzo& Harper 1982;FoxMorrow
1983; Nüñez-FarfánDirzo1988;Alliende1989;for applications ofsimilarherbivoryindices).
Meanvaluesofherbivorylevelwerecomputed for control and marked whorls of all marked plants (exceptfor a plant without caterpillar records and thus lacking marked whorls), and compared using pairedt-tests.As expected,averageherbivorylevels were significantlygreaterinmarkedwhorlsthanin control ones(t=5.48,N=27,P0.0001).This indicatesthat,althoughoursamplingscheme was incomplete(onlytwoobservation nightsaweek),the observations providedareliable assessmentofthe distribution ofcaterpillars withinplants.
DATAANALYSIS
Foramong-plant comparisons, plantmeanswere computedfor variablesmeasuredrepeatedlywithin plants(architectural traitsandherbivorylevel).How ever,measurementsfromcontrol andmarkedwhorls should nothavethesameinfluenceonplantmeans. Controlwhorlsarearandomsubsetof allthose remaining unmarked onaplant, whilemarked ones representacompleteenumerationofasubsetthatis determined bythe presence of caterpillars on par ticulardates,anditis potentiallynonrandomifcater pillarsselecttheleafwhorlsonwhichtheyfeed.We thereforecalculatedweightingfactorsforcontroland markedwhorlsineachplant(FandFM,respectively), andusedthemto computeweightedplantmeans. Weightingfactorswerecalculatedforeachplantas
F=[(WT— WM)/WT)]/ W,and
FM =(WM /WT)/ WM,
whereWM =numberofmarkedwhorls,W=num berofcontrolwhorlsandWT=totalnumberofleaf whorlsontheplant.
Nonparametric statisticalmethodswere used wheneverdatadeparted fromnormalityornormality testscouldnotbe performedreliablyduetosmall samplesizes. Statisticalanalyses,unlessotherwise stated, wereconducted usingprocedures intheSAS package(SASInstitute1989). Thesignificanceof differences amongplantmeanswastestedwith Kruskal—Wallisanalysisofvariance(NPAR1WAY pro cedure). Relationships between variables were assessedusingregressionmethods (REG procedure), and the significance of regression coefficients was testedusingrandomization methods(Noreen1989; Manly1991)whenvariablesdidnotfulfilnormality requirements. Within-plant differences between marked and control whorls were analysed using pairedt-tests.
Results
J Ui
AMONG-PLANTVARIATION
-I
Plants differedboth insizeand architectural traits. There was broad variation in height (range 45—
140cm), number of leaf whorls (3—275), and both major (30—380cm)and minor (10—170cm)plant diameters.Plantsalsodifferedsignificantlyin mean length(H27=180.0,P<0.0001)and meanbasal diameter(H27=222.7,P<0.0001)ofstems,mean
numberofleavesperwhorl(H27=150.7,MEAN STEMDIAMETER (mm)
P0.0001), and mean branching order of leaf whorls(H27=120.9,P<0.0001)(dataforcontrol andmarkedwhorlscombinedforeachplant).
Werecorded337noctuidlarvaeover22 obser vation nights. Both mean herbivory levelsand her bivoreloadsdifferedwidelybetweenplants(Fig.2). Therewasstatisticallysignificantindividualvariation inboththenumberofcaterpillarsrecordedperobser vationsession(H27=133.4,P0.0001)and inthe proportionofleafarearemovedbytheendofthestudy (H27 =118.4, P< <0.0001).These two variables werepositivelycorrelated across plants (r=0.508, P=0.007). Afewplants had both high herbivory levelsandcomparatively fewlarval records(Fig.2), andthesignificance oftherelationshipincreases dramaticallyafterremovingthese outliersfromthe analysis(r,=0.764,P=0.00003).Noinformation is available toexplaintheoccurrence oftheseoutliers, butthe overallsignificantassociationacrossplants betweenherbivoreloadandherbivorylevelindicates, at least, that (1)our nocturnal observation scheme wassufficienttodetectindividualvariation incater pillarabundance; (2)observeddifferencesamong plantsinleafdamagewereconsistentwithdifferences intheabundanceofherbivores;and(3) eithercanbe usedtodescribeindividualvariation intheincidence ofherbivores.
Toevaluatetheinfluenceofplantfeaturesonher bivoreload and herbivory level,weconducted sep aratemultiplelinearregressionsfor eachdependent variable(herbivore loadandherbivorylevel)usinga stepwise method. The independent variables con-
Fig.3Relationship betweenmean herbivory level(%leaf arearemoved)and meanbasalstemdiameterofindividual Daphnelaureolaplants.
sideredweretotalnumberofleafwhorls,thesquare root oftheproduct ofmajor and minor diameters, plantheight,andthe weighted plantmeansfor stem basal diameter, stem length, and number ofleaves andbranching orderofleafwhorls.Thesignificance levelof those variables showing the largest effects werecomputed usingrandomizationmethods. Only totalnumberofleafwhorlsandthemeanbasaldiam eterofstemshadsignificanteffectsonherbivorylevel andherbivoreload(Table1).Acrossplants,herbivore load wasdirectly related to number ofleafwhorls and inversely(although onlymarginally significant) tomeanbasalstemdiameter. Herbivory level was inversely relatedtomeanbasalstemdiameter(Fig.3), withdamagebeingpredictablysmallestamongplants withthethickeststems.Noneoftheotherplanttraits considered exhibited significant relationships with eitherherbivorylevelorherbivoreload.
WITHIN-PLANT VARIATION
Thedifferencebetweenmarkedandcontrol whorlsin averagestembasaldiameter, stemlength,branching order, andnumberofleaveswastestedusingpaired t-tests.Foreachcharacteristic tested,data pairswere therespectivewithin-plant averagesformarked and control whorls.Onaverage,leafwhorlswherecater pillars had been recorded had significantly more leaves,lowerbranchingorder,andshorterstemsthan control ones(Table 2).Meanbasaldiameter wasnot differentincontrol andmarkedwhorls.Theseresults suggestthatcaterpillarsshowsomewithin-plantselec tionoffeedingsitesbasedonstructuralcharacteristics ofleafwhorlsandtheirsupportingstems.
Discussion
AMONG-PI.ANTVARIATION
Theproportionofleafarearemovedbyinvertebrate herbivores hasoften beenusedtoinfervariation in
Fig.2Individualvariationinmean(+SD)herbivoreload theincidenceofherbivores but simultaneous direct (caterpillarsrecorded/observationday)andherbivorylevel countsoftheherbivoresthemselvesarerare(e.g.Fritz (%leafarearemoved)inDaphnelaureola plants. 1990;Moran Whitham 1990;DudtShure1994;
Table2 Differencesinarchitecturaltraitsbetweencontrol(noNoctuidlarvaerecorded)andmarked(larvaerecorded)leaf whorls.Significancewastested bycomparison ofcontrol and marked meanvaluesperplant usingpaired1-tests(N=27 plants)
Population average
Variable / Control / Marked / PStembasaldiameter(mm) / 11.2± 3.9 / 10.4± 2.9 / 1.27 / 0.1122
Stemlength(cm) / 78.1± 20.4 / 63.5± 18.3 / 4.60 / 0.0001
Branchingorder / 2.3±0.5 / 2.0±0.3 / 4.01 / 0.0005
Numberofleaves / 16.2± 3.2 / 17.9± 3.4 / 3.52 / 0.0016
Nuñez-FarfánDirzo1994). Conversely,studies analysingherbivoreloadshavenotoftenrelatedthem toproportionallossesinplantfoliage(Cottan eta!.
1986;KarbanCourtney1987;Boecklenetal.1990).
The significantrelationshipfoundacrossindividual plantsbetweenherbivoreload(numberoflarvaerec orded)andherbivorylevel(percentageleafareaeaten) indicatesthatestimatesoffoliarlossesattheendof theseasonadequately reflectedthedefoliation inten sityexperienced byindividual plantsofD.laureola. Thus, at leastinthisspecies,measurements ofleaf arearemovedmaybereliablyusedtoassessthedis tribution ofinvertebrate herbivoresbothamongand withinplants.
Significantdifferenceswere foundamongplantsin levelsofnaturalherbivory.Nevertheless,theobserved rangeofplantmeans(0.1—12.8%)isconsiderablynar rowerthanextremesofdefoliationlevelsusedinmost experimental studies (20—100%;Heichel Turner
1984; Elmqvist Gardfjell 1988; Marquis 1988).
Meanpercentage(±SD)ofleaf arearemovedinour studyfromD.laureolaplants(4.3±3.3)wassimilar tothosereportedinotherstudiesexaminingherbivory levelsunderfieldconditions, whichfound herbivory means (3.2± 1.5%; Marquis 1988)and ranges of variation(1—9%,DudtShure1994;5—9%and0.9—
7%,Cottan eta!.1986)muchsmallerthanthoseused
inexperimental studies.Thesefindingshighlightthe needforconducting pilotstudiesunder natural con ditionspriortoundertakingartificial defoliation experiments.Defoliationtreatmentsshouldthen be chosen to becomparableto herbivory levelscom monlyfacedbyplantsinthefield,rather than only totheextremefigures experiencedduringherbivore outbreaks.
Individualvariations inherbivorylevelscausedby insectlarvaeareusuallylinkedtooviposition choice bytheparentinsects(Rausher1983). Inthisstudy, differencesamong D.laureo!a plants weresig nificantlyrelatedtoplantsizeandstructuralfeatures. Intensityofherbivoryincreasedwithnumberofleaf whorls,anddeclinedwithaveragebasaldiameter of stems.Thelarvaeofthenoctuidspeciesinvolvedare notspecialistonD.!aureola.Ovipositionistherefore not likelyto behighlyspecific,withfemaleslaying eggsatgroundlevelinherbaceousplantsclosetoone
ofthehostspecies.FemalemothsovipositingnearD.
!aureolaplantsmighteasilydiscriminateamongthese plantsonthebasisoftheiroverallsize,asreportedfor otherlepidopteran species(Forsberg1987;Karban & Courtney1987);itseemslesslikelythattheycoulduse thebasaldiameterofstemsasacueinhostselection. LarvaeofT.flammeaand N.jantherarelyappear to movebetweenD.!aureolaplants(C.Alonso,personal observation), solarvalselectionbetweenplantscan not explain observed patterns. Differential sur vivorshipofcaterpillars(ZangerlBerenbaum1993) onplantswithstemsofdifferentthicknesses,perhaps relatedtostructure-dependent difficultiesfacedby younglarvaeinreachingleaf whorls,couldpartly explainsubsequentdifferences inbothfrequencyof caterpillars anddamageexperiencedbytheplants.
The modest defoliation levelsexperienced by D.
!aureoladuring thisstudy are representative forthe speciesinthestudyregion(C.M.Herrera,J.L.Yela, unpublished data). Such levels may or may not decreaseplantreproduction orsurvival,butevol utionary responses tothecorrelation betweenplant traits and damage would require that the traits involved were heritable. Number and diameter of stems, the traits correlated with herbivory, pre sumablydependmoreonplantage,developmentand locationinthehabitatthanonintrinsicplantfeatures. Noctuidmothlarvaethereforeseemunlikelytoactas selectiveagentson D.!aureola,atleastforthetraits consideredhere.
WITHIN-PLANT VARIATION
Withinplants,larvaeoccurredmostfrequentlyinleaf whorlscharacterized bylowerbranchingorder,shor terstems,andgreater numberofleaves.Sincecater pillarsarepresumablyunableto detectdifferences amongleafwhorlsinchemicalorphysicalproperties offoliage,ifany,untiltheyhavereachedtheleaves (Schultz1983), theseresultsindicatethatstructural traitsofstemsareusedascueswhileaccessingfeeding sites. Larvaedidnotappeartodiscriminateamong stemsatground level, astherewerenosignificant differencesbetweenthebasaldiametersofcontroland markedwhorlsofthe sameplant,insteadselection occurredatstemforks,favouringtheshortestpossible routetoaleafwhorl(lowerbranchingorder).
Foliageis patchilydistributed withinD.laureola plants, occurring in the form of dense clusters of leavesseparated bya network of rewardlessstems. Thenonrandom,architecture-relateddistribution of larvaeamongleafwhorlsmaythusreflect(I)aselec tiveresponsetodifferencesintheamount offoliage, aimed at increasing the food value of the patch chosen;(2)abehavioural responsetothecontrasting costsassociatedwithforagingalongstemsofdifferent length,directedatdecreasingthe movementcosts requiredtoreachthefoodpatch;or(3)acombination ofthesetwoeffects. Thefirstexplanation seems unlikely,sinceall leafwhorlsprovideavirtually unlimited food supply to larvae (the maximum amount offoliagethatanindividuallarvacaningest duringafeedingboutis someordersofmagnitude smallerthantheamount offoliageavailable).Thisis supported bytheobservation that96.2% ofmarked whorls had mean herbivory levels <30%,and no whorlwaseverfoundwithanherbivorylevel70% (despitepotential re-visitationofthesameleafwhorl ondifferentdates).In contrast, larvaetendedtooccur mostfrequentlyinwhorlswith shorterstems,sug gestingaroleformovementcostsinexplainingtheir distributionwithinplants.
Schultz(1983)proposedthatcaterpillarfacetrade offsamongfeedingcosts,predation risks,andmove mentcosts,andsuggestedthat thelastfactor isthe mostimportantinnocturnalcaterpillars.Thissugges tionisconsistent withour interpretationofwithin- plantlarvaldistribution,andalsowiththeforaging behaviourexhibitedbyT.flammea. Green-coloured larvaeofthisspecies, cryptictovisuallyorientated enemies,remainedfeedingonthesameleafwhorlfor several consecutivenights,thusreducingmovement coststoaminimum.Whenrisksincreasedasa conse quence ofcolour change, the foraging pattern was alteredtoadailymovementbetweenfeedingsitesin leaf whorlsandhidingplacesbeneaththeplants.If reducingthemovementactuallydiminishesforaging costs and, subsidiarily, reducesthedurationof the larvalperiodandalsodecreasespredation risk,there couldbeselectionfortheuseofarchitectural traitsas cuesforaccessing foragingsites.Giventhepol yphagousbehaviourofT.fiammeainthisregion,itis surprisingthatthe speciesdisplaysfinelytunedbehav iourwhenfeedingonbothD.laureolaandotherplant species(whenfeedingon Narcissuslongisparhus,T. flammeaalsoshowsacolour-dependentactivitypat tern;C.M.Herrera,unpublisheddata).Thusthepol yphagous condition does not necessarily mean reduced capabilitiestoadjustindividualforaging behaviour to the characteristics of different host plants.
CONTRASTS BETWEENAMONG- AND WITHIN- PLANT VARIATION
Correlatesofherbivoryloadhavebeenfoundtodiffer attheamong-andwithin-plant levels.Amongplants,
Table 3Percentage of total population variance inarchi tecturaltraitsofleafwhorlsthat wasaccountedforbyvari ationamongandwithinplants
Variable Amongplants Withinplants
Stembasaldiameter 36.0 64.0
Stemlength 21.9 78.1
Branchingorder 22.0 78.0
Numberofleaves 25.6 74.4
Average 26.4 73.6
differencesinherbivoreloadarerelatedtovariation innumberof whorlsand(marginally)tomeanstem diameter,whilewithinplantstheyarerelatedtovari ationinstemlength,branchingorder,andnumberof leavesperwhorl.Thisdiscrepancymaybeattributed todifferencesbetweenthechoicesexertedby ovi positing moths (among plants) and by the larvae (withinplants),andprobablydependonthe relative amounts ofvariabilityexhibitedbythestudiedtraits atthetwoscales.For thearchitectural traits exam ined,thewithin-plant component accounted for roughlythreetimesasmuchvarianceastheamong- plantcomponent(Table3), thusprovidingcon siderableopportunitiesforlarvaetotakeadvantage ofstructural cues.Ovipositing females,ontheother hand,weremostlikelyconstrained touseothercues intheirselection,suchasthenumberofleafwhorls. Insectsmaynotuse thesameplantfeatureswhen decisionsaretakenatdifferentlifehistorystages,or maybeunabletodoso.Ultimately. theimplications ofthisphenomenon willdepend onwhether or not discriminationamongconspecific plants(Rausher& Papaj 1983;Forsberg 1987;Karban 1992)orwithin plants(Whitham 1983) hasdemographical conse quencesfortheinsectspeciesinvolved.Inthecaseof D.laureola,furtherstudiesareneededtoassessthese demographical consequences.
Acknowledgements
We areparticularlyindebtedtoJoséL. Yelafor enthusiasticsupport andencouragement, andfor teachingussomanyimportantdetailsofthenatural historyofnoctuidmoths.He,M.C.Anstett,P.Jord ano,J.M.vanGroenendaelandtwo anonymous reviewersprovidedusefulcommentsandcriticismson earlier versionsof themanuscript. P.Jordano also drewFig.1forus.AliciaPrietoandRocjoRequerey providedtechnicalassistance.TheAgenciadeMedio Ambienteauthorized our workinCazorla and pro videdinvaluablefacilities.Thisstudywassupported byDGICYTgrant PB9I-0114andapredoctoral fel lowshipfromtheMinisterio deEducaciOnyCiencia toC.A.
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