Relationships between abiotic environment, plant functional traits, and animal body size at Mount

15 Pages·2017·1.52 MB·English

RESEARCHARTICLE
Relationships between abiotic environment,
plant functional traits, and animal body size at
Mount Kilimanjaro, Tanzania
DavidSchellenbergerCosta1*,AliceClassen2,StefanFerger3,MariaHelbig-Bonitz4,
MarcellPeters2,KatrinBo¨hning-Gaese3,5,IngolfSteffan-Dewenter2,MichaelKleyer1
1 DepartmentofBiologyandEnvironmentalSciences,UniversityofOldenburg,Oldenburg,Germany,
2 DepartmentofAnimalEcologyandTropicalBiology,UniversityofWu¨rzburg,Wu¨rzburg,Germany,
3 SenckenbergGesellschaftfu¨rNaturforschung,BiodiversityandClimateResearchCenter,Frankfurt,
Germany,4 InstituteofExperimentalEcology,UniversityofUlm,Ulm,Germany,5 InstituteforEcology,
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Evolution&Diversity,GoetheUniversityFrankfurt,Frankfurt(Main),Germany
a1111111111
a1111111111 *[email protected]
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Abstract
Theeffect-responseframeworkstatesthatplantfunctionaltraitslinktheabioticenvironment
toecosystemfunctioning.Oneecosystempropertyisthebodysizeoftheanimalslivingin
OPENACCESS
thesystem,whichisassumedtodependontemperatureorresourceavailability,among
Citation:SchellenbergerCostaD,ClassenA,Ferger
others.Forprimaryconsumers,resourceavailabilitymaydirectlyberelatedtoplanttraits,
S,Helbig-BonitzM,PetersM,Bo¨hning-GaeseK,et
al.(2017)Relationshipsbetweenabiotic whileforsecondaryconsumerstherelationshipisindirect.Weusedplanttraitstodescribe
environment,plantfunctionaltraits,andanimal resourceavailabilityalonganelevationalgradientonMountKilimanjaro,Tanzania.Using
bodysizeatMountKilimanjaro,Tanzania.PLoS
structuralequationmodels,wedeterminedtheresponseofplanttraitstochangesinprecipi-
ONE12(3):e0174157.https://doi.org/10.1371/
tation,temperatureanddisturbancewithandassessedwhetherabioticconditionsorcom-
journal.pone.0174157
munity-weightedmeansofplanttraitsarestrongerpredictorsofthemeansizeofbees,
Editor:EricGordonLamb,Universityof
moths,frugivorousbirds,andinsectivorousbirds.Traitsindicatingtissuedensityandnutri-
Saskatchewan,CANADA
entcontentstronglyrespondedtovariationsinprecipitation,temperatureanddisturbance.
Received:August25,2016
Theyhaddirecteffectsonpollinationandfruittraits.However,theaveragebodysizesofthe
Accepted:March3,2017
animalgroupsconsideredcouldonlybeexplainedbytemperatureandhabitatstructure,not
Published:March20,2017 byplanttraits.Ourresultsdemonstrateastronglinkbetweentraitsandtheabioticenviron-
Copyright:©2017SchellenbergerCostaetal.This ment,butsuggestthattemperatureisthemostrelevantpredictorofmeananimalbodysize.
isanopenaccessarticledistributedunderthe Community-weightedmeansofplanttraitsandbodysizesappearunsuitabletocapturethe
termsoftheCreativeCommonsAttribution
complexityofplant-animalinteractions.
License,whichpermitsunrestricteduse,
distribution,andreproductioninanymedium,
providedtheoriginalauthorandsourceare
credited.
DataAvailabilityStatement:Allrelevantdataare
withinthepaperanditsSupportingInformation Introduction
files.
Plantsaretheprimaryproducersofterrestrialecosystems.Alargebodyofliteratureinplant
Funding:Thisresearchwasmadepossiblebya
ecologyhasemergedaroundplantfunctionaltraits,i.e.plantpropertiesindicatingafunctional
grantfromtheGermanScienceFoundation(grant
relationshipwiththeabioticenvironmentorwithbioticcomponentsofecosystems[1–2].The
numberKL756/5-1)intheframeworkofthe
notionofthetranslationofchangesintheabioticenvironmenttoecosystemfunctioning
CollaborativeResearchUnitFOR1246"Kilimanjaro
ecosystemsunderglobalchange:Linking mediatedbyplantfunctionaltraitsisdescribedbytheeffect-responseframework[3].It
PLOSONE|https://doi.org/10.1371/journal.pone.0174157 March20,2017 1/15
Environment,planttraits,andanimalbodysize
biodiversity,bioticinteractionsandbiogeochemical assumestraitsrespondtochangesintheabioticenvironmentandaffectecosystemfunction-
ecosystemprocesses(KiLi)". ing,includinganimalcommunitycomposition.Manystudiesinvestigatedthesimultaneous
Competinginterests:Theauthorshavedeclared roleofplantresponseandeffecttraitsinrelationtopropertiesofthevegetationcompartment
thatnocompetinginterestsexist. ofecosystems,suchasstandingbiomass,productivity,andlitterdecomposition[4–7].Rarely,
however,havestudiesaddressedthelinksbetweenabioticenvironmentalconditions,plant
functionaltraitsandothertrophiclevels.Recently,Lavoreletal.[8]proposedanextensionof
theeffect-responseframework,employingamultitrophicperspective.
Bodysizeisoneofthemostprominentcharacteristicsofanimalspecies.Ithasimplications
forthelifehistoryandtheroleofspecieswithinecosystems[9].Therelationshipsofbodysize
tometabolicrate,growth,mortality,locomotion,andnutrition,amongothers,haveledto
manyhypothesesconcerningthevariabilityandrangeofbodysizeswithinorganismgroups
inrelationtoenergeticconstraints,texturaldiscontinuityofhabitats,bioticinteractionssuch
aspredationandcompetition,andbiogeographicalandphylogeneticlimitations[10–11].
Onthelevelofindividualspecies,observationsoncloselyrelatedextantandfossiltaxahave
ledtotheformulationofseveralrules,thebest-knownofwhichisBergmann’srule[12].It
statesthatamongcloselyrelatedspecies,taxaincolderclimatestendtobelargerthanthose
fromwarmerregions.Thereasonforthispatternisseenintherelativelylowerheatlossof
largeranimalscomparedtosmallerones.PreviousworkdiscussedBergmann’sruleforendo-
andectothermicanimals(see[13]foranoverview).Forthelatter,higherfrostresistance,
largerenergyreservesforsurvivingstarvationperiods,andlongergrowthanddevelopment
timeatlowtemperatureshavebeenbroughtforwardtoexplainnegativerelationshipsoftem-
peratureandsize[14–16].Amoregeneralexplanationcanbederivedfromthenotionthat
specificmetabolicrate,i.e.metabolicratedividedbybodymass,islowerforlargeanimals
comparedtosmallones,resultinginamoreefficientuseofenergy[17].
BecauseofmuchdiscordancewithBergmann’srule[18–20],theassumptionoftempera-
turebeingthemaindriverofanimalbodysizehasbeenchallengedandresourceavailability
wasproposedasanotherimportantdeterminantofbodysize[19,21–22].Resourceavailability
dependsonprimaryproductivity;directlyforherbivores[23],andindirectlyforpredatoryani-
mals[24–26].
Here,wepresentanovelapplicationoftheeffect-responseframeworkusingplantfunc-
tionaltraitstopredictvariationinthebodysizeofanimaltaxaalonganelevationgradientat
MountKilimanjaro,Tanzania.Weaskhowplanttraitsrespondtovariationsinprecipitation,
temperatureanddisturbancebylanduseandhowtheyaffectmeananimalbodysize.Func-
tionaltraitswereselectedtoexpressresourceavailabilityformoths,bees,frugivorousbirds,
andinsectivorousbirds.Mothsrelyonplantleavesduringgrowthanddevelopment,andnec-
taratthereproductivestage.Beesareentirelydependentonpollenandnectar.Frugivorous
birdsfeedonfleshyfruits.Insectivorousbirdsonlyindirectlydependonplantfunctionaltraits,
astheyconsumeinsects,whichrelyonplantsfornutrition.Wehypothesizedthatalargersup-
plyofleaves,nectar,fruits,andinsectsshouldresultinanincreaseinthebodysizeofmoths,
bees,andbirds,respectively.Leafbiomasswasapproximatedbythetotalplantbiomassofa
community,nectarquantitybytheproportionofinsect-pollinatedplants.Supplyoffruitswas
recordedinthefieldandinsectavailabilityforbirdswasapproximatedbythecumulativebody
sizeofbeesandmothsineachplot.HerbivoresoftenpreferleaveswithalowC/Nratio[27].
Toaccountforthis,weincludedavariableaggregatingleafC/Nratioandtoughnessinthe
analysisofmothbodysize,expectinglowerC/Nratiostobeassociatedwithlargerbodysize.
Theeffectofthesefactorsonthecommunity-weightedmeanbodysizeofmoths,bees,frugivo-
rousbirds,andinsectivorousbirdswasinvestigatedforeachgroupseparately.
Ofthemultiplefactorspossiblyaffectingbodysizedistributions,temperatureasaproxyfor
energeticconstraintsandhabitatstructurewereadditionallyconsidered.Habitatstructurewas
PLOSONE|https://doi.org/10.1371/journal.pone.0174157 March20,2017 2/15
Environment,planttraits,andanimalbodysize
describedbytotalplantbiomass.Formoths,thisfactorexpressedbothresourcesandhabitat
texture,correspondingtodifferencesbetweenopenareasandforests.Forbeesandmoths,the
morepatchydistributionofresourcesexpectedinforestsshouldleadtoincreasesinbodysize
[28].Forbirds,onthecontrary,multilayercanopiesareanobstacletomobilityandmayfavor
smallerspecies[29].
Structuralequationmodelingwasappliedtoinvestigatethecomplexrelationshipsbetween
plantfunctionaltraits,totalplantbiomass,theabioticenvironment,andthebodysizeofthe
differentanimalgroups.Thisimpliedadditionalhypothesesconcerningtherelationshipsof
plantfunctionaltraitsandtheabioticenvironment(Table1).Totalplantbiomassandleafeco-
nomicswereexpectedtoincreasewithprecipitationanddecreaseorincreasewithdisturbance,
respectively[30–31].Leafeconomicssummarizesseveralcorrelatedplantfunctionaltraits
describingaplantstrategygradientfromfastgrowthandnutrientturnovertopersistenceand
Table1. Hypothesesdefiningthestructuralequationmodels. ThestructuralequationmodelsrelatingplanttraitstoanimalbodysizeCWMswereimple-
mentedaccordingtoexpectedrelationshipsbetweenabioticenvironment,plantfunctionaltraits,animalbodysizeCWMs,andanimalcumulativebodysize.
Model Response Predictor Hypothesized Explanation
Relationship
M,B, Disturbance Temperature + Anthropogenicactivitiesarestrongestatlowelevationscloseto
F,I settlements
M,B, BodysizeCWM Temperature - Moreefficientenergyuseoflargeranimalsincoldenvironments[12,
F,I 17]
I Cumulativebodysize* Temperature - Moreeffectiveenergyuseoflargeranimalsincoldenvironments[12,
17]
M,B, Leafeconomics Precipitation + Tougherleavesconservewater[32]
F,I
M,B, Totalplantbiomass Precipitation + Watersupplylimitsprimaryproductivity[30]
F,I
M,B, Leafeconomics Disturbance + Fastgrowthandturnovernecessary[33]
F,I
M,B, Totalplantbiomass Disturbance - Biomassremoval
F,I
B BodysizeCWM Disturbance + Largespeciescanbetterexploitfragmentedhabitats[34–35]
M,B Insect-pollinated Leafeconomics + High-leafeconomicsplantsinstudyareaaremostlyinsect-pollinated
plantsCWM weeds
F Bird-dispersedfruits Leafeconomics - High-leafeconomicsplantsmostlyproducesmallwind-dispersed
CWM seeds
M BodysizeCWM Leafeconomics + LowerC:Nratio,higherfoodquality[36]
M,B Proportionofinsect- Totalplantbiomass + Reducedwindspeedthroughpersistentfoliageinforestsmakeswind-
pollinatedplants pollinationlesseffective[37]
F Bird-dispersedfruit Totalplantbiomass + Advantageoflongertransportdistancesoflargeseedsthrough
CWM animalsinforests[38]
M,B, BodysizeCWM Totalplantbiomass +/- Increaseinresourcesformoths[19,23],longerforagingroutesfavor
F,I largeinsects,forbirds,smallerspeciescanbettertravelincanopies
[10]
M,B Cumulativebodysize* Totalplantbiomass + Higherprimaryproductivityresultinginlargeranimalbiomass[39–40]
M,B BodysizeCWM Proportionofinsect- + Increaseinresourcesthroughmorebiomass[19,23]
pollinatedplants
F BodysizeCWM Bird-dispersedfruit + Increaseinresourcesthroughmorebiomass[19,23]
CWM
I BodysizeCWM Cumulativebodysize* + Increaseinresourcesthroughmorebiomass[19,23]
Abbreviations:M:mothmodel,B:beemodel,F:frugivorousbirdmodel,I:insectivorousbirdmodel.
*Mothsandbees.
https://doi.org/10.1371/journal.pone.0174157.t001
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Environment,planttraits,andanimalbodysize
nutrientretention[32].Fastgrowthandnutrientturnoverareessentiallylinkedtohigh
resourcesupplyanddisturbancefrequencies.Totalplantbiomassshouldpositivelyaffectnec-
tarandfruitavailability,whiledisturbanceshouldberelatedtohighnectar,butlowfruitavail-
ability.Wemadeuseofauniquedatasetincludingclimaticdata,planttraits,plantbiomass,
andanimalabundancedatasampledsynchronouslyonsixtyplotsatMountKilimanjaro.Our
studysitescoveredawiderangeoftropicalhabitatsvaryingstronglyinmeantemperature,
precipitation,andhumandisturbanceandarethereforeidealfortestingtheoriesaboutthe
influenceofbothtemperatureplantfunctionaltraitsonconsumercommunitytraits.
Materialsandmethods
Studyregion
MountKilimanjarowasselectedasthestudysystem.ItislocatedinNorthernTanzaniaat
3.1˚S37.4˚E.Itisthehighestfree-standingmountainintheworldandcoversanareaof
approx.4000km2.Theelevationgradientrangesfromthelowlandsat800ma.s.l.tothepeak
at5892ma.s.l.Precipitationvaluesrangefrom550mm•a-1to3600mm•a-1,withthehighest
amountofrainfalloccurringatmid-elevationsintheforestbeltandthelowestamountsinthe
alpinezoneandtheplainssurroundingthemountain[41].Anthropogenicdisturbanceis
expresseddifferentlyuponthemountain.Fires,occasionaltimberextraction,andsmall-scale
collectionofotherforestproductsaffecthigherelevations,whileagricultureinvolvingfertiliz-
ersandherbicideapplicationispracticedinlowlandareas.
Datacollection
AllnecessarypermitsweregrantedfromtheTanzanianCommissionforScienceandTechnol-
ogy,theTanzanianWildlifeAuthority,andTanzaniaNationalParks(340-ER-NA-96-44,
TNO/HQ/C.10/13/VOL.III).
Collectionofplantfunctionaltraitsanddataonanimaltaxatookplaceon60plots,each
of0.25ha,inthetwelvemajorvegetationtypesatthemountainbetweenAugust2010and
November2012(S1Table).Plotsweredistributedequallyamongvegetationtypes,fiveplots
belongingtoeachtype.
Undisturbedplotsatlowelevationsaround1000ma.s.l.weredominatedbyannualgrasses
anddrought-toleranttrees,whereasweedscharacterizecultivatedareas.At1600m,coffee
plantationsandagroforestrysystemsharboredmostlydicotyledonousweeds.Grasslandswere
dominatedbyseveraltaxaofPoaceae.Woodylifeformsaccountedforthelargestpartofplant
biomassfrommiddletoupperelevations,withatransitionfromrainforesttocloudforest
characterizedbysmalltreesandhighlichenabundanceataround3000m.Alpinevegetation
occurredupto4500mandwasmainlycomposedofshrubs,perennialherbs,andgrasses.
Meantemperaturewasderivedfromseveralyearsofcontinuousmeasurementswithauto-
maticdataloggerscoveringthetimeperiodofourdatacollection[42].Annualprecipitation
datawasderivedfromtheKilimanjarorainfallmodel[43].Disturbancewascalculatedasa
compositemetricincludingtheeffectsoflanduseatlocalandlandscapescales([44],S1File).
Plantfunctionaltraitswerecollectedfromthemostabundantvascularplantspecies,
accountingfor80%oftotalplantbiomassoneachplot.Abundancewasdefinedaspercent
coverdeterminedfromvegetationsurveys.Coverwasestimatedforeachstratumdepending
onthevegetationstructureoftheplot.Coverofallstratawassummedupforeachspeciesand
standardizedtoaplottotalof100%.Plantfunctionaltraitswerechosentoindicatevegetative
growth,persistence,andreproductivecharacteristics.Fifteenindividualsperspeciesweresam-
pledfromdifferentplotswithintheelevationaldistributionrangeforspecificleafarea(SLA),
leafdrymattercontent(LDMC),stemspecificdensity(SSD),leafnitrogencontent(leaf
PLOSONE|https://doi.org/10.1371/journal.pone.0174157 March20,2017 4/15
Environment,planttraits,andanimalbodysize
N ),leafphosphoruscontent(leafP ),fruitingfrequency,fruitnumber,andfruitsize.
mass mass
Additionally,thetotalplantbiomassperplotwascalculatedwithallometricequationsusing
completetreeandundergrowthinventories(detailsin[45]).Samplingandprocessingofplant
materialfollowedKleyeretal.[46].
Percentagecovervaluesfromvegetationsurveyswereusedtocalculatecommunity-
weightedmeantraitvaluesforeachplot(CWM,[47]).Todescribeleafpalatabilityformoths,
thefirstaxisofaprincipalcomponentsanalysisusingSLA,LDMC,SSD,leafN andleaf
mass
P wasextractedandtermed"leafeconomics"(forcorrelationsofvariablesseeS2Table).
mass
Thetraitsusedarerelatedtotheworldwideleafeconomicsspectrum[32,48],reflectingagra-
dientfromplantswithfastresourceuseandnutrientturnovertothosewithslowandpersis-
tentgrowth.Theformerarecharacterizedbysofttissueswithhighnitrogencontentandare
generallypreferredbyherbivores,duetotheireasierdigestionandhighernutritionalvalue
[32].Alldatawerecollectedonthesameplotswithinacommontimeframetoavoidcon-
foundingeffectsofspatialandtemporalvariability.
Toquantifynectarandfruitavailabilityontheplots,pollinationanddispersalsyndromes
wereextractedfromtheFloraofTropicalEastAfrica[49].Nectaravailabilityisdifficultto
quantify,asnectaramountsperflower,floweringtimes,andtheproportionoftissueinvested
inflowersarehighlyvariablebetweenspecies.Thisresourcewasapproximatedbytheabun-
dance-weightedproportionofinsect-pollinatedplantspecies.Toassessfoodresourcesforfru-
givorousbirds,averagefruitnumbersperindividualplantsampledinthefield(fn),average
fruitsize(fs),fruitingfrequency(ff),andrelativeabundance(ra)wereusedtocalculatethe
bird-dispersedfruitCWM(bdc)accordingtothefollowingformula:
X
bdc¼ ra(cid:15)meanðfnÞ(cid:15)meanðfsÞ(cid:15)ff (cid:15)d
bdc
Thesumistakenoverallplantspeciesintherespectiveplot.δ equals1forspeciespro-
bdc
ducingfruitsconsumedbybirdsand0otherwise.Birdswereobservedthroughpointcounts,
bothinthedryandwetseasontoincludetemporalvariation(see[50]formethodology).Bees
weresampledwithpantrapsontheforestfloorandinthecanopyofwoodyvegetation.Sam-
plingwasrepeatedseveraltimestoaccountfortemporalvariation(see[44]).Mothswere
caughtwithanautomaticlighttrapwithasuperactiniclighttube(6watt,FritzWeberEntomo-
logiebedarf,Stuttgart,Germany)aslightsource.Thiswasrepeatedindryandwetseasons.
BodymassofbirdswasderivedfromDunningJr[51]andusedasaproxyforbodysize[52].
Withinanimalgroupsofsimilarbodystructure,bodylengthandsizearehighlycorrelated
[53].Forbees,bodysizewasapproximatedbythehighlycorrelatedintertegulardistance(ITD,
[54]).MothbodylengthwasmeasuredusingabinocularLeicastereomicroscopewithacali-
bratedocularmicrometer.
Aswithplanttraits,animalbodysizewasweightedbyspeciesabundance,yieldingabody
sizeCWMforeachtaxonorguild,toavoidgivingrarespeciesthesameweightingasabundant
ones[21].Forsimplicity,inthefollowingcommunity-weightedmeanbodysizeisreferredto
asbodysize.Cumulativebodysizesofmothsandbeesasfoodresourcesforinsectivorous
birdswerecalculatedasthesumoftheindividualbodysizesofallindividualsoftherespective
groupssampled.Forplotmeansandrangesofplantfunctionaltraits,animalbodysize,cumu-
lativebeeITD,andcumulativemothbodylengthseeS3Table.Analyseswerealsoperformed
withcommunitymeansnotweightedbyabundance,butastherewerenosubstantialdiffer-
ences,onlyCWMresultsarepresented.
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Environment,planttraits,andanimalbodysize
Dataanalysis
Structuralequationmodelingwithmixedeffectmodelswasappliedtorelatemeanbodysizeto
environmentaldataandplantfunctionaltraits[55].Weusedthedirectional-separation-test
(d-sep,[56])toconfirmorrejecttheassumedrelationshipsbetweenvariablesinthestructural
equationmodel.d-sepworksbytestingindividualregressionsbetweenallpairsofvariablesof
themodel.Directrelationships,i.e.variablesconnectedbyarrows,shouldturnouttobesignif-
icant.Indirectrelationships,i.e.variablesconnectedbyseveralarrowsthroughothervariables,
shouldnotbesignificant.However,forindirectrelationships,notthevariablesthemselves,
buttheirresidualsfromregressionsontheirdirectconnections,aretested.Thisapproach
deliversP-valuesfortheindividualrelationshipsbetweenvariablesandanoverallmodelfit.
Weappliedmixedeffectmodelsforregression,becausetheyofferthepossibilityofincluding
plot-specificandvegetationtype-specificrandomeffects[55].Weassumedlinearrelationships
betweenallparameterswithinthelimitedrangesofthevariablesinvestigated.Variableswere
standardizedpriortoanalysis.ToobtaincorrectP-values,wefollowedtherecommendations
ofBarretal.[57]andincludedrandomslopesandrandomintercepts.Allcalculationswere
doneinR[58].Formixedeffectmodels,thelmerfunctioninRpackagelme4wasused[59]
withGaussianerrorstructureobtainingmaximumlikelihoodestimates(optionREML=F).
Separatemodelswererunforeachanimalgrouptokeeptotalvariablenumberslowwith
regardtothenumberofobservations.Foreachgroup,thehypothesizedmodelincludingall
predictedinterdependencieswastestedfirst.Then,animprovedmodelomittingthenon-sig-
nificanttermsataP-valueof0.05wasrun.Coefficients,P-valuesandR2valuesforbothinitial
and"significant"modelsarereported.Table1liststhedetailedhypothesesdefiningthestruc-
turalequationmodels.
Results
Therewerelargedifferencesintheindividualrelationshipsandeffectsizesinferredfromthe
structuralequationmodels(Fig1,seeS4Tableunstandardizedcoefficients,individualP-val-
ues,andconditionalR2values).Overall,expectationsconcerningtheabioticenvironmentand
plantfunctionaltraitswereconfirmedbytheanalyses,butrelationshipsbetweenplantfunc-
tionaltraitsandbodysizeweremuchweakerandnotalwaysaccordingtothehypotheses.For
individualvariables,explainedmarginalR2,i.e.theproportionofvarianceexplainedbylinear
regressions,wasbetween0.16and0.58,whileconditionalR2valuesexpressingthesummed
effectoflinearregressionsandaccountingforvegetationtype-differencesrangedfrom0.16to
0.89(S4Table).Overallmodelprobabilityvaluesrangedfrom0.41to0.81.Exclusionofnon-
significanttermsfromtheinitialhypotheses(dottedlinesinFig1,S4Table)hadapositive
effectonmodelR2values,especiallyforbirds.
Relationshipsbetweentheabioticenvironmentandplanttraitsandbiomasswerethesame
forallmodels.Leafeconomicsandtotalplantbiomasswerepositivelyrelatedtoprecipitation.
Disturbancehadanegativeeffectontotalplantbiomass,butapositiveoneonleafeconomics.
Theproportionofinsect-pollinatedplantswasexplainedbytotalplantbiomassandleafeco-
nomics,bothwithpositivecoefficients.Bird-dispersedfruitCWMwasnegativelyrelatedto
leafeconomicsandpositivelytototalplantbiomass.
Mothandfrugivorousbirdbodysizewereexplainedbytemperaturealone,whilebodysizes
ofbeesandinsectivorousbirdswerealsorelatedtototalplantbiomass.Bothmothandfrugivo-
rousbirdbodysizedecreasedwithincreasingtemperature(Fig1Aand1C).Ashypothesized,
beebodysizeincreasedwithdisturbance,butdecreasedwithtemperature(Fig1B).Contraryto
ourexpectation,insectivorousbirdbodysizewaspositivelyrelatedtotemperature(Fig1D).
Totalplantbiomassaffectedbeebodysizepositively,butwasnegativelyrelatedtoinsectivorous
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Environment,planttraits,andanimalbodysize
Fig1.Structuralequationmodelshowingtherelationshipbetweentheabioticenvironment,disturbance,plantfunctionaltraitsand
community-weightedmeansofanimalbodysizeformoths,bees,insectivorousandfrugivorousbirds.Standardizedcoefficientsaregivenfor
positive(solidlines)andnegative(dashedlines)relationships.Double-headedarrowsdenotecorrelationsofpredictors.Dottedlinesindicate
hypothesizedrelationshipsthatwerenotsupportedbythedata,i.e.whichturnedouttobenon-significantintheregressionmodels.R2valuesare
marginal,i.e.givefixedeffectexplainedvariance.SeeS4Tableforunstandardizedcoefficients,individualP-values,andconditionalR2values.(a)
Moths,(b)Bees,(c)Frugivorousbirds,(d)Insectivorousbirds
https://doi.org/10.1371/journal.pone.0174157.g001
birdbodysize,inlinewithourhypotheses.Resourceavailabilityhadnopositiveeffectoninsec-
tivorousbirdbodysize,asindicatedbythelackofasignificantrelationshipwithcumulative
bodysizeofmothsandbees.Cumulativebodysizeofmothsinturnwaspositivelyrelatedto
temperature,whilecumulativebodysizeofbeesdecreasedwithincreasingtotalplantbiomass.
Discussion
Inourstudy,wemodeledtheresponseofplantfunctionaltraitstoenvironmentalgradients
andtheireffectonbodysizesofdifferentanimaltaxa.Weassumedthatbodysizesdepend
directlyontemperatureanddisturbanceandindirectlyonplanttraitsandtotalplantbiomass
asindicatorsofresourceavailabilityorhabitatstructure.Temperatureaffectedthebody
sizeofallanimalgroupsconsideredandtotalplantbiomassaffectedbodysizesofbeesand
insectivorousbirds.Planttraitswereunrelatedtothebodysizeofanyofthegroups,suggesting
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Environment,planttraits,andanimalbodysize
thateitherresourceavailabilitymaynotbethemaindriverofbodysize,orcommunity-
weightedmeansofplanttraitsdidnotreflectresourceavailabilityadequately.
Environment–plantfunctionaltraits
Precipitationanddisturbancewerelinkedtoincreasesinleafeconomics.Thisvariableaggre-
gatedasuiteofcorrelatedtraitssuchasspecificleafarea,leafnitrogenandphosphoruscon-
tent,leafdrymattercontent,andstemspecificdensity.Itindicatesagradientrangingfrom
relativelyslowgrowthandconservationofresourcestofastgrowthandrapidturnoverof
resourcesinleavestermedtheleafeconomicsspectrum[32].Highvaluesofleafeconomics
occurredinfast-growingplantswithhighlypalatabletissues,whilelowvalueswerefoundin
slow-growingplantsinvestinginlong-lastingstructuralcomponents.Whethertraitsindicating
theleafeconomicsspectrumarelinkedtoprecipitationonglobalscalesisstillequivocal[60].
AtMountKilimanjaro,precipitationwasthestrongestdriveroftotalplantbiomassandleaf
economics,whereastemperaturehadalmostnoeffect,incontrasttofindingsonaglobalscale
[60].Thestrongpositiveeffectofdisturbance(logging,grazing)onleafeconomicsisconcor-
dantwithotherstudiesworldwide(e.g.[31,61]).
Thepositiveandnegativerelationshipsbetweenleafeconomicsandtheproportionof
insect-pollinatedplantsandbird-dispersedfruitCWM,respectively,wereexpectedduetothe
functionaldifferencesbetweenundisturbedsavannaandforestsontheonehandandagricul-
turalareasontheother.AtMountKilimanjaro,grazedandploughedhabitatsmostlyharbor
fast-growingdicotyledonousweedswithratherhighleafeconomicsvalues.Theseweedsare
pollinatedbyinsects,butproducemostlywind-dispersedseeds,insteadofbird-dispersed
fruits.Seeddispersalbyanimalsisoftenstronglyaffectedbydisturbance,butpollinationisnot
[44,62].LargedifferencesbetweenmarginalandconditionalR2valueswereevidentintotal
plantbiomass,intheproportionofinsect-pollinatedplants,andinbird-dispersedfruitCWM.
Thiscouldbecausedbynon-linearrelationshipsofpredictorsandtraitsorfactorsnot
accountedforbythepredictors[63].
Environment–animalbodysize
Bergmann’sruleanditsrecentformulationsintheframeworkofthemetabolictheoryofecol-
ogy[17]predictdecreasesinbodysizewithincreasingtemperature.Mothsandfrugivorous
birdswereinaccordwiththisexpectation,inlinewithpreviousstudies(e.g.[14,20,64]).Con-
versely,insectivorousbirdsizeincreasedwithtemperature,contrarytomostotherstudieson
birds[65].UnlikepatternsofbodysizealongelevationalgradientsintheAlps[66],meanbody
sizeofbeesatKilimanjarodidnotfollowBergmann’srule.Beesofsmallestbodysizeoccurred
inthealpineHelichrysumshrublandsituatedathighestelevations,suggestingincreasedextinc-
tionprobabilitiesforlarge-bodiedbeespeciesinlow-energyhabitats[67].Theonlyothervari-
ablefoundtoincreasebeebodysizewasdisturbance,probablybecausemostwildbeesthat
visitcropsorruderalplantsindisturbedhabitatsnestinnaturalhabitats[34]andisolated
resourcesindisturbedhabitatscanbetterbeexploitedbylarge-bodiedbeesspecieswithlarger
foragingranges[35].However,forsocialspecies,whichaccountedforasmallproportionof
beespeciesrichness,butforaconsiderablepartofthespecimens,KaspariandVargo[68]
notedthatcolonysizeratherthanindividualbodysizeshouldfollowBergmann’srule.Never-
theless,thiscouldnotbetested,ascolonysizedatawerenotavailable.
Plantfunctionaltraits–animalbodysize
Thefoodavailabilityhypothesis[21]andthetexturaldiscontinuityhypothesis[10]justified
theexpectationofastronginfluenceofplantfunctionaltraitsandtotalplantbiomasson
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Environment,planttraits,andanimalbodysize
animalbodysize.However,wedidnotfindanysignificantrelationshipbetweenbodysizeand
CWMsoffunctionaltraitsthatindicateresourceavailability.Assumingthe"resourceavailabil-
ityhypothesis"werecorrect,ashortcomingoftheapproachpresentedheremightbethatthe
traitsusedwerenotpreciseenoughindicatorsofthe"true"foodresources.Weusedleafand
stemtraits,thatindicatedagradientfromcarbon-rich,hardtissuetosoft,nutrient-richtissue
asanindicatorformothfoodavailability[69].This,however,maynotadequatelyreflectthe
amountofpalatabletissueformoths,asLepidopteratendtobespecializedtoparticularhost
plants[70].Inmoths,acorrelationbetweenbodysizedistributionsandsizedistributionsof
floralresourceshasbeenobserved[71],indicatingthatnectarquantitymayhavepredictive
potentialforbodysize.Still,theCWMofinsect-pollinatedplantsmaybeatoocoarseestimate
ofthenectaractuallyavailableforbeesandmoths,asflowerrewards,inflorescencesizesand
nectarcontentscanvarystronglyacrossplantspecies[72].
Totalplantbiomasshadasignificanteffectonthebodysizeofbees.Rainforestswerechar-
acterizedbythehigheststructuralcomplexity,indicatingthatlargerdistancestofoodsources
selectforlargerbeespecies[28,44].Mothshowever,didnotrespondinthesameway,possibly
becausethecaterpillarlifestageexertsastronginfluenceonbodysizeandleavesarearather
abundantresource,whichdonotforcecaterpillarstoforagelongdistancesinbothopenland-
scapesandforests.
Insectivorousbirdbodysizewasnotsignificantlyrelatedtoresourceavailability,probably
becausethelatterwasapproximatedbythecumulativebodysizesofallmothandbeeindividu-
alsperplot.Thesemeasuresshouldbegoodproxiesofoverallinsectabundanceinmostvege-
tationtypes,butprobablynotathighelevations.Beeabundanceinthealpinezonewashigh,
butotherinsectswereratherrare(pers.obs.).Forleafherbivores,thiswasevidencedbythe
virtualabsenceofdamagedfoliageathighelevations.Includingotherinsecttaxainourstudy
wouldlikelychangeourestimateofthefoodavailabilitypatternforinsectivorousbirds.Thus,
amoreprecisemeasurementofcumulativeinsectbodysizeshouldalsoincludeotherrelevant
groups,e.g.DipteraandColeoptera.
Inaddition,theuseofcommunity-weightedmeans(CWMs)ofbodysizesmayaffectthe
result:Whiletheunweightedbodysizesofspeciesmaydecrease,changesinabundancesmay
resultinincreasesofthebodysizeCWMs[73].Thisisevidencedbythechangesinmothand
beecumulativebodysizewithtemperatureandwithtotalplantbiomass,whichwereopposed
tothoseofthecorrespondingCWMs.Thus,increasesordecreasesinaveragebodysizecanbe
counterbalancedbychangesinoverallabundanceoflarge-bodiedandsmall-bodiedspecies.
Thiscouldbeanotherreasonforthepositiverelationshipbetweeninsectivorousbirdbodysize
andtemperature,meaningthatathigherelevations,smallspeciesmayhavemoreindividuals
thanlargeones,ascomparedtolowerelevations.Inaddition,bodysizeCWMsdonotaccount
forthedispersionofbodysizesinhabitats.Dispersion,ratherthanmeans,maylikelyrespond
toincreasingstructuralcomplexityofhabitatsorincreasingdiscontinuityofresourceavailabil-
ityacrossscales[74–75].
Bodysizedistributionsprobablyalsodependonotherfactorsnotconsideredinourstudy
[10].Largebodysizemaybecausedbysexualselectionorelevatedfecundityoflargeindividu-
als[76].Theinterplayofsexualselectionforlargerindividualsandnaturalselectionforsmaller
onescancomplicatepatterns[77].Blanckenhorn[76]proposedselectiveforcesfavoringsmall
bodysize:Largerindividualsneedmoreresources,maybepreferredbyparasitesandpreda-
tors,andmayhavereducedfecundityifreproducinglate.However,quantifyingtheseparame-
tersinacomparativeempiricalapproachacrossmultipletaxa,orguildsalongmultiple
environmentalgradients,isdifficult.
Weusedproxiesofplanttraitstoindicateresourceavailabilityasapredictorofbodysize.
Thiswasnecessarybecauselogisticconstraintsdidnotallowforquantifyingtruenectar
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Environment,planttraits,andanimalbodysize
availability,orinsectivorousbirdfoodsupply,amongothers.Theselectionofproxieswasnot
arbitrary;higherproportionsofinsect-pollinatedplantsshould,onaverage,yieldmorenectar
thanlowerones.Highercumulativebodysizesofmothsandbeesshouldindicatehigheravail-
abilityofotherpreyinsectseatenbybirds.Theserelationships,whileplausible,maynotalways
hold:Incertainhabitats,chemicallyorphysicallywell-defendedinsectscouldhavealarger
cumulativebodysizethantheinsectsfromotherlocalitieswithoutincreasingresourceavail-
abilityforbirds.Abundantinsect-pollinatedplantspecieswithheavyallocationtovegetative
comparedtoreproductivebiomasscouldprovidelessnectarthanrareinsect-pollinatedspecies
withstrongerallocationtoreproductivebiomass.Eventually,thismayleadtoadivergence
betweentheproxiesandtheactualtraitsmeasured.Whilethisconstraintneedstobeconsid-
ered,thetestofhypotheseswithproxiesmayleadtonewinsights,iftherelationshipsofprox-
iesandtruevariableshold.Thishasbeenseenintheuseof"soft"insteadof"hard"traitsto
predictplantphotosynthesisandcarbonallocation[78],orthecommonuseofprecipitation
insteadofsoilwateravailabilitytopredictvegetationpatterns[79].
Insummary,majoreffectsofallabioticpredictorsonplantfunctionaltraitscouldbecon-
firmed.Temperatureaffectedanimalbodysizes,albeitindifferentdirections.Themore-food
hypothesiswas,however,hardlysupportedbyourdata.Ifthishypothesiswerecorrect,we
shouldhavefoundsomecouplingbetweenleaftraitsindicatingpalatabilityandbodysizesof
animals,particularlyherbivorousguilds.Thereareseveralpossibleexplanations.Firstly,the
traitsdescribingresourceavailabilitymaynothavebeengoodenoughproxiestorevealthe
truerelationships.Secondly,usingthecommunity-weightedmeans(CWMs)oftraitsand
bodysizesmayhavelumpedtogethermanyspeciesthatdonotnecessarilydependonthe
sameresources.Mothcaterpillarsareknowntobehost-specific[70],andtherelationshipof
plantsandpollinatorsischaracterizedbyalargevarietyofinteractionsfromco-dependent
plant-animalspeciespairstogeneralistspecies[80].Also,frugivorousandinsectivorousbirds
arelikelytobespecializedtosomedegree,makingtheuseofCWMsinappropriate.
TheconceptbehindCWMsisthatenvironmentalconditionsexertstrongselectiveforces
onatraitandfilteronlyasmallsubsetofthetotaltraitexpressionsofthegeographicalspecies
pool,leadingtoconvergenceofthetraitvaluesaroundameandescribingthissubset[81–82].
ThelargeenvironmentalvariationalongtheslopesofMt.Kilimanjaro,resultinginhabitatsas
diverseasalpinevegetation,rainforests,savannaandfieldsfavoredthefilteringofplanttraits
asevidencedbyourstructuralequationmodel.CWMshavebecomeincreasinglypopularin
plantfunctionaltraitstudies,mainlybecausetraitmeansperplotcanbedirectlylinkedto
environmentalvariablescharacterizingtheplot,thusmakingstatisticalanalysesrelativelyeasy.
Plant-animalinteractions,however,seemtobetoospecies-specifictoallowtheuseofcommu-
nity-weightedmeans,bothinplantsandanimals.Forfuturework,itappearsmorepromising
tostudypairwisematchingofplantandanimaltraitsinecologicalnetworks[83–85]andto
correlatethesematcheswithenvironmentalconditions.
Conclusion
Weinvestigatedtheresponseofplanttraitstotheenvironmentandtheireffectonthecommu-
nity-weightedmeanbodysizeofmoths,bees,frugivorousbirds,andinsectivorousbirdsat
MountKilimanjaro,Tanzania.Plantfunctionaltraitswereselectedtoreflectresourceavail-
abilityforanimals.Structuralequationmodelingconfirmedtheresponseofplantfunctional
traitstotheabioticenvironment.However,notplantfunctionaltraits,buttemperatureand
totalplantbiomasswerethemaindriversofmeanbodysizeinourstudysystem.Temperature
—bodysizerelationshipswerenotalwaysinlinewithexpectations,indicatingmultiplefactors
shapethebodysizedistributionoftheanimalgroupsinvestigated.Wefoundnosupportfor
PLOSONE|https://doi.org/10.1371/journal.pone.0174157 March20,2017 10/15