An Experimental Test of Buffer Utility as a Technique for Managing Pool-Breeding Amphibians

28 Pages·2017·2.22 MB·English

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New Hampshire Agricultural Experiment Station
New Hampshire Agricultural Experiment Station
Publications
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Jessica S. Veysey Powell
University of New Hampshire, Durham
Kimberly J. Babbitt
University of New Hampshire, Durham, [email protected]
Follow this and additional works at: https://scholars.unh.edu/nhaes
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Veysey Powell JS, Babbitt KJ (2015) An Experimental Test of Buffer Utility as a Technique for Managing
Pool-Breeding Amphibians. PLoS ONE 10(7): e0133642. https://dx.doi.org/10.1371/journal.pone.0133642
This Article is brought to you for free and open access by the New Hampshire Agricultural Experiment Station at
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RESEARCHARTICLE
An Experimental Test of Buffer Utility as a
Technique for Managing Pool-Breeding
Amphibians
JessicaS.VeyseyPowell*,KimberlyJ.Babbitt
DepartmentofNaturalResourcesandtheEnvironment,UniversityofNewHampshire,Durham,New
Hampshire,UnitedStatesofAmerica
* [email protected]
a11111
Abstract
Vegetatedbuffersareusedextensivelytomanagewetland-dependentwildlife.Despite
widespreadapplication,bufferutilityhasnotbeenexperimentallyvalidatedformostspe-
OPENACCESS cies.Toaddressthisgap,weconductedasix-year,landscape-scaleexperiment,testing
howbuffersofdifferentwidthsaffectthedemographicstructureoftwoamphibianspeciesat
Citation:VeyseyPowellJS,BabbittKJ(2015)An
ExperimentalTestofBufferUtilityasaTechniquefor 11ephemeralpoolsinaworkingforestofthenortheasternU.S.Werandomlyassigned
ManagingPool-BreedingAmphibians.PLoSONE eachpooltooneofthreetreatments(i.e.,reference,100mbuffer,30mbuffer)andclearcut
10(7):e0133642.doi:10.1371/journal.pone.0133642
tocreatebuffers.Wecapturedallspottedsalamandersandwoodfrogsbreedingineach
Editor:BenediktR.Schmidt,UniversitätZurich, poolandexaminedtheimpactsoftreatmentandhydroperiodonbreeding-populationabun-
SWITZERLAND
dance,sexratio,andrecapturerate.Thenegativeeffectsofclearcuttingtendedtoincrease
Received:February17,2015 asforest-bufferwidthdecreasedandbestrongestforsalamandersandwhenotherstress-
Accepted:June30,2015 orswerepresent(e.g.,atshort-hydroperiodpools).Recapturerateswerereducedinthe
30m,butnot100m,treatment.Throughouttheexperimentforfrogs,andduringthefirstyear
Published:July21,2015
post-cutforsalamanders,thepredictedmeanproportionofrecapturedadultsinthe30m
Copyright:©2015VeyseyPowell,Babbitt.Thisis
treatmentwasonly62%and40%,respectively,ofthatinthereferencetreatment.Frogsex
anopenaccessarticledistributedunderthetermsof
theCreativeCommonsAttributionLicense,which ratioandabundancedidnotdifferacrosstreatments,butsalamandersexratioswere
permitsunrestricteduse,distribution,and increasinglymale-biasedinbothcuttreatments.Bythefinalyear,therewereonaverage,
reproductioninanymedium,providedtheoriginal
onlyabout40%and65%asmanyfemalespredictedinthe100mand30mtreatments,
authorandsourcearecredited.
respectively,comparedtothefirstyear.Breedingsalamandersatshort-hydroperiodpools
DataAvailabilityStatement:Thedatausedinthis
wereabout10%asabundantinthe100mversusreferencetreatment.Ourstudydemon-
studyareavailablefromtheDryaddatabase(doi:10.
stratesthatbufferspartiallymitigatetheimpactsofhabitatdisturbanceonwetland-
5061/dryad.547rp).
dependentamphibians,butbufferwidthandhydroperiodcriticallymediatethatprocess.We
Funding:ThisworkwassupportedbygrantstoKJB
providethefirstexperimentalevidenceshowingthat30-m-widebuffersmaybeinsufficient
fromtheNationalResearchInitiativeoftheUSDA
CooperativeStateResearch,Education,and formaintainingresilientbreedingpopulationsofpool-dependentamphibians,atleastduring
ExtensionService(http://www.csrees.usda.gov/ thefirstsixyearspost-disturbance.
funding/rfas/nri_rfa.html)grantnumbers:2003-35101-
12922and2007-35101-18281;andtheNortheastern
StatesResearchCooperative(http://nsrcforest.org/).
Theprojectwasalsosupportedbyfellowshipsto
JSVPfromtheRobertandPatriciaSwitzer
Foundation(http://www.switzernetwork.org/)the
UniversityofNewHampshireGraduateSchool(http://
PLOSONE|DOI:10.1371/journal.pone.0133642 July21,2015 1/26
AmphibianForestBufferExperiment
www.gradschool.unh.edu/),andtheUniversityofNew Introduction
HampshireCooperativeExtension(http://extension.
Vegetatedbuffershavebeenusedextensivelyforseveraldecadestoprotectwetlandsacrossa
unh.edu/).Partialfundingwasprovidedbyagrantto
KJBfromtheNewHampshireAgricultural varietyoflandscapes.Bufferswereoriginallydesignedtofilterwaterpollutantsandmaintain
ExperimentStation(http://www.colsa.unh.edu/aes). waterquality[1,2].Inthiscontext,15–30m-widebuffersareoftensufficienttoremovenitro-
ThisisScientificContributionnumber:2593.This gen,phosphorus,andsedimentfromrunoffbeforeitenterswetlands[3–5].Overthelasttwo
workwasalsosupportedbytheUSDANational
decadespolicy-makershaveincreasinglyreliedonbufferstoconservewetland-dependentwild-
InstituteofFoodandAgricultureMcIntire-Stennis
life[1–3].Lackingwildlife-specificdata,theyassumed15–30m-widewaterqualitybuffers
Project,accessionnumber:0226124.Thesefunders
hadnoroleinstudydesign,datacollectionand wouldbenefitwildlife[3,6,7].Suchnarrowbuffersmaybeinsufficientformaintainingviable
analysis,decisiontopublish,orpreparationofthe populationsofmanywetland-dependentspecies,however,becausethesespeciesregularlyuse
manuscript. habitatthatextendsfartherfromwetlandsthantypicalwater-qualitybuffers[8–10].
CompetingInterests:Theauthorshavedeclared Thismaybeespeciallytrueforamphibiansthatbreedinephemeralpools.Thesemi-annual
thatnocompetinginterestsexist. dryingcycleofephemeralpoolspreventsestablishmentofpredatoryfishpopulations,making
thesepoolsextremelyproductiveamphibianhabitat.Duringthenon-breedingseason,these
amphibiansrangeacrossthesurroundinglandscape,usingadditionalwetlandsanduplands
forforaging,shelter,estivation,andhibernation[11–13],sometimesmigratinghundredsof
metersintoterrestrialhabitat[14–16].Land-usesthatalterthehabitatqualityofbreeding
poolsand/oradjacentuplandscanpotentiallyhavestrongnegativeeffectsonlocalandregional
populationpersistence.
Historically,ephemeralpoolsreceivedlittlepolicyprotectionintheUnitedStates(U.S.).
FederalprotectionostensiblyfallsundertheCleanWaterAct,butistenuousgivenrecent
SupremeCourtdecisionsandhasneverofficiallyincludedabuffer[17].Somestates,counties,
andmunicipalitiessupplementfederallawbyimplementingmorestringentlocalpolicies.
However,only15of50stateshavesubstantialwetlandprograms,mostofwhichdonotinclude
protection,letalonebuffers,forephemeralpools[18].FourofsixNewEnglandstatesprovide
aregulatorybufferforephemeralpools(mean±SD:23±29m;range:0–76m),butthesebuff-
erstypicallyonlyapplytoasubsetofpoolsandprojecttypes[19–22]andaresubstantiallynar-
rowerthanthe164-290-mbuffersthatscientistsrecommend,basedonamphibianmigration
data(e.g.,[23]).
PreviousobservationalandmodelingstudiesconcludethatregulatorybuffersintheU.S.are
inadequateforprotectingpopulationsofephemeral-pool-breedingamphibians[10,23,24].In
mostofthesestudies,however,buffereffectswereneverexplicitlytested,butratherestimated
fromspecies’migrationdistancesinundisturbedhabitat.Thus,verylittleisactuallyknown
abouthowthedemographicsandbehaviorofephemeral-pool-breedingamphibiansdifferin
unbufferedandbufferedsystems,andwhatoptimalbufferwidthsarefordifferentspecies.
Overall,experimentalconfirmationoftheneedforwiderbuffersisseverelylacking.Though
somepolicy-makersexpressinterestinexpandingbufferstoaccommodatewildlifehabitat
needs[25–27],theyhesitatetosupportpolicychangeswithoutsolid,experimentalevidence
demonstratingtheutilityofbuffersforwildlife[28,29].
Toaddressthisneed,weconductedasix-year,landscape-scaleexperimentandexamined
howbuffersofdifferentwidthsaffectbreeding-adultdemographyfortwoamphibianspeciesat
ephemeralpoolsinanindustrialforestofthenortheasternU.S.Westudiedspottedsalaman-
ders(Ambystomamaculatum)andwoodfrogs(Lithobatessylvaticus)becausetheyusesimilar
macrohabitat,butdifferinmicrohabitatpreferencesandkeydemographictraits,andmaythus
requiredifferentconservationstrategies.Inparticular,spottedsalamandersutilizesmall-
mammalburrowsextensivelyasrefugia[13,30],canliveupto32yearsinthewild(meanadult
ageinonenorthernpopulationwas8.8years;[31,32]),andusuallybreedmultipletimesin
theirlives[33].Bycontrast,woodfrogsseekrefugeinotherwetlandsduringthesummer[11],
overwinterinleaflitter[11,34],haveamaximumlifespanof5–6years[35,36],andtypically
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breedonlyonceafterreachingsexualmaturity[35,37],butaremorefecundthanspottedsala-
manders[10].
Foreachspecies,weexaminedwhethertheabundance,proportionrecaptured,andsexratio
ofbreedingadultsdifferedacrossbuffertreatmentsand/orvariedwithtimeasdisturbedforest
aroundexperimentalbuffersregenerated.Wehypothesizedthat:
1. Forbothspecies,breeding-adultabundanceandrecaptureproportionwouldbepositively
correlatedwithbufferwidth.Weexpectthisbecausewiderbuffersprovideagreaterareaof
high-qualityhabitatinproximitytobreedingpoolsandshouldsupportalarger,morestable
breedingpopulationthanpoolswithnarrowerbuffers.
2. Forbothspecies,breeding-adultsexratio(i.e.,theproportionofmalesinthebreedingpop-
ulation)wouldvarywithbufferwidth.Malesofbothspeciesclusterclosertobreedingpools
thandofemales,suchthatgenerally,buffersshouldprovidemorehabitatformales,and
femalesshouldbedisproportionatelyimpactedbydisturbancesbeyondthebuffer[38–40].
Duetoalackoffine-grainresolutioninexistingknowledgeofthespatialdistributionof
bothsexesaroundpools,however,wedidnotpredictdirectionalityintherelationship
betweensexratioandbufferwidth.
3. Forbothspecies,anynegativeimpactstobreeding-adultabundance,recaptureproportion,
orsexratiointhebufferedtreatmentswouldrecoverwithtime(i.e.,berestoredtovalues
similartothoseinthereferencetreatment,afterdeviatingfromreference-treatmentvalues
atsomepriortime),asdisturbedforestsregenerated.
4. Woodfrogs,beingmorefecund,wouldrecoverfasterthanspottedsalamanders.
Methods
Studysite
Weconductedthisresearchinanindustrialforest,privatelyownedbyInternationalPaper/
SustainableForestTechnologies,ineast-centralMaine,U.S.(45°0’52”N,44°48”32”N;68°
28’11”W,67°53’10”W).Theforestispredominantlyeasternhemlock(Tsugacanadensis)and
northernhardwood(Fagusgrandifolia,Acersaccharum,Betulaalleghaniensis)atlowereleva-
tions,andbalsamfir(Abiesbalsamea)andredspruce(Picearubens)athigherelevations.Mod-
eratehills,wetlands(includingnumerousephemeralpools),anddirtloggingroadsare
common.In2002,weidentified300ephemeralpoolsinthislandscapeandchose40ofsimilar
size(i.e.,0.1–0.3ha)andadjacentforest(i.e.,uncutwithin1000m)forin-depthinspection.In
spring2003,wesurveyedeggmassesatthese40poolsandidentified35withbreedingpopula-
tionsofbothwoodfrogsandspottedsalamandersandhydroperiodsofatleastfivemonths
post-iceout.Werandomlychose12ofthe35poolsforthisstudy.Inspring2004,welearned
thatoneofthe12poolshadapermanentinflowandremovedthatpoolfromthestudy.
Buffercreation
Werandomlyassignedeachoftheremaining11poolstooneofthreetreatments:reference
(i.e.,uncut;N=3),100mbuffer(N=4),or30mbuffer(N=4).FromSeptember2003to
March2004,thelandownerusedclearcuttingtocreateexperimentalbuffersatthe100mand
30mtreatmentpools.Thecuttingremovedallmerchantabletrees(cid:1)5cmdiameteratbreast
heightandslash,thoughasmallquantityofwoodydebrisremained.Aftercutting,poolsinthe
twobuffertreatmentshad,respectively,a100-mor30-m-wideuplandbufferencirclingthe
poolanda100-m-wideconcentricclearcutaroundthebuffer(Fig1).Weselectedbufferwidths
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Fig1.Experimentaldesignimplementedat11naturalephemeralpoolsineast-centralMaine,USA.Undisturbedbuffersofeither100m(left;n=4)or
30m(right;n=4)wereleftadjacenttopoolsand100-m-wideclearcutswerecreatedaroundthebuffers.Forestbeyondtheclearcutwasundisturbed.No
cuttingoccurredatreferencepools(notshown;n=3).
doi:10.1371/journal.pone.0133642.g001
typicalofthoseinexistinglaws,BestManagementPractices(i.e.,voluntaryforestryanddevel-
opmentpoliciesdesignedtoprotectwaterandsoilquality),andtheliterature(e.g.,[12,41,
42]).
Amphibiansampling
Duringsummerandfall2003,weencircledeachofthe11poolswithadriftfence/pitfalltrap
array[43].Weusedplasticsiltfencingthatwas91cmtallandburied8–10cmdeepandposi-
tionedfences5mupgradientofeachpool’shighwatermarktominimizefloodingrisk.We
buriedpairsof5.7literaluminumcansonoppositesidesofthedriftfenceat10mintervals
[43].Topreventamphibiandesiccation,weplacedamoistenedspongeinthebottomofeach
trap.
From2004through2009,weopenedtrapsinthespringafterice-outandclosedtrapswhen
apoolwasdryforatleastsevenconsecutivedaysor,inthefall,whenamphibianswerenolon-
geractiveduetohardfrosts.Weuprootedsectionsofthefenceduringthewintertoallow
movementofnon-focalspecies.Wecheckedpitfalltrapsdailyduringperiodsofhighamphib-
ianactivity(i.e.,April-MayandJuly-September)andeveryonetofivedaysduringperiodsof
lessamphibianactivity(i.e.,Juneandlatefall).Duetopoorroadconditions,wecouldnot
accessone30m-bufferpoolin2009.However,ourstatisticaltechniquewasrobusttomissing
data[44],allowingustousetheotherfiveyearsofdatafromthispool.
Wecapturedandcountedallamphibiansenteringandexitingthepoolsandsexedalladult
amphibiansexitingpools.Weassumedallnon-gravidadultspottedsalamandersandwood
frogsleavingapoolhadbredinthatpoolandmarkedeachwithapool-specifictoe-clip[45].
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Afterprocessingeachanimal,wereleaseditontheoppositesideofthefencefromthepointof
capture.From2005to2009,wecountedthenumberofrecapturedindividualsateachpool.A
pool-specifictoe-clipwasnecessarybecausesomepoolswereincloseenoughproximitythat
inter-pooldispersalwaspossible.However,giventhestudy’sdurationandtheproblemsassoci-
atedwithcuttingmultipletoes[46–48],wedidnotmarkanimalsseparatelyforeachyearthey
werecapturedoreventoindicatetheinitialcaptureyear.Itisthuspossiblethatafewanimals
werecountedmorethanonceinasingleyear.Tominimizethechanceofthisoccurring,we
tookthefollowingprecautions.First,ifanindividualreturnedtoapoolinthesameyearthatit
wasfirstmarked(indicatedbyafreshtoecut),weonlyincludeditsinitialvisitforthatyearin
ouranalyses.Second,weonlycountedandmarkednon-gravidfemales.Similarly,though
closeddrift-fencearraysarehighlyefficientattrappingadultambystomatidsandwoodfrogs
[40,49,50],itispossiblethatsomeindividualstrespassed(i.e.,climbedoverorunder)the
fenceandwereundetectedbyourcounts.Tolimitpotentialtrespass,weregularlyinspected
andmaintainedourfences.Weneversawadultwoodfrogsorspottedsalamandersclimbing
fencesduringinspections.Iftrespassdidoccur,ouramphibiancountswouldbeconservative.
Bycontrast,anymultiplecountsofthesameindividualinagivenyearwouldsomewhatinflate
ourrecapturecounts.Overall,however,thenumbersofanimalsthattrespassedorwere
countedmorethanonceinayearwouldbeverysmallcomparedtothetotalnumberof
amphibianstrapped.Furthermore,wehavenoreasontoexpectthatmultiple-countortrespass
rateswoulddifferacrosstreatmentsoryears.Consequently,ouranalysesshouldremainvalid,
despiteanymultiplecountsortrespass.
Hydroperiodsampling
Wecalculatedhydroperiodforeachpoolineachyearasthenumberofdaysapoolheldwater
betweenice-out(i.e.,<75%ofthepoolwascoveredinice)andthedaythepooldried
completely.Tofacilitatestatisticalanalyses,weassignedahydroperiodenddateofOctober
28thtopoolsthatdidnotdryinagivenyear.Wechosethisenddatebecausewestillhadevi-
denceofpersistentwaterinthesepoolsonthisdate,butitwassufficientlylateintheyearthat
mostamphibiansatourstudypoolswereinactive.
Statisticalanalyses
Toassesstherelativeimpactsofforestrytreatmentandhydroperiodonthesizeandcomposi-
tionofspottedsalamanderandwoodfrogactive-breedingpopulations,wedevelopedgeneral-
izedlinearmixedeffectsregressionmodelsusingthe“glme”functioninthecorrelatedData
libraryofS-Plus8.0(InsightfulCorporation,Seattle,WA,USA2007)andthe“glmer”function
inthelme4library[51]ofR2.13.0[52].Wedefinetheactive-breedingpopulationasalladults
thatmigratetoapoolandattempttobreedinagivenyear.Thus,ourresultsapplytoasubset
ofeachspecies’totallocalpopulationanddonotaccountforadultsalamandersthatskip
breedinginagivenyearorjuveniles.Fortherestofthispaper,werefertoourstudypopulation
simplyas“breedingadults.”
Wemodeledboththesexratioandproportionofrecapturedadultsforeachspeciesusing
mixed-effectslogisticregressionwithalogitlink.Wedefinedsexratio,withineachstudyyear
andatagivenpool,as:Numberofbreedingmales/(Numberofbreedingmales+Numberof
breedingfemales).Wedefinedtheproportionofrecapturedadults,withineachstudyyearand
atagivenpool,as:Numberofrecapturedbreedingadults/(Numberofrecapturedbreeding
adults+Numberofnew-capturedbreedingadults).Notethatthenumberofrecapturedbreed-
ingadultsinagivenyearissimplyacountofthenumberofmarkedindividualsthatreturned
tobreedataparticularwetlandinthegivenyear,withoutdistinguishingbetweenadults
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markedindifferentyears.Thisdefinitionofrecapturedadultsderivesfromourmethodof
markingindividualswithapool-specifictoe-clip,butnotayear-specifictoe-clip.Wemodeled
breeding-adultabundanceforeachspecieswithmixed-effectsPoissonregression,usingalog
link.Wedefinedbreeding-adultabundanceasthetotalnumberofadults(i.e.,recaptured
adults+new-capturedadults)activelybreedinginagivenyearatagivenpool.
WetreatedyearandpoolIDascrossedrandomeffects[44]inallmodels.Wemodeledthe
variance-covariancestructureforeachregressiontoaccountforinter-yearcorrelationatindi-
vidualpoolsandheterogeneousvarianceacrossgroups.Weusedlikelihoodratioteststoopti-
mizeeachregression’svariance-covariancestructure.Weaccountedforintra-poolcorrelation
inthesalamanderabundancemodelusingafirst-orderauto-regressiveprocesswithyearasthe
timevariable.Wedidnotneedtodefineacorrelationstructureforanyothermodel.Wespeci-
fiedthevariancestructureforallmodelsexcepttheproportion-of-recaptured-salamander
model.Forsalamandersexratio,weassigneddifferentvariancestoeachstudyyear.Forsala-
manderabundance,weallowedthevariancetoincreaseexponentiallyasafunctionofmean-
poolhydroperiod(i.e.,themeanhydroperiodforeachpoolacrossthesixstudyyears).Forthe
proportion-of-recaptured-frogsmodel,weassigneddifferentvariancestoeachtreatment.For
frogsexratio,weallowedthevariancetoincrease,withineachstudyyearseparately,asapower
functionofthemodel’sfittedvalues.Forfrogabundance,weallowedthevariancetoincrease,
withineachtreatmentseparately,asapowerfunctionofthestandarddeviationofpoolhydro-
period(ascalculatedforeachpoolacrossthesixstudyyears).Intheirfinalforms,allmodels
satisfiedtheassumptionsofPoissonorlogisticregression.
Ourpredictorvariableswere:buffertreatment,mean-poolhydroperiod,standarddeviation
ofpoolhydroperiod,aninteractionbetweentreatmentandmean-poolhydroperiod,andapair
ofnumericdummyvariablesrepresentinganinteractionbetweentreatmentandstudyyear.
Weusedthefirstdummyvariable(dv.cut)todistinguishwhetherapoolwassubjectedtoclear-
cuttingornot.Weusedtheseconddummyvariable(dv.30m)toindicatemarginalimpactsto
the30mtreatmentpools.Weuseddummyvariablestorepresenttheeffectsofstudyyearfor
severalreasons.First,weneededtocapturepotentialchangesovertimeastheclearcutsregen-
erated.Second,wecouldnottreatyearasacategoricalvariableasthisrequiredestimatingfive
coefficientsforyearaloneandourmodelswouldnotconvergewiththisformulation.Whenwe
usedthetwodummyvariables,bycontrast,weonlyneededtoestimatetwocoefficients,we
couldsimultaneouslyinvestigateatreatmentXyearinteraction,andourmodelsconverged.
Becauseofthisformulation,however,wecouldnottestforsignificantdifferencesbetweenspe-
cificyears.Rather,weusedtheestimateddummy-variablecoefficientstodescribedifferences
intheaverageratesofchangeovertimeacrosstreatments.WeusedANOVAstodeterminethe
overallsignificanceofeachfixedeffectandt/ztests(forS-plusandRmodels,respectively)to
testthesignificanceofthedifferenttreatmentlevels(α=0.05).Weusedtreatmentcontraststo
comparethereferencetreatmenttoeachrespectivecuttreatment(i.e.,bydefault,therewasno
directcomparisonbetweenthe100mand30mtreatments;[44]).WeappliedasequentialBon-
ferroniproceduretoadjustα-levelsduringaposthoccomparisonbetweenthe100mand30m
treatmentsfortheproportion-of-recaptured-frogsmodel(i.e.,theonlymodelwithasignificant
overalltreatmenteffect,butnosignificantdifferencesidentifiedbythedefaultinter-treatment
comparisons).
Basedonourexperimentaldesign;priorknowledgethathydroperiodstronglyinfluences
bothspecies’populationdynamics;andadesiretoapproximatenaturalconditions,whereall
predictorssimultaneouslyactonthespecies’populations,wedecidedaprioritoretainallpre-
dictorsineachmodel,whetherornottheywerestatisticallysignificant.Weallowedtwoexcep-
tionstothisrule.First,basedonanaprioridecision,wedroppedthehydroperiodinteraction
fromamodelwhenitwasnotsignificantandrefitthemodelfortheremainingfixedeffects.
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Wemadethisdecisionprincipallybecause,unlikefortheotherpredictors,wehadnostrong
priorreasontothinktheremustbeatreatmentXhydroperiodinteraction.Ultimately,weonly
includedthehydroperiodinteractioninthesalamander-abundancemodel.Second,we
droppeddv.30mfromtheproportion-of-recaptured-frogsmodelbecausedv.30mandtreat-
mentwereconfounded,withbothpredictorseffectivelycancelingeachotherout.Forthis
modelalone,weinvestigatedthepossibilityofconfoundingfactorsbecauseraw-datagraphs
suggestedaverystrong30m-treatmenteffect,whichwasnotsupportedbythefullmodel
results.Wetestedforconfoundingfactorsbysystematicallydroppingeachpredictorinturn
fromthemodelandassessingmodelfit(usinggraphsoffittedversusobservedvalues).Ulti-
mately,weretainedtreatmentinsteadofdv.30mbecauseareducedmodelwithtreatmentpro-
videdabetterfitthanonewithdv.30m.However,thereduceddv.30mmodelsuggestedthat
theproportionofrecapturedfrogsinthe30mtreatmentdecreasedduringthestudy.Wedid
nottestforconfoundingfactorswiththeothermodels,aswehadnoevidencetosuggestthat
thismightbeaproblemforanyothermodel.
Ethicsanddatadepositionstatements
WeconductedalloftheresearchinaccordancewiththerulesoftheInstitutionalAnimalCare
andUseCommitteeattheUniversityofNewHampshire(IACUC-UNH).IACUC-UNH
approvedourresearchprotocol,asdetailedinpermits:020601and050604.Noneofthecap-
turedspecieswereprotectedorendangeredunderfederalorstatelaw.Weconductedthe
researchonprivateland,withpermissionfromthelandowner.Forthesereasons,noadditional
permitsorpermissionwereneededtoconductthiswork.Thedatausedinthisstudyareavail-
ablefromtheDryaddatabase(DOI:10.5061/dryad.547rp).
Results
From2004to2009,wecaptured3624breedingspottedsalamanders,including2811(78%)
new-capturesand812(22%)recaptures,and1518(42%)femalesand2099(58%)males.Simi-
larly,wecaught6521breedingwoodfrogs,including5478(84%)new-captures,1014(16%)
recaptures,2427(37%)females,and4072(63%)males.Breeding-adultabundance,especially
forwoodfrogs,washighlyvariablewithinandacrosspoolsandyears(Table1,Fig2).For
example,atonereferencepool,annualbreedingwoodfrogabundancerangedfrom70to568,
withameanof214frogs,whileatadifferentreferencepool,breedingwoodfrogabundance
rangedfrom54to161,withameanof95.Hydroperiodalsovariedwidelyinthisforest-
ephemeralpoolecosystem.Onesemi-permanentpoolnevercompletelydriedduringthesix-
yearstudy.Bycontrast,meanhydroperiodatanotherpoolwasaboutfourmonths,butvaried
byasmuchas49days.
Spottedsalamanders
Breeding-salamandersexratioincreasedsignificantlyinbothcuttreatmentsduringthestudy,
andamarginallysignificanttrendindicatedthattherateofincreaseinthe30mtreatmentwas
slightlylowerthanthatinthe100mtreatment(Table2;Fig3).Forexample,at100m-buffer
pools,wepredictedapproximatelyequalnumbersofeachsexin2004,but2.3malesperfemale
in2009.Similarlyat30m-bufferpools,wepredictedapproximately1.3malesperfemalein
2004,but2malesperfemalein2009.Atreferencepools,bycomparison,wepredicted1.1
malesperfemale,onaverage.Inbothcuttreatments,themale-biasedsexratioswereprinci-
pallydrivenbyadecreaseinthenumberofbreedingfemales.
Theproportionofrecapturedbreedingsalamanderswassignificantlylowerinthe30mver-
susreferencetreatmentrightafterthecut,butsignificantlyincreasedwithtime,sothatby2008
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AmphibianForestBufferExperiment
Table1. Meanandvariabilityofpredictorandoutcomevariablesat11naturalephemeralpoolsineast-centralMaine,USA. Populationparameters
representactivelybreedingadultsonly.
Mean±SE Range
Meanhydroperiod(days) 125.96±5.98 44.83–197.00d
SDhydroperiod(days)a 31.81±1.58 6.32–48.76
SpottedSalamander WoodFrog
Mean±SE Range Mean±SE Range
Abundance
Reference 36.44±3.95 10–66 135.61±27.58 54–568
100m 76.29±16.46 0–242 88.21±13.35 7–215
30m 49.52±7.96 9–157 86.35±11.18 14–221
Proportionrecapturedb
Reference 0.31±0.04 0.06–0.51 0.23±0.04 0.05–0.67
100m 0.23±0.04 0.00–0.63 0.19±0.03 0.03–0.42
30m 0.19±0.03 0.00–0.48 0.09±0.01 0.00–0.18
Sexratioc
Reference 0.55±0.04 0.10–0.78 0.62±0.04 0.31–0.88
100m 0.59±0.04 0.17–1.00 0.60±0.03 0.14–0.76
30m 0.58±0.03 0.38–0.81 0.62±0.03 0.32–0.86
aStandarddeviationofthepoolhydroperiod.
bProportionrecaptured=numberofrecapturedbreedingadults/(numberofrecapturedbreedingadults+numberofnew-capturedbreedingadults).
cSexratio=numberofbreedingmales/(numberofbreedingmales+numberofbreedingfemales).
dSomepoolsdidnotdryinsomeyears.Tofacilitateanalyses,weassignedsuchpoolsalate-fallhydroperiodenddate.Meanhydroperiodwascalculated
usingthecappedenddates.
doi:10.1371/journal.pone.0133642.t001
(i.e.,fiveyearspost-cut),recaptureproportionsat30m-bufferpoolswerepredictedtobeonly
slightlylessthanthoseatreferencepools(Fig4).Thesetrendswerelargelydrivenbymaleand
femalenew-captureabundance,whichtendedtobehigherinthe30mtreatmentduringthe
firstthreeyearsofthestudy,butsimilarinthetwotreatmentsbythestudy’send.Toalesser
extent,thesetrendsmayalsobeexplainedbyrecapturenumbersinthe30mtreatment,which
werelowimmediatelyafterthecut(butalsoduringthelasttwostudyyears.Infact,werecap-
turednofemalesfromthe30mtreatmentin2009.However,fewsalamanderswererecaptured
inanytreatmentin2009andwecouldonlytrapatthreeofthefour30m-bufferpoolsin2009).
Therewasnosignificantdifferenceintheproportionofrecapturedsalamandersbetweenthe
referenceand100mtreatments.
Atshorthydroperiodpools,wefoundsignificantlyfewerbreedingspottedsalamandersin
the100mtreatmentthanthereferencetreatment(Fig5).Forinstance,ifpoolmeanhydroper-
iodwere45days(theminimummeanhydroperiodobserved),thenumberofbreedingsala-
mandersata100m-bufferpoolwaspredictedtobeonlyabout12%oftheabundanceata
referencepool.However,abundanceincreasedsignificantlywithmeanhydroperiodatthe
100m-bufferpools,suchthatforeachadditionaldayapoolheldwater,thenumberofbreeding
adultswaspredictedtoincreasebyabout3%.Wefoundnosignificantdifferenceinbreeding
salamanderabundancebetweenthereferenceand30mtreatments.
Woodfrogs
Thoughwefoundnodifferenceinrecaptureproportionbetweenthereferenceandeithercut
treatment,thesignificantresultsofthepost-hoctestshowedthattheproportionofrecaptured
breedingadultsinthe30mtreatmentwaspredictedtobe,onaverageandforthedurationof
PLOSONE|DOI:10.1371/journal.pone.0133642 July21,2015 8/26
AmphibianForestBufferExperiment
Fig2.Breedingspottedsalamanderandwoodfrogabundance.Shownforpopulationsat11naturalephemeralpoolsineast-centralMaine,USA,across
thesixstudyyears.Eachpoolislabeledwithanidentifyingnumberandtheappliedforestrytreatment.Experimentalforestrytreatmentswere:reference
(uncut),100mundisturbedbuffer,30mundisturbedbuffer.
doi:10.1371/journal.pone.0133642.g002
thestudy,62%ofthatinthe100mtreatment(Fig6).Thiswaslargelybecausebothmaleand
femalerecaptureswerescarceinthe30mtreatment.Infact,nofemaleswererecapturedat
30m-bufferpoolsin2008.Buffertreatmentwasnotasignificantpredictorofbreedingwood
frogabundanceorsexratio,butwedidfindsignificantlymorebreedingwoodfrogsatpools
withlongerhydroperiods.Foreachadditionaldayapoolheldwater,abundancewaspredicted
toincreasebyafactorof0.7%.
Discussion
Thisisthefirstlandscape-scaleexperimenttoexplicitlytestwhethervegetatedbuffersarean
effectivetoolformanagingephemeral-pool-breedingamphibiansandtocomparethevariable
impactsofdifferentbufferwidthsonbreeding-amphibiandemography.Wefoundbuffer
width,timesincecut,andpoolhydroperiodwereallimportantfactorsindeterminingbreed-
ing-adultresponsetoclearcuttingofterrestrialhabitataroundephemeralpools.Contraryto
ourhypotheses,breeding-populationabundanceandbufferwidthwerenotpositively
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