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Research
Letter
Assessing
the
consistency
of
hotspot
and
hot-moment
patterns
of
wildlife
road
mortality
over
time
Rodrigo
Augusto
Lima
Santos
a,b,c,d,∗,1,
Fernando
Ascensão
e,f,1,
Marina
Lopes
Ribeiro
c,
Alex
Bager
g,
Margarida
Santos-Reis
d,
Ludmilla
M.S.
Aguiar
a,baCursodePós-Graduac¸ãoemEcologia,InstitutodeCiênciasBiológicas,UniversidadedeBrasília,Brasília,DF,Brazil bLaboratóriodeBiologiaeConservac¸ãodeMorcegos,DepartamentodeZoologia,UniversidadedeBrasília,Brasília,DF,Brazil cInstitutoBrasíliaAmbiental(IBRAM),Brasília,DF,Brazil
dCentreforEcology,EvolutionandEnvironmentalChanges(Ce3C),FaculdadedeCiências,UniversidadedeLisboa,Lisboa,Portugal eCentrodeInvestigac¸ãoemBiodiversidadeeRecursosGenéticos(CIBIO/InBio),UniversidadedoPorto,Vairão,Portugal
fCentrodeEcologiaAplicada“ProfessorBaetaNeves”(CEABN/InBio),InstitutoSuperiordeAgronomia,UniversidadedeLisboa,Lisboa,Portugal gDepartamentodeBiologia,UniversidadeFederaldeLavras,Lavras,MG,Brazil
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received10August2016 Accepted6March2017 Availableonline27March2017 Keywords: Roadsegments Roadkill Aggregations Scaleeffect Mitigations
a
b
s
t
r
a
c
t
Spatialandtemporalaggregationpatternsofwildlife-vehiclecollisionsarerecurrentlyusedtoinform whereandwhenmitigationmeasuresaremostneeded.Theaimofthisstudyistoassessifsuch aggre-gationpatternsremaininthesamelocationsandperiodsovertimeandatdifferentspatialandtemporal scales.Weconductedbiweeklysurveys(n=484)on114kmofnineroads,searchingforroadcasualties (n=4422).Aggregationsweresearchedusingdifferentlengthsofroadsections(500,1000,2000m)and timeperiods(fortnightly,monthly,bimonthly).Ourresultsshowedthathotspotsandhot-momentsare generallymoreconsistentatlargertemporalandspatialscales.Wethereforesuggestusinglongerroad sectionsandlongertimeperiodstoimplementmitigationmeasuresinordertominimizethe uncer-tainty.Wesupportthisfindingbyshowingthattheproportionalcostsandbenefitstomitigateroadkill aggregationsaresimilarwhenusingdifferentspatialandtemporalunits.
©2017Associac¸˜aoBrasileiradeCiˆenciaEcol ´ogicaeConservac¸˜ao.PublishedbyElsevierEditoraLtda. ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(
http://creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction
Roadshavea varietyofecologicaleffects ontheir surround-ingenvironment,andoneofthemoststudiediswildlife-vehicle collisions(WVC)(Formanetal.,2003; Reeetal.,2015).Several researchershave demonstratedthatroadkillsareoftenspatially andtemporallyaggregated,hereafterreferredasWildlife-Vehicle Aggregations(WVA).WVA aregenerallyrelated tospecies’ bio-logicaltraits(e.g.mating),roadfeatures(e.g.trafficvolume),the surroundinglandscapeorclimateconditions(Gunsonetal.,2011; Maloetal.,2004;Smith-PattenandPatten,2008).Therefore,WVA mayindicatepreferentialtargets(hotspotsandhot-moments)for implementing mitigation measures (Malo et al., 2004; Morelle etal.,2013;Reeetal.,2015).TheidentificationofWVAisoneof
∗ Correspondingauthor.
E-mailaddress:rodrigosaantos@gmail.com(R.A.LimaSantos). 1Theseauthorssharethefirstauthorship.
theapproachesmostusedbyresearchersanddecisionmakersto implementmortalitymitigationonroads(Santosetal.,2015).
Mitigationmeasuresmustbeplannedtoensureeffectiveness, duetothehighcostofinstallationandmaintenance(Reeetal., 2015).Thus,itisnecessarytodeterminethebestspatialscale(s)at whichputativepredictorsindicatelocationsofWVA(Langenetal., 2007;Reeetal.,2015).Ideally,WVAneedtobespatiallyrestricted inlength,sinceshortroadsectionscanbemoreeasilymitigated byfaunalpassagesanddriftfencingthanwhenWVAsegmentson roadaredistributedoverabroaderextentoftheroad(Langenetal., 2007).Ontheotherhand,understandingtheroleofseasonalityon roadmortalityallowstheidentificationofpossibleWVAincertain periods(hot-moments),anddecisionmakerscandirectmitigation measurestowardtheperiodofhigherWVA,whichwillreducethe costs(Sullivanetal.,2004).
The aim of this study was to investigate if the spatial and temporal patterns of WVA were similar during the same time periodforthedifferenttaxonomicgroups. IfWVAoccur consis-tentlyin thesamelocation andtime period, i.e.do notchange overtime,mitigationmeasuresappliedthereinwillprobablybe http://dx.doi.org/10.1016/j.pecon.2017.03.003
1679-0073/©2017Associac¸˜aoBrasileiradeCiˆenciaEcol ´ogicaeConservac¸˜ao.PublishedbyElsevierEditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
more cost-effective(Costa etal., 2015).Additionally,we evalu-atedhowdifferentroadsegmentlengthortimeperiodaffected theconsistencyofspatialandtemporalWVA patterns.We con-siderthathighercorrelationofWVApatternsbetweenconsecutive years indicatehigherreliability inusing suchlocationsas miti-gationtargets.Hence,weevaluatehowcost-benefiteffectiveness couldvarywhentargetingmitigationtoshort/longroadsectionsor timeperiods.Cost-benefitanalysiscanbecomplexinroadecology (Costaetal.,2015).Here,weadoptasimpleapproachwherewe countthenumber ofcasualtiesthatcouldhavebeenprevented
if road mitigation was implemented in WVA (assuming full effectiveness).
Materialsandmethods
Studyarea
WeconductedthestudyinBrasília(FederalDistrict),locatedin theCerradobiomeofCentralBrazil.Atotalof114kmpertaining toninedifferentroadsweresurveyed.Moredetailsofthestudy
500 m
A
B
5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 5 4 4 3 3 2 2 1 1 AprilMayJune July AugustSeptemberOctoberNovemberDecemberJanuaryFebruary
March AprilMayJuneJulyAugust
SeptemberOctoberNovemberDecemberJanuaryFebruary
Mar h April June
August October DecemberFebruary
0 50 100 150 200
Fortnightly Monthly Bimonthly
Amphibians Reptiles Birds Mammals 0 30 60 Road section Time period Year Year 90 0 20 40 1000 m 2000 m Amphibians Reptiles Birds Mammals
Fig.1.Locationofwildlife-vehicleaggregations(WVA)peryearandclass,alongthe114kmofroadsurveyed(A)andalongtheyear(B).Eachverticalpanelpresentsthe locationswhenusingdifferentspatial(A)ortime(B)unitstodetectWVA.
area,includingweatherconditions,traffic,roads,protectedareas monitoredandamapareprovidedinText1inAppendix1. Datacollection
Weconductedroadsurveysbiweekly(twosurveys/week)for5 years,surveyingall114kmbycampaign(i.e.,allroadtypeswere surveyed equally), between April 2010 and March 2015, total-ing480roadkillsurveys.Twoobserversandonedriversearched forroadkillsin avehicletravelingatca.50km/h. Theobservers recorded thelocation of carcasses using a hand-held GPS (5m accuracy).Carcasseswereremovedafterdatacollectiontoavoid pseudo-replication and recounting carcasses. Domestic animals werenotconsideredinfurtheranalyses.
Dataanalyses
WVCrecordswereaggregatedbyclass(amphibians,reptiles, birds,andmammals)andyear,andseparatedatasetsforthe spa-tialandtemporalinformationwerecreated.Forthespatialdataset, weaggregated therecords byroad segmentsof500, 1000 and 2000mlength.Thetemporal datasetwasaggregatedusing fort-nightly,monthlyandbimonthlytimeperiods.Weconsideredayear ofsurveyasthetimebetweenAprilandMarchofthefollowingyear. Hereafterwewillrefertothesectionlengthsandtimeperiods asunits.
Foreachclassandyearofsurveyweassumedthattheobserved numberofroadkillsperunitwouldfollowarandomPoisson dis-tributionwithamean()equaltothetotalnumberofroadkills dividedbythetotalnumberofunits.Theprobabilityofanyunit havingxnumberofcollisionswastherefore:
p(x)= x x!e
Ameanvalue()foreachtaxawascalculated,andconsidering roadkillsperyear.Asthemean()variedacrosstaxa,each500mof roadsectionwiththreeormorecollisions,couldbedefinedasWVA forAmphibians.Roadsectionswithfourormorecollisionswere classifiedasWVAforReptiles,tobirdssevenormorecollisions,and formammalswiththreeormore.TheseminimumvaluesforWVA detectionincreasedforlongerroadsections(1000mand2000m) scales.Forhot-moments,periods(fortnight)withfiveormore colli-sionscouldbedefinedasWVAforAmphibians.ForReptiles,periods (fortnight)withthirteenormoreroadkillswereclassifiedasWVA, andtobirdsthirtythreeormoreroadkills.Theseminimumvalues forWVAdetectionincreasedforlongertimeunits(monthlyand bimonthlytimeperiods).
WeconsideredaunittobeaWVAwhenp(x)>0.95.Weusedthe falsediscoveryratetoreducethelikelihoodofdetectingfalseWVA (TypeIerror)duetomultipletesting (Benjaminiand Hochberg, 1995). We usedthesame approach of Maloet al., (2004)as it permitseasycomparisonamongsamplingschedulesusingafixed spatialscale.Besides,thismethodseemstoperformbetterthan otherstodetectfatalityhotspots (Gomes etal.,2009).We then transformedtheconsecutiveunitsintoabinaryvariableof pres-ence/absenceofWVA.Hence,foreachyearthereisahot-moment andahotspotevaluationforeachtaxonomicclass.
ThesimilarityofWVApatternsovertimewasassessedusing correlationtestsbetweenconsecutiveyearsusingthePhi coeffi-cient(rPhi)(Zar,1999).ThePhicoefficientmeasuresthedegreeof
associationbetweentwobinaryvariables,anditsinterpretation issimilartothecommoncorrelationcoefficients.Thisprocesswas performedforeachaggregationunit(spatialandtemporal).Finally, thecost-benefitanalysiswasperformedforeachtaxa, yearand unit,byrelatingtheproportionofroadsectionsortimeperiods that were classified as WVA with the proportion of casualties
potentiallyavoidedifthoseWVAweremitigated.Allcalculations andplotswereperformedusingRsoftware(RCoreTeam,2015) andtheRpackagesHmisc,vcd,cowplotandggplot.
Results
Werecorded4422non-domesticroad-killedanimals,ofwhich 5%wereamphibians(n=274,9species),15%reptiles(n=690,34 species),71%birds(n=3009,91species),and9%mammals(n=448, 24species)(TablesS1andS2inAppendix1).Wedetectedseveral WVAinallclassesforallspatialandtemporalunitsconsidered, exceptformammalshot-moments(Fig.1AandB).
1.00
A
B
0.75 0.50 0.25 0.00 1.00 0.75 0.50 0.25 0.00 Birds Phi coefficient Phi coefficient 500 m 1000 m 2000 m Mammals Reptiles Amphibians Birds Reptiles AmphibiansFortnightly Monthly Biomonthly
Fig.2.Phicorrelationsbetweenconsecutiveyears,perclassandaccordingtothe spatial(A)ortemporal(B)unitusedtodetectWVA.
500 m
A
B
0.6 0.75 0.50 0.25 0.00 0.75 0.50 0.25 0.00 0.75 0.50 0.25 0.00 0.75 0.50 0.25 0.00 0.0 0.1 0.2 0.3 0.4 0.5 0.4 0.2 0.0 0.6 0.4 0.2 0.0 0.6 0.4 0.2 0.0 0.6 0.4 0.2 0.0 0.0 0.1Proportion of road with mitigation Proportion of road with mitigation
Propor
tion of casualties potentially a
v
oided
Propor
tion of casualties potentially a
v oided 0.2 1000 m 2000 m Amphibians Amphibians Reptiles Reptiles Birds Birds Mammals Mammals
Fortnightly Monthly Biomonthly
Fig.3.Cost-benefitassessmentusingtherelationbetweentheproportionofcasualtiesthatcouldpotentiallybeavoidedwiththeproportionofroad(A)oryear(B)that wouldbemitigated.Linesrepresentthegainintheproportionofcasualtiesrelativelytoincreaseinmitigation.Thestraightlinerepresentsthe1:1gain,i.e.whenincreasing themitigationin1%onewouldexpectanincreaseinavoidedcasualtiesof1%;thefollowinglinesrepresent,respectively,thegains1:2,1:3,1:4and1:5.
Regardingthespatialdataset,whenusingunitsof500mand 1000m,mostWVAwereidentifiedonlyonceineachclass(Fig.1A). However,this patternwasnotconsistentacrosstheclasses.For example,when usinga unitof 1000m, we detectedonly 4%of sectionsthat wereWVA foramphibiansinmorethanoneyear, whileforbirdsthisproportionascendedto14%.Nevertheless,we foundoveralllow correlationvalues (rPhi<0.5)between
consec-utiveyearsinWVApatternsforallclassesforthesesmallerunit lengths(Fig.2A).Conversely,when usingthelongerunitlength (2000m)thenumberofsectionsthatwereclassifiedasWVAmore thanonceincreased,e.g.9%foramphibiansand23%forbirds. Like-wise,thesimilarityinWVApatternswashigher,particularlyfor amphibiansandreptiles,withvaluesofrPhiwellabove0.5(Fig.2A
andFigureS1inAppendix1).Surprisingly,thesameWVAsections thatoccurred(km10and38forroadsplitin2000m,Fig.2A)for alltaxawerelocatedinfour-laneroads(FigureS2inAppendix1). Thecost-benefitevaluationsuggestsasimilarpatternacrossunit length,withineachclass.Forexample,5–10%mitigationoftheroad couldpotentiallyresultinanavoidanceof20–50%ofcasualtiesfor amphibians,reptilesormammals.Infact,fortheseclasses,when
usingaunitlengthof2000m,therelationoftheproportionof casu-altiespotentiallyavoided(benefit)wasgenerally4foldgreaterthan theproportionofroadmitigated(cost);whileforbirdsthebenefit was2foldgreater(Fig.3A).Hence,planningmitigationusinglarger roadsectionsisapparentlymoreeffectiveasitincorporatesmore WVAfromdifferentyears,andyetdoesnotrepresentadecreasein thecost-benefitrelation.
Regarding the temporal dataset, we found higher similarity inWVApatternsinconsecutiveyearswhenusingthethree dif-ferent timeunits,exceptformammals,which wasmore evenly distributedthroughouttheyear(Fig.1B).Highercorrelationswere detectedwhenusinglongertimeunits(bimonthly),particularlyfor amphibiansandbirds(medianrPhi>0.75)(Fig.2B).Theperiodsof
highestroadkillforamphibianswerebetweenOctoberand Novem-ber;forreptilesbetweenFebruaryandMay(andpeaksatDecember and January);andfor birdsbetweenOctober andMarch.These aggregationperiodswereconsistentlyhighlightedinthedifferent units(Fig.2BandFigureS3inAppendix1).Ingeneral,usinglonger timeunitstodetectWVAwerealsoaseffectiveasshorterunits. Forexample,applyingmitigationforabouttwoandhalfmonths
(20%ofyear) wouldpotentiallyavoidca. 50–75%ofroadkillsof amphibians.Forreptiles,theidentificationofWVAusinglonger timeunit(bimonthly)highlighted2–6monthsofhighermortality, whichisprobablyrelatedtothediversityofspeciesincludedinthis classthathavedifferentpeaksofmovementandtherefore mortal-itythroughouttheyear(e.g.turtlesandlizards).Inallcases,the relationbetweentheproportionofcasualtiespotentiallyavoided wastwofold(ormore)theproportionof yearundermitigation (Fig.3B).Therefore,theuseoflongertime-periodsispreferable asitpotentiallyincludesWVAsfromdifferentyearsandagaindoes notrepresentadecreaseinthecost-benefitrelation.
Discussion
Inthisstudyweaimedtoassesstheconsistencyof hotspots andhot-momentsovertime,i.e.,wequestionedifasignificant pro-portionofWVAoccurinthesamesites/periods,andifthepattern wasmaintainedatdifferentspatialandtemporalscales.Ourresults showedthatWVApatternsaremoreconsistentwhenusinglarger spatialandtemporalunits.Moreover,althoughintuitivelyonemay thinkthatmitigationplansshouldtargetwelldefinedandshort roadsectionsortimeperiodstoincreasethecost-benefitresources, weshowthattheproportionalcostsandbenefitsaresimilar regard-lessofthespatialandtemporalunitsusedtodetectWVA.Although moreresourcesarerequiredwhenmitigatinglongersectionsor timeperiods,thenumberofcollisionspotentiallyavoidedisalso higher.Thesepatternsarewellillustratedbythenumeroussections classifiedasWVAwhenusingsmallerspatialortimeunits,many ofwhicharenotconsistentacrosstheyears.Hence,largerunits mayguaranteemorereliableinformationonwhereandwhento allocatemitigationmeasures.Importantly,withineachWVA, mit-igationshouldfocusonbroad-scaleoftheroadsectionorperiod asroadkillsmayoccuratdifferentpointsormomentsindifferent years.
Mitigationmeasuresfocusedonsinglepointlocations(e.g., cul-verts)isunlikelytobesufficienttomaintainthelong-termviability ofpopulations(Patricketal.,2012).We suggestthatmitigation shouldfocusbroad-scalemeasuresdeployedatlongerroad sec-tionsandtimeperiods,althoughthesearemoreexpensivetobuild andmaintain(Beaudryetal.,2008;Patricketal.,2012).Few meas-urescanbeimplementedat largescales,suchas thereduction ofspeed limits(Hobday and Minstrell,2008), velocityreducers anddriftfencesconnectingtofaunalunderpasses(Ascensãoetal., 2013;Reeetal.,2015).Differentstrategiescanbeadopted,which willdependon thefinancial resourcesavailable and thetarget species.Forinstance,manysmallcrossingsundergroundcanbe implementedifturtlesarethetargetspecie(Beaudryetal.,2008). Also,ourresultshighlightedthefour-lanesectionsaspriority sec-tionstomitigate,suggestingthatthe“true”WVAisareflectance ofhightraffic,sincetheseroadssegmentsshowsthehighesttraffic volumesinourstudyarea.
ThetemporalanalyzesrevealedastrongassociationofWVAof amphibians,reptilesandbirdswiththerainyseason(Octoberto Marchinourstudyarea).Thisperiodcorrespondstotheoccurrence ofmigratoryeventsand/orbreedingseasonformanyspecieshere recorded(Sick,2001;Coelhoetal.,2012).Previousworkshavealso reportedincreasedmortalityratesduringwarmandwetseasons, whiledryorcoldseasonsgenerallypresentlowervalues(Coelho etal.,2012;Langenetal.,2007;Morelleetal.,2013).Identifying hot-momentsofWVCusinglargertemporalperiodsmayprovide importantinformationtoimplementshort-timemitigation meas-uressuchastemporaryroadclosureorspeedreduction(Hobday andMinstrell,2008;Sullivanetal.,2004).Thelackofaggregation periodsformammalsmaystemfromthefactthatthedatasetwas composedmostlybyhighlymobileandgeneralistspecies.These
traitsleadtoamoreuniformdistributionofroadkills,which mini-mizesthechancesofoccurringWVA.
Itshouldbenotedthatbothspatialandtemporalvariationof roadkillsmayberelatedtodifferencesinvehicletrafficduringthe yearorfluctuationsinpopulationabundance(Coelhoetal.,2012; Smith-PattenandPatten,2008).Unfortunately,toourknowledge, suchdatadoesnotexistforourstudyarea.Also,weworkedatthe taxonomiclevelofClass,therebyprecludingmorespecific analy-ses.Byanalyzingatthespecieslevel,suchpatternscouldprobably bemorestableovertime.However,thiswouldrequirealarge vol-umeofroadkilldataforsinglespecies,whichisratherunfeasible andnotpossibleforourdataset.Finally,wechosenottoanalyze scalesgreaterthan2000m,asthecostsofimplementingmitigation measureswouldbecomeexcessive.
Conflictsofinterest
Theauthorsdeclarenoconflictsofinterest.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.pecon.2017.03.003.
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