Fine-scale movement of the European hedgehog: an ......SHANAHAN ET AL.: SPOOL-AND-THREAD TRACKING OF...
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New Zealand Journal of Ecology (2007) 31(2): 160-168 ©New Zealand Ecological Society Available on-line at: http://www.newzealandecology.org/nzje/ Fine-scale movement of the European hedgehog: an application of spool-and-thread tracking Danielle F. Shanahan 1 *, Renaud Mathieu 3, 4 and Philip J. Seddon 2 1 School of Integrative Biology, University of Queensland, St Lucia 4072, Australia 2 Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand 3 Spatial Ecology Research Facility, School of Surveying, University of Otago, PO Box 56, Dunedin, New Zealand 4 CSIR – NRE, Earth Observation Research Group, Building 33, PO Box 395, Pretoria 0001, South Africa *Author for correspondence (Email: [email protected]) Published on-line: 3 December 2007 ___________________________________________________________________________________________________________________________________ Abstract: The European hedgehog is a significant predator species of rare and endangered ground-nesting birds in the riverbeds of the Waitaki Basin, South Island, New Zealand. Studies focusing on the movements and habits of this species have generally been limited to broad-scale radio-tracking studies or incidental trap-catch data. Within our study, we aimed to investigate the finer scale movement patterns of the hedgehog in relation to vegetation structure by using spool-and-thread tracking. We captured 30 hedgehogs (15 female, 15 male) within the study area, and spool-and-thread-tracked the movements of each over a single night. Only two of the 30 animals moved onto the gravel areas of the riverbeds where birds nest – hedgehogs may therefore not target birds’ nests as a primary prey source, but rather as a secondary prey item. The movement paths were all non-random, and males demonstrated particular linearity in their tracks. This straighter and more directed movement may be due to more intensive mate search at this time of the year. We also assessed habitat use using a very high resolution habitat map (derived from Ikonos 4-m-resolution satellite image). Dense grassland was the most selected habitat type, perhaps because insect prey are at a higher density in this vegetation type. Hedgehogs (particularly males) also used boundaries of all habitat types significantly more than the centre of habitat patches. We found the spool-and-thread tracking technique does have limitations: (1) it could be inappropriate for animals exhibiting a significant escape response; (2) the data do not include a temporal dimension. However, these problems were not considered relevant for this study. Fine-scale studies such as this can provide increased power when investigating the ecology of species at a scale relevant to trap placement. ___________________________________________________________________________________________________________________________________ Keywords: animal movement; Erinaceus europaeus occidentalis; Ikonos; introduced predators; Waitaki Basin Introduction Introduced mammalian predators have been implicated in the decline or extinction of many of New Zealand’s endemic species (King 1984; McLennan et al. 1996; Murphy et al. 2004). The European hedgehog (Erinaceus europaeus occidentalis) preys upon many ground-nesting bird species. One such species is the critically endangered black stilt or kaki (Himanotopus novaezelandiae), which nests on the braided riverbeds of the Waitaki Basin, South Island, New Zealand (BirdLife International 2003). Despite extensive control operations targeting introduced mammals in the Waitaki Basin, hedgehogs continue to have an adverse effect on the nesting success of ground- nesting river birds (Keedwell et al. 2002). Feral cats (Felis catus), hedgehogs and ferrets (Mustela furo) have been held responsible for 43, 20 and 18%, respectively, of nest losses of kaki and other birds in the riverbeds (Sanders & Maloney 2002). Further information on the fine-scale movement behaviour of species such as the hedgehog may enable us to improve our understanding of the ecology of the species with reference to the risk to ground-nesting birds. This information has been identified as essential for determining optimum placement of traps and poison bait stations; but such knowledge may also support alternative management strategies (such as habitat manipulations) that may discourage predators from important conservation areas (Norbury et al. 1998; Ragg & Moller 2000). Few studies have investigated movement behaviour of hedgehogs in relation to landscape and vegetation structure and composition; those that have, have generally used broad-scale techniques such as radio tracking (e.g. Morris 1986; Baker 1989; Moss 1999), or incidental trap- catch data analysed post hoc (Cameron et al. 2005). Such studies have provided information that can aid predator management at a broad scale (e.g. Berry 1999; Ragg &
Transcript of Fine-scale movement of the European hedgehog: an ......SHANAHAN ET AL.: SPOOL-AND-THREAD TRACKING OF...
160 NEWZEALANDJOURNALOFECOLOGY,VOL.31,NO.2,2007
New Zealand Journal of Ecology(2007)31(2): 160-168©NewZealandEcologicalSociety
Availableon-lineat:http://www.newzealandecology.org/nzje/
Fine-scalemovementoftheEuropeanhedgehog:anapplicationofspool-and-threadtracking
DanielleF.Shanahan1*,RenaudMathieu3,4andPhilipJ.Seddon21SchoolofIntegrativeBiology,UniversityofQueensland,StLucia4072,Australia2DepartmentofZoology,UniversityofOtago,POBox56,Dunedin,NewZealand3SpatialEcologyResearchFacility,SchoolofSurveying,UniversityofOtago,POBox56,Dunedin,NewZealand4CSIR–NRE,EarthObservationResearchGroup,Building33,POBox395,Pretoria0001,SouthAfrica*Authorforcorrespondence(Email:[email protected])
Publishedon-line:3December2007
___________________________________________________________________________________________________________________________________
Abstract: The European hedgehog is a significant predator species of rare and endangered ground-nesting birds in theriverbedsoftheWaitakiBasin,SouthIsland,NewZealand.Studiesfocusingonthemovementsandhabitsofthisspecieshavegenerallybeenlimitedtobroad-scaleradio-trackingstudiesorincidentaltrap-catchdata.Withinourstudy, we aimed to investigate the finer scale movement patterns of the hedgehog in relation to vegetation structure byusingspool-and-threadtracking.Wecaptured30hedgehogs(15female,15male)withinthestudyarea,andspool-and-thread-trackedthemovementsofeachoverasinglenight.Onlytwoofthe30animalsmovedontothegravelareasoftheriverbedswherebirdsnest–hedgehogsmaythereforenottargetbirds’nestsasaprimarypreysource,butratherasasecondarypreyitem.Themovementpathswereallnon-random,andmalesdemonstratedparticularlinearityintheirtracks.Thisstraighterandmoredirectedmovementmaybeduetomoreintensivematesearchatthistimeoftheyear.Wealsoassessedhabitatuseusingaveryhighresolutionhabitatmap(derivedfromIkonos4-m-resolutionsatelliteimage).Densegrasslandwasthemostselectedhabitattype,perhapsbecauseinsectpreyareatahigherdensityinthisvegetationtype.Hedgehogs(particularlymales)alsousedboundariesofallhabitattypes significantly more than the centre of habitat patches. We found the spool-and-thread tracking technique does have limitations: (1) it could be inappropriate for animals exhibiting a significant escape response; (2) the data do notincludeatemporaldimension.However,theseproblemswerenotconsideredrelevantforthisstudy.Fine-scalestudiessuchasthiscanprovideincreasedpowerwheninvestigatingtheecologyofspeciesatascalerelevanttotrapplacement.___________________________________________________________________________________________________________________________________
Keywords: animal movement; Erinaceus europaeus occidentalis; Ikonos; introduced predators; Waitaki Basin
IntroductionIntroducedmammalianpredatorshavebeenimplicatedinthedeclineorextinctionofmanyofNewZealand’sendemic species (King 1984; McLennan et al. 1996; Murphyetal.2004).TheEuropeanhedgehog(Erinaceus europaeus occidentalis) preysuponmanyground-nestingbirdspecies.Onesuchspeciesisthecriticallyendangeredblackstiltorkaki(Himanotopus novaezelandiae),whichnestsonthebraidedriverbedsoftheWaitakiBasin,SouthIsland, New Zealand (BirdLife International 2003).DespiteextensivecontroloperationstargetingintroducedmammalsintheWaitakiBasin,hedgehogscontinuetohaveanadverseeffectonthenestingsuccessofground-nestingriverbirds(Keedwelletal.2002).Feralcats(Felis catus),hedgehogsandferrets(Mustela furo)havebeenheldresponsiblefor43,20and18%,respectively,ofnestlossesofkakiandotherbirdsintheriverbeds(Sanders
& Maloney 2002). Further information on the fine-scale movementbehaviourofspeciessuchasthehedgehogmayenableustoimproveourunderstandingoftheecologyofthespecieswithreferencetotherisktoground-nestingbirds. This information has been identified as essential for determiningoptimumplacementoftrapsandpoisonbaitstations; but such knowledge may also support alternative managementstrategies(suchashabitatmanipulations)thatmaydiscouragepredatorsfromimportantconservationareas (Norbury et al. 1998; Ragg & Moller 2000).
Fewstudieshaveinvestigatedmovementbehaviourof hedgehogs in relation to landscape and vegetationstructure and composition; those that have, have generally usedbroad-scaletechniquessuchasradiotracking(e.g.Morris 1986; Baker 1989; Moss 1999), or incidental trap-catchdataanalysedpost hoc(Cameronetal.2005).Suchstudieshaveprovidedinformationthatcanaidpredatormanagement at a broad scale (e.g. Berry 1999; Ragg &
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Moller2000).However,thereissomeindicationfromthese broad-scale studies that fine-scale habitat structure may play an important role in determining the areasindividualswillmovethrough.Forexample,Doncasteret al. (2001) used radio-telemetry to demonstrate thatdisplaced hedgehogs showed an attraction to habitatedges,whichwerebelievedtoactascorridorstobetterlocations.Also,inanincidental-trap-catchstudyCameronet al. (2005) found that hedgehogs were most oftencaughtintrapswithin2.5moflowvegetativecover,andalongpredatorpathways.Thesestudiesraisequestionsas to whether these structures are used continuouslyduringforagingtrips,orwhetherthereareotherareasorlandscapestructuresthattrappingeffortsshouldfocusonmore to improve trap efficacy. This is the basis for the currentresearch.
Refining the scale at which animal movement studies arecarriedoutpresentstechnicalandlogisticalproblems.Highlyaccurateanimalmovementdataincombinationwithextremelyhigh resolutionhabitat informationarerequired.Options forgathering theappropriateanimallocationdataincludeGlobalPositioningSystem(GPS)collars,asusedformedium-tolarge-sizedmammalssuchas ungulates and wolves (Merrill 2000; Johnson et al. 2002).CommerciallyavailableGPSdevicesare,however,not yet light-weight enough for use on many of NewZealand’sintroducedpredators,andareoftentoocostlyforwildliferesearchprojects.Spool-and-threadtrackinghasbeendevelopedasaninexpensiveyeteffectivemethodoftracking fine-scale movements of small mammals (Miles et al. 1981; Woolley 1989). In an environment such as theWaitakiBasin,thethreadprogressivelyunravelsandsnags on boulders or vegetation as the animal moves,recording every turning point. In combination withGPS technology, spool-and-thread tracking can yieldaccuratemovementdatainaformcompatiblewithGISsoftware.High-resolutionsatelliteimagery(upto1m)isnowavailablecommercially(e.g.Ikonos,Quickbird),andthisprovidesauser-friendlyoptionfordevelopingprecisehabitatmaps.
Thisstudyaimstobegintodevelopandtestpossibleresearch techniques for investigating the fine-scale habitat use of small mammals in New Zealand braided riversystems,whilebuildingonknowledgeofhedgehoghabitatuse.Weemployspool-and-threadtrackingtoobtainhigh-accuracymovementdata,andcombinethesewithhabitatmapsgeneratedfromhigh-resolutionsatelliteimagery.Indoingsowealsoaimtoaddressthreemainquestionsinrelationtohedgehogmovementpatterns:(1) Is a very fine-scale movement study meaningful? i.e. at a very fine scale, do hedgehogs move in a random fashion or are there obvious patterns in this movement?(2)Whichhabitatorvegetationtypesappeartobeimportantfor foraging hedgehogs? How often do hedgehogs frequent the gravel/rocky areas where river birds nest? (3) Do hedgehogs exhibit expected use of landscape
structure,includingvegetationedgesandlinearvegetationstrips?
MethodsStudy areaThestudywascarriedoutintheOhauandTekapobraidedriverbeds, Waitaki Basin, South Island, New Zealand(44°16–20´ S; 170°5–13´ E; Fig. 1). The river areas used inthisstudycompriselongstretchesofrockyandgravelareas interspersed with low-flow waterways. The water flow is heavily controlled by hydro dams located at the head of each river, and hence flooding events are rare. Both rivershaveheavyinvasive-weedinfestations,particularlysweetbriar(Rosa rubiginosa)andwillow(Salixspp.).Ninespeciesofnativebirdusetheareaasbreedinghabitat(Byrom2002), severalofwhich requireopenareasofgravelsorrockstonestsinglyorincolonies.
Habitat mapThehabitatmapusedinthisstudywasderivedfromaclassified Ikonos satellite image, with 4-m resolution. The final habitat map was composed of six vegetation classes, consideredtorepresentthemajorstructuralelementsofthevegetationinthebraidedriverbeds.Thisnumberofclasses also provided a very high level of classification accuracy.TheIkonosimagewascapturedon1December2002. The vegetation structure was unlikely to havechanged significantly in the period between the image capture and the tracking element of this study (springand summer 2003), as no flood events were experienced intheriverbedsintheyearsbetweenand,asmentioned,water flow is heavily controlled. The final classes of the
Figure 1.Locationofthestudysite–WaitakiBasin,SouthCanterbury,SouthIsland,NewZealand.
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habitat map were: water; rocks and gravels; low-density vegetation on bare soil; dense grassland; medium- to high-density shrubs; high dense willow trees.
Animal trapping and trackingThirty hedgehogs (15 males, 15 females) were live-trappedusing20Holden live-capturebox traps (MikeHolden; www.traps.co.nz) and 50 Grieves cage traps (GrievesWroughtIronandWirework,302TravisRoad,Christchurch, New Zealand). A total of 1500 ha wastrapped between September 2003 and January 2004(spring/summer).Blocksofapproximately300haweretrappedfor10-dayperiods,withtrapsspaced150–250mapart.Trapswerebaitedwithfreshrabbit(Oryctolagus cuniculus),andcheckedwithin3hofsunrise.Trappedhedgehogswereprovidedwithdrygrassandwater,andleftinsituuntildusk.Non-targetspecieswereimmediatelyreleased.
Spool-and-thread tracking devices were attached,andtheanimalswerereleasedatdusk.Aspool-and-threadbackpack (weight 12.4 g, average 1.9% of hedgehogbodyweights)wasgluedtothespinesontherearofeachhedgehog(Fig.2).Backpacksweremadebyplacingtwospoolssidebysideandenclosingthemwithinheat-shrinkplastic. Each spool (manufactured by Danfield (UK), distributedinNewZealandbyTexspecNZ,Auckland)containedapproximately450mofthread.Itisunknownwhatproportionoftheanimal’sentirenightlymovementthis length encompasses, but no hedgehogs were everfound still attached to the string. The total availablethreadlengthwasapproximately900m,whichisclosetoestimatesofhome-rangelengthforhedgehogsintheregion(Moss1999).Theendofthethreadwastiedofftoa fixed object and the animal was released at dusk. The resultingtrailwasfollowedandmappedthenextmorningusing a GeoExplorer 3 GPS unit (Trimble). Locationfixes were recorded every five seconds. Tracks were then downloadedanddifferentiallycorrectedwithinTrimble
Figure 2. European hedgehog with a spool-and-threadbackpackattached.
GPS Pathfinder Office 2.90, using the base station from theSchoolofSurveying,UniversityofOtago,Dunedin,NewZealand(190kmfromthestudysite).Themaximumhorizontalerrorofthetrackswas3m.
Analysis of animal movement pathsThreeaspectsofhedgehogmovementpatternandhabitatselectionwereinvestigatedtoaddressthethreequestionsposed in the introduction: (1) test for random movement; (2) preferred habitat types; and (3) use of habitat edges.
Test for random movementThis analysis was carried out to determine whetherhedgehogsmove through their landscapeat randomatthis fine scale, or whether the animals make directional choices due to other, potentially measurable fine-scale factors(e.g.habitatstructure).Thelinearityofalltrackswas compared with that of random walk tracks. Onethousand random tracks were generated for each realtrackusingAnimalMovementAnalysis1.1(Hooge&Eichenlaub1997),anextensiontoArcView3.2®(ESRI):eachdistancetravelledbetweenturningpointswasequaltothatobservedinthetrueanimaltracks,andrandomturninganglesweresubstitutedfortheactualanglestaken.Thisprocessprovideddatathatenabledustocomparerandommovementpatternsdirectlywithrealpatterns.Thestatisticscomparedbetweenrealandrandomtrackswere(a)R2:theaveragestraight-linedistanceeachconsecutiveturningpointmovedawayfromthegeometriccentre(startpointof the track); and (b) LI: the total linear distance between thestartandendpointsofananimal’spath,dividedbythe total length of the path (linear paths yield LI = 1; and LI<1isnon-linear).
Foreachindividual,observedR2andLIvalueswerecomparedwiththerangeofvaluesgivenbytherandomtracks.Observedvaluesthatfellintherangeofthetop5%ofvaluesfortherandomwalktracksindicatedthemovement path was significantly more linear than would be expected if turning angles were made at random.Alternately,iftheR2andLIofanobservedanimaltrackwasinthelower5%oftherangeofvaluesfortherandomwalk tracks, then the observed track was consideredsignificantly more constrained than would be expected from a random track (i.e. it exhibits higher site fidelity) (Hooge&Eichenlaub1997).
Preferred habitat typesCompositionalanalysis(Aebischeretal.1993)wasappliedtothedatatodetermineifthehedgehogspreferredmovingwithin certain habitat types. This statistical techniquecomparestheproportionofeachhabitatavailabletoananimalwiththeproportionactuallyused.Inthisstudythehabitat‘used’wasconsideredthatwithinabufferof15meithersideofthetrack.AsrecommendedbyEricksonetal.(2001),thisbuffersizeaccountsforalltheeffortassociated
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with the habitat map and animal tracks.This includesmaximumerrorinthegeo-referencingoftheIkonosimage(± 6 m); horizontal precision of the animal tracks recorded ontheGPS(± 3 m after differential correction); and a pixelsizeof4m(satelliteimageresolution).Thehabitat‘available’toananimalwasconsideredthecircularareaaroundthestartpoint,witharadiusofthestart-to-endpointlengthoftherealanimaltrack(i.e.thetotalareathatthespool-and-threadtrackcouldhaverecorded).Rawdataonthetotalareaofhabitattypesboth‘used’and‘available’toeachanimalwereconvertedintoapercentageoftheareacovered.Compositionalanalysisrequiresthateveryhabitat isused, sozerovaluesweresubstitutedwithasmallpositivevalue,smallerthanthesmallestproportionof used habitats (Aebischer et al. 1993); this substitute valuewas0.001.
Use of habitat edgesThis section of the analysis aims to provide quantifiable evidence for fine-scale preferential boundary use as described by broad-scale habitat-use studies (Baker 1989; Pascoe 1995; Ragg & Moller 2000; Doncaster et al. 2001). A landscape index (edge-density index; edge length (km) per hectare; Haines-Young & Chopping 1996) was used to quantifytheamountofeachhabitatboundarywithinthe15-mbufferzoneofeachanimaltrack.ThismeasurementwasgeneratedwithinPatchAnalyst1.0,anextensiontotheArcView®GISsystem(Elkieetal.1999).Anedge-densityvaluewasgeneratedforeachhabitattypeinthebufferzoneofeachobservedanimaltrack,andtocreatepaireddata,theindexwasalsogeneratedinthesamewayfor10randomwalktracks(i.e.10foreveryrealtrack).Tentrackswereusedbecausetheasymptoteofthemeanwasreachedbythispointinmultiplesimulationsofedge-densitymeasuresforeachrealtrack,indicatingthatthisnumberprovidedacceptablerepresentationofedgeuseexpectedfromrandommovement.
The mean index values for the real tracks werecomparedwiththemeanvaluesfromtherandomtracks,usingpairedt-tests,onet-testforeachhabitattype.Thoughrepeatedt-testscanincreasetheTypeIerror,otheranalyseswereconsideredinappropriateforthistypeofdata.Asthelengthoftheedgeofonehabitattypeisnotindependentofthelengthofotherhabitattypes(i.e.edgesbetweentwoside-by-sidehabitattypesaremeasuretwice:onceforeachhabitattype),themeasurementscannotbeconsideredindependent. We attempted to carry out a RepeatedMeasures anova to deal with this non-independence; however,asthedegreesoffreedomwithinthistestarereduced, the sample sizes in this studywerenot largeenough.Thet-testswerethusappropriatefordetectingoveralldifferencesbetweentherealandtherandomdataforeachhabitattype,anddifferencesbetweenthehabitattypeswasnotinvestigated.Summarystatisticsdetailingthedistanceofeachturningpointwithinthetracksfromthenearesthabitatedgewerealsogenerated.
ResultsGeneral observationsHedgehogs often did not follow existing animal runs; rathertrackswouldoftenamblethroughdensegrass(Fig.3). Short-term nests (where the animal had obviouslyremainedforaportionofthenight–stringwasballedup and a flat hollow had been made in the vegetation) werecommonlyfoundwithindenselonggrass,andmorepermanentnests(twicefoundwithyoung)weregenerallylocatedinlongergrassrightatthebaseofsweetbriarorothershrubs.
Test for random movementTheR2valuesfor12females(80%offemales)and13males (87.5% males) were significantly higher than for the corresponding random tracks.The LI values for 7females (46.7%females)and14males (93.7%males)were significantly higher than the corresponding random values.Therefore, there is evidence that generally themovementsofbothmaleandfemalehedgehogsaremoredispersed(non-random)thanwouldbeexpectedhadtheybeenmovingrandomly.
Preferred habitat typesOnlytwomalehedgehogswererecordedasmovingoutontotherocky/gravellyareaswhereriverbirdsnest.Thewaterwasneverenteredbyanimals,andisonlyrecordedinthehabitatrankingsbelowasitappearsasahabitattype in the buffer zone when animals walk alongsideriversorlakes.
Male hedgehogs showed non-random habitatselection(WilksΛ =0.2353,χ2
5=21.70,P<0.001).Arankingmatrixorderedthehabitattypesinthesequence:densegrassland>medium-tohigh-densityshrubs>highdensewillowtrees>low-tomedium-densityplantsonbare soil >>> rocks/gravels >>> water (Table 1; >>> denotes a significant difference between two consecutively rankedhabitattypes).Rocks/gravelsandwaterwereusedsignificantly less than all other habitat types.
Forfemalehedgehogs,comparisonofhabitat-usewithhabitatavailabilitygaveWilksΛ =0.5022(χ2
5=10.33,P=0.066).Arankingmatrixorderedthehabitattypesinthesequence:densegrassland>medium-tohigh-densityshrubs>highdensewillowtrees>low-tomedium-densityplantsonbaresoil>water>rocks/gravels(Table1).
Use of habitat edgesThemeanedge-densityvaluesformalehedgehogsweresignificantly higher (t-tests, P < 0.001; Fig. 4b) than the values for the random tracks for five habitat types. All habitat types produced a P-value at this level ofsignificance. The habitat edges used more than randomly were:rocks/gravels,highdensewillowtrees,medium-tohigh-density shrubs, dense grassland, and low-density
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Figure 3. Examplesof female and malehedgehog spool-and-threadtracksmappedwith the six-classhabitat map derivedf rom an Ikonossatellite image of4-mresolution.
Table 1. Resultsfromthecompositionalanalysisofhedgehogspool-and-threadtracksfromSeptember2003toJanuary2004.Habitat types are ranked in order of relative preference (1, highest; 6, lowest) with significant differences between consecutively rankedhabitattypesmarked*(P<0.05,fromrandomisationtests).___________________________________________________________________________________________________________________________________
Rankingofhabitattypesforeachgroup
Femalehedgehogs Malehedgehogs
Habitattype n=15 n=15___________________________________________________________________________________________________________________________________
Water 5 6*Rocks/gravels 6 5*Low-densityvegetationonbaresoil 4 4Densegrassland 1 1Medium-tohigh-densityshrubs 2 2Highdensewillowtrees 3 3___________________________________________________________________________________________________________________________________
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plantsonbaresoil.Thisindicatesthatmalehedgehogsusehabitatedges,orecotonalboundaries,morethanexpectedhadtheirmovementsbeenrandom.
Themeanedge-densityvaluesforfemalehedgehogswere significantly higher (t-tests,P < 0.01; Fig. 4a) than thevaluesfortherandomtracksforthreehabitattypes.Thesehabitatedgeswere:highdensewillowtrees,low-tomedium-densityplantsonbaresoil,androcks/gravels.Thisindicatesthatfemalehedgehogsusetheboundariesofonlysomehabitattypesformovement.
DiscussionSpool-and-thread trackingThis study successfully used fine-scale movement data from spool-and-thread tracking and high-resolutionsatellite imagery toquantifyhabitatand landscapeuseby hedgehogs. This methodology provided quantifiable evidenceforhedgehogmovementpatternsthatpreviouslycould only be inferred from broad-scale movement
Figure 4.Bargraphsofedgeuseofdifferenthabitattypes(W=water,R=rocks/gravels,T=highdensewillowtrees,S=medium-tohigh-densityshrubs,G=densegrassland,B=low-densityvegetationonbaresoil)for(a)femalehedgehog(darkgrey)andcorrespondingrandomwalktracks(white),and(b)malehedgehog(darkgrey)andcorrespondingrandomwalktracks(white).Edgeuseisrepresentedbyedge-densityvaluesofeachhabitattypeina15-mbufferzonearoundeachtrack.n= 15 for all groups. * denotes a significant difference between themeansatP<0.05.Errorbarsarestandarderrors.
studies.Theseresultsmustbeinterpretedalongsidethelimitationsofthesetechniques,andinthissensethisstudyalsoprovidesessentiallessonsontheinterpretationandmethodology of future fine-scale movement studies.
Theinterpretationofthemovementdataispotentiallyconfoundedbyescapebehaviourexhibitedbytheanimal.Afterhandling,thehedgehogsusedinthisstudygenerallyburrowedintonearbygrassyareasandremainedthereatleastuntiltheresearcherleft.Tracksdidnottendtoexhibitlonglinearmovementsaswewouldexpecttoobserveifanimalswereattemptingtoescape,thusweconsideredtheeffectoftheescaperesponsetobenegligible.Thisstudyalsooriginallyattemptedtousethespool-and-threadtechniqueonferrets(Mustela furo),amoremobilespecies.Whenreleasedtheseanimalstendedtorundirectlyintotheundergrowthawayfromtheresearcher.Longstraightmovements were initially observed in the tracks. Asthe length of the escape behaviour was very difficult to quantify,thetrackswereconsiderednon-representativeofnormalferretmovement.Thesewerethereforediscardedfromthestudy.Thisexperiencedoesprovideavaluablelessoninunderstandingthelimitationofthespool-and-threadtechnique.Thesuitabilityofthetechniqueforthespeciesunderstudyshouldbeconsideredcarefully.
The lack of a temporal dimension to the animalmovementpathsinthisstudycouldalsoplacelimitationson the interpretation of this data, though the powerand fine-scale information gained can be considered a beneficial compromise. The spool-and-thread tracks give noindicationastotheamountoftimeananimalspentineachhabitatpatch,sotheimplicitassumptioninthisstudy is thatpresence in ahabitat type is an adequatemeasureofrelativeintensityofuseforactivepurposes(suchas foragingormate search).Recordingonly themovementpathofananimalmayhoweveravoidamajorbiasthatcanexistinconventionalmethodswhererestinghabitattypesmaybecomeover-represented.Thismaybeofparticularproblemwhenusing, for example, radio-trackinginahedgehogstudy.Thecaptureresponseofahedgehogistoremaininonepositionuntilthethreathasbeenremoved,andtheanimalsarenocturnal,commonlyrestinginoneplaceforthelengthofaday.Radio-trackingstudies commonly obtain only a single-position fix over thecourseofa24-hourperiod,potentiallyonlyobtaininglocationdatafornestingorrestinghabitattypes.
Thespooland thread techniquedidprovideveryfine-scale data for this study, and was inexpensive and accurate. There are currently no modern trackingtechniques that can provide the researcher with moreaccurate information. The spool-and-thread techniquealsohasthepotentialtoallowtheresearchertoanswerfurther specific questions on how the animal physically movesthroughthevegetation,andwhattrackingmarks(e.g.characteristicburrowing throughvegetation)mayidentifythepresenceoftheanimalinanarea.
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Hedgehog movement patternsOurstudyprovidesevidencethatinvestigatingmovementatthescaleofthisstudyisappropriate,aspatternsandnon-randommovementwereobservedacrossthelengthofthetracks.Thisanalysisalsorevealedotherinterestinginformationrelevanttohedgehogecology.Malehedgehogtracks tended to be more linear than female tracks.Previous observational (non-quantitative) studies onhedgehogmovementpatternshavealsoshownadifferencebetweenthesexes.Wroot(1984),foundthatover70%offemalehedgehogmovementswereshort,withfewrapidmovements.Howevermalestendedtomoveinamorerapidandlinearstylethanfemales,allowingthemtocovermoreground.ThehedgehogsinthisstudyweretrackedbetweenSeptemberandJanuary,muchofwhichisthehedgehog-breedingseasoninNewZealand(Novemberto January in the Waitaki Basin; Moss & Sanders 2001). Thismayexplainthedifferencesinmovementpatternsbetweenthesexes.Campbell(1973)suggestedthatduringthebreedingseasonmalehedgehogsarelikelytosearchactivelyformates,andarethereforemorewide-rangingthanfemales.Femalehedgehogsalsooftenhavelittersinanestatthistimeofyear(twofemalesinthisstudyweretrackedbacktonestscontaininglitters),andmaybelesslikelytotravellongdistancesawayfromtheiroffspring.Thiscould reduce themeansquareddisplacementandlinearitymeasurements,resultinginvaluesclosertothoseoftherandomtracks.Thistendencytostaywithinamoreconfined area than males is supported by the smaller home-rangesizeoffemalehedgehogsobservedinmanystudies(Moss&Sanders2001).
The hedgehogs in this study tended to remainpreferentiallyindensegrassland.Cameronetal.(2005)also found that hedgehogs were best trapped in lowvegetation rather than bare substrate, supporting ourfinding. The most obvious potential explanation for the preference for grassland is a possible higher relativeabundance of insect prey species in this habitat type.PreviousstudiesinEuropehavefoundstronglinksbetweenhedgehog distribution and the density of invertebrateprey species (Micol et al. 1994; Cassini & Föger 1995). Further studies on invertebrate distribution within thestudyareawouldberequiredtotestthis.Thepresenceofsuitablenestsiteshasalsobeenlinkedwithhedgehogdistribution(Parkes1975),andMoss(1999)foundthatshrubsandtussockorgrasslandwasthepreferrednestinghabitat.Theselectionofdensegrasslandbyhedgehogsinourstudythereforesupportsthis.
Finally,thisstudyprovidedsupportforthehypothesisthat fine-scale landscape structure is important tohedgehogs. In this case habitat boundaries were usedpreferentiallyovermorecentralareas,thoughthispatternwasmuchstrongerinmalesthaninfemales.Wesuggestthat the use of edge habitat is linked with directionalmovementorsearchbehaviouratahigherlevel(e.g.inthesearchformatesratherthanforinsectprey)inwhichanimals may be required to move greater distances at
greaterspeeds.Theoverallhigherlinearityofthemalehedgehogtrackssupportsthis.Doncasteretal.(2001)andMorris(1986)alsodocumentedhedgehoguseofhabitatboundaries, suggesting that the edgesof densehabitattypesallowlessencumberedmovement.
Interestingly,inthisstudyonlytwomalehedgehogswererecordedtomoveoutintotherockyorgravelareaswhere ground-nesting river birds have been known tonest. As the proportion of nests taken by hedgehogsis high (20%; Sanders & Maloney 2002), our result is unexpected.Thiswouldsuggesthedgehogsrarelyseekbirds’eggsasprey.Astudyofthedietofhedgehogsintheareaalsodemonstratedthatonly4%ofthegutsstudiedcontainedtheremainsofbirds’eggs(Jonesetal.2005).ThisresultandthelowfrequencyofanimalstravellingontothegravelriverbedssupportthehypothesisproposedbyJonesetal.(2005)thatalargehedgehogpopulationcouldbesupportedprimarilybyinvertebratebiomass,andsmallpopulationsofnativebirdsandlizardsmaysufferasasecondarypreysource.
ConclusionThecombinationoftheveryhighresolutionhabitat-mapandtheaccuratespool-and-threadtrackingdataenabledquantification of fine-scale habitat-use by hedgehogs. This research generally supported habitat-use findings fromotherradio-trackingandincidental-trap-catchdata.This,initself,addstothebodyofinformationonhabitatpreferences of the European hedgehog that could beusedtoenhancetrappingprocedures.Wedemonstratedthat it is worth following hedgehogs at this fine scale, andthismayalsobeimportantforotherspecies.Radio-trackingdatacanleavetheresearcherwithanumberof‘unknowns’resultingfrominaccuraciesanderrorswithinthetechnique.Spool-and-threadtrackinghasenabledustofollowmomentbymomenteachdecisiontheanimalhasmadewithregardstoitshabitatandenvironment.Thispoweraloneallowsustosupportobservationsmadeusingothertechniques,butalsoenhancesourunderstandingoftheecologyandbehaviourofthespeciesatamuchmoreintimatescale.
AcknowledgementsWethanktheDepartmentofConservation,Twizel,forlogisticalsupportandLandcareResearch,Alexandra,fortheloanofequipment.ChrisJonesprovidedadviceondrafts and fieldwork, and numerous field assistants made the field season possible. Des Smith provided statistical adviceandreadmanuscriptdrafts,andEsbenKristensonandClairCameronalsoprovidedstatisticaladvice.IanJamiesonandGrantNorburycommentedonearlyversionsofthemanuscript.ThisprojectwasfundedbyanOtagoUniversityResearchGranttoPJS.
167SHANAHANETAL.:SPOOL-AND-THREADTRACKINGOFHEDGEHOGS
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