CEC Research | https://doi.org/10.21973/N3BW8D Fall 2017 1/7
Mechanismsofnichepartitioningbetween
desertrodentsintheEastMojave
CieraCastillo1,KatherineHernandez2,IsabellePanza1,AlexaRowland1
1UniversityofCalifornia,SantaCruz,2UniversityofCalifornia,SanDiego,
ABSTRACT
Understandinghowspeciescancoexistenceinlightofcompetitionhaslongbeena focus of ecological study. One of the many mechanisms that have beenproposedtoexplainhowspecieswithoverlappingnichescancoexististhattheydiffer intheirbehavioralpatterns. Inourstudywe investigatedhowrodents inthegeneraDipodomysandPeromyscuspartitiontheirhabitatspatiallyandhowtheir behaviors differ. To test for behavioral differences and spatial habitatpartitioning in these species we monitored animals using traps and wildlifecameras and mapped their habitat use in space. We found that Peromyscusdisplayed risk averse strategies in their foraging and habitat use whereasDipodomysdisplayedmorevariability intheirmovementsandweremorelikelyto take risks. These interspecific patterns help us to better understandcommunity ecology as a whole, and can have broad implications for thestructureofcommunitiesinsensitivedeserthabitats.
INTRODUCTION
Co-occurring species can avoid costlycompetitive interactions via spatial,temporal, or resource-associated nichepartitioning (Schoener 1974; Wiens1977), and which can further lead tostablespeciescoexistence(AlbrechtandGotelli 2001).The mechanisms allowingfor coexistence of species, andparticularly the number of speciescoexisting inagivenhabitat,havebeenexploredfrommanyviewpoints(Hafner1977). Horn and MacArther (1972)
found that spatial partitioning canenablecoexistenceonasingleresource.Schroder and Rosenzweig (1974)trapping records suggest that onespeciesofDesertKangarooratprefersagrassier habitat, and D. merriami ahabitat dominated by creosote bush.Theirfindingssuggestthatthesespeciesavoid competition through habitatselection which is one mechanism ofspeciescoexistence.Desert rodents have been
fundamentalinexploringtheunderlyingmechanics of community ecology given
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thecoexistenceofadiverseassemblageof granivorous rodent species within arelatively simple desert ecosystem(Kotler and Brown 1988). The MojaveDesert houses diverse assemblages ofgranivorous rodents, including severalspeciesofDipodomys,Peromyscus,etc.,andwhosecommunitiesvarywithmicroand macro-habitat heterogeneity(Stevens and Tello 2009). Coexistenceamongthesegranivorousrodentsoccurvia a combination of mechanisms,including spatial and temporal nichepartitioning (Kotler and Brown 1988).Dipodomys and Peromyscus are twomajor genera of nocturnal rodents intheMojavewhosediets relyonheavilyon seeds and whose ranges oftenoverlap (Kotler and Brown 1988;Stevens and Tello 2009). The use ofshared habitat, resources, and similarforaging behavior between Dipodomysand Peromyscus presents a uniquesysteminwhichtoexplorethedifferentmechanisms leading to speciescoexistence. Despite an abundance ofstudies examining niche partitioning indesert rodents, there remains a lack ofclarification on the mechanismsunderlying the coexistence ofDipodomys and Peromyscus in theMojaveDesert.In this study, we investigated spatial
and temporal mechanisms of nichepartitioning in Peromyscus andDipodomys. Specifically we ask ifPeromyscus and Dipodomys exhibitspatial partitioning in home range andhabitat use. Secondly, we ask ifPeromyscus and Dipodomys exhibittemporal partitioning in foraging
behavior. Understanding how thesemechanisms contribute to thecoexistence of these two competingrodent genera can help furtherelucidate the outcomes of sharedniches, and furthermore adds to ourunderstanding of community-levelpatternsofresourceuse.
METHODS
StudyArea
We conducted our study at theSweeney Granite Mountains DesertResearch Center in the East MojaveDesert,California,U.S.A.(34°48’20”N,115° 39’ 50” W). We conducted ourstudy during the fall (November 3–8,2017), with average nightlytemperatures ranging from 13 degreesC.Ourstudyareawas located inmixeddesert scrub, with dominant plantspecies consisting of creosote bush(Larreatridentata),Mojaveyucca(Yuccaschidigera), and buckhorn cholla(Cylindropuntiaacanthocarpa).
ExperimentalDesign
Inordertoinvestigatemechanismsofspatial partitioning among PeromyscusandDipodomys rodents, we conductedmark-recapture sampling and trackedindividual movements over the courseof 5 nights. We conducted mark-recapture samplingwithin a 48m x 48m grid containing 25 plot pointsregularly spaced 12 m apart. At eachplot point, we placed a Sherman livetrap filled with cotton balls to providewarmth and baited with oats andpeanut butter to attract granivorousrodents. We set traps at sunset (~5
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p.m.) and returned at approximately8:30 p.m. each night to check traps.Captured rodents were identified tospecies. Rodents not identified asDipodomys or Peromyscus werereleased without further processing asthey were not focal in our study.Rodents identified as Dipodomys orPeromyscus, were processed accordingto the following protocol. First, werecorded life history characteristics toassist with species identification,including theweight of the rodent, thelengthofitstail(andlengthoftheapicaltuft for Dipodomys), the length of thehind foot, the length of the ear, andcountof toes forDipodomys. Secondly,we individuallymarkedrodentsat theirfirst capture with a unique patternconsisting of a series of dots and linesmarkedusingblackandbluesharpieonthe ventral side of the tail. Lastly, wedippedtherodentintoabagofUVdust(pink for Peromyscus and yellow forDipodomys), and released them at thespot where they were captured. WefollowedtheirpathwithaUVflashlight(51LED,395nM)foras faraspossible,often ending at the entrance of aburrow.Werecordedtheseburrowsona gridded map, giving each burrow auniqueidentifierandspatialcoordinatesin reference to our trapping grid. Wealsorecordedtheestimateddistance(asthecrowflies)oftheUVtrail,whichweusedasaproxyforforagingdistanceorhome range. On subsequent trappingdays, recaptured individuals wereidentifiedbytheiruniquesymbolontheventralsideof thetailbutnoother lifehistory characteristics were recorded.
However, recapped individuals wereagaindipped inUVdustandassociatedinformation regarding burrow locationanddistancetraveledwererecordedasadditional information on the homerangeofthatindividual.Inordertoinvestigatetemporalniche
partitioning between Dipodomys orPeromyscus and to monitor rodentforaging behavior, we placed fourcamera traps inanareaaway fromourgrid, each with a quarter cup of bait(oats and peanut butter). We setcameratrapstorecord30svideoswith5 seconds gaps in-between triggeredrecordings. For three consecutivenights, we turned on the camerasaround4:45p.m., baited camera traps,and returned the following morningaround 9 a.m. to retrieve the cameramemory cards. From the camera trapdata, we recorded the time of a visitfrom Peromyscus and Dipodomys, thefrequency of visits, and duration of avisitmeasuredasthenumberoftimesacamera was consecutively triggered byanindividualbeforeitleftthesite.DatafromcameratrapsandSherman
live trapswereanalyzed in JMPusingat-test comparing number of recordingsof an individual per visit, and by chi-squared test comparing the averageproportionofvisitstothebaitedcameratrap sites between genus. Recaptureratesbetweengenuswasanalyzedusingachi-squaredtest.We used R Statistical Software
programming (v. 3.4.2) to conduct allspatial visualization and analyses. Inordertovisualizeindividualhomerangewe generated Minimum Convex
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Polygons (MCP; package ggConvexHull)aroundlocationswhereindividualsweretrapped by Sherman live trapping orfollowed by UV dust trails to burrows.We calculated home range using MCPmethodology (package “adehabitatHR”,function mcp, percent = 100). Homerangeestimatesrequiredaminimumoffive tracking points, and thus,wewereonly able to estimate home range forfour Peromyscus and a singleDipodomysindividual.
RESULTS
Estimated spatial territories ofPeromyscus and Dipodomys indicatedlittleoverlapbetweenhomerangesandhabitatusebetweenthetwogenera(Fig
1). Home range estimates rangedbetween5m2and400m2inPeromyscusand 22 m2 for a single Dipodomysindividual.We found significantlymorevisits to baited camera traps byDipodomysthanbyPeromyscus(N=52,P = 0.04; Fig 2). We also found thatthere was a marginally significantdifference in duration time of visitsbetween Dipodomys and Peromyscus;specifically, Dipodomys were observedspendingmoretimeforagingatcameratrapsitesthanthePeromyscus(P=.08,t=-1.8;Fig3).Finally,Dipodomyshadasignificantly lower rate of recapturesthanPeromyscus (N = 48, P = 0.03; Fig4).
Figure1.HabitatpartitioningbetweenDipodomysandPeromyscus.Gridrepresentsourstudyplotinthefieldwithdimensionsextendedfrom48mx48mto60mx60minordertoincorporateburrowsfoundoutsideofourstudyplot.Eachunitrepresentsameter.Minimalconvexpolygonsweremadefollowingindividualsfromtrapsat
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whichtheywerecapturedtotheburrowstheyretreatedto.Shadedregionsarethecalculatedhomerangesofanindividual.
Figure2.Averageproportionofvisitstocameratrap sites between Dipodomys andPeromyscus.Shadedbarsarerepresentationsofmean from individualswithstandarderrorbarsincluded.
Figure3.AveragedurationofDipodomysandPeromyscusvisitstobaitedcameratraps.Shadedbarsarerepresentationsofmeanfromindividualswithstandarderrorbarsincluded.
Figure4.AveragedurationofDipodomysandPeromyscusvisitstobaitedcameratraps.Shadedbarsarerepresentationsofmeanfromindividualswithbinomialerrorbarsincluded.
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DISCUSSION
Our results suggest that coexistencebetween Dipodomys and Peromyscusmay be mediated by spatial nichepartitioning of habitat. Dipodomys areconsiderably larger than Peromyscus,and consequently have much largerhome ranges (what are their estimatedhome ranges?) that exceed the areaofourtrappinggrid(Scheibe1984).Largergranivores, suchasDipodomys, tend tocreate “patches” in which they forage,relative to their size and otherneighboring rodents (Price and Brown1983). Our results concur with thesestudies, and suggests that spatialpartitioning does occur betweenDipodomys and Peromyscus, and thattheir foraging territories are likelyrelativetotheirrespectivesizes.In addition,we did not find evidence
fortemporalnichepartitioningbetweenDipodomys and Peromyscus. However,we did find that Dipodomys wereapproaching and spending a longeramountoftimefeedingatcameratrapsthan Peromyscus. This suggests adifference in foraging behavior thatextends beyond temporal or spatialpartitioning. Peromyscus, by having aspecific territory and visible hesitancywhile feeding, ismore riskaverse in itsforaging behavior than Dipodomys.However, Peromyscus were recapturedfar more than Dipodomys, and wereconsistently recaptured in the sameareaofthegrid.Thus,thedifferencesinforaging behavior between these twogenera also likely contribute to furtherniche partitioning. Peromyscus may bemore likely to give up a food resource
notdirectlywithin their territory,whileDipodomys may not. Thus, Dipodomyslikely are able to exploit high-riskforaging opportunities whilePeromyscus may be restricted to low-risk foraging opportunities. Theseobservations concur with a study byPrice and Brown (1983), which foundthat the interspecific differences inmicrohabitat affinity in desert rodentscontributes to their coexistence andgenuscharacterization.Overall, our study reveals complex
niche partitioning in Dipodomys andPeromyscus. The two genera have verysimilar diets and life histories, but bypartitioningtheirhabitatthroughspatialandbehavioralmeanstheymayreducecostly energy expenditures associatedwithcompetition.Becauseof theharshconditions and limited resources in theMojavedesert, it isunsurprisingto findsuch dynamic interactions arise.However, it is worth researching howsimilarmulti-levelpartitioningoccurs inother desert habitats, where otherrodentgeneratakeprecedence.
ACKNOWLEDGEMENTS
This work was performed at theUniversity of California’s SweeneyGranite Mountains Desert ResearchCenter,doi:10.21973/N3S942.
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