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Isopod assemblage in response to management techniques and environmental variables on Nantucket Island Andrew McCandless, Steven Murschel, Ashley Smithers 9 December 2014 Introduction: The sandplain grassland and heathland of the coastal Northeast are part of a rare and protected habitat type. Many species of plants, birds, and insects rely on these remaining sandplain habitats. Managers have focused on using burning and mowing as ways to control invasive species and maintain current plant communities. With any rare landscape, as the area recedes and goes through landscape changes, the species endemic to the area decline (Foster & Motzin 2003). Research on landscape management impacts on biota primarily focuses on the impacts to charismatic creatures such as birds (Zuckerberg & Vickery 2006; Van Dyke et al. 2004); less research is focused on the lower order species such as invertebrates. Research on the role of soil invertebrates is important in altered grassland ecosystems because of their role in decomposition processes and potential assistance in restoration of degraded lands (Snyder & Hendrix 2008). Terrestrial isopods are arthropods (class: Crustacea) that live in moist humid environments in soil and leaf litter (Heeley 1941). They are important for creating microbiomes and microclimates within ground litter (Souty‐Grosset et al. 2005) by eating plant material as well as moving decaying material from the soil surface to deeper moisture microsites in the soil (Hassall et al 1987). In grassland communities, isopods improve soil and maintain litter decomposition (Curry 1994). Isopods act as bioindicators for habitat quality of grassland (Longcore 2003; Souty‐Grosset et al 2005; Snyder Hendrix 2008). Invertebrate responses to burning and mowing management activities in grasslands have been reported in several studies (Anderson et al. 1989; Chambers and Samways 1998; Dunwiddie 1991). These different management techniques can be used to change the microclimates of isopod habitats. Springett (2006) identified changes in litter microclimates in response to fire and reduction of the number of forest isopods. A better understanding of the impacts on isopod communities of Nantucket Island will help drive management decisions on the sandplain grassland and heathland.
Transcript of Isopod assemblage in response to management techniques and …web.pdx.edu/~scm6/Isopod_Magic.pdf ·...
IsopodassemblageinresponsetomanagementtechniquesandenvironmentalvariablesonNantucketIsland AndrewMcCandless,StevenMurschel,AshleySmithers 9December2014 Introduction:
ThesandplaingrasslandandheathlandofthecoastalNortheastarepartofarareandprotectedhabitattype.Manyspeciesofplants,birds,andinsectsrelyontheseremainingsandplainhabitats.Managershavefocusedonusingburningandmowingaswaystocontrolinvasivespeciesandmaintaincurrentplantcommunities.Withanyrarelandscape,asthearearecedesandgoesthroughlandscapechanges,thespeciesendemictotheareadecline(Foster&Motzin2003).Researchonlandscapemanagementimpactsonbiotaprimarilyfocusesontheimpactstocharismaticcreaturessuchasbirds(Zuckerberg&Vickery2006;VanDykeetal.2004);lessresearchisfocusedonthelowerorderspeciessuchasinvertebrates.
Researchontheroleofsoilinvertebratesisimportantinalteredgrasslandecosystemsbecauseoftheirroleindecompositionprocessesandpotentialassistanceinrestorationofdegradedlands(Snyder&Hendrix2008).Terrestrialisopodsarearthropods(class:Crustacea)thatliveinmoisthumidenvironmentsinsoilandleaflitter(Heeley1941).Theyareimportantforcreatingmicrobiomesandmicroclimateswithingroundlitter(Souty‐Grossetetal.2005)byeatingplantmaterialaswellasmovingdecayingmaterialfromthesoilsurfacetodeepermoisturemicrositesinthesoil(Hassalletal1987).Ingrasslandcommunities,isopodsimprovesoilandmaintainlitterdecomposition(Curry1994).
Isopodsactasbioindicatorsforhabitatqualityofgrassland(Longcore2003;Souty‐Grossetetal2005;SnyderHendrix2008).Invertebrateresponsestoburningandmowingmanagementactivitiesingrasslandshavebeenreportedinseveralstudies(Andersonetal.1989;ChambersandSamways1998;Dunwiddie1991).Thesedifferentmanagementtechniquescanbeusedtochangethemicroclimatesofisopodhabitats.Springett(2006)identifiedchangesinlittermicroclimatesinresponsetofireandreductionofthenumberofforestisopods.AbetterunderstandingoftheimpactsonisopodcommunitiesofNantucketIslandwillhelpdrivemanagementdecisionsonthesandplaingrasslandandheathland.
Figure1:Aconceptualmodelofthestudy.Wehypothesize(HypothesisA)thatmowing,burning,andcontroltypemanagementsaffectisopodassemblagewithmowingandburningaffectingisopodassemblagesignificantlydifferentlyfromthecontrol.Wefurtherhypothesize(HypothesisB)thatthenon‐managementvariables(litter,woodysteamcount,%groundcoveranddistancetoocean)affectisopodassemblagewithlitterbeingthebestpredictor.
UsingunpublisheddatafromMcKenna‐Foster(2009)wherespiderandisopodassemblage
insitesrepresentingthreemanagementtypes:burning,mowing,andnotreatmentweremeasured,weaddressedthequestion:whatistheeffectoftreatment(fire,mowing,andnotreatment)onisopodassemblage?Wetestedthehypothesisthatisopodassemblageinareasmanagedwithburningandmowingwouldbesignificantlydifferentfromareaswithnotreatment(Figure1,HypothesisA).Thenwelookedattheimpactthatplantlitter,percentgroundcover(grass,heath,andbareground),distancetoocean,andwoodystemcounthadonisopodassemblage.Thesecondquestionweaddressedwas:Whatistherelativeimportanceofthesemeasuredenvironmentalvariablesonisopodassemblage?Thesecondhypothesiswetestedwasthatlitterwouldhavethelargesteffectonisopodassemblageoftheenvironmentalvariables(Figure1,HypothesisB).
Methods: Studylocation
SmoothHummocksCoastalPreserve(SHCP)locatedonNantucketIslandisasandplaingrasslandandheathlandpreservemanagedbytheNantucketIslandLandBank.NantucketIslandislocated30milessouthofCapeCod,Massachusetts(Figure2).SandplaingrasslandandheathlandarefoundonNantucket,Tuckernuck,andMartha’sVineyardislands.ThestudysiteSHCPwasapproximately345hectaresofsandplaingrassland,sandplainheathland,andpitchpinestands.
Eightmanagementsites(Figure2)locatedonSHCPwereselectedforthisstudy.The
siteswereselectedbasedonthemanagementtheyreceived,theyvaryinsize,andtheyhaveunpavedroadswithsomemowedvegetationbreaks.Onsiteswithmanagementregimesofmowingandburning,themanagementactivityhadbeendoneatleastoncebetween1998‐2008.Datacollectedforresearchonarthropodswerecollectedin2008fromtheresearchplotsplacedineachmanagementsite.Threeofthesiteshadburningasthemainmanagement
Figure2:LocationofNantucketIslandoffthecoastofmainlandMassachusetts(topleft),thelocationofSHCPonNantucketIslandisoutlined(topright),andthelocationoftheeightmanagementsitesatSHCP(bottom).Circlesindicateplotlocationsfoundineachmanagementsite.MapbyAndrewMcKenna‐Foster.
activity,fourofthesitesweremanagedwithmowing,oneofthemowedsiteswasmowedeveryyear,andtheeighthsitehadnotreatmentsandactedasthecontrol.
Figure3.Experimentaldesign.Therewerethreetypesoftreatmentsite:burn(n=3),mow(n=4),andcontrol(n=1).Eachtreatmentsitehadthreeplots.Eachplothadonetransectandeachtransectconsistedofsevenpitfalltraps.Thetotalnumberofpitfalltrapspermanagementtypevaried,burn(n=63),mow(n=84),andcontrol(n=21).Anesteddesignwithmanagementsites,plots,transects,andpitfalltraps
Threeplotswereplacedineachoftheeighttreatmentsites(Figures2and3).Plotsusedforthestudywerechosenbyvisitingrandomlyorderedsitesandchoosingonesthathadspecificcharacteristics:80%grasscoverand20mfromthenearestroad.Toensureindependencebetweensites,eachplothadtobewithinitsownsandplaingrasslandpatchandseparatedbyshrubbyvegetationfromtheothersites.Eachoftheplotshadonetransect(n=24)andsevenpitfalltraps(n=168).Thesevenpitfalltrapsweresetthreemetersapartinatransectparalleltothesoutherncoastline.Thepitfalltrapswerefivecmdiameterpolypropylenecupsfilledwith50:50propyleneglycol.PitfalltrapsweresetandsampledforsevendaysinbothMayandAugust2008.
Eachtransectwasplacedneararandomlyselectedpointintheplot.Eventhoughrandom,theresearchersrequiredthatoneendofeachtransectwasatleast0.5morgreaterfromthenearestheathplant.Sixenvironmentalvariablesweremeasuredalongeachtransect:dryleaflitteringrams,grass,heath,andbaregroundpercentcover,andwoodystemcountpersquaremeter.DistancetotheoceanwascalculatedusingGISandmeasuredinmeters.
Investigatingtheeffectsofmanagementtreatmentonisopodassemblage
Todeterminewhethermanagingsandplaingrasslandandheathlandbymowingorcontrolledburninghadasignificantimpactonisopodassemblage,aKruskal‐Wallisranksumonewayanalysisofvariancetestwasperformed.Thistestwasappropriateduetothenon‐
parametricnatureofthedataandbecausethistestcouldhandleourvaryingsamplesizes.Toaccountforthenestedexperimentaldesign,themeanaverageoftheisopodcountsforallpitfalltrapswithinamanagementsitewasusedastheisopodcountforthatmanagementsite.
Modelingisopodassemblagecountbasedonmeasuredenvironmentalvariables
Alinearmodelwasgeneratedtopredictpooledisopodassemblageacrosssitesbasedonsixpredictorvariables(drylittermass,percentcoverofgrass,heath,bareground,distancetotheocean,andwoodystemcount).Priortomodelconstruction,dataweresplitrandomlyintotwosubsets.Thefirstsubsetcontained~75%ofthedata(131observations)andthesecond~25%(44observations).Buildingthemodelbasedon75%ofthedata(themaindataset)enabledmodelvalidationusingtheremaining25%(thetestdataset).
Diagnosticplotsforafullmodelofthemaindataset(includingresidualsvs.fittedvalues,standardizedresidualsvs.theoreticalquantiles,standardizedresidualsvs.fittedvalues,andstandardizedresidualsvs.leverage)wereusedtovisuallydeterminetheneedtotransformtheresponsevariable‐isopodassemblage.Basedonvisualanalysisofacorrelationmatrixfromthemaindataset(AppendixFigureA),distancetooceanandwoodystemcountwerelogtransformedtoreducetheimpactofoutliers.ABox‐Coxpowertransformationwasusedtodeterminethetransformationmostlikelytoresultinnormallydistributedresidualsfortheresponsevariable(AppendixFigureB).Thedataweretransformedanddiagnosticplotsforasecondfullmodelwerecheckedvisually(AppendixFigureC).Havingdeterminedthemodelmetnecessaryassumptions,twomethodswereemployedtodetermineaminimumadequatemodel. Anallsubsetscriterion‐basedprocedurewasusedfollowedbyastepwise/criterion‐basedhybridproceduretotakeadvantageofthecriteriaMallow’sCP,adjustedR2,theBayesianInformationCriterion(BIC),andtheAikaikeInformationCriterion(AIC).Weselectedthenumberofvariablesandtheparticularvariablesforaminimumadequatemodelbasedonthesefourcriteria.AnANOVAtestwasusedtodeterminewhethertheminimumadequatemodelwassignificantlydifferentfromthefullmodel.Varianceinflationfactors(VIF)werecalculatedtotesttheminimumadequatemodelforissuesofmulticollinearity.Theminimumadequatemodelwasscaledusingz‐scores,andascaledminimumadequatemodelwasproduced.Z‐scoringwasusedtostandardizethepredictorcoefficients.Oncestandardized,predictorcoefficientswerecompareddirectly(whereasthiswaspreviouslyimpossibleduetounitdifferences)enablingananalysisoftherelativepredictivepowerofeachpredictorvariable.Lastlythepredictionsofthemodelweretestedusingthetestdatasetandassessedbycalculatingtherootmeansquaredeviation. Results: Theeffectsofmanagementtreatmentonisopodassemblage(Figure1,HypothesisA) Isopodassemblagevariedwidelywithintreatmentgroupswithstandarddeviationsbeinggreaterthanthemeansinallthreetreatmenttypes(Table1).Sitestreatedbymowingappeartohavesignificantlyhigherlevelsofisopodsthancontrolsitesorthosetreatedbyburning(Figure4).Sitesmanagedbyburninghadameanisopodassemblageof78thatwasveryclosetothecontrolmeanof81.Mowedsiteshadameanapproximately60%greaterwithacountof126isopods.Thestandarddeviationforsitesmanagedbyburningwas176,162formowedsites,and117forthecontrolsite(Table1).
Table1:Descriptivestatisticsofisopodassemblage(numberofindividuals)givendifferentmanagementtreatments(Burnn=3,Mown=4,Controln=1).
Burn Mow Control Minimum 0.0 0.0 5.0 Maximum 902.0 747.0 419.0 Median 14.0 68.0 22.0 Mean 78.4 126.4 80.8 Standarddeviation 176.4 162.0 117.1 Variance 31112.9 26255.8 13714.2
Figure4:Barchartofthemeanofisopodassemblage(numberofindividuals)givendifferentmanagementtreatments(Burnn=3,mean=78.4,SE=+/‐22.2,Mown=4,mean=126.4,SE=+/‐17.7Controln=1,mean=80.8,SE=+/‐22.1). Thenon‐parametricKruskal‐Wallistestdidnotindicateasignificantrelationshipbetweenisopodassemblageandanyoneofthemanagementtypes(Χ2=1.22,df=2,p=0.54).Thenullhypothesisthattherewasnorelationshipbetweenburningandmowingmanagementandisopodcouldnotberejectedbasedonthedatacollected. Modelingisopodassemblagebasedonmeasuredenvironmentalvariables(Figure1,HypothesisB) Histograms,scatterplots,andSpearmancoefficientsofeachvariableindicatedstrongcorrelationsbetweenthefourpredictorsl.stem(thelogofwoodystemcount),heath,grass,litterandtheresponsevariableisopod.Potentialmulticolinnearitywasseenbetweengrassandheath,grassandl.ocean(thelogofthedistancetotheocean),grassandl.stem,andheathandl.stem(Figure5).VisualassessmentofFigure5showsapotentiallystrongpositivecorrelationbetweenl.isopod(1+thelogofisopod)andlitter,andanotherpositivecorrelationbetweenl.isopodandl.stem.
Thefullmodelshowingl.isopodasafunctionoflitter,grass,heath,ground,l.ocean,and
l.stemyielded:
. 0.04 ∙ 0.02 ∙ 0.01 ∙ 0.12 ∙ 0.12 ∙ . 0.57 ∙ . 2.18
Thismodelindicatedthat1+naturallogofisopodcountwouldincreaseaslitter,thenaturallogofthedistancetotheocean,and1+naturallogofthewoodystemcountincreased.1+naturallogofisopodcountwoulddecreaseifthepercentcoverofgrass,heath,orbaregroundincreased (R2=0.4953, p=2.2x10‐16, α=0.05). Thefullmodelpasseddiagnosticsillustratingresidualswerenormallydistributed,withoutunderlyingstructure,andtherewerenoinfluentialoutliersaccordingtotheCook’sdistancemetric(AppendixFigureC).
Anallsubsetscriterion‐basedprocedureusingthecriteriaMallow’sCP,adjustedR2,andtheBICindicatedthatamodelwiththreepredictorsandoney‐interceptshouldbeselected(AppendixFigureD).ThiswasdeterminedbytheindicationthatamodelwithfourparameterswouldhavethelowestMallow’sCP,highestadjustedR2,andlowestBIC.Astepwise/criterion‐
Figure5:Correlationmatrixshowinghistograms,scatterplots,andSpearmancorrelationsbetweentheresponsevariableisopodassemblageandthesixpredictorvariables.
basedhybridmethodalsosuggestedthatamodelwiththreepredictorsandoney‐interceptshouldbeselectedandbothmethodsindicatedtheselectionofaminimumadequatemodelwithl.stem,bareground,andlitterasthethreepredictors.
Theminimumadequatemodelbasedonanallsubsetscriterion‐basedprocedureandastepwise/criterion‐basedhybridapproach:
. 0.56 ∙ . 0.10 ∙ 0.05 ∙ 1.46
Theminimumadequatemodel(R2=0.4989)wassignificantoverallwithap‐valueof2.2x10‐16(α=0.05).Thismodelindicatedthatl.isopodwillrisewithanincreaseinl.stemandlitteranddecreasewithanincreaseinground.Thetestformulticollinearitybetweenthevariablesl.stem(VIF=1.1),ground(VIF=1.0),andlitter(VIF=1.1)intheminimumadequatemodelrevelednoissuesofmulticollinearity.Normalityofresidualsandtheabsenceofhighleverageoutliersweredeterminedbyvisualinspectionofdiagnosticplots(AppendixFigureE).TheANOVAbetweenthefullandminimumadequatemodel(p=0.7369)indicatedthatthenullhypothesisofthetest(thatthetwomodelsarenotdifferent)couldnotberejectedthusthesimplerminimumadequatemodelwasmaintained.
Thez‐scoredminimumadequatemodelenabledadirectcomparisonoftherelativeinfluenceofeachpredictor:
. 0.51 ∙ . 0.23 ∙ 0.28 ∙ 1.34 10 16
Thez‐scoretransformationoftheminimumadequatemodelhasthesameR2valueandsignificanceastheminimumadequatemodel.Thisstandardizationofthemodelcoefficientsshowedl.stemwasthemostimportantofthemeasuredpredictorsofl.isopodandwasapproximatelyasimportantasthetwootherpredictorscombined.Groundandlitterwereverysimilarintheirpredictivepoweronl.isopodwithanincreaseingroundleadingtoadecreaseinl.isopodwhileanincreaseinlitterleadstoanincreaseinl.isopod.Ourmodelshowedtherelativeimportanceratiooftheenvironmentalvariablesonisopodassemblagetobe0.51woodystemto0.23baregroundpercentageto0.28dryleaflitter(i.e.a0.51:0.23:0.28proportion). Theminimumadequatemodelpredictedthetrendseeninboththemaindata(subsetof131observations)andthetestdata(subsetof44observationsnotusedtoconstructthemodel)(Figure6).Therootmeansquaredifferenceofl.isopodwas1.50andtheuntransformedrootmeansquaredifferencebetweentheminimumadequatemodelpredictionsandobservedisopodassemblagewas4.48.
Discussion: Managementtreatmentonisopodassemblage Whilemanagementwasfoundtonotimpactisopodassemblage,thisislikelytheresultofalackofanalyticalpowerduetothesmallsamplesize,unequaltreatmentreplication,andtheuseofarankbasedtest.Theapparentdifferencesinmeanbetweenmowedtreatmentareasandcontrolledorburnedareas(Figure4)providesfurtherevidencethatfindingnorelationshipbetweenisopodassemblageandtreatmentwasduetoalackofstatisticalpower.Otherresearchconductedacrossarthropodtaxasuggestsotherarthropodpopulationswereeithernotgreatlyinfluencedbymowingand/orburningtreatmentsortherewashighvariationofresponsestotreatmentsacrosstaxaandseason(Dunwiddie1991).Currentresearchshowsarthropods,includingisopods,generallyadapttomanagementactions(Warrenetal.1987;Dunwiddie1991).Further,observedmanagedsitesweremowedandburnedindifferentyearspotentiallyprovidingisopodassemblagetheopportunitytoreboundfromtreatmentmakingtheaffectsundetectablebythe2008observation.
Theknownrelationshipbetweenmanagementandenvironmentalpredictorsofisopodassemblagefurthersupportsthesuppositionthatalackofsignificanteffectswasduetolowpowerofanalysis.Mowingandburningaretoolsusedtomaintainthevegetationonsandplaingrasslands(Neill2006).Burningconsumesleaflitteranddestroysisopodmicrohabitats(Springett2006).Mowingshouldleadtoanincreaseinlitterthatmayalsoimpactisopodhabitatavailability.Usingmowingandburningmayindirectlybenefitisopodcommunitiesbyimprovingthehabitatrequirementsthatisopodsrelyon.
Figure6:Left:Minimumadequatemodelpredictionwiththemaindatashowingtheobservedversuspredictedvaluesofl.isopod.Thelineshowsthepredictedl.isopodvaluesbasedontheminimumadequatemodel.Right:Minimumadequatemodelpredictionwiththetestdataof44pointsinitiallysubsettedfromtheobservationsandnotusedintheconstructionoftheminimumadequatemodel.Thelineshowsthepredictedl.isopodvaluesbasedontheminimumadequatemodel.
Isopodassemblageinrelationtosignificantenvironmentalvariables Whilethehypothesisthatleaflitterwouldbethestrongestpredictorofisopod
assemblagewasnotsupported,leaflitterwasthefoundtobeofsecondhighestimportanceafterwoodystemcount.Woodystemsweremainlyrepresentedbylivingblackhuckleberryandsomebayberry.Understandingthewoodystem/isopodassemblagerelationshipisakeytoanalyzingisopodpopulations.Otherstudieshavehighlightedtheimportanceofhabitatstructure,includingconnectivelandscapepropertiesandvariationsinfaunaandsoiltypes,onisopodpopulationdynamics(Davis1984;Souty‐Grossetetal.2005).Isopodspeciesmayrelyonwoodystemsforprotectionfromsolarradiationandcapturedrainfall(Volketal.2000).Balancingthehabitatmosaicofheathandshrubswithingrasslandsmaybenefitisopodsandthegrasslandcommunity.FurtherupkeepofinterspersedwoodystemswithintheNantucketmanagedecosystemwouldlikelybenefitfloraandfaunainteractionsandisopodpopulations.
Thesignificantinfluenceoflitterandbaregroundcomparedtoothermeasuredenvironmentalfactorshighlighttheimportanceofdetritusasafoodsourcefortheisopodcommunity.Isopodsshowverystrongpreferencesfordetritus(Hassalletal.1987;Zimmer2002),andinthisecosystemgroundlitteriscomprisedmostlyofdecomposinggrasses(Burkeetal.1998).ThenegativerelationshipbetweenbaregroundandisopodassemblagefurtheremphasizestheimpactofvegetationandsubsequentgroundlitteronNantucketisopodassemblage.Maintaininganaturallydecomposingecosystemwithhighflorarichnessandscatteredvegetationdiversitywillmostbenefitisopodassemblageinthisecosystem. Althoughmorethan90%ofisopodscountedappearedtobeofthesamemorphologicaltype,thefewuniqueindividualssuggestsidentifyingisopodstothespecieslevelmayidentifyisopodspeciesindicativeofparticularimportantmicrohabitats.Specieslevelidentificationcouldhelpimproveunderstandingofecosysteminteractions.Rarepredatoryisopodshavebeenshowntobestrongbioindicatorsofsuccessfulrestorationeffortsinterrestrialshrublands(Longcore2003),andknowingisopodspeciescouldbetterinformlandscapemanagement.Identificationofvegetationspeciesmayalsoshowisopodpreferenceandcontributetoabetterunderstandingoftheimportanceoflittercomposition.
Theroleofenvironmentalvariablesnotmeasuredinourstudy,suchassoilmoisture,whichhasbeenshowntohaveasignificanteffectonisopodassemblage(Warburg1987),mayhavelimitedthepredictivepowerofourlinearmodel.Awiderangeofinteractingfactorscontributetothecreationofmicrohabitatsandmicrobiomeswithinshrubsscatteredthroughoutgrasslands.Shrubswithingrasslandsactasareasofintensenutrientfluxfromroot/leafturnoverandexcessexcrementfromsurroundingfauna(Vetaas1992).Terrestrialisopodshaveundergoneevolutionarychangestodwellonland,anddespitesomespecieslivinginaridregions,allspeciesneedprotectionfromdesiccation(Schmidt&Wägele2001).Scatteredshrubs(woodystems)ingrasslandscreatemoist,hospitablemicroclimatessimilartotheeffectscatteredtreescanhaveinalandscape(Manningetal.2006).
Themodelvalidationindicatedthatthemodeldoesagoodjobofdescribingrelatedindependentdata.Therootmeansquaredifferenceof4.48forisopodassemblageisassessedbasedonitscomparisonwiththemeanisopodassemblageof101.Adifferenceof4.48issmallincomparisontothemeanof101indicatingtheapplicabilityofthelinearmodeltoindependentdata.
Study‐wideconsiderations
Asstrongpredictorsofisopodassemblage,alterationsinmanagementthatimpactleaflitterandwoodystemsshouldaffectisopodassemblage.Furtherstudyonagreaternumberof
independentgrasslandandheathlandsitesthathavebeenmowedorburnedshoulduncovertherelationshipbetweenisopodassemblage,managementaction,andthesignificantpredictorvariablesfromourminimumadequatemodel.Managementaffectsleaflitter(Andrewetal.2000),groundcover,andwoodystems,andthosethreepredictorsindicateisopodassemblage.Thattheaffectofmanagementonisopodassemblagewasnotfoundinourstudywhileleaflitter,percentbareground,andwoodystemswerefoundtobesignificantpredictorsofisopodassemblageisalmostcertainlyduetooursamplingdesignandprovidesaclearcalltofurtherresearch.Bibliography: Anderson,R.C.,Leahy,T.,&Dhillion,S.S.(1989).Numbersandbiomassofselectedinsectgroupsonburnedandunburnedsandprairie.AmericanMidlandNaturalist,151‐162.
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Appendix:
FigureA:Correlationmatrixshowinghistograms,scatterplots,andSpearmancorrelationsbetweentheresponsevariableisopodassemblageandthesevenpredictorvariables.
FigureB:ResultsofBox‐Coxpowertransformationonisopodasafunctionoflitter,grass,heath,ground,l.ocean,andl.stemindicatingalogtransformationwouldbeappropriate(asseenbythe95%confidenceintervallinesenveloping0).
FigureC:Diagnosticstatisticsoffullmodel.Topleftshowsthattheresidualsarenormallydistributedaboutazerolineandshownostructure,thetoprightshowsthenormalityofthestandardizedresiduals,thebottomleftshowsthestandardizedresidualshavenounderlyingstucture,andthebottomrightshowsthattherearenohighleverageoutlierswithCook’sdistancesofgreaterthan0.5.
FigureD:Plottingtheresultsofanallsubsetscriterion‐basedproceduremodelingl.isopodasafunctionofbetween1and6variablesand1y‐intercept.TopleftshowsthelowestMallow’sCpformodelswith1‐6variables+1y‐intercept.ToprightshowsthehighestR2andbottomleftthelowestBayesianInformationCriterionformodelswith1‐6variables+1y‐intercept.Eachcriterionindicatesthatamodelwith3predictorsand1y‐interceptwouldbethebest.
FigureE:Diagnosticplotsoftheminimumadequatemodel.Topleftshowsthattheresidualsarenormallydistributedaboutazerolineandshownostructure,thetoprightshowsthenormalityofthestandardizedresiduals,thebottomleftshowsthestandardizedresidualshavenounderlyingstucture,andthebottomrightshowsthattherearenohighleverageoutlierswithCook’sdistancesofgreaterthan0.5.
