Drivers and Patterns of Ground-Dwelling Beetle Biodiversity across Northern Canada

16 Pages·2015·0.43 MB·English

RESEARCHARTICLE
Drivers and Patterns of Ground-Dwelling
Beetle Biodiversity across Northern Canada
CrystalM.Ernst*,ChristopherM.Buddle
DepartmentofNaturalResourceSciences,McGillUniversity,Sainte-Anne-de-Bellevue,Quebec,Canada
* [email protected]
Abstract
Manymacroecologicalpatternsofbiodiversity,includingtherelationshipbetweenlatitude
andspeciesrichness,arewell-described.Datacollectedinarepeatable,standardized
mannercanadvancethedisciplinebeyondthedescriptionofpatternsandbeusedtoeluci-
dateunderlyingmechanisms.Usingstandardizedfieldmethodsandahyper-diversefocal
taxon,viz.Coleoptera,weaimto(1)describelarge-scalelatitudinalpatternsoftaxonomic
OPENACCESS
diversity,functionaldiversity,andassemblagestructureacrossnorthernCanada,and
Citation:ErnstCM,BuddleCM(2015)Driversand (2)determinewhichclimatic,spatial,andhabitatvariablesbestexplainthesepatterns.We
PatternsofGround-DwellingBeetleBiodiversity
collectedterrestrialbeetlesattwelvelocationsinthethreenorthernmostecoclimaticzones
acrossNorthernCanada.PLoSONE10(4):
e0122163.doi:10.1371/journal.pone.0122163 inNorthAmerica:northboreal,subarctic,andhigharctic(51–81°N,60–138°W).Afteridenti-
fyingbeetlesandassigningthemtoafunctionalgroup,weassessedlatitudinaltrendsfor
AcademicEditor:GregorioMoreno-Rueda,
UniversidaddeGranada,SPAIN multiplediversityindicesusinglinearregressionandvisualizedspatialpatternsofassem-
blagestructurewithmultivariateordinations.Weusedpathanalysistotestcausalhypothe-
Received:October24,2014
sesforspeciesandfunctionalgrouprichness,andweusedapermutationalapproachto
Accepted:February5,2015
assessrelationshipsbetweenassemblagestructureand20possibleclimaticandenviron-
Published:April22,2015 mentalmechanisms.Morethan9,000beetleswerecollected,representing464species
Copyright:©2015Ernst,Buddle.Thisisanopen and18functionalgroups.Speciesandfunctionaldiversityhavesignificantnegativerela-
accessarticledistributedunderthetermsofthe tionshipswithlatitude,whicharelikelyexplainedbythemediatingeffectsoftemperature,
CreativeCommonsAttributionLicense,whichpermits
precipitation,andplantheight.Assemblageswithinthesameecoclimaticzonearesimilar,
unrestricteduse,distribution,andreproductioninany
medium,providedtheoriginalauthorandsourceare andthereisasignificantrelationshipbetweenassemblagestructureandlatitude.Species
credited. andfunctionalassemblagestructurearesignificantlycorrelatedwithmanyofthesamecli-
DataAvailabilityStatement:Weprovidealistofall maticfactors,particularlytemperaturemaximaandminima.Atalargespatialextent,thedi-
taxainS1Table,andacompletedatasetofindividual versityandassemblagestructureofnorthernbeetlesshowstronglatitudinalgradientsdue
specimenrecordsisavailablethroughCanadensysat
tothemediatingeffectsofclimate,particularlytemperature.Northernarthropodassem-
http://doi.org/10.5886/5dvj8642.Thedataset
blagespresentsignificantopportunitiesforbiodiversityresearchandconservationefforts,
associatedwiththismanuscriptisnowpublishedand
hasbeenmadepubliclyavailable,thankstothestaff andtheirsensitivitytoclimatemakethemidealtargetsforlong-termterrestrial
atCanadensys.Thedatawillsoonbeintegratedinto diversitymonitoring.
CBIFandGBIFaswell.Thewebpage,linkedtothe
DOIinthemanuscript,ishttp://data.canadensys.net/
ipt/resource.do?r=northern-beetle-specimens.
Funding:ThisworkwassupportedbyaNational
ScienceandEngineeringResearchCouncilof
Canada(NCERC)StrategicProjectGrant(Ecological
StructureofNorthernArthropods:Adaptationtoa
PLOSONE|DOI:10.1371/journal.pone.0122163 April22,2015 1/16
BiodiversityofNorthernBeetles
ChangingEnvironment)awardedtoC.Buddle,T. Introduction
Wheeler,andD.Currie(UniversityofToronto),its
Macroecologistshavesuccessfullydescribedlarge-scalespatialpatternsofbiodiversityandspe-
supportingpartnersandcollaborators,andaNSERC
CGS-DAlexanderGrahamBellCanadaGraduate ciesdistributions.Amongthem,thelatitudinalgradientofspeciesrichness,inwhichfewerspe-
ScholarshipawardedtoC.Ernst.Thefundershadno ciesarefoundathighlatitudescomparedtoattheequator,hascaptivatedresearchersformany
roleinstudydesign,datacollectionandanalysis, decades[1].Overthepastdecade,therehasbeenincreasedinterestinelucidatingthecausal
decisiontopublish,orpreparationofthemanuscript. mechanismsbehindlatitudinaldiversitypatterns[2–5].Thesearchforlikelymechanismshas
CompetingInterests:Theauthorshavedeclared beenchallenging,however,andalthoughabroadrangeofclimatic,evolutionary,biotic,and
thatnocompetinginterestsexist. spatialhypotheseshavebeenputforth,reviewedby[3],nosinglefactorhasbeenidentifiedasa
keymechanism.Itishighlylikelythatthenumberofspeciesfoundatdifferentlatitudes,and
thewaythesespeciesassembleoverspaceandtime,istheresultofmultipleinteractingecologi-
calandevolutionaryfactors[6,7].Despitethechallengeofteasingaparttherelativecontribu-
tionsofdifferentfactorstopatternsofdiversity,climate—inparticular,temperature—hasbeen
broadlyrecognizedasakeyelementinbothterrestrialandaquaticsystems[8–10]andisworthy
ofadditionaltesting.
Recently,someworkershavebegunincludingalternativeorcomplimentarygenetic,mor-
phologicalorfunctionalmeasuresofdiversityalongsidethetraditionaltaxonomicmetricof
speciesrichness[11].Functionmayplayaparticularlyimportantroleininfluencingspeciesdi-
versitypatterns,asspeciesrichnessmaybelimitedbybioticinteractions,differencesinlifehis-
torytraits,andenvironmentalgradientsthatinfluencenicheavailability[12].Theinclusionof
functionaldiversityinmacroecologicalstudiesmayultimatelyyieldstrongertestsofbiodiversi-
tytheories[5,12,13].
Inadditiontoexpandingthelensthroughwhichweexaminediversitypatternsbeyondtax-
onomy,wecanalsousetheframeworkestablishedbyearliermacroecologicalstudiesasaplat-
formfromwhichtobroadenthescopeoffuturework.Someauthorshaveidentifiedavenuesof
researchthatholdgreatpromisefortheadvancementofmacroecologicaltheory,namely,the
inclusionof:(1)standardized,repeatablefaunalsampling,(2)broadertaxonomicorfunctional
scopes,(3)broaderecologicalscopes(e.g.,multiplehabitatsorbiomes),and/or(4)underrepre-
sentedyetecologicallysignificantbiomes(aquaticsystemsandpolarregions)andtaxa(inver-
tebrates,non-vascularplantsandfungi)[3,5].Whilesomeoftheseadvancementspresent
logisticalchallenges,workingtoovercomethemmaygeneratesufficientquantitativedatato
testandsupportgeneralizablestatementsaboutlarge-scalepatternsandprocessesofdiversity
[14,15].Understandingtheunderlyingprocessesresponsibleforpatternsofdiversitywillpro-
videpowerfulmechanismsforpredictingtheeffectsofclimatechangeandotheranthropogenic
disturbancesonbioticcommunitiesandtheircomponentspecies[5].
Testinglarge-scalebiodiversitypatternsandprocessesrequiresusingafocaltaxonthatisdi-
verse,easilysampled,representativeofdifferentprocessesandfunctions,taxonomicallywell
understood,andsensitivetoenvironmentalorecologicalchanges[16].Beetlesareanideal
studytaxon:theyareoneofthemosttaxonomicallyandfunctionallydiversegroupsofanimals
[17],andarethemostabundant,diverseandecologicallysignificantepigeic(i.e.,livingpre-
dominantlyonthegroundsurface)insecttaxoninnorthernsystems[18,19].Beetlesarealso
easilycaughtusingpassivetrappingmethodsthatcanbestandardizedandthuspermitcom-
parisonsofassemblagesacrosstimeandspace[20–22],andshowrapidresponsestoenviron-
mentalchange[23–25].Thesefactorsmakenorthernbeetlesidealfocaltaxaforconductinga
comprehensiveexaminationofterrestrialdiversityandassemblagestructure.
Theoverallobjectiveofourresearchwastoconductastandardized,species-levelassess-
mentofbothtaxonomicandfunctionalbiodiversitypatternsacrossmultiplebiomes,using
beetlesasafocaltaxon.Ourspecificgoalswereto:(1)quantifylatitudinalpatternsindiversity
andassemblagestructure(taxonomicandfunctional),and(2)establishtheclimaticand/or
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environmentalfactorsassociatedwithtaxonomicand/orfunctionalassemblagestructure
acrossalargegeographicextent,whichwastwelvelocationsinthethreenorthernmostecocli-
maticzonesofNorthAmerica(Table1).Wehypothesizedthat:(1)beetleswillconformto
classicallatitudinalgradientsofdiversity,withgreaterspeciesandfunctionalrichnessatlower
latitudes;(2)speciesrichnessisdirectlyinfluencedbyclimaticvariables,whichareinfluenced
bylatitude;(3)speciesandfunctionalassemblagestructureswilldisplaylatitudinalgradients
ofsimilarity;(4)variationsinassemblagestructurewillbebestexplainedbyclimatic,rather
thanspatialorbiotic,variables.
MaterialsandMethods
Studyregionandsamplinglocations
In2010and2011,aspartofalargerresearchproject[26,27]wecollectedterrestrialbeetlesat
twelvelocationsinthethreenorthernmostecoclimaticregionsofnorthernCanada[28]:four
locationswereinthenorthborealregion,fourinthesubarcticandfourinthehigharctic
(Table1).Theextentofthestudyconsistsofalatitudinalgradientof51–81°Nandalongitudi-
nalgradientof60–138°W.Permitsweregrantedbythefollowingagenciestoconductsampling
inthenorthernterritories:NunavutResearchInstitute(ScientificResearchLicence),Nunavut
DepartmentoftheEnvironment(WildifeResearchPermit),Yukon’sDepartmentofTourism
andCulture(ScientistsandExplorersLicence),AuroraResearchInstituteoftheNorthwest
Territories(ScientificResearchLicence),ParksCanadaAgency(ResearchandCollectionPer-
mit;fortheLakeHazenlocation,whichfallsintheQuttinirpaaqNationalParkofCanada,
NU).Innoinstancedidourworkinvolvethecollectionendangeredorprotectedspecies.In
lightofthisfact,andsincenoothersamplingwasconductedinprovincialparksorwildlifere-
serves,nospecificpermitswererequiredtosampleonpubliclandintheotherlocations.
Ateachlocation,weestablishedthreereplicateswithinabout15kmofeachother.Incon-
siderationofthefactthathabitatselectionbynorthernarthropodsishighlydependenton
moistureandvegetation[29,30],eachreplicatecontainedtwobroadlydelimitedandecolog-
icallydistincthabitats.“Mesic”habitatsarecharacterizedbyhigherelevationsandwell-drained
soil,while“wet”habitatshavesaturatedorpoorlydrainedsoils,andcanbefoundinadjacent
low-lyingregions.Themesicvegetationwasadiscontinuouscoverofdwarfshrubs,perennial
forbs,andlichens.Wethabitatscontainedcontinuouscoverofmoss,sphagnum,saxifrages
Table1. Names,coordinates,andecoclimaticzonesofsamplinglocations,anddatesofsampling.
SamplingLocation Latitude,Longitude EcoclimaticZone SamplingPeriods(dates) SamplingYear
1 2 3
LakeHazen,NU 81.82975,-71.32244 HighArctic 19–23.vii 23–28.vii - 2010
BanksIsland,NWT 73.22412,-119.55255 HighArctic 7–11.vii 11–15.vii 15–19.vii 2011
CambridgeBay,NU 69.12177,-105.41688 HighArctic 7–11.vii 11–15.vii 15–19.vii 2011
Iqaluit,NU 63.76144,-68.57352 HighArctic 17–21.vii 21–25.vii 25–29.vii 2010
Kugluktuk,NU 67.78157,-115.27824 Subarctic 22–26.vi 26–29.vi 29.vi-2.vii 2011
TombstoneMtns.,YT 64.60629,-138.35637 NorthBoreal 21–24.vi 24–27.vi 27.vi-01.vii 2011
Churchill,MB 58.73573,-93.79789 Subarctic 1–5.vii 5–9.vii 9–13.vii 2010
Schefferville,QC 54.75970,-66.71120 Subarctic 30.vi-3.vii 3–6.vii 6–9.vii 2010
NormanWells,NWT 65.29112,-126.62262 Subarctic 7–11.vi 11–14.vi 14–17.vi 2011
Yellowknife,NWT 62.52110,-113.38174 NorthBoreal 7–11.vi 11–15.vi 15–18.vi 2011
GooseBay,NFLD 53.21199,-60.45062 NorthBoreal 15–18.vi 18–21.vi 21–24.vi 2010
Moosonee,ON 51.28034,-80.64252 NorthBoreal 15–19.vi 19–23.vi 23–25.vi 2010
doi:10.1371/journal.pone.0122163.t001
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andsedges.Inordertoensureconsistencyofsamplinginbothhabitatsacrossalllocations,we
establishedallreplicatesinopenareaswithnotreecanopycover;weencounteredsomedwarf
blackspruceinsomeofthemoresouthernsites.
Insectsamplingandspecimenprocessing
Inallreplicatedhabitatsateachlocation,wesetninepitfallandnineyellowpantrapsinthree
transectsasa30x75mgrid.Wedeployed108trapsperlocation,and1296trapsintheentire
samplingdesignandweservicedthemthreetimesovertwoweeks(seeTable1forcollection
dates,andforacompletedescriptionoftrappingandcollectionprotocols,see[26]).Toaccount
forphenology,wesampledthesouthernmostlocationsfirst,andthenorthernmostlast,sothat
insectactivitywouldbeapproximatelyequalatalllocations.Weplacedspecimensin95%etha-
nolandreturnedthemtothelaboratoryforsorting,andidentifiedadultbeetlestospeciesor
morphospeciesusingtraditionalmorphologicalapproaches.
Weassignedeachbeetletoafunctionalgroupbasedonitspredominantfoodsourceand
modeoffeeding[14].Sincebiomassintegratesfunctionalcharacteristicsofassemblagessuch
asenergyandnutrientflow(Saint-Germainetal.2007;Wangetal.2009),weuseditasthe
metrictodescribeassemblagesfunctionally(i.e.,ratherthanabundance).Weestimatedthe
biomassofeachbeetleusinglength:biomassregressions[31,32],usingmeasuredbodylength
oraknownaveragelengthofcommonspeciescollectedpreviously[26].
Voucherspecimensofallspeciesarenowcontainedinatleastoneofthefollowingcollec-
tions:LymanEntomologicalMuseum(Sainte-Anne-de-Bellevue,Québec,Canada),Canadian
NationalCollectionofInsects,ArachnidsandNematodes(Ottawa,Ontario,Canada),Canadi-
anMuseumofNature(Ottawa,Ontario,Canada).
Environmentalvariables
Wequantitativelyassessedtheplantcommunityateachlocationbyrandomlyestablishingfive
1m2quadratsinandadjacenttothetrapgridineachhabitatreplicate.Tocharacterizetheveg-
etationineachquadratweuseda%coverclassificationsystem,theBraun-Blanquetscale[33].
Weassignedplantstoaclass(forbs,shrubs,graminoids,mosses,lichens),gaveeachclassa
Braun-Blanquetscoreof1to6,anddeterminedthemeanscoreforeachclassforeachlocation.
Wemeasuredthemaximumheightofthevegetationofanyclass(MaxVegHt)ineachquadrat,
anddeterminedanaverageoftheseheightsforeachlocation.
Climatedataareavailableonline(CanadianNationalClimateDataandInformationAr-
chive(http://climate.weatheroffice.gc.ca).Weusedclimatenormals(calculatedusingatleast
15yearsofdatatakenbetween1981–2010)recordedattheweatherstationclosesttoeachloca-
tiontoobtainthefollowing:meanannualtemperature(MeanTemp),meanmaximumtemper-
ature(MaxTemp),meanminimumtemperature(MinTemp),meantemperatureofwarmest
month(WarmMean),meantemperatureofcoldestmonth(ColdMean),meantotalannual
precipitation(TotPrecip),meandegreedaysabovezero(DDA0),meandegreedaysbelowzero
(DDB0),meanwindspeed(Wind),meanannualdayswithsunshine(SunDays),meanannual
totalsunshinehours(SunHrs),andnumberoffrost-freedays(Frost).Giventheirproximity,
weconsideredallreplicatesatthesamelocationtohavethesameclimateconditions.
Dataanalyses
WeconductedallanalysesusingR,version3.1.1(RCoreTeam,2014).Wedescribedthetotal
abundanceofbeetlesateachlocation.Whileoursamplingwasstandardized,theresultingsam-
plesizeswereunequalandthereweremanyrarespecies.Therefore,inadditiontocalculating
theobservedspeciesrichness,observedfunctionalgrouprichness,Simpson’sdiversity,and
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BiodiversityofNorthernBeetles
Pielou’sevenness,wealsousedtheChao1index[34],anasymptoticestimator,togenerateun-
biasedestimatesofexpectedspeciesrichnessateachlocation.Theanalyseswereperformed
usingtheveganpackage[35].
Totestwhetherbeetleswouldconformtopredictedlatitudinalgradientsofdiversity,we
usedlinearregressionstotesttherelationshipbetweeneachdiversityindexandlatitude.The
indiceswerelog-transformedpriortorunningthemodelstoensuredatametassumptionsof
normality.Weremovedextremeoutlierstoimprovemodelfit:IQAforobservedspeciesrich-
ness,Chao1andfunctionalrichness,andNORforevennessandSimpson’sindex.
Inadditiontoidentifyinglatitudinaldiversitypatterns,wewantedtodetermineiflatitude
hasanindirecteffectondiversitythatismediatedbyotherbiotic,climaticorspatialfactors.
Confirmatorypathanalysisisonemethodoftestingmultivariatecausalhypothesesthatcannot
betestedthroughrandomizedexperiments[36].WeusedShipley’sd-separationtestofcausal
hypotheses[36,37]toanalysetherelationshipsbetweenlatitude,diversity,andmechanistic
factors.Inthisanalysis,thecausalhypothesesarerepresentedbyasetofstructuralequations
thatarevisualizedasapathmodel(ordirectedacyclicgraph;DAG),andthesecausalrelation-
shipsimplyindependencerelationsbetweenotherpairsofvariables(abasisset),eitherdirectly
orafterconditioningonothervariables.Usingthepackageggm[38]wedevelopedthreealter-
nativeDAGswithhypothesisedrelationshipsbetweenlatitude,speciesandfunctionaldiversity,
andseveralmechanisticvariables,thengeneratedthebasissetsoftheconditionalindependen-
ciesresultingfromthemodels.
Theinitiallistofmechanismsconsideredincludedtheclimateandvegetationvariablesde-
scribedintheprevioussection.Inordertoavoidissuesassociatedwithautocorrelationandto
achievethemostparsimoniousDAG,wereducedthenumberofvariablesinthemodelbycon-
sideringthestatisticalandecologicalrelationshipsbetweenthevariables.Firstwevisualized
thedatawithscatterplotsofallpairwisecombinationsofvariables.Alltemperatureandsun-
relatedvariableswerecorrelated,soMeanTempwasselectedasaproxyforalltemperature
measures.ThereweresomemissingWindvaluessothisvariablewasomitted.Precipitation
wasmoderatelycorrelatedwithMeanTemp,butsinceitcapturesaverydifferentcomponentof
climate,andbecausemoisturecanbeaconstraintfornorthernarthropods[18,30],itwasre-
tained.AllvegetationclassBraun-Blanquetscoreswereretained,aswasMaxVegHt.Next,we
examinedthevarianceinflationfactorsusingtheviffunctionintheHHpackage[39].Somein-
flationfactorswerehighduetorelationshipsbetweendifferentvegetationclasses,suggesting
weshouldselectonerepresentativevegetationvariable.Weadditionallyperformedstepwise
AICmodelselection,whichselectedMeanTemp,MaxVegHtandTotPrecipasindependent
variables.Thesestepsandresultsledustoconcludetheselectionwiththesethreeecologically
significant,noncolinear(VIFbetween2.2and3.5)variables.
Weconsideredthreeplausiblealternativemodelstotesttherelationshipbetweenlatitude
andspeicesrichness(Fig1A–1C).Theseincludedthesimplestmodel(Fig1A),whereonlylati-
tudeactsonthemediatingeffects,andtwomodelswhereinteractionsbetweenmediatingfac-
torswereconsidered.InmodelB,temperature(T)isdirectlyinfluencedbylatitude,whichin
turnaffectsplantheightandprecipitation.InmodelC,precipitationadditionallyinfluences
vegetationheight.ForeachDAG,wetestedallhypothesizedindependencesbetweenthesevari-
ablesandthenconductedoveralltestsofthemodelsusingFisher’sCtests.Weperformeda
separatepathanalysisforfunctionalgrouprichness.Again,wedevelopedthreeplausiblemod-
els,usingthebest-fitDAGforspeciesrichnessasastartingpoint(Fig1,Model1),becauseit
suggestedthatinteractionsbetweentemperature,vegetationandprecipitationwerenotimpor-
tant.Sincefunctionalgrouprichnessislikelytoincreaseasmorespeciesareadded(asshown
earlier,thereisapositiverelationshipbetweenspeciesandfunctionalrichness),weincluded
speciesrichnessinthemodel.Thethreemodelswedevelopedweredesignedtodetermine
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BiodiversityofNorthernBeetles
Fig1.Mapofthe12studylocations(NorthPoleAzimuthalprojection),showingthespatial
distributionoffunctionalgroups.FGswerepooledintotrophicgroups,andthepiechartsshowthe
proportionofthetotalsitebiomassrepresentedbyeachtrophicgroup:carnivore(black),herbivore(white),
omnivore(grey)anddetritivore(diagonallines).Piechartsizesaregraduatedaccordingtotheproportionof
theentirestudy’sbeetlescollectedatthatsite.Higharcticsites:HAZ(LakeHazen,NU);BAN(BanksIsland,
NWT);CAM(CambridgeBay,NU);IQA(Iqaluit,NU).Subarcticsites:KUG(Kugluktuk,NU);TOM
(TombstoneMtns.,YT);CHU(Churchill,MB);SCH(Schefferville,QC).Northborealsites:NOR(Norman
Wells,NWT);YEL(Yellowknife,NWT);GOB(GooseBay,NFLD);MOO(Moosonee,ON).
doi:10.1371/journal.pone.0122163.g001
whether:a)latitudeactsdirectlyonfunctionalrichness,b)theeffectsoflatitudearemediated
byspatialorbioticvariables,and/orc)speciesrichnessprovidesasecondlevelofmediation
(i.e.,thattheeffectsoftemperature,vegetationandprecipitationonfunctionalrichnessare
solelyoradditionallymediatedbyspeciesrichness).
Wehypothesizedthatassemblageswoulddemonstratespatial(latitudinal)gradientsofsim-
ilarity.Totestthis,wevisualizedspeciesandfunctionalassemblagesateachlocationwithnon-
metricmultidimensionalscaling(NMDS)ordinations,usingtherich[40]andvegan[35]pack-
ages.Non-metricmultidimensionalscalingisanindirectordinationapproachthatmaximizes
therankordercorrelationbetweendistancesinadistancematrix,andthefunctionweused
(metamds)usesmultiplerandomstartingconfigurationstofindastableglobalsolutionforthe
ordination.Assemblagesthataremoresimilartoeachotherarearrangedmorecloselyinthe
ordinationspace.Inthiscase,wegeneratedtheordinationsusingBray-Curtisdistancematri-
cesoflog+1transformedabundances(species)orbiomasses(functionalgroups).Becausethe
speciesassemblagesatHAZwerecomposedofonlyasinglespecies,weomittedthelocation
fromtheNMDSanalyses.Wewerealsointerestedintherelationshipbetweenspeciesand
functionalassemblages,sowecomparedthecongruenceofthetwoordinationsusingProcrus-
tesrotationanalysis,andassessedthecorrelationbetweenthenusingapermutationalstatistic
calculatedbythefunctionprotest[41].
Totesttherelationshipsbetweenassemblagestructureandspatial,climaticandbioticfac-
tors,theenvfitfunctionintheveganpackagewasusedtofiteachfactorontheordinationsas
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vectors.Thedirectionofeachvectorindicatesthedirectioninwhichthelinearchangeinthe
variableisthefastest,andthelengthofthevectorisproportionaltothestrengthofthecorrela-
tionbetweenthevariableandthepositionoftheassemblagesinordinationspace.Thisfunc-
tionprovidesanobjectiveinterpretationoftheresultsofunconstrainedordinationanalyses
andgeneratesameasureoffitaswellasasignificancevaluebasedonapermutationtest(1000
permutations).
Results
Wecollected9062beetles:2832inthehigharctic,3275inthesubarcticand2955inthenorth
borealzone.Abundancesvariedbetweenlocations,rangingfrom14individualscollectedfrom
thenorthernmostlocation(LakeHazen,NU),to1696individualsfromCambridgeBay,NU,
alsointhehigharctic(Fig1).Therewasnorelationshipbetweenlatitudeandabundance.Thir-
ty-fourfamiliesand155generawererepresentedinthecollection.Over80%ofthecollection
wascomprisedofthreefamilies:Carabidae(6221individuals,68.8%oftotalnumberofspeci-
mens),Staphylinidae(870,9.6%),andCryptophagidae(247,2.7%).Wefoundstaphylinidsat
alllocations,carabidsatalllocationswiththeexceptionofLakeHazen,andcryptophagids
onlyfromlocationsinthesubarcticandborealzones.WeprovidealistofalltaxainS1Table,
andacompletedatasetofindividualspecimenrecordsisavailableathttp://doi.org/10.5886/
5dvj8642.
Species-levelidentificationsweredonewheneverpossible,thoughwefocusedourefforts
onidentifyingthemostabundantandwidespreadtaxa(e.g.,Carabidae,Staphylinidae)and
thosethataretaxonomicallywellknown.Intotal,weidentified464speciesandmorphospe-
cies,andrichnessrangedfromasinglespeciesobservedinLakeHazen,to115inthenorthbo-
reallocationMoosonee,ON.Amongthesampleswere15newprovincialand/orterritorial
(cid:1)
records(denotedbyanasterisk, ,nexttothespeciesnameinS1Table).Mostspecieswere
foundatthreeorfewerlocations,butothersweremorewidespread.Forexample,inthehigh
andsubarcticzones,Pterostichushaematopus,P.brevicornis,P.caribouandAmaraalpina
wereparticularlyabundantandwidelydistributed;together,thesefourspeciesrepresented
44.0%ofthetotalcatch.
Eighteenfunctionalgroupswereidentified,representingdiversespecialistandgeneralisther-
bivoresandcarnivores,omnivores,andsaprophages,withdifferentfoodsourcesandmodesof
feeding(S2Table).Functionalgrouprichnessrangedfrom1inLakeHazento13groupsintwo
ofthesubarcticlocations(NormanWellsandYellowknife,NWT)andoneboreallocation,
Moosonee.CarnivorescomprisedthemajorityofthebiomassinallsitesexceptGooseBay,
NFLD,whichwastheonlylocationwhereherbivoreshadthegreatestproportionofbiomass
(Fig1).Strictherbivoreswerelargelyabsentfromthesubarcticandhigharcticlocations,but
werewellrepresentedintheborealsites.AlthoughMoosoneehadveryhighspeciesandfunc-
tionalrichness,highnumbersoftwoverylargepredaciouscarabidbeetleswerecaught(Cara-
busmaeanderandC.melanarius),andthesedominatedtheoverallbiomass(Fig1).Omnivores
wereexceptionallyprominentinthehigharctic,representedprimarilybytheopportunistically
predaciousgranivore,Amaraalpina(Fig1).
Diversity
Bothobservedspeciesrichness(P=0.002,R2=0.6345,F=18.36,df=9)andexpectedspecies
richness(P=0.002,R2=0.6411,F=18.86,df=9)exhibitedstrongnegativerelationships
withlatitude.TheSimpson’sindexsimilarlydeclinedsignificantlywithlatitude(P=0.0043,
R2=0.5723,F=14.38,df=9),butevennesshadnosignificantspatialpattern(P=0.06,
R2=0.3078,F=5.002,df=8).Functionalgrouprichnesshadahighlysignificantnegative
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relationshipswithlatitude(P=0.0007,R2=0.7101,F=25.49,df=9).Functionalgroup
richnessandspeciesrichnesshaveaverystrongpositivelinearrelationship(P<0.0001,
R2=0.9307,F=148.6,df=10).
Inourpathanalysisofspeciesdiversity,weoptedtoincludeMeanTemp,TotPrecip,and
MaxVegHt,asmediatingeffects.Fisher’sCtests(whichsimultaneouslytestallindependencies
inaDAG)indicatedthat,whileallthreemodelsweregoodfitswithnoimportantdependencies
missing,thefirstandsimplestmodel(Fig2A)providedthebestoutcome(Fishers’sC=3.637,
P=0.888).Thepathanalysisindicatesthatspeciesrichnessisindirectlyaffectedbylatitude.
MeanTemp,MaxVegHtandTotPrecipalldeclineaslatitudeincreases.MeanTempandMax-
VegHthavepositiveeffectonspeciesrichness,whileTotPreciphasanegativeeffect.The
strengthanddirectionoftherelationshipsdeterminedbythepathanalysisforthebest-fit
modelareshowninFig2(Model1).Forfunctionaldiversity,Fisher’sCtestsindicatedthatall
threemodels(D-F)wereacceptable,butmodelFwasthebestfit(Fisher’sC=5.099,P=0.984).
InthispathanalysiswefoundthatMeanTempandspeciesrichnessbothhadapositiveeffect
onfunctionalrichness,andthatlatitudehadanindirectnegativeeffectthroughthesemediating
factors(Fig2,Model2).Relationshipsbetweenfunctionalgrouprichness,andMaxVegHtand
TotPrecip,didnotimprovethemodel.
Fig2.Threealternative(A,B,andC)directedacyclicgraphs(DAG)ofhypothesizeddirectandindirecteffectsonspeciesrichness(S).Model1:
resultsofbestfitpathmodel(derivedfrommodelA).D,EandFarealternativeDAGsofhypothesizedeffectsonfunctionalgrouprichness(F).Model:results
ofthebestfitpathmodel(frommodelF).Thedirectionofthearrowindicatesthedirectionoftherelationship.Solidlinesindicateasignificantrelationship.
EstimatedcoefficientsandP-valuesareshownforeachrelationship.Latitude(L),TotPrecip(P),MeanTemp(T)andMaxVegHt(V)
doi:10.1371/journal.pone.0122163.g002
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BiodiversityofNorthernBeetles
Assemblagestructure
SolutionswerereachedfortheNMDSordinationsofthespeciesassemblages(stress=0.079913,
Fig3a)andthefunctionalassemblages(stress=0.05501,Fig3b).WiththeexceptionofCAM
andBAN,whosespeciesassemblagesessentiallyoverlapintheordinationspace,alllocations
displayeddistinctspeciesandfunctionalassemblages.Thearrangementofthelocationsinthe
ordinationsindicatedthatthoseinthesameecoclimaticzonehadsimilarassemblages,and
thattherewerecleardelimitationsbetweenzones.Functionalassemblagesfromthewestern
andeasternpartofthecontinentshowaslighttendencytoassembleclosertogether,evenbe-
tweenecoclimaticzones;thisismorepronouncedforlocationsinthenorthborealandsubarc-
tic.Agradientofsimilaritywasevidentbetweenecoclimaticzones:assemblagesinthe
Fig3.Non-metricmultidimensionalscalingordinationsshowingsimilaritiesbetweena)species,and
b)functionalassemblagesfromateachlocation.Shadedpolygonsareusedtodelimittheordination
spaceoccupiedbysitescontainedwithinthesameecoclimaticzone.Spatial,climatic,andbioticvariables
thataresignificantlycorrelatedwiththeassemblagesareplottedontheordinationspaceasvectors(for
clarity,non-significantvariablesareomitted,andonlythemostimportanttemperature-relatedvariableis
includedforeachordination);thelengthofthevectorindicatesthestrengthofthecorrelation.(HAZhasbeen
omittedasanoutlerfrombothordinations,asitcontainsonlyone,uncommonspecies).
doi:10.1371/journal.pone.0122163.g003
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BiodiversityofNorthernBeetles
subarcticweremorelikethosefoundinthehigharctic,whilenorthborealassemblageswere
moresimilartosubarcticthantohigharcticassemblages.Speciesassemblagestructurewassig-
nificantlycorrelatedwithlatitude(S3Table).AccordingtotheProcrustesrotationanalysis,the
speciesandfunctionalordinationswerecorrelated(Procrustessumofsquares=0.36157,Pro-
crustescorrelation,r=0.7964,P=0.001,999permutations).
Speciesandfunctionalassemblagestructureweresignificantlycorrelatedwithalltempera-
ture-drivenclimaticfactors,withtheexceptionthatfunctionalassemblageswerenotrelatedto
DDBO(S3Table).BothassemblagetypeswerealsosignificantlyrelatedtoFrost.Speciesas-
semblagestructurewasadditionallysignificantlyrelatedtoTotPrecip,whilefunctionalassem-
blagealsohassignificantrelationshipswithSoilD,VarSoilDandGramCov.Otherwise,the
plantcommunitycompositionisnotrelatedtoeitherspeciesorfunctionalassemblagestruc-
ture.Variablesthataresignificantlyrelatedtoassemblagestructureareplottedasvectorsin
Fig3(onlythetemperaturefactorwiththestrongestrelationshiptoassemblagestructureis
shown,forclarity).
Discussion
Atalargespatialextent,thediversityandassemblagestructureofnorthernbeetlesshowstrong
latitudinalgradients,primarilyduetomediatingeffectsofclimate,particularlytemperature.
Ourresearchspannedalatitudinalgradientof30°,includedthreeecoclimaticzones,andused
standardizedfieldsamplingofterrestrialbeetlestouncoverbiodiversitypatternsacrossalarge
geographicextent.Macroecologicalstudieswithextentsthatrangelessthan20°oflatitudeare
unlikelytoshowclearspatialpatternsofspeciesrichness,evenifsuchpatternsexist[3];our
findingsshouldthereforeprovideanaccurateportrayalofspatialbiodiversitypatternsinthe
farnorth.Wefoundthatbeetlespeciesrichness(observed,predicted,andasmeasuredbythe
Simpson’sindex)exhibitsclassicalnegativerelationshipswithlatitude.Thisalignswithgeneral
observationsofbeetlerichnessdrawnfrompresence/absencedatainarcticandnorthernboreal
regions[19]andwithsimilarpatternsdescribedforotherinsecttaxaatacontinentalscalein
temperateregions,includingantsinEurope,andgrasshoppers,butterfliesanddungbeetlesin
NorthAmerica[3].
Evennessshowedatendencytodeclinewithincreasinglatitude,butthegradientwasnon-
significant.Speciesrichnessandevennesscanbe,andoftenare,similaralonglatitudinalgradi-
ents,buttheymayalsoshownoorevennegativerelationshipswitheachother,astheymayre-
flectdifferentaspectsofspatialvariationinspeciesassemblagesanddifferentresponsesto
ecologicalfactors[42,43].Alternatively,ifassemblagesaremorespecies-richbecausetheypos-
sessgreaternumbersofrarespecies,thenwemightexpectlatitudinaldecreasesinrichnessto
beaccompaniedbygreaterevenness[3].Whilethemajorityofourrarespecies(singletonsand
doubletons)wereindeedlocatedinthemoresoutherlylocations,thisdidnottranslatetoapos-
itivelatitudinalevennesstrend.
Inourstudy,functionalgrouprichnessshowedaverystrongclassicalspatialgradient,de-
creasingsharplywithincreasinglatitude.Studiesonlarge-scalelatitudinalpatternsofanimal
functionaldiversityarescarceandtheirconclusionsarevariable.Stevensetal.[14],forexam-
ple,foundthatNewWorldbatfunctionaldiversity,richnessanddominancestronglyincreased
towardstheequator,andmammalianfunctionaldiversitygenerallyappearstodisplaythe
samepattern[15].Conversely,ananalysisofinvertebratescollectedfrom1000streammoni-
toringstationsintheUSshowedthatfunctionalrichnessdecreasedonlyweaklywithincreased
latitude[44].
Specificfunctionalgroupsorguildsmaydisplaydifferentlatitudinaldiversitypatterns.For
example,alargesurveyofthefreshwaterdetriviorousshredderguildrevealedmuchhigher
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