Ecology and Evolution 2017; 7: 771–779  | 771www.ecolevol.org

Received:6July2016  |  Revised:13October2016  |  Accepted:29October2016DOI:10.1002/ece3.2640

O R I G I N A L R E S E A R C H

Plant invasion and speciation along elevational gradients on the oceanic island La Palma, Canary Islands

Manuel J. Steinbauer1,2† | Severin D. H. Irl1† | Juana María González-Mancebo3 |  Frank T. Breiner4,5 | Raquel Hernández-Hernández3 | Sebastian Hopfenmüller6 |  Yohannes Kidane1 | Anke Jentsch7 | Carl Beierkuhnlein1

ThisisanopenaccessarticleunderthetermsoftheCreativeCommonsAttributionLicense,whichpermitsuse,distributionandreproductioninanymedium,providedtheoriginalworkisproperlycited.©2016TheAuthors.Ecology and EvolutionpublishedbyJohnWiley&SonsLtd.

†Equalcontribution.

1DepartmentofBiogeography,BayCEER,UniversityofBayreuth,Bayreuth,Germany2SectionEcoinformatics&Biodiversity,DepartmentofBioscience,AarhusUniversity,Aarhus,Denmark3DepartamentodeBotánica,EcologíayFisiologíaVegetal,UniversidaddeLaLaguna,SanCristóbaldeLaLaguna,Tenerife,Spain4SwissFederalResearchInstituteWSL,Birmensdorf,Switzerland5DepartmentofEcologyandEvolution,UniversityofLausanne,Lausanne,Switzerland6DepartmentofAnimalEcologyandTropicalBiology,UniversityofWürzburg,Würzburg,Germany7DepartmentofDisturbanceEcology,BayCEER,UniversityofBayreuth,Bayreuth,Germany

CorrespondenceManuelJ.Steinbauer,SectionEcoinformatics&Biodiversity,DepartmentofBioscience,AarhusUniversity,Aarhus,Denmark.Email:steinbauer@bios.au.dk

Funding informationEuropeanUnion’sHorizon2020,Grant/AwardNumber:641762;Net-Biometransnational;GermanResearchFoundation(DFG);UniversityofBayreuth;DanishCarlsbergfondet,Grant/AwardNumber:CF14-0148.

AbstractEcosystems that provide environmental opportunities but are poor in species andfunctionalrichnessgenerallysupportspeciationaswellasinvasionprocesses.Theseprocessesareexpectednottobeequallyeffectivealongelevationalgradientsduetospecificecological,spatial,andanthropogenicfilters,thuscontrollingthedispersalandestablishment of species. Here, we investigate speciation and invasion processesalongelevationalgradients.Weassessthevascularplantspeciesrichnessaswellasthenumberandpercentageofendemicspeciesandnon-nativespeciessystematicallyalongthreeelevationalgradientscoveringlargepartsoftheclimaticrangeofLaPalma,CanaryIslands.Speciesrichnesswasnegativelycorrelatedwithelevation,whilethepercentageofCanaryendemicspeciesshowedapositiverelationship.However,thepercentageofCanary–Madeiraendemicsdidnotshowarelationshipwithelevation.Non-native species richness (indicating invasion) peaked at 500m elevation andshowedaconsistentdeclineuntilabout1,200melevation.Abovethatlimit,nonon-nativespecieswerepresentinthestudiedelevationalgradients.Ecological,anthropo-genic,andspatialfilterscontrolrichness,diversification,andinvasionwithelevation.With increase in elevation, richness decreases due to species–area relationships.Ecologicallimitationsofnativeruderalspeciesrelatedtoanthropogenicpressureareinlinewiththeabsenceofnon-nativespeciesfromhighelevationsindicatingdirec-tionalecologicalfiltering.Increaseinecologicalisolationwithelevationdrivesdiversi-fication and thus increased percentages of Canary endemics. The best preservedeasterntransect,includingmaturelaurelforests,isanexception.ThehighpercentageofCanary–Madeiraendemicsindicatesthecloudforest’senvironmentaluniqueness—andthusecologicalisolation—beyondtheMacaronesianislands.

K E Y W O R D S

alienspecies,altitudinalgradient,colonization,diversification,diversity,endemism,exotic,high-elevationecosystems,islandbiogeography

772  |     STEINBAUER ET Al.

1  | INTRODUCTION

Elevational gradients on isolated mountains (especially on oceanichigh-elevationislands)poseauniqueopportunitytostudyimportantecological processes, including evolutionary dynamics (Steinbaueretal.,2016)andplantinvasion(Alexanderetal.,2011;Daehler,2005).For both processes, oceanic high-elevation islands offer a uniquesetting provided by the high rates of species evolved in situ (neo-endemism;Whittakeretal.,2007).

Islandstendtobemorepronetoinvasionthanareaswithhigherdegreesofconnectivityonthecontinent(island susceptibility hypothe-sis,Simberloff,1995)duetolowspeciesrichness(i.e.,ahigherrelativeecological opportunity), functional diversity, and reduced competi-tiveness of species compared to those of themainland (Sol, 2000).However,recentstudiesindicatethattheislandsusceptibilityhypoth-esisisnotgenerallyvalid.Presumably,theisolationofislands(smallerregional speciespool) also impedes theestablishmentofnon-nativespecies(Etherington,2015;Jeschke,2008;Vila,Pino,Montero,&Font,2010),whichnowadaysmainlyoccursviadirectanthropogenicvectors.

Introductionsofnon-nativespeciesarenotequallyprobablealongelevationalgradients.Accordingly,themajorityofnon-nativeplantspe-ciesisexpectedinlowerelevationsandlessextremehabitats,whereclimaticconditionsaremorefavorable,humandisturbancesaremostfrequent,and isolation is lowest (Becker,Dietz,Billeter,Buschmann,&Edwards,2005;Pauchardetal.,2009).Extremeclimaticconditionsof high-elevation ecosystems and low anthropogenic disturbancespreventtheestablishmentofnon-nativespecies(butseeexceptions,e.g.,Irl,Jentsch,&Walther,2013;Alexanderetal.,2016).Hence,itisprobablethatthemajorityofintroducednon-nativesarepre-adaptedtomid-orlowelevations(Pauchardetal.,2009).Accordingly,thelow-elevationalienfloramaybethemainspeciespoolforthealienfloraofhigh-elevationecosystems(McDougalletal.,2011).Incontrasttohigh-elevationspecialists,invasivenon-nativespeciestendtobegen-eraliststoleratingawiderangeofclimaticconditions(Alexanderetal.,2011;Higgins&Richardson,2014),althoughthedistributionpatternin the area of introduction depends on the interaction of traits oftheinvadingspeciesandtheenvironmentalconditions(Haideretal.,2010).

Onmountainousoceanic islands, low-elevationecosystemshavelarger spatial extent than high-elevation ecosystems—a typical fea-tureofmountainsystems(butseeElsen&Tingley,2015).Especiallytropicalandsubtropicalislandspossessabroadclimaticrangeduetowindward/leeward effects and thus a comparatively high absolutenumberofhabitats(Hortal,Triantis,Meiri,Thebault,&Sfenthourakis,2009;Irletal.,2015).Asaconsequence,mostspeciesonoceanicis-lands—non-natives, natives, or endemics—will be found in low- andmid-elevationecosystems(Zobeletal.,2011).Inordertounderstandinvasionprocesses,however,itisofhighinterestwhetherthepercent-ageofnon-nativespeciesincreaseswithelevation.Thepercentageofnon-nativespeciescanbeseenasanindicatorforinvasionprocessesindependent of overall richness (c.f.,Gillespie,Claridge,&Roderick,2008)althoughitisstillsusceptibletodifferencesindisturbances.

Recent studies suggest that diversification, as indicated by twofrequentlyusedindices(i.e.,thepercentageofsingle-islandendemicspeciesandthepercentageofarchipelagoendemicsspecies;Emerson&Kolm,2005;Whittakeretal.,2007),increaseswithelevation—apat-ternwhich has large-scale support for islands aswell as continen-tal mountains (Steinbauer etal., 2016). However, this trend can bemodifiedonan intra-insular levelbyotherenvironmental influencessuchasdifferentaspectsofclimate(esp.precipitation;Irletal.,2015)andshouldbestrongerthemorepronouncedenvironmentalchangesalongtheelevationalgradientare.Additionally,diversificationissup-portedbygeneticisolation(andhenceatendencytowardspeciation)andtheavailabilityofecologicalopportunitiesduetolowspeciesrich-ness(Hughes&Eastwood,2006;Steinbauer&Beierkuhnlein,2010).

Directional ecological filtering along elevational gradients(Alexander etal., 2011) causes isolation of high-elevation ecosys-tems(Gillespie,2004).Particularlyonoceanicislands,high-elevationecosystems tend to bemore isolated from the nearest ecosystemswithcomparableenvironment than their low-elevationcounterparts(Steinbaueretal.,2016).Thelatterarelessdistantfromsimilarhabitatsonotherislandsandonthemainland(Steinbauer,Irl,&Beierkuhnlein,2013).Inconsequence,high-elevationsystemsformislandswithinis-lands(Fernández-Palacios,Otto,Thebaud,&Price,2014).Elevationalgradientsthusprovideanexcellentnaturalsettingtoinvestigatetheeffectofisolationonspeciationandinvasionprocesses.

Here,wetestwithaspecificallydesignedfieldstudythefollowingpredictionsusingvegetationsurveysalongthreeelevationgradientsrangingmorethan2000monLaPalma(CanaryIslands).Basedoncur-rentknowledge,weexpect(1)thatthevascularplantspeciesrichnessaswellasthenumberofendemics(endemictoLaPalma,endemictotheCanaryIslandsorsharedbetweenMadeiraandtheCanaryIslands,respectively)decreases,whilethepercentageofendemicspeciesin-creaseswithelevation.(2)Theabsolutenumberofnon-nativespeciesdecreases,whilethepercentageofnon-nativespeciesincreaseswithelevation, if theavailabilityofunoccupiedhabitatsathigh-elevationecosystems significantly enhances the establishment of non-nativespecies.Thecontrary istobeexpected, iftheextremeenvironmen-talconditionsathighelevationsposeamoreeffectivefilterfornon-nativespeciesthanthealreadybypassedgeographicalisolation.

2  | MATERIALS AND METHODS

2.1 | Study area

LaPalmaisoneofthemostwesternandyoungestislandsoftheCanaryarchipelago(~1.7Ma;Carracedoetal.,2002)expressingcontinuousvol-canicactivity(Prietoetal.,2009).Theislandcovers706km²andreachesamaximumelevationof2426ma.s.l.attheRoquedelosMuchachos(Figure1).ThenorthernpartofLaPalmaiscomprisedbytheCalderadeTaburientecomplexandistheoldestandsteepestpartoftheisland,thusofferingalsosteepenvironmentalgradients(Irletal.,2015).

Owingtoitsnorthwesternposition,LaPalmareceivesfairamountsofprecipitationinwinter,yetextensivedryperiodsarecommononthewesternsideoftheislandandathighelevations(Irl&Beierkuhnlein,

     |  773STEINBAUER ET Al.

2011). Besides elevation, the dominating climatic influence factorsarehumidtradewindsregularlyblowingfromthenortheast(Garzón-Machado,Otto&delArcoAguilar,2014)anddividingtheislandintoahumidwindwardsideandadroughtandfire-proneleewardside.Thus,themainprecipitationgradientoftheislandrunsalonganortheast–southwestaxisreachingfromthewesterncoast(approx.120mm/a)tothehigh-elevationnortheasternslopesoftheCalderadeTaburiente(approx.1,300mm/a;AEMET,2012).Abovethecloudbank,athermalinversion layer exists that impedes an orographic rise of moist airmasses eliminating the tradewind influence.Above the thermal in-version layer,highsolar radiationandhighdaily temperatureampli-tudesarecommon.There,wintersnowappearsregularlyandevenicestormscanoccur(Garzón-Machadoetal.,2014).

2.2 | Sampling design

Threetransectsforfloristicinventorywereinstalledacrossthecom-pleteelevationalgradient.Onereachedthecalderarimfromthedrywestcoast,onefromthemorehumidnorthcoast,andonefromthenortheastthroughthelaurelforestbelt,thus,systematicallydesignedtocoverallvegetationzonesandthecompleteprecipitationgradientofferedbyLaPalma(Figure1).Withineachtransect,weinstalledplots(10×10m) atpreselectedelevational levels.Criteria forplot selec-tionwere accessibility and a slope not higher that 35°, but alwaysuninfluenced by roads. Along the western and northern transects,weestablished21plotsdistributedevenlybetweensevenelevationlevels spanning100mand separatedby300-m intervals (note: theintervalbetweenthesecondhighestandthehighestelevationlevelonly measured 200m, because a maximum elevation of 2,400ma.s.l.couldnotbeexceeded).Threereplicatesper levelwereestab-lished,which is in accordancewith guidelines for optimal samplingalong environmental gradients (Schweiger, Irl, Steinbauer, Dengler,&Beierkuhnlein,2016).DatasamplingonthewesternandnortherntransectstookplaceinFebruary2009.Alongtheeasterntransect,24plotsdistributedacross12elevationlevelswith200-mintervalswereinstalled.Thus,tworeplicateswereestablishedper level.Datasam-plingfortheeasterntransecttookplacefromFebruarytoApril2013.AllrawdataareprovidedinonlineAppendix.

Werecordedallfloweringplantspeciesandclassifiedthemassingle-island endemics, archipelago endemics (i.e., endemic to the CanaryIslands), and non-native and nonendemic (including definitely native,possiblynativeandprobablynative)accordingtoAcebesGinovésetal.(2010).Inourstudy,thegroupofnon-nativesconsistsofallsub-classesgivenbyAcebesGinovésetal.(2010),thatis,differentiatedintointro-ducedspecies,probablyintroducedspeciesandintroducedinvasives.

As the Canary Islands share a high number of endemic specieswithMadeira,wealsoclassifiedCanary–Madeira endemics.ThisgroupincludesalltheCanaryendemicsplusthespecieswithagloballyre-stricted distribution range to the Canary Islands andMadeira. TheCanary Islands andMadeira currently are located at a distance lessthan400kmandwereevencloserduringtimeperiodswithlowersealevel(Fernández-Palaciosetal.,2011).Wefurtheridentifiedasubsetof Madeira–Canary endemics by excluding those species that onlyoccurredontheCanaries—thusfocusingonlyonendemicspeciesin-habitingbotharchipelagos(furtheronreferredtoasMadeira–Canary twins).

2.3 | Statistical analysis

Therelationbetween indices (forrichness,endemism,and invasion)with elevationwas assessed using generalized linearmixedmodels(R package lme4 version 1.1-7, Bates,Maechler, Bolker, &Walker,2015). Fullmodels includedelevation,elevation2, transect aswell astheinteractionsbetweentransectandelevationasexplanatoryvari-ables (y~elevation+elevation2+transect+transect:elevation). In afurther step, elevation, elevation2, and/or the interaction betweentransectandelevationwasremoved ifnotsignificantlycontributingto theexplanatorypowerof themodel. The laterwas testedusingaFtest(ANOVA)betweenamodelincludingandoneexcludingthefocalvariable (backwardselection;Table1providesanoverviewonfinalmodelspecifications).Asvegetationplotsarereplicatedperel-evationallevel,weincludedablockfactorintherandomstructureofthemodelstoaccountforthisspatialclustering.

Poisson error distribution (log-link function) was used for allrichness-based indices, while binomial error distribution (logit-link function) was used for percentage values. The binomial error

F IGURE  1 MapofLaPalma.LaPalmaisthenorthwesternmostislandoftheCanaryIslandssituatedoffthecoastofMorocco(Africa)intheAtlanticOcean.Themapillustratesthelocationofthewestern(circles,n = 21),northern(squares,n = 21),andeasterntransects(triangle,n = 24)coveringalargeelevationalgradientofmorethan2400mindetail.Duetothepredominanttradewinddirectionfromthenortheast,precipitationincreasesfromwesterntoeasterntransect,especiallyatmid-elevations

774  |     STEINBAUER ET Al.

distribution forpercentagevalueshas theadvantage toaccount forthefactthataparticularpercentagevalueismorepreciseifitisbasedonalargernumberofobservations(herethenumberofspecies).Notethatpercentageofendemicspeciesiscalculatedexcludingnon-nativespecies to get a cleaner signal for diversification. Patterns remain,however,ifnon-nativeswereincluded.

Goodness-of-fitstatisticsfortheimplementedmodelswereper-formedcalculatingmarginalandconditionpseudo-R2asimplementedinRpackagepiecewiseSEM (version1.1.3;Lefcheck,2015)basedonNakagawa and Schielzeth (2013). All statistical analyses were per-formedinRversion3.2.0(RDevelopmentCoreTeam,2015).

3  | RESULTS

3.1 | Diversification along an elevation gradient

Plantspecies richnessshowedahighlysignificantdecreasewith in-crease in elevation (p<.001). The northern transect showed bothhigherspeciesrichness(p > .01)andasteeperslope(Figure2a),asin-dicatedbyasignificantinteractionwithelevation(p > .05).Plantspe-ciesrichnessdecreasedfromvaluesatthelowestelevation(approx.100ma.s.l.).Richnessvaluesreachedfrommorethan30speciesonthenorthern througharoundhalfof thaton thewesternandeast-erntransectstolessthanthreespeciesinthesummitregion(approx.2,300melevation).Trendsremainedsimilarifnativespeciesrichnesswasanalyzed(non-nativespeciesnotincluded), indicatingtheirhighshareintotalspeciesnumbers(Figure2b).

Canary archipelago endemic richnesswas low atmid- and highelevationswithfewerthanfourspeciesperplot.Canaryarchipelagoendemicrichnessgenerallydecreasedwithelevationforalltransects,with similar declines from about ten species at 100m elevation toabout four speciesat thesummit (p < .001 forelevation,p < .01 for

elevation2,nointeractionbetweenelevationandtransect,Figure2c).ThenumberofCanaryarchipelagoendemicswassignificantlyhigheron the northern and western transects, with the difference beingabouttwotothreespecies.ThisdifferencedisappearsifthedefinitionofendemismincludessharedCanary–Madeiraarchipelagoendemics(Figure2h).

ThepercentageofCanaryarchipelagoendemicsdisplayedamin-imum around 750m for all transects (p < .001)with overall highestpercentage values (100%) in high elevations. Thewestern transectexpressedonaveragehigherpercentagevaluesthanthenorthernandeasternones(Figure2f,p < .001).ThepatternswitchesifthedefinitionofendemismincludedMadeira.TheproportionofCanary–Madeiraar-chipelagoendemicsishighestintheeasterntransectfollowedbythewesterntransect(p < .01).ThepercentageofMadeira–Canaryarchi-pelagoendemicsshowsamid-elevationminimumat1,000–1,400minteractingwithanoverallincreasewithelevationforthewesternandnortherntransectsthatisnotpresentontheeasterntransect(interac-tionp < .05,Figure2k).Theeasterntransectalsodifferedbyshowingamuchhigherproportionofendemicsreachingupto80%–100%.

Theoverallnumberofsingle-islandendemicspeciesislow(n = 14),makingtheir interpretationtentative.Single-islandendemicrichnessperplotshowedsignificantdifferencesbetweentransects (no inter-action), slightly declined on thewestern transect and increased onthenortherntransect(p < .01)butdidnotchangealongtheelevationgradientontheeasterntransect.Thenumberofsingle-islandendem-icsperplot increased from theeastern transect through thenorth-erntransecttothewesterntransect(p < .001,Figure2d).Overall,thewesterntransecthadnineandthenortherntransecteightsingle-islandendemicspeciescomparedtofourspeciesontheeasterntransect.

Thepercentageofsingle-islandendemicsdifferedsignificantlybe-tweentransects,withthewesterntransecthavingthehighestvalues.Thepercentageofsingle-islandendemicspeciesshowedanincrease

TABLE  1 Modelstructurefordifferentdependentvariables

Variable Model structure Family R2

MargR2

Cond

Overallrichness y~elevation***+transectn.sig+elevation:transect*+(1|block) Poisson .74 .79

Nativerichness y~elevation***+transectn.sig+elevation:transect*+(1|block) Poisson .72 .77

%Natives y~elevation*+elevation2*+transectn.sig+(1|block) Binomial .81 .81

Archipelagoendemics y~elevation***+elevation2**+transect*+(1|block) Poisson .40 .48

%Archipelagoendemics y~elevation***+elevation2***+transect*+(1|block) Binomial .71a –

Islandendemics y~elevation*+transect***+(1|block) Poisson .44 .56

%Islandendemics y~elevation2*+transect***+(1|block) Binomial .32a –

Canary–Madeiraendemics y~elevation***+elevation2*+transectn.sig+(1|block) Poisson .59 .59

%Canary–Madeiraendemics y ~elevation***+elev.2***+transect**+elev.:transect*+(1|block) Binomial .66a –

Madeira–Canarytwins y~elevation***+elev.2***+transectn.sig+elev.:transect**+(1|block) Poisson .83a –

%Madeira–Canarytwins y~elevation**+elevation2**+transect***+(1|block) Binomial .82 .83

Non-natives y~elevation*+elevation2**+transect**+(1|block) Poisson .77a –

%Non-natives y~elevation***+elevation2*+transectn.sig+(1|block) Binomial .50a –

Formodelsmarkedwithan“a”attheR2Marg,pseudo-R2wascalculatedafterremovingtherandomstructure(blockeffect)fromthemodelasthefunction

piecewiseSEMfailedtoprovidepseudo-R2valuesforthemixedeffectmodel.Significanceofexplanatoryvariables(basedonF-statistics)isindicatedwithstars(p < .05*;p < .01**;p < .001***).Marginal(R2

Marg)andconditional(R2

Cond)

pseudo-R2werecalculatedbasedonRpackagepiecewiseSEM(Lefcheck,2015).

     |  775STEINBAUER ET Al.

withelevationforthewesternandnortherntransectsbutnotfortheeasterntransect(overallp < .05,elevation2,Figure2g).

Canary–Madeiratwins(endemicspeciesoccurringonMadeiraandtheCanaryIslands)showedaunimodaldistributionthatpeakedatdif-ferent locationsalongtheelevationalgradient (p < .01forelevation,p < .001forelevation2,interactionp < .05,Figure2i).Forthewesterntransect,therelationshippeakedatlowelevations,whileonthenorth-ern andeastern transects, the relationshippeaked shortlybeloworabove500melevation,respectively.

TheshareofCanary–MadeiratwinsintheCanaryflorawashigh-estatmid-elevations(p < .001,nointeraction,Figure2l).Asexpected,thepercentagewashighestontheeasterntransect,whichtranscendsthecloudforestzone(p < .001).

3.2 | Invasion along elevational gradients

Non-native species richness reached its maximum around 500m(Figure2j) and afterward decreased for all transects with elevation(p < .001, no interactions), with an above average number of non-native species on the northern slopes. Non-native species richnessranged from zero to five species on the northern transect and zeroto two in thewestern andeastern transects.Aboveanelevationofapproximately1,200m,non-nativespeciesweremissing.Thepercent-ageofnon-nativesdisplayedasimilarmaximumat500mfollowedbyadecreasewithelevation(p < .05),withoutdifferencesbetweenislandaspect(Figure2m).Thepercentageofnon-nativesspannedfrom10%at500mtozeroaround1,200mandabove.

F IGURE  2 Totalspeciesrichness(a),nativespeciesrichness(b),single-islandandarchipelagoendemicrichness(candd),theirrespectivepercentagevalues(e–g).ThenumberofspeciesendemictoMadeiraandtheCanaryarchipelago(includingCanaryonlyendemics,h),aswellasMadeira–Canarytwins,whichareendemictobothislandgroups(excludingCanaryonlyendemics)(i)andnon-nativesspeciesrichness(j)aswellastheirpercentages(k–m).Regressionlinesresultfromgeneralizedlinearmixedeffectmodels

Western transect

Northern transect

Eastern transect

(a) (b) (c) (d)

(e) (f)

(g)

(h) (i) (j)

(k)

(l) (m)

Elevation asl [m]

Nat

ive

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Elevation asl [m]

# A

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Elevation asl [m]

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Elevation asl [m]

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Elevation asl [m]

# M

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# M

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ativ

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% N

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% M

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Elevation asl [m]

% M

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Elevation asl [m]

Ove

rall

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richn

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010

2030

0 1000 2000

010

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0 1000 2000

7080

9010

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040

80

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4060

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0 1000 2000

2040

6080

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60

0 1000 2000

01

23

45

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46

0 1000 2000

01

23

45

0 1000 2000

510

15

0 1000 2000

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4  | DISCUSSION

4.1 | Species diversity and diversification along elevational gradients

Inour study,plant species richnessaswell as thenumberofarchi-pelago (Canary and Canary–Madeira) endemic species decreasedwithelevation,acommonpatternwhichwaspreviouslyfoundonLaPalma(Irl,Steinbauer,Epperlein,etal.,2014)andotherislands(Trigas,Panitsa,&Tsiftsis,2013).Adecreaseinsurfaceareaofindividualel-evationalzoneswithincreaseinaltitudeismaderesponsibleforthiseffect(Lomolino,2000;butseeElsen&Tingley,2015).Furthermore,area is often seen as a surrogate for habitat diversity (Hortal etal.,2009). Larger areasmight support larger population sizes,which inturndecreasestheriskofextinction(MacArthur,1972).However,asweusedastandardizedsamplingdesignwithequallysizedplots,thereisnoeffectof(plot)areaonspeciesrichnessinourstudy.Rather,localspecies richnessmay be enhanced due to “sampling” from a largerspeciespoolgenerallyavailableatlowelevations(Kargeretal.,2011).Harshenvironmentalconditionswithclimaticextremeeventsathighelevationsmightadditionallydecreasetotalandnativespeciesrich-nessanddensity(habitatharshnesshypothesissensuDefeo,Gomez,&Lercari,2001).

Thegeographic isolation that reduces thenumberof individualspre-adaptedtohigh-elevationecosystemsarrivingonanislandmayen-hancediversificationbygeneticisolation(nonadaptiveallopatricspe-ciation).Speciescommunitiesfilteredbyisolationmayfurtherprovideecologicalopportunity for adaptive radiation tofill gaps in resourceusagethatareleftunoccupied(Steinbaueretal.,2016).Opportunity-drivendiversification is supportedbyphylogenetic studies (Bennett&O’Grady,2013).BothmechanismscanhelptoexplaintheincreaseinthepercentageofCanaryarchipelagoendemicsaswellastheper-centageofsingle-islandendemicswithelevation,expressingahigherdegreeofendemisminhigh-elevationecosystems(Steinbauer,Otto,Naranjo-Cigala,Beierkuhnlein,&Fernández-Palacios,2012;Verboom,Bergh,Haiden,Hoffmann,&Britton,2015). In contrast to richness-based indices (like the number of endemics), the percentage of en-demicspeciesindicatestowhichdegreelocalspeciespoolsonoceanicislandsoriginate from insituspeciation (thealternativeoriginbeingcolonizationfromotherareas,Irletal.,2015).

In summary, two explanations for the observed pattern in thethreeelevationalgradientsareinaccordancewiththesuggestedrel-ativeimmaturityofhigh-elevationecosystemscomparedtolowerel-evations (Fernández-Palaciosetal., 2014). First, in lowerelevations,species cannot realize their complete potential distribution due tobioticinteractions.Suchcompetitiveinteractions,however,decreasewithelevation,whichisparticularlytrueforhigh-elevationecosystemsowingtotheirlowspeciesrichness(butseeIrletal.,2012).Second,thespatial isolationofhigh-elevationspeciesorcladesprovidesthepossibilityfordiversification(speciationenhancedbyisolationaswellasbyecologicalopportunity;Steinbaueretal.,2016).Theimmigrationofpopulationsfromotherislandsdecreaseswithelevationduetolon-gerdistancestosourceregionsandsmallerareas(Merckxetal.,2015;

Steinbaueretal.,2013).Inaddition,locallydisturbancesareanaddi-tionalkeyfactorinfluencingthehigherpercentageofCanaryarchipel-agoandsingle-islandendemics.Particularly,plotsinthelowerpartsofthewesternandnortherntransects(0–800ma.s.l.)haveahighfrac-tionofnativespeciesthatareassociatedwithhabitatsmodifiedand/ordisturbedbyhumans (ruderalspecies),whichmight influencethepercentageofendemics.

On theeastern transect,which isbetterpreservedcompared tothenortherntransect(especiallythelaurelforest),theproportionofCanary–Madeiraendemicsdoesnot increasewithelevation. Incon-trast,single-islandendemicspeciesarevirtuallyabsentfromthistran-sect, not showingdifferencesbetween lowerandhigherelevations,andCanaryarchipelagoendemicsrichnessislow.Onereasonforthismaybethefactthatthelaurelforestsharesalargenumberoffloralel-ementswithMadeira(asisreflectedinthenumberofCanary–Madeiraarchipelagoendemics;Figure2i).Thishighlightsthatdispersalvectors(andthusisolation)differbetweenthemorehumidlaurophyllouseast-ernvegetationandthedrierwesternpartsofLaPalmaindicatingthatdispersalcharacteristicsvarybetweendifferentvegetationunitsevenwithinasingle island(Nogalesetal.,2015).However,futurestudieswillneedtoidentifywhetherdispersaltraitsdifferbetweenvegetationzonesonislandsandhowthisinfluencesdiversification.

Finally,ecologicalisolation,notonlygeographicalisolation,mightberesponsibleforthehighdiversificationinthelaurelforests.SincetheonsetoftheMediterraneanclimate, laurelforestsonlypersistedinMacaronesia,wheretheoceanicenvironmentbufferedtheclimaticoscillationsofthePleistocene(Fernández-Palaciosetal.,2011;Patiñoetal.,2016).Insummary,wesuggestthatenvironmentalfiltering,an-thropogenicdisturbance,andspatialandecologicalisolationofcom-munitiestobethemainfactorsresponsiblefortheobservedchangesinspatialtrendswithelevation.

4.2 | Elevational threshold for non- natives along the elevational gradients

Non-nativespeciesrichnessonLaPalmashowsaclearpeakat500m,probablyassociatedwiththemaincentersofpopulationandagricul-ture in this elevational zone (del Arco Aguilar, González-González,Garzón-Machado,& Pizarro-Hernández, 2010). Beyond that, eleva-tionexhibitsastrongnegativeinfluenceonnon-nativespeciesrich-nessandthepercentageofnon-nativespecies.Aboveabout1,200ma.s.l., non-native species weremissing in this study. Environmentalconditions above this threshold seem to be unfavorable for non-nativeplantspeciesonLaPalma,atleastatthisstateofthe(dynamic)invasionprocess.Inthefollowing,wemakeseveralassumptionsfortheexistenceofthisthreshold:

1. The1,200-m threshold corresponds verywellwith the transitionfrom cultivated lowland vegetation (i.e., plots located mainly inextensivelymanagedrangelandorrecentlyabandonedcultivation)to the pine forests (although cultivation can reach higher ele-vations in other areas of La Palma). Above 1,200m elevation,only extensive forestry is practiced in the sampled area due to

     |  777STEINBAUER ET Al.

increasing steepness and deteriorating climatic conditions. As aconsequence,humandisturbancesassociatedwithagricultureandthe probability of an anthropogenic introduction of non-nativespeciesaregreatlyreducedabovethe1,200-mthreshold.However,historiclanduseinformofherding,fire,andhuntinghasaffectedandstillaffectsthewholeenvironmentalgradient(Garzón-Machadoetal., 2010; Irl, Steinbauer, Messinger, etal., 2014).

2. Pinus canariensisforestsarehighlydominantabovethisthreshold(1,200–1,500m). This vegetation type has the highest fire fre-quencyontheCanaryIslands.Onlynon-nativespecieswitharela-tivelyhighfiretolerancecanpotentiallyestablishinthevegetationzoneof thepine forest; thus,firemightactasa strongfilter forpotential invaders particularly asmany fire-prone ecosystems inotherpartsof theworldarenonforested.However,firecanalsocauseashort-termincreaseinnativeandnon-nativeannualplants(mostlyruderals;GonzálezGómezetal.,2011).Athighelevations,firetogetherwithharshconditions(Kueffer,Pyšek,&Richardson,2013),suchashighsolarradiation,strongdiurnaltemperatuream-plitude,extensivesummeraridity,andwintersnowandicestorms(Garzón-Machadoetal.,2014),probablyfilterpotentialinvadersinthesummitscrub.

3. The sampling design of this study avoided plant communities ofpurehumanoriginsuchasroadsidecommunities,whichareoftenappliedforquestionsregardingplantinvasioninmountainsystems(see,e.g.,Alexanderetal.,2011;Arévaloetal.,2010;Ottoetal.,2014).RoadshavebeenshowntobeveryeffectivevectorsofplantinvasionandhomogenizationontheneighboringislandsofTenerife(Arévaloetal.,2010;Haideretal.,2010)andinothermountainousareasoftheworld(Alexanderetal.,2011).RoadsidecommunitiesathigherelevationsexistonLaPalmaaswell,wherenon-nativespeciessuchasReseda luteolaL.reachup intothehighesteleva-tions (unpublisheddata). In general, roadside communitiesmighthelp to identifymaximumelevations reachedbynon-nativespe-cies,indicatingphysiologicallimitsofnon-natives(e.g.,temperaturethresholds;Poll,Naylor,Alexander,Edwards,&Dietz, 2009), yetfocusingonmorenaturalcommunitiesisabettertooltodeterminethestateof invasibilityandmighthelp to identifyhowbiologicalconstraintssuchasinterspecificcompetitionlimitplantinvasion.

Introducedplantstendtobepre-adaptedtolow-elevationsitesbe-causeintroductionusuallytakesplaceatlowelevations.Thelowamountof non-native high-elevation specialists and the filtering effect of theP. canariensisforest(Alexanderetal.,2011)leadtotheabsenceofnon-nativespeciesinhigh-elevationecosystems.However,thismaybeonlyatransitionalsituation,asglobalclimatechangehasthepotentialtoalterthehigh-elevationharshnessfilteron islands, creatingpossibilities forinvasionatincreasingelevationsinthenearfuture(Harteretal.,2015).

Besides increasing temperatures, increasing moisture availabilityhas been shown to facilitate the invasion process (Maron&Marler,2007; Stohlgren etal., 1999).We observed highest values for non-nativespeciesrichnessonthenortherntransect.Thissupportsenvi-ronmentalfiltering resulting in lowernon-nativespecies richnessonthewesternsideoftheisland.Ontheotherhand,thedenseevergreen

canopyofthelaurelforestontheeasterntransectpreventsmostlightfrompenetratingtotheground(Delgado,Arroyo,Arévalo,&Fernández-Palacios,2007),whichpotentiallymightpreventtheestablishmentofheliophyllousnon-nativespecies.Generally,thetotalnumberofnon-nativeplants and their contribution to thevegetation structure andcompositionarelowonLaPalmacomparedwithmanyoceanicislands(Daehler,2005;Macdonald,Thébaud,Strahm,&Strasberg,1991).Thisfact, togetherwith the identifiedspatialpatterns,maycontribute tocopingandmanagementstrategiesinordertomitigatepossiblefuturenegativeimpactsofinvasivespeciesatanearlystage.

4.3 | Theoretical concept of invasion and speciation on oceanic islands

Invasionofnon-nativeplantspeciesanddiversificationofnativeanden-demicplantsaregenerallyseenastwoindependentprocesses.However,theunderlyingenvironmentaldriversonoceanicislands’elevationalgra-dients,thatis,environmentalfiltering,degreeofhumaninfluence,andgeographicfilteringintheformofspatialandecologicalisolation,seemto influence both processes, but in contrasting directions. Aiming todissectthepresumedinterrelationofunderlyingenvironmentaldriverswiththeseprocesses,Levin(2003)suggestedusingislandsasresearchobjectsbecauseoftheirexcellentsettingforthispurpose.

Consistent with phylogenetic findings (Merckx etal., 2015),Steinbaueretal.(2016)foundelevation-drivenisolationtopositivelyinfluence speciation. This seems to be a general phenomenon onhigh-elevationislandsbutalsoappliestomountainsonthecontinent,highlighting its importanceforglobalbiodiversitypatterns.Althoughanthropogenicinfluencesaffectcurrentspeciesdistributionpatterns,forexample,via theoccurrenceof ruderal species, theevolutionarysignalisstillstronginthespatialgradientsdetectedinourstudy.Thelower immigration ratesathigherelevations results in small speciespools,whereas the degree of endemism, as a proxy for diversifica-tion,ishigh(Merckxetal.,2015).Acombinationofdriverseffectivelyimpedes colonization of high elevations by natural immigration andcolonization.Amongthesedriversareharshclimaticconditions,de-creasingareaandisolationcausedbytheeffectivegeographicalisola-tionfromcomparableecosystems(Steinbaueretal.,2013,2016).

Atthecurrentstageofinvasion,high-elevationecosystemsseemtobebetterprotectedfrominvasionprocessesthanlowelevationsonLaPalma.Recentsynthesesshowthatthisisageneralphenomenonbecauseharshnesstendstodecreasetheinvasibilityofasystem(inva-sion syndrome;Kuefferetal.,2013).However,changesinlanduseandclimateinthenearfuturemightalterinvasionfiltersathighelevations,posingathreattotheseenvironments(Alexanderetal.,2016).Whilemany lowlandsystemsarepresumablynonrestorable,high-elevationsystemsstilloffertheopportunitytopreventlarge-scalealterationsbynon-nativespecies,ifimmediateprotectionmeasuresaretaken.

ACKNOWLEDGMENTS

WewouldliketothankDr.FélixMedinaandFranciscoPrietooftheConsejería deMedioAmbiente delCabildo Insular de LaPalma for

778  |     STEINBAUER ET Al.

theirsupportandJulioLealPérezforhishelp inthefield.TheEliteNetworkofBavariawithintheframeoftheGlobalChangeEcologystudyprogramsupportedthefieldsurvey.ThisworkcontributestotheprojectECOPOTENTIALfundedfromtheEuropeanUnion’sHorizon2020 research and innovation program under grant agreement no.641762. This work further contributes to the project MOVECLIMunder the frameworkof theNet-Biome transnational first callwithsupportoftheAgenceNationaledelaRecherche,theConseilrégionaldelaRéunion,theConseilrégionaldelaGuadeloupe,theGovernmentofAzores,andtheGovernmentofCanarias.Additionally,thispubli-cationwas fundedby theGermanResearchFoundation (DFG) andthe University of Bayreuth in the funding program Open AccessPublishing.MJSwaspartlysupportedbytheDanishCarlsbergfondet(CF14-0148).

CONFLICT OF INTEREST

Nonedeclared.

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How to cite this article:Steinbauer,M.J.,Irl,S.D.H.,González-Mancebo,J.M.,Breiner,F.T.,Hernández-Hernández,R.,Hopfenmüller,S.,Kidane,Y.,Jentsch,A.andBeierkuhnlein,C.(2017),PlantinvasionandspeciationalongelevationalgradientsontheoceanicislandLaPalma,CanaryIslands.EcologyandEvolution,7:771–779.doi:10.1002/ece3.2640