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Aim Exotic species are a major threat to biodiversity and have modified native communities worldwide. Invasion processes have been extensively studied, but studies on species richness and beta diversity patterns of exotic and native species are rare. We investigate such patterns among exotic and native fish communities in upland and lowland rivers to explore their relationship with environmental drivers. Location Northern Italy. Methods Exotic and native fish beta diversity patterns were investigated separately in lowland and upland sites using Local Contribution to Beta Diversity (LCBD) and Species Contribution to Beta Diversity (SCBD) analyses. To examine the main environmental variables affecting the LCBD, a Boosted Regression Trees (BRT) method was used. Community dispersion among and within stream orders was investigated with the PERMDISP test. Results In lowland sites, exotic species richness was higher than native species richness, especially in large rivers and drainage canals. An opposite trend was found in upland sites, where native species richness was higher than exotic species richness, especially in large rivers. No clear LCBD patterns were found along stream orders in the lowland, whereas higher stream orders in the upland showed the highest LCBD. Its patterns in upland and lowland sites were related to a number of factors, such as total suspended solids and total phosphorus. Community dispersion among stream orders did not show a relationship with environmental heterogeneity. SCBD values were positively correlated with species occupancy in the study area, and native species showed higher SCBD values than exotic species only in the uplands. Main conclusions Large rivers in the uplands are important in maintaining native fish diversity and should be protected against invasive fish. In contrast, most lowland rivers have suffered from biological homogenization. Some rare native species can show low contribution to beta diversity, but still need conservation actions due to their risk of local extinction.
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Diversity and Distributions. 2019;25:983–994.    
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 983
wileyonlinelibrary.com/journal/ddi
Received:1June2018 
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  Revised:4D ecember2018 
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  Accepted:23Januar y2019
DOI:10.1111/ddi.1290 4
BIODIVERSITY RESEARCH
Diversity patterns of native and exotic fish species suggest
homogenization processes, but partly fail to highlight
extinction threats
Anna Gavioli1| Marco Milardi1| Giuseppe Castaldelli1| Elisa Anna Fano1|
Janne Soininen2
ThisisanopenaccessarticleunderthetermsoftheCreativeCommonsAttributionLicense,whichpermitsuse,distributionandreproductioninanymedium,
providedtheoriginalworkisproperlycited.
©2019TheAuthors.Diversity and Distributions PublishedbyJohnWiley&SonsLtd
1DepartmentofLifeSciencesand
Biotechnology,UniversityofFerrara,
Ferrara,Italy
2DepartmentofGeosciencesand
Geography,UniversityofHelsinki,Helsinki,
Finland
Correspondence
MarcoMilardi,DepartmentofLifeSciences
andBiotechnology,UniversityofFerrara,
Ferrara,Italy.
Email:marco.milardi@gmail.com
Editor:FranzEssl
Abstract
Aim: Exoticspeciesareamajorthreattobiodiversityandhavemodifiednativecom-
munities worldwide.Invasionprocesses have beenextensivelystudied,butstudies
onspeciesrichnessandbetadiversitypatternsofexoticandnativespeciesarerare.
Weinvestigate suchpatterns among exoticand nativefishcommunities in upland
andlowlandriverstoexploretheirrelationshipwithenvironmentaldrivers.
Location: NorthernItaly.
Methods: Exoticandnativefishbetadiversitypatternswereinvestigatedseparately
inlowland anduplandsites using Local ContributiontoBeta Diversity(LCBD)and
SpeciesContributiontoBetaDiversity(SCBD)analyses.Toexaminethemainenvi-
ronmentalvariablesaffectingthe LCBD,aBoosted RegressionTrees(BRT)method
wasused.Communitydispersionamongandwithinstreamorderswasinvestigated
withthePERMDISPtest.
Results: Inlowlandsites,exoticspeciesrichnesswashigherthannativespeciesrich-
ness,especiallyinlargerivers and drainagecanals.Anoppositetrend wasfoundin
uplandsites,wherenativespeciesrichnesswashigherthanexoticspeciesrichness,
especiallyinlargerivers.NoclearLCBDpatternswerefoundalongstreamordersin
thelowland,whereashigherstreamordersintheuplandshowedthehighestLCBD.
Itspatternsinuplandandlowlandsiteswererelatedtoanumberoffactors,suchas
totalsuspendedsolids and totalphosphorus.Communitydispersionamongstream
ordersdidnotshowarelationshipwithenvironmentalheterogeneity.SCBDvalues
werepositivelycorrelatedwithspeciesoccupancyinthestudyarea,andnativespe-
ciesshowedhigherSCBDvaluesthanexoticspeciesonlyintheuplands.
Main conclusions: Largeriversintheuplandsareimportantinmaintainingnativefish
diversityandshouldbeprotectedagainstinvasivefish.Incontrast,mostlowlandriv-
ers have suffered from biological homogenization. Some rare native species can
showlowcontribution to betadiversity,butstill needconservation actions dueto
theirriskoflocalextinction.
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1 | INTRODUCTION
Theimportanceofbiodiversityfor ecosystem functioning andresil-
ience,aswellasforhumansthroughthesupplyofecosystemservices
(e.g.,food,pestcontrol,fisheries),iswidelyacknowledged(Cardinale
etal., 2012;Hooperet al.,2005;Worm et al., 2006). Nevertheless,
biodiversityconstantlydeclinesworldwide(Butchartetal.,2010)and
todefinemanagementplansthatcanhaltthisdeclineitisnecessary
to under stand biodive rsity tren ds in space and time (R ichardson &
Whittaker,2010). A common approach to detect these biodiversity
trends is to m easure variatio ns in taxonomica l diversity (Chia rucci,
Bacaro, & Scheiner, 2011; Colwell & Coddington, 1994). In 1960,
Whittakerproposedthetaxonomicaldiversitycouldbedefinedasthe
result of threecomponents: alpha(localdiversity),beta(variationof
communitycompositionamongsites)andgammadiversity(regional
diversity;Whittaker,1960,1972).Inrecentyears,moreattentionhas
beenfocusedonbetadiversity(Andersonetal.,2011)duetoitsabil-
itytoidentifyhumanimpactsondiversity(e.g.,agriculture,speciesin-
vasionandclimatechange)atmultiplescales(Socolar,Gilroy,Kunin,&
Edwards,2016).Differentmeasuresofbetadiversityhavebeenpro-
posed (e.g., Baselga, 2010; Tuomisto, 2010), and recently,Legendre
and De Cáceres(2013) proposed amethod thatnot only estimates
theoverallbetadiversity,butalsoquantifiestheLocalContributionto
BetaDiversity(LCBD)bysinglesitesandtheSpeciesContributionto
BetaD ive rsit y(S CBD)b yin div idu als pecies. Bot hLCBDandSCB Dcan
alsobeconsideredasmeasuresoftheuniquenessofsitesandspecies
for a region a nd have been use d to investigate sp ecies distr ibution
shiftsinfishcommunities(Kuczynski,Legendre,&Grenouillet,2017)
and other taxa such as diatom communities (Jyrkänkallio‐Mikkola,
Siljander,Heikinheimo,Pellikka, &Soininen,2018)and streaminver-
tebr ate s(Heino&G nro os,2017;Sor, Legendre,&Lek,2018 ;Tonkin,
Heino,Sundermann,Haase,&Jähnig,2016).
Despitetheimportanceofdiversitymeasuresinexplainingtax-
onomic al biodiversi ty, the main shor tcoming of thes e measures is
thatallspeciesaretypicallyconsideredequally,withouttakinginto
account evolutionary or ecological differences between species
(Chiarucciet al., 2011).Forexample, takingintoaccountthenative
orexoticstatusof a specieshasimportant implicationsintermsof
manageme nt and conser vation, also consi dering that the i nvasion
sensitivityofthecommunitycouldberelatedtodiversitymeasures
such as speciesrichness (Hooper et al., 2005). Invasionsof exotic
speciescanoftencauseanativespecies’declinethroughpredation,
hybridization, competition and indirect effects (Blackburn et al.,
2014;Simberloffetal.,2013).
Freshwaters are particularly susceptible to exotic species in-
vasions, and in such ecosystems, exotic species areconsidered one
of the main c auses of biodiver sity loss (Dud geon et al., 200 6). For
instan ce, in fish communit ies, exotic specie s constitute one of th e
majordriversofextinctionintheMediterraneanregion(Crivelli,1995)
and can cause taxonomic homogenization (i.e., taxonomic similar-
ity across communities),par ticularlyin the Nearctic and Palearctic
regions ( Villéger, Blanchet, Beauchard, Obe rdorff, & Brosse, 2011,
2015). There i s also evidence th at only few introd uced exotic spe-
cies(e.g.,commoncarp,Cyprinus carpio L.)drivethistrend(Toussaint,
Beauchard, Oberdorff, Brosse, & Villéger, 2016). There are many
studiesfocusingontheeffectsofexoticspeciesonnativeones(e.g.,
Milardietal.,2018);however,large‐scalediversitypatterns innative
and exotic sp ecies commun ities are stil l understu died, especi ally in
freshwaters(someexceptions:Kuczynskietal.,2017;Leprieur,Olden,
Lek,&Brosse,2009;Maceda‐Veigaetal.,2017).
Toinvestigatethesepatterns,wefocusedonfishbiodiversityin
riversandstreamsinNorthernItaly,oneofthemostheavilyinvaded
areasinthecountry.Insomestretchesoftheserivers,theinvasion
of exotic fis h, and a correspon ding decline of native s pecies, oc-
currednearlytwentyyearsago(Castaldelli,Pluchinottaetal.,2013).
Here,we(a)investigatedhowspeciesrichness (i.e.,alphadiversity)
andthe uniquenessofcommunity composition (i.e., betadiversity,
LCBD)varyamongexoticandnativefishspeciesfromheadwatersto
lowlandrivers,thatis,acrossstreamorders.Secondly,we(b)inves-
tigatedthe relative influenceofmainwater physico‐chemical vari-
ablesontheuniquenessofthecommunitycompositionatsites(i.e.,
LCBD).Wealso(c)examinedthevariationinexoticandnativecom-
munity within stream orders and studied whether wecould relate
withinstreamordervariationincommunitiestothedegreeofwater
physico‐chemicalheterogeneity.Finally,we(d)analysedthespecies
contributiontobetadiversity(i.e.,SCBD)underthehypothesisthat
native species mightcontribute moretobeta diversitythan exotic
ones,whichtendtohomogenizecommunities.Wealsoexaminedif
arelationshipbetweenspeciesoccupancyandspeciescontribution
tobetadiversityexisted.
Our results can helpto understand spatial clines innative and
exotic species diversityand howtheseclines respond todifferent
water physico‐chemical variables. Such information would in turn
be useful to improve management and conservation actions in
freshwaters.
2 | METHODS
2.1 | Study area
The stu dy area is locat ed in Northe rn Italy and i ncludes the l arg-
est rive r basin in Italy, the Po Ri ver basin (71,00 0km2). The area
hosts m ore than 17 million of inh abitants an d is impacted by ag-
ricultu ral activitie s and livestock fa rming. The stu dy region has a
KEY WORDS
Alienspecies,betadiversity,biodiversityconservation,freshwater,invasions,non‐native
species,speciesdiversity,speciesrichness
    
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GAVIOLI e t AL.
Mediterranean continentalclimate,withanannualaverageprecipi-
tationof1,036mmandameanairtemperatureof12°C.Therivers
networkconsidered includethePoRiverin allitscourse,theOglio
River,oneofthemostimportantlefttributariesofthePoRiver,and
therighttribu tariesintheEmilia‐Romagnaregion.Asarefe re nceex-
ternaltothePoBasin,weincludedtheBrentaRiver,locatedonthe
north‐eastofthePoBasin,andtorrentsandriverssouthof the Po
Basin,untilthesouthernmostborderoftheEmilia‐Romagnaregion.
In the upla nd rivers, org anic material orig inating from villa ges
andsmalltownsandlivestockfarmsisthemainsourceofpollution.
Conversely,ahighdegreeofurbanizationandintensive agriculture
characterizethe lowlandrivers,wherehighnutrientloads haveled
toeutrophication(Castaldelli, Soanaetal., 2013; Soana, Racchetti,
Laini, B artoli, & Vi aroli, 2011). Tos upport agr icultural ac tivities , a
complexnet wo rkofdr ainagecanalshasbeenestab lishedinthelow-
lands.Thissystemiscompletelyhuman‐regulatedwithhydrological
manageme nt directed to dr ainage or irriga tion supply (Ca staldelli,
Pluchinotta et al., 2013; Milardi, Chapman, Lanzoni, Long, &
Castaldelli,2017).Overall,a totalof337 sampling sitesin105 wa-
tercourseswere sampled between 1999 and2010andincluded in
this study,coveringa wide rangeof freshwater habitats, different
altitudinalzonesandenvironmentalconditions(Figure1).Weconsid-
eredthatcommunityturnoverwouldnotbearelevantfactorinour
study,duetothefactthatfishcommunitiesaretypicallymoretem-
porallystablethanotheraquaticcommunities(Korhonen,Soininen,
&Hillebrand, 2010).Furthermore,the study areawas already ina
lateinvasionstage(Milardietal.,2018),sincelossofnativespecies
andexoticinvasionoccurredmainlypriorto1997(Castaldelli,Soana
etal.,2013),thatis,beforethedataanalysedherewerecollected.
2.2 | Stream surveys
Fi s h d atawer e c ollectedw i t h i namoni t o r i ngprog r a m m e forthec o m -
pilationoftheofficialFishInventoriesoftheEmilia‐Romagnaregion
(Emilia‐Romagna Region, 2002, 2005, 2008), the Padova province
(Padova Pr ovince, 2010), the Po Rive r (Po River Water Auth ority,
2008)andtheOglioRiver(OglioRiverWaterAuthority,2016).Fish
samplingwasperformedtypicallyfromApriltoSeptemberbyelec-
trofishing.Insitesofhigherwaterdepthandconductivity(e.g.,lower
stretchesoftherivers),electrofishingwascombinedwiththeuseof
nets.Formoredetailsonfishsamplingmethods,seeAschonitisetal.
(2018),Gaviolietal.(2018),Milardietal.(2018).
FIGURE 1 MapofsamplingsitesintheNorthernItaly,altitudinalgradientandLocalContributiontoBetaDiversityforupland(darkgrey
circles)andlowlandsites(lightgreycircles)calculatedforthetotalfishcommunity.PoRiverbasin,BrentaRiverbasinandRomagnarivers
basinareshown
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Fish spec ies were classif ied according to Kot telat and Freyhof
(2007), t aking into account recent taxonomic dete rminations and
common names as listed inFishBase (Froese& Pauly,2017). Each
species was categorized asnative orexotic:a species was consid-
eredasnativewhennaturallypresentinItalianwatercourses,andas
exoticwhenintroducedbyhumans,irrespectiveofthetimeelapsed
sincetheintroduction.Fishspeciesabundancewasexpressedusing
Moyleclasses(Moyle&Nichols,1973)rangingfrom1(lowerabun-
dance, 1–2individualsper site) to5(higherabundance, morethan
50individualspersite).Hybridspecimensoruncertainspecieswere
excludedfromthisstudyinordertoavoidtaxonomicasymmetries.
Typically, in European rivers, fish communities change from
Salmonidae to Cyprinidae dominated, along an altitude gradient,
fromheadwaterstolargeriversatlowelevation(Aarts& Nienhuis,
2003).Takingintoaccountsuchcommunityshifts,studysiteswere
dividedintotwogroups:lowlandsites(sitesbelow100mabovesea
level)anduplandsites(sitesabove100mabovesealevel).Thislimit
isnotabsoluteanditisnotastrongphysicalbarrierforfishspecies,
butitwaschosenbasedonearlierstudiesintheregion(Aschonitiset
al.,2018;Milardietal.,2018)andseparatetypicallowlandimpacted
environmentsfromthelessimpactedones,locatedintheuplands.
Water physico‐chemical sampling was performed with stan-
dard methods in proximity to the fish sampling sites by Regional
Environmental ProtectionAgency(ARPA) forPoRiver,BrentaRiver
and Emilia‐ Romagna rivers a nd by Oglio River Water A uthority fo r
theOglioRiver.Eightwaterphysico‐chemicalvariableswereincluded
as follows: water temperature (°C), electrical conductivity (μS/cm),
chemical oxygen demand (COD [O2mgL−1]), biological oxygen de-
mand(BOD5 [O2mgL−1]),t otal suspend ed solids (mg/L ), total phos-
phorus(PmgL−1),ammonia(NmgL−1)andnitratenitrogen(NmgL−1).
2.3 | Stream order analysis
The stream order of each sampling site was calculated from
Digital Elevation Model (DEM) data (ISPR A, Italian Institute for
Environme ntal Protect ion and Researc h)t hrough the Arc GIS 10.1
software.Inordertoharmonizetheelevationmodel,theDEMlayer
wasfirstresampledinto10mpixelsize.Then,usingtheHydrology
Spatial AnalystTool,the flowdirection and theflow accumulation
based on DEM layer were cal culated. Finally, for th e entire river
networkgeneratedbyflowaccumulation,thestreamorderwiththe
Strahlermethod(Strahler,1957)wascalculated. This procedurere-
sulted reliablefor uplandstreams,while in thelowland,itwasless
accurate possibly duetothefactthatinthe lowlands, theflowdi-
rectionandmagnitudehavebeenmodifiedbyhumans.TheStrahler
stream orderwas thusmanuallychecked andrevised whenneces-
saryinlowlandriversandstreams.
InordertobalancethenumberofriverssampledineachStrahler
streamorder,riversweregroupedintofourclassesbasedonstream
order:class1—riverswith1and2Strahlerstreamorder,class2—riv-
erswith 3 and4streamorder, class 3—rivers with5and 6 stream
order and class4—riverswith Strahler stream orderhigherthan 6.
Asthedrainageandirrigation canalslocatedinlowlands couldnot
beassignedintoanynaturalclass,theywereassignedintoaseparate
classcalled“Drainage.”
Overall,intheuplands,sixsamplingsiteswereincludedinstream
orderclass1,41instreamorderclass2 ,55instreamorderclas s3 ,6in
streamorderclass4andnositesweresampledindrainagecanals.In
thelowlands,nosamplingsiteswereincludedinthefirststreamorder
class, 17were included in stream order class 2,53 instream order
class3,94inthestreamorderclass4and40inthedrainagecanals.
2.4 | Statistical analysis
All sta tistical an alyses were pe rformed fo r lowland (20 4 sampling
sitesbelow100mofaltitude)andupland(133samplingsitesabove
100mofaltitude)sitesseparately,takingalsointoaccountthedis-
tinctionbetweenexoticandnativefishspecies.
2.4.1 | Species richness and local contribution to
beta diversity in exotic and native fish species
Tostudytheuniquenessoffishcommunitycompositionacrosssites,
theLocal ContributiontoBetaDiversity(LCBD)wascalculatedfor
eachsamplingsiteusingthebeta.divfunctionin“adespatial”Rpack-
age(Dray etal., 2018) based on Legendre andDe Cáceres (2013).
This method calculates the Total Beta Diversity (BDTo ta l) from the
totalvarianceof asitebyspeciescommunitytable. The LCBD was
derived by partitioningtheBDTot a lintothelocalcontributions, and
the sum of th e LCBDs for all sites is equ al to 1. For this metric ,
highervaluesofLCBDofasiteindicateanunusualspeciescompo-
sition compared withthe averagecommunity in thedata. From an
ecologicalpoint ofview,the LCBD values represent thedegreeof
uniquenessofthesamplingunitsintermsofcommunitycomposition
(Legendre&DeCáceres,2013).
To investigate how LCBD and richness varies across stream
orderclasses,theKruskal–Wallis(KW) test(R function kruskal.test)
wasapplied.Thechoice ofKruskal–Wallistest was due to the fact
thatdatadidnotmeetallassumptionsofANOVA,testedwithad.tes t
functionin“nortest”package(Gross&Ligges,2015).
2.4.2 | Relative influence of main water physico‐
chemical variables on the local contribution to
beta diversity
Amachinelearningmethod,BoostedRegressionTreesanalysis(BRT;
Elith,Leathwick,&Hastie,2008),wasusedtoinvestigatehowLCBD
wasinfluencedbywaterphysico‐chemicalvariables.BRThasbeen
consideredtobe an efficient methodtodescribeanynonlinearre-
lationshipsbetweenvariables(e.g.,thresholds)anditautomatically
incorporates interactions between variables. This approach dif-
fersfrom traditionalregressionmethodsasBRTanalysiscombines
together alarge number ofsimple tree models using the boosting
techniquetoimprovethepredictiveperformance.BRTanalysisfur-
thercalculatestherelativeinfluenceofpredictorsonresponsevari-
able.Theeffectofpredictorsisshowedthroughthefittedfunctions
    
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thatprovideausefulbasisforinterpretation,althoughtheyarenot
perfectrepresentationincase ofstronginteractions between pre-
dictors(Elithetal.,2008).BRTwasperformedwithGaussiandistri-
bution,bagfractionof0.75andshrinkageof0.001intheRsoftware
package“gbm”(Ridgeway&Southworth,2017).
2.4.3 | Variation in exotic and native community
dispersion among and within stream orders
Inordertoinvestigatethedegreetowhichthereiscommunitystruc-
tural va riation within a s tream order c lass, a test of hom ogeneity
of dispersion (PERMDISP) was used (Anderson, 2006; Anderson,
Ellingsen,&McArdle,2006)withafunctionbetadisperint he“vegan”
Rpackage(Oksanenetal.,2017).Throughtheaveragedissimilarity
from individual observations to their group centroid, this test cal-
culatesthedegreeofdispersion,thatisbetadiversity(whenbased
onpresence–absencedata)andthecommunitystructuralvariation
(whenbasedonabundancedata;Andersonetal.,2006;Heinoetal.,
2013)withinstreamorderandtestifitdiffersamongstreamorders.
ThePERMDISPanalysiswasrunusingGowerdissimilaritiesonfish
abundance data and Sørensen dissimilarity on presence/absence
data. Moreover,wealsoinvestigated thedegree of waterphysico‐
chemical dispersion within stream order classes using Euclidian
distances. A permutation test with 999 permutations (permutest
function)was usedto compare the degree of within group disper-
sionsamonggroups.Alinearregressionanalysiswasusedtotestthe
nullhypothesisofnorelationshipbetweenthedistanceofcentroid
basedonabundancedata(i.e.,communitiesstructuralvariation)and
the dist ance to centroid of wate r physico‐chemic al variables (i.e .,
water physico‐chemical heterogeneity) across sites (Heino et al.,
2013).
2.4.4 | Differences in species contribution to beta
diversity between native and exotic species and the
relationship with species occupancy
We calculate d the Species Co ntribution to B eta Diversit y (SCBD)
that showsthe degree of variation ofaspeciesacrossthe consid-
ered area(Legendre & De Cáceres, 2013). It can be consideredas
a measure of t he relative imp ortance of e ach species in af fecting
betadiversity(Heino&Grönroos,2017).Linearregressionwasused
to investig ate the relation ship between t he SCBD values a nd the
number ofsites occupiedfor each species and theKruskal–Wallis
testwasusedtoinvestigatedifferenceinSCDBvaluesbetweenthe
lowlandsanduplands.
AllstatisticalanalysiswasperformedinRsoftware,version3.4.3
(RCoreTeam,2017).
3 | RESULTS
Atotalof60fishspecieswereobservedinthestudyarea,with38
native and 2 2 exotic species. I n the upland site s, fish communit y
wascomposed of24native speciesand11exoticspecies,whereas
in the lowla nd sites, 38 nati ve and 22 exotic speci es were found
(SupportinginformationTableS1).
Minimum,maximum,averagesandstandarddeviationsofwater
physico‐chemicalvariablesandaltitudeforlowlandanduplandsites
are reported in Supporting information Appendix S1: Appendix A.
Variation of water physico‐chemical variables alongst reamorder
classesareshowninSupportinginformationAppendixS1:Appendix
B. In summary, lowland sites showed the highest anthropogenic
pollution,withthehighestvaluesofammoniaandnitratenitrogen,
chemicaloxygendemand(COD), biologicaloxygendemand (BOD5)
and total p hosphorus. A lso, electric al conductiv ity, mainly due to
brackishwaters,andtotalsuspendedsolidswerehigherinthelow-
landsitesthanuplandsites.Duetothealtitudinalgradient,thelow-
estwatertemperaturesweredetectedintheuplandsites.
3.1 | Species richness and local contribution to beta
diversity in exotic and native fish species
Exotic fi sh species richn ess was higher in low land sites than upl and
sites, where only few exotic species were recorded (Figure 2). The
exotic species richness showed significant differences among stream
orderclassesinthelowlands(KWχ2=53.7,df=3,p<0.001)andinthe
uplands(KWχ2=71.2,df=3,p<0.001)withapositivetrendtowards
higher streamorders(Figure2a,b).Native speciesshowed significant
differencesamongstreamorderclassesinbothlowlands(KWχ2=54.0,
df=3,p<0.001) anduplands(KW χ2=71.2,df=3,p<0.001). Inthe
lowlands,nativerichnesspeakedin streamorderclass3andwaslow-
estindrainagecanals, whereasintheuplandsrichnesswashighest in
streamorderclass4andlowestinclass1(Figure2a,b).
Considering all fish species,BDtotalforlowland and upland sites
were0.631and0.607,respectively.The distributionofLCBD values
consider ing all species is sh own in Figure 1. The h ighest values of
LCBDinthelowlandsitesoccu rredinthePoRiverD eltaandinSouth
East areaofEmilia‐Romagna region. In upland sites, LCBDs showed
a high spatial variability across the studied area. According to the
Kruskal–Wallistest,LCBDvaluesdidnotshowsignificantdifferences
amongs tre amorderclassesinthelowl andscon sider ingnati vespeci es
(Figure3a;K Wχ2=1.7,df=3,p>0.05).Whereasconsideringexotic
species,LCBDvalues showedasignificantdifference amongstream
orderclasses(KWχ2=9.0,df=3,p<0.05)moreevidentbetweenthe
stream orderclass3anddrainage canalsclass(Figure3a). In the up-
lands,LCDBvalues showed significantdifferencesconsideringboth
native(Figure3b;KWχ2=24.7,df=3,p<0.001)andexotic species
(Figure3b;KWχ2=65.6,df=3,p<0.001)alongstreamorderclasses,
reachingthehighestvaluesinlargerivers.
3.2 | Relative influence of main water physico‐
chemical variables on the local contribution to
beta diversity
According toBRTanalysis, thetotal suspended solids and thetotal
phospho rus were retained a s the most impor tant factor affecting
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LCBDvalues for bothexoticandnative speciesinthelowlandsand
intheuplands, respectively(Figure 4).Asevident inthefittedfunc-
tions,thesepredictorsshowednegativerelationshipswithrespective
LCBD.Thechemicaloxygendemand(COD)wasthesecondmostim-
portantpredictoramongnativespecies,forbothlowlandandupland
sites,anditshowedapositiverelationshipwithLCBD.Amongexotic
species,a secondimportantfactorwastotalphosphorus inlowland
sites,havinganegativeinfluenceonLCBDatlowphosphoruslevels.
Inuplandsites,nitratenitrogenhadthehighestinfluencebeingposi-
tivelyrelatedwithLCBD,havingaclearthresholdabovewhichLCBD
notablyrises.
3.3 | Variation in exotic and native community
dispersion among and within stream orders
According to P ERMDISP analyse s, within strea m order disper-
sion varie d significa ntly among st ream order clas ses (Figure 5)
forexoticspeciesbothinlowlands(F(3,200)=26.8,p<0.01,)and
uplands(F(3,129)=119.3,p<0.01,),butalsofornativespeciesin
thelowlands(F(3,200)=20.2,p<0.01)anduplands(F(3,129)=61.4,
p<0.014). Based on the pairwise comparisons, within stream
orderdispersiondifferedsignificantlyinlargerriversregardless
toaltitudinal zones and indrainage canals network. Stream or-
dersdidnotdifferintheirwaterphysico‐chemicalheterogeneity
(Supporting informationAppendixS1:AppendixC) eitherinthe
lowlands(F(3,200 )=0.4,p>0.05)orintheuplands (F(3,1 29)=0.5,
p>0.05). Accordingtolinearregressionanalysis, withinstream
water physico‐chemical heterogeneity had no significant rela-
tionship withcommunitydispersioneitherfornative and exotic
speciesintheuplands(R2=0.004,p > 0.05; R2=0.014,p>0.05,
respectively) or for exotic ones in the lowlands (R2=0.0001,
p>0.05).However,aweakbutsignificantrelationshipwasfound
fornativespeciesinthelowlands(R2=0.024,p<0.05).
FIGURE 2 Boxplotsrepresentingthevaluesofexotic(orange)andnative(green)fishspeciesrichnessinthelowlands(a)anduplands(b)
alongstreamorderclasses
FIGURE 3 BoxplotsrepresentingLocalContributiontoBetaDiversity(LCBD)valuesforexotic(orange)andnative(green)fishspecies
alongstreamorderclassesinthelowlands(a)anduplands(b)
    
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3.4 | Differences in species contribution to beta
diversity between native and exotic species and the
relationship with species occupancy
SCBD showed a linear positive relationship with the number of
sitesoccupiedforeach species(Figure6,bothfor exotic (R2=0.91,
p<0.001) and native spe cies (R2=0.90, p<0.001) in the lowland
sites(Figure6a)andintheuplandsites(Figure6b;R2=0.74,p < 0.001
fornativespecies;R2=0.31,p<0.001forexoticspecies).SCBDval-
uesandspecies occupancyforeachspeciesaregiveninSupporting
information Appendix S1: Appendix D. According to the Kruskal–
Wallistest, no differenceswerefound in SCBDvalues between ex-
oticandnativecommunitiesinthelowlandssites(KWχ2=2.4,df=1,
p>0.05), whereas in theuplandsites SCBDvalueswere higherfor
nativespeciesthanexoticones(KWχ2=8.3,df=1,p<0.01).
4 | DISCUSSION
Large‐scalediversitystudiesfocusingsimultaneouslyonexoticand
nativespeciesdiversityinfreshwaterecosystemsarestillrelatively
rare,althoughexoticspeciesmayplayastrongroleinnativespecies
diversit y loss. This s tudy investig ated diversit y patterns and t heir
driversamongexoticandnativestreamfishspecies.
4.1 | Species richness and local contribution to beta
diversity in exotic and native fish species
An increase of species richness from headwaters to lowland
rivers wa s previously fo und not only in fis h (Beecher, Dott , &
Fernau, 1988; Chea, Lek , Ngor, & Grenouillet, 2017) but also
in other taxa such as macroinvertebrates and diatoms (Finn,
Bonada , Múrria, & Hughes , 2011; Stenger‐Kovács , Tóth, Tóth,
Hajnal, &Padisák,2014)suggesting ageneraldiversitypattern.
Differentmechanismshavebeenproposedtodrivethispattern,
includingwatertemperature,rivermorphology (e.g.,depthand
width)andhabit atdiversity(Allan&C astillo,20 07).Inourstud y,
onlyexoticspeciesrichnessincreasedwithstreamorderclasses
withthehighestexoticrichnessinthelargestriversanddrainage
canalsnetwork.Incontrast,nativerichnessshowedanincrease
acrosss trea morderonl yintheup lands,wher easinthelow lan ds,
native spe cies richness d ecreased in lar ge rivers and dra inage
FIGURE 4 BoostedRegressionTreesummaryshowingtherelativeinfluenceofwaterphysico‐chemicalvariablesonLocalContribution
toBetaDiversity(LCBD)valuesforlowland(a)andupland(b)sites.Thecurvesoffittedfunctionforthemostimportantvariablesarealso
showninthepanelsontheright.COD:chemicaloxygendemand
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canalsnetwork.Anthropogenicdisturbance(e.g.,pollution,river
modificationsandflowregulation) couldpartly explain lowna-
tivespeciesrichnessinlowlandrivers,andparticularlyinthear-
tificialdrainage network,butalso pastexoticspeciesinvasions
could have played a central roleinshapingthis distribution. In
fact,exoticspecieshavepushedmostlowlandsnativespecieson
theedgeofloc alext inc tioninsever alsitesanddisplacedmos tof
themonthe boundaryof theirnaturaldistribution to the high-
estreachoftherivers(Milardietal.,2018).However,upstream
river scannotprovid esuitablehabitatsfora llsuchnativespecies
andcannotcompletelycompensatethelossofnativespeciesof
thelowlands.
Thedeclineinnativespeciesrichnesswasmoreevidentindrain-
agecanalsnetwork where direct effects (e.g., predationand com-
petition)andindirecteffects(e.g.,changesinwaterquality)ofsome
successful exotic invaders (e.g., Silurus glanis and Cyprinus carpio)
were amplified due to the lower habitat complexity (Castaldelli,
Pluchinottaetal.,2013).
Converse ly to richness pa tterns, LCBD di d not show clear
differences among stream order classes in the lowland sites
for eithe r native or exotic speci es, suggesting t hat fish com-
munities in different stream orders had typically similar de-
greeof uniqueness. Thisresult indicateda similar community
composition across sites in the lowlands, probably driven by
the mostwidespread exotic species suchas thecommon carp
orthecruciancarp(Carassiusspp.).Thesetwospeciescanalso
promotehomogenizationincommunitiesespeciallyinPalearctic
regions (Toussaint etal., 2016; Villéger, Blanchet, Beauchard,
Oberdorff, & Brosse, 2011). Upland sites (high stream order
class)contributedstronglytobetadiversityofexoticandnative
species,suggesting that largeriversathigher elevationscould
provideregionallyuniquehabitatsandconditions.Interestingly,
highexoticspeciesLCBDvaluesinlargeuplandriverscanbethe
resultofanearlyinvasionprocessfromwidespreadexoticcom-
munit iesinthelowlands(Milardietal.,2018)andthusunderline
aneed forconservationand possibly restorationof suchsites
(Legendre&DeCáceres,2013).Theseresultssuggestthatnot
onlyheadwaterstreamsrequireconservationattentionforna-
tivefishspecie s,assuggestedinoth erstudie s(Mat thews,1986;
Paller,1994),but thatlargeriversintheuplands canalsocon-
tributetoregionaldiver si tybyharbour inguniquenativespecie s
communities.
FIGURE 5 Boxplotsshowingmeandistancetocentroidsalong
streamorderclasses,basedonGowerdissimilaritiesofnative
(green)andexotic(orange)fishspeciesinthelowlands(a)andinthe
uplands(b)
FIGURE 6 RelationshipbetweenSpeciesContributiontoBeta
Diversity(SCBD)andfishspeciesoccupancy(numberofsites)
forexotic(orangepoints)andnative(greenpoints)species,in
thelowland(a)andtheupland(b)sites.Pleasenotethatscalesin
occupancyfornativeandexoticspeciesinpanelb)aredifferentfor
exotic(down)andnative(up)species
    
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4.2 | Relative influence of main water physico‐
chemical variables on the local contribution to
beta diversity
Differentwater physico‐chemicalvariableswereprovenimportant
forLCBD inlowland versusuplandsiteswhenconsideringbothex-
otic and nat ive species. The l arge import ance of total suspen ded
solids in explaining LCBD (withnegati verelationship) reflect snot
only anth ropogenic effe cts but also t he effects of e xotic ecosys-
tem‐engineeringspecies such as crucian or common carp. In fact,
thesespeciescanincreasewaterturbiditythroughtheresuspension
ofsedimentswhilefeeding,inturncausingaphytoplanktonbiomass
increaseandlossofsubmergedvegetation,whilebeingabletotoler-
atehighturbiditythemselves(Crivelli,1995).Asaconsequence,fish
communitytendto changereflectingthisenvironmentalshift,with
for example a loss of clear water species with thewater turbidity
rise.
In upland s ites, LCBD was mainl y driven by total ph osphorus,
suggesting a strong role of nutrients on beta diversity patterns.
Nutrientscanaffectbetadiversitypromotingthepresenceofhighly
tolerantspeciesandnegativelyaffectingthemostsensitivespecies.
SimilarresultswerefoundalsoinFinnishlakes,wherespeciesrich-
ness of eutrophication‐tolerant species increased towards higher
nutrientloads(Olinetal.,2002).
Other authors suggested a strong influence of morphological
factors(e.g.,waterdepth,width, flowconditions or substratumty-
pology)ondiversitypatterns,suchassubstratefeaturesondiatoms
(Jyrkänkallio‐Mikkola,Heino,&Soininen,2016),macroinvertebrates
(Heinoetal.,2013)andfish(D'Ambrosio,Williams,Witter,&Ward,
2009).Unfortunately,inourdataset,dataonmorphologicalfeatures
were not avail able, and the i nvestigation of t heir role in af fecting
LCBD was not possible. However, supporting the importance of
waterchemistry,alsoMaceda‐Veigaetal.(2017)recentlyfoundthat
salinizationandnutrientpollution(suchasnitrate,nitrite,phosphate
supply)constituteoneofthemajorthreatstonativefish,inaddition
tohydrologicalfeatures.
4.3 | Variation in exotic and native community
dispersion among and within stream orders
Thedegreeofwaterphysico‐chemicalheterogeneitydidnotvary
amongstreamorderclasseseitherinuplandsorinthelowlands,
sugge s t i ng t hatwatercon d i t i o ns d on ot d i f feram o n g s t reamor d e r
classes.Wealso didnotfindarelationshipbetweencommunity
dispersionandheterogeneityinwaterphysico‐chemicalvariables
(exceptfornative communitiesin thelowlands). The absenceof
suchrelationshipswasalsofoundbyHeinoetal.(2013)instream
macroinvertebrate communities and by Jyrkänkallio‐Mikkola et
al. (2016) for diatoms. Different explanations already proposed
forsuchapatterncould alsobeapplicable here:(a) fishspecies
distributionsmaynothavebeenrelatedonlytowaterconditions
butalsotodispersalprocesses,(b)patternsaredifficulttoseeat
thecommunitylevel due to the species‐specificresponses and
(c) the lack of imp ortant ha bitat descri ptors such as rive r mor-
pholog y (Heino et al., 2 013). Although on ly the water physi co‐
chemical descriptors were considered in this study, we expect
that water physico‐chemical patterns might reflect also other,
mo regene r al, stre ama lter atio nsd uef ore xamp let oag ricultu reo r
farmanimals(Allan&Castillo,2007).Thus,weconcludethatthe
degree of co mmunity disp ersions doe s not strongly d epend on
thelevelofwaterphysico‐chemicalheterogeneitywithinstream
orderclasses.Theonlyexceptionwasnativespeciescommunities
in the lowlands, which showed a weak relationshipwit hwater
physico‐chemicalheterogeneity,perhaps indicating theirsome-
whathighersensitivitytowaterqualityvariationsduetoanthro-
pogenicpressuresortoexoticfishspeciespresenceasdiscussed
above.
4.4 | Differences in species contribution to beta
diversity between native and exotic species and the
relationship with species occupancy
As we hypot hesized, native s pecies had hig her SCBD value s than
exotic species, butonly in theuplands.The fact that SCBDvalues
didnot oftendiffer between nativeand exoticspecies in thelow-
landscouldbetheresultofsimplifiednativecommunities,composed
bythefewnativespeciesmostresilient to the invasionprocess,as
previous ly suggested by oth er studies in the s ame area (Lanzon i,
Milardi, Aschonitis, Fano,&Castaldelli, 2018; Milardi etal., 2018).
It may also ind icate that exotic sp ecies communiti es are spatially
structured, with different species dominating communities across
sites(Clavero & García‐Berthou, 2006).One morereasoncould be
thepositiverelationshipbetweenSCBDandthespeciesoccupancy,
suggesting that themost widespreadfish species (whichare often
exoticspecies,too)canstrongly affectbetadiversity.Forexample,
competitionand predation mechanisms as well as thefact that ex-
oticspeciesareabletochangeenvironmentalconditionscanconcur
toexcludenative speciesfrom afish community.However,theex-
pectedpositiverelationshipbetweenabundancebasedSCBDvalues
andspeciesoccupancywaspreviouslyfoundalsoinstreaminsects
by Heino and Grönroos (2017) sugge sting that specie s with high
SCBD valuesare expectedtohave relatively highlocal abundance
andhighsitesoccupancy.Ourresultsalsoconfirmthepredictability
ofSCBD values from species occupancy and abundance (Heino &
Grönroos,2017;daSilva,2018).
However,perhapscounterintuitively,itisnotalwaystruethat
species withhigh SCBD(i.e., highcontribution tobeta diversity)
areimportant to preservediversity whenexotic or nativestatus
was considered. For example, in lowland sites, the most wide-
spread exo tic species such as t he stone moroko (Pseudorasbora
parva),thecrucian,carpandthecommoncarpshowedhighSCBD
but,duetothehomogenizationeffect(Toussaintetal.,2016)and
their ability to modify the environment (Breukelaar, Lammens,
Breteler, & Tatrai, 1994; Chumchal, Nowlin, & Drenner, 2005;
Alain J. Crivelli,1983), theycan negatively affect native species
diversit y. Of consequences, the high SCBD values can help to
992 
|
   GAVIOLI et AL.
identifythemostabundantandwidespreadexoticspecieswhich
couldhavenegativeeffectonnativecommunities.Likewise,high
SCBDvalues innative speciescanidentifyspeciesthatnotneed
conservation measures due to thehighabundanceand wide dis-
tribution, suchas thechub (Squalius squalus)or the Italian bleak
(Alburnus alborella).Contrarily,lowSCBDvaluescanidentifyrare
native species, that for the low abundanceand restricted distri-
bution that require major conservation measures such as the
Italian nase(Chondrostoma soetta),and theSouthEuropeannase
(Protochondrostoma genei),classifiedasendangered byIUCN and
includedintheHabitatDirective(AnnexII).Itisalsopossiblethat
some low SCBD values were due to the low sites occupancy of
speciesattheedgeoftheirdistributionsuchastherainbowtrout,
Oncorhynchus mykiss,inthelowlandsortheEasternmosquitofish,
Gambusia holbrooki,intheuplands.
Taking into account t hese aspects , these analysis outputs re-
quiredaccurateconsiderationastothegeographicalrangeandthe
exoticornativespeciesstatus.Weencouragefutureresearchinthis
field to update the informationavailableand to better understand
themajordrivesofit.
5 | CONCLUSIONS
Duetothelossofnativefreshwaterbiodiversityworldwide(Strayer
&Dudgeon,2010),theneedfortheidentificationofpriorityareasfor
conservation (Hermoso, Clavero,&Kennard, 2012) andthelimited
conservationresourcesavailable;therearethreemainimplications
forfutureconservationstrategies found in thispaper: (a) not only
headwatersrequireconservationmeasuresbutalsolargeuplandriv-
ersareimportantincontributingtonativefishdiversity.Thesesys-
temsresultedinalowexoticspeciespresence,promotingzoneswith
highnative diversity.(b)Inuplandsites,native species showedthe
highestcontributiontobetadiversity,butthispatternwasnotfound
inlowlandsites,which shows the importance ofprotecting native
comm uni tie si nup lands ite s,whilesuggest ingageneralhomogeniza-
tionprocessinthelowlandcommunities.(c)Somerare nativespe-
cies that ar e restric ted to few sites ca n show low contrib ution to
betadiversity,butsuchspeciesmaystillneedconservationactions
duetotheir risk of localextinctions.Thissuggests tointerpretthe
resultsofSCBDcarefully,becausetheabundanceofrarespeciesis
typicallyunderestimated.
ACKNOWLEDGEMENTS
We thank LL.D. V.E.Manduca and Dr. M.Rizzoli of the Fisheries
Bureau of the Emilia‐Romagna Region for providing the Fish
Inventoriesdatainthecontextofalong‐termresearchcollaboration.
Th e O glioR i v e rWa t erAut h o r i t y(Consorzio dell'Oglio,inItalian)isalso
acknowledgedforprovidingfishandwaterqualitydatafortheOglio
River.Wealso thank Dr.R.Spaggiariand Dr.S.Franceschini ofthe
Emilia‐Rom agna Region Environm ental Protect ion Agency (A RPA
EMR), the Piemonte Region Environmental Protection Agency
(ARPA‐Piemonte)andthe VenetoRegionEnvironmental Protection
Agency(ARPAV)forprovidingthewaterqualitydatabase.
DATA ACCESSIBILITY
Fish data u sed in this stud y are shown in Supp orting info rmation
TableS1.DataavailablefromtheDryadDigitalRepository:https://
doi.org/10.5061/dryad.83g8j8f
ORCID
Marco Milardi https://orcid.org/0000‐0001‐6104‐294X
Janne Soininen https://orcid.org/0000‐0002‐8583‐3137
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BIOSKETCH
Theauthorshaveabackgroundinspeciesdiversitydistributions
(J.S.),fishandinvasionecology(A.G.,G.C.,M.M.)aswellasgen-
eralecology(E.A.F.).Thesedifferentresearchlines were joined
toinvestigatethespatialdistributionoffishspeciesanditsdriv-
ers,consideringtheexotic/nativespeciesstatus.
Authorcontributions:J.S.andG.C.conceivedtheidea,A.G.col-
lectedthedatasetandanalysedthedata,A.G.andJ.S.ledmanu-
scriptwriting,M.M.,G.CandE.A.Fprovidedmajorinputonthe
manuscript.
SUPPORTING INFORMATION
Additional supporting information may be found online in the
SupportingInformationsectionattheendofthearticle.
How to cite this article:GavioliA,MilardiM,CastaldelliG,
FanoEA,SoininenJ.Diversitypatternsofnativeandexotic
fishspeciessuggesthomogenizationprocesses,butpartlyfail
tohighlightextinctionthreats.Divers Distrib. 2019;25:983–
994. ht tps://doi.org/10.1111/ddi.12904
... These nativeinvasive species with broad niches may contribute less to beta diversity than species with narrow or intermediate niches, because the latter species may occur in environmentally restricted conditions and contribute more to beta diversity (Heino and Grönroos, 2017). Therefore, exploring the contribution of each species to beta diversity may provide a platform for understanding the formation of community diversity patterns (Heino and Grönroos, 2017;Gavioli et al., 2019). ...
... In this study, the RA of most native species across sites was higher than that of native-invasive species, especially in regions where native fish species were completely predominant. Previous studies have shown that species with high total abundance across sites contribute most to the abundance-based β-diversity (Heino and Grönroos, 2017;Gavioli et al., 2019). When the identities of native and native-invasive species are considered, the high SCBD values can reveal which native-invasive species are the most abundant and widespread, whereas the low SCBD values can identify rare native species. ...
... When the identities of native and native-invasive species are considered, the high SCBD values can reveal which native-invasive species are the most abundant and widespread, whereas the low SCBD values can identify rare native species. Although some rare native species have a relatively lower contribution to β-diversity, such species may still need conservation actions owing to their local extinction risk (Gavioli et al., 2019). Meanwhile, the native-invasive species with high abundance should be strictly controlled to mitigate the ecological consequences of low-head dams. ...
Article
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Low-head dams are ubiquitous human disturbances that degrade aquatic ecosystem function worldwide. The localized effects of low-head dams have been relatively well documented; however, most previous studies have ignored the concealed process caused by native-invasive species. Based on fish assemblage data from the first-order streams of four basins in the Wannan Mountains, we used a quantitative approach to assess the effects of low-head dams on fish assemblages by distinguishing between native and native-invasive species using occurrence- and abundance-based data, respectively. Low-head dams significantly decreased native fish alpha diversity while favoring native-invasive fish. The opposite pattern between the two fish types partly masked changes in the whole fish assemblage. Meanwhile, the establishment of widespread native-invasive species and the loss of native species driven by low-head dams influenced the interaction network structure. The degree to which local fish assemblages were altered by low-head dams, i.e., beta diversity (β-diversity) was significantly higher for abundance-based approaches than for occurrence-based ones, suggesting that the latter underestimated the effects of low-head dams. Furthermore, the species contribution to β-diversity of native species was significantly higher than that of native-invasive species in both impoundments and free-flowing segments for abundance-based data. In communities or regions where native fish species are predominant, our results suggest that understanding which species contribute to β-diversity will offer new insights into the development of effective conservation strategies by taking the identities of native and native-invasive species into account.
... Since no geographically detailed data on introduction, early establishment and subsequent spread of the invasion was available, our investigation focused on the late-invasion stage. We chose our putative spatial drivers of fish invasion with guidance from previous studies that highlighted the importance of climate 28 , geography 29 , habitat fragmentation 30 and anthropogenic pressure (in particular eutrophication) 23 in shaping the spread of invasive freshwater species. Our study aimed to test the hypothesis that spread and biological invasions would be mainly driven by anthropogenic impact causing habitat degradation, and that natural variables would play a secondary role. ...
... The temporal and spatial mismatch between environmental and biotic variables, given the nearly impossible task to monitor all variables at a similar temporal and spatial resolution, is a common problem of studies in this field 38 . While perhaps unavoidable, temporal and spatial mismatches can be minimized, using improved interpolation and minimal extrapolation, and considering timescales comparable to species assemblages turnover time 29,39 . More detailed spatial and temporal information on introduction history could provide information to further interpret the patterns observed. ...
... In summary, the dataset included 3777 sites sampled 1999-2014, recorded a total of 99 different fish species (35 of which were exotic and already established, even if some are restricted to areas with thermal springs), spanned > 11 degrees of longitude (~ 1200 km) and 10 degrees of latitude (~ 1100 km), covering streams at altitudes -2.7-2500 m above sea level. Community turnover was not a relevant factor in our study, because fish communities are typically stable over these timescales and the data was collected in a restricted timeframe within each area 29,39 . Furthermore, time elapsed since last introductions was sufficient to analyze distribution patterns after major invasions had already occurred see e.g. ...
Article
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We analyzed the large-scale drivers of biological invasions using freshwater fish in a Mediterranean country as a test case, and considering the contribution of single species to the overall invasion pattern. Using Boosted Regression Tree (BRT) models, variation partitioning and Redundancy Analysis (RDA), we found that human factors (especially eutrophication) and climate (especially temperature) were significant drivers of overall invasion. Geography was also relevant in BRT and RDA analysis, both at the overall invasion and the single species level. Only variation partitioning suggested that land use was the second most significant driver group, with considerable overlap between different invasion drivers and only land use and human factors standing out for single effects. There was general accordance both between different analyses, and between invasion outcomes at the overall and the species level, as most invasive species share similar ecological traits and prefer lowland river stretches. Human-mediated eutrophication was the most relevant invasion driver, but the role of geography and climate was at least equally important in explaining freshwater fish invasions. Overall, human factors were less prominent than natural factors in driving the spread and prevalence of invasion, and the species spearheading it.
... However, generalization is possible when community dynamics are affected by large scale gradients, such as for instance the upstream-downstream gradients in rivers, or gradients related to sources of human disturbance Schlosser, 1987). Moreover, local dynamics can be influenced by spatial connectivity associated for instance with the topology of stream networks, anthropogenic barriers and habitat FCUP Modelling biodiversity patterns and processes to support conservation in stream networks 198 Ch6 fragmentation Crabot et al., 2020;Erős & Lowe, 2019;Hugueny et al., 2010), as it affects meta-community mass effects mediated by dispersal (Heino et al., 2015;Tonkin et al., 2018), as well as the spread of invasive species Gavioli et al., 2019;Milardi et al., 2019;. Therefore, spatial modelling of community dynamics requires establishing relations with environmental variables predicting variation in dynamics metrics, and accounting for spatial variables reflecting the effects of connectivity. ...
... The spread of exotic species may also have affected community dynamics, due to temporal changes in their own prevalence and abundance, but also due to eventual negative effects on native species Gavioli et al., 2019;Milardi et al., 2019;Zanden et al., 2015). This is supported by faster changes in sites with higher proportion of exotic species, and by the high coefficients of variation in the abundance of exotic species when compared to most native species. ...
... In contrast, lower orders at low elevation generally correspond to warm water streams with richer communities dominated by cyprinids and exotics, which during the dry summer months are often reduced to a series of disconnected pools, and thus where fish communities may vary widely from year to year in association with droughts and floods . Other possibility is that elevation acted as a surrogate for increasing human disturbance in the FCUP Modelling biodiversity patterns and processes to support conservation in stream networks 214 Ch6 lowlands driving higher variability in fish communities, which may be mediated by the increasing prevalence and abundance of exotic species Gavioli et al., 2019;Milardi et al., 2019). This idea is supported by the positive relation observed between the proportion of exotic fish and community variability, and by the inverse relation between the prevalence of exotic crayfish and elevation also found in the watershed . ...
Thesis
Full-text available
Biodiversity is not evenly distributed across our planet. Freshwater ecosystems hold a disproportionate amount of biodiversity when compared with other biomes, though it only covers a small portion of our planet surface. Because water is essential for human activities, population growth and economic development put an enormous pressure on freshwater ecosystems. Besides the direct anthropogenic pressures, such as over exploitation, damming, habitat modification and pollution, the freshwater species usually present restricted distributions, to a watershed or a region, and face the threat of hundreds of invasive species that have been introduced to freshwater ecosystems. Due to all these factors, freshwater biodiversity is among the most threatened of our planet. Stream networks deserve special attention because they are particularly threatened and rich in biodiversity. Streams networks are linear bodies of water with a dendritic, or tree shape, organization, flowing from the headwaters to a single outlet. The distribution of organisms in stream networks are not random, resulting from several processes that work at different scales, like climate, hydrology and biotic interactions. The diversity and abundance of fish and many other organisms are usually associated with an increase in stream order, but there are also dispersal processes that should be taken into account. The distribution of some species, like invasive species, is often more a reflection of spatial processes, such as multiple introductions and posterior expansion, than environmental filters that limit the distribution. Headwaters can function as refuges from adverse biotic interactions for species that support water intermittency. Stream communities, like fish, usually persist in time in a state of dynamic equilibrium, varying between alternate states with no discernible direction of change. Deviations from this equilibrium may reflect disturbances to the community from natural states, like droughts or floods, or from anthropogenic sources. For proper conservation and management of stream networks, it is essential to understand the drivers of the spatial patterns and dynamics of stream biodiversity. Species distribution models (SDM’s) are the set of tools used to derive spatially-explicit predictions of environmental suitability, by relating species occurrences to relevant environmental data. Due to their nature and the nature of the stream network habitats, the development of SDM’s for organisms that are associated with streams is challenging. Aquatic organisms are rarely available for direct observation, and even with the help of standard techniques, like electrofishing, it is fair to assume that we will fail to detect some of the species present at any given location. This issue, known as imperfect detectability, is a common source of bias in SDM’s, and tends to be ignored by freshwater researchers. Accounting for spatial autocorrelation (SAC) improves SDM performance, but the dendritic structure of stream networks, together with strong environmental gradients, create spatial dependences with complex structures that are not completely described by Euclidean distances. Biotic interactions, such as competition or predation, are also a potential source of mismatch between the actual and the predicted distribution of species, particularly if the interactions are between invasive species and native species. Long term monitoring of communities is essential to understand the impact of anthropogenic pressures in stream ecosystems, but usually rely on data collected on any given number of discrete locations. A spatial continuous view of the temporal dynamics would be essential to study such pressures, and of value to plan conservation and management actions. The main aim of this thesis is to develop new tools and frameworks to help ecologists and conservationists to obtain a more realistic depiction of the distribution of species, and the temporal dynamics of communities at the riverscape scale. I mainly focused on solutions to the issues related to dealing with imperfect detectability, accounting for SAC in stream networks, accounting for biotic interactions, and extrapolating the community temporal dynamics to a continuous spatial prediction. To address these issues, I have collected data on the distribution of fish, crayfish, and amphibians on a specific study system, the Sabor River, a Mediterranean watershed in the Northeast of Portugal. To describe the distribution of fish species with data collected in a comprehensive electrofishing survey, while accounting for imperfect detectability, we extended the time-to-detection occupancy-detection model to include interval-censored observations, because it is difficult to ascertain the exact time-to-detection of a species when sampling fish with electrofishing techniques. Using a Bayesian hierarchical framework, we modelled the probability of water presence in stream segments, and the probability of species occupancy conditional on water presence, in relation to environmental and spatial variables. We also modelled time-to-first detection conditional on occupancy in relation to local factors, using a modified interval-censored exponential survival models. To account for SAC, we included a spatial autocovariate term in the estimation of the probability of water presence and the probability of species occupancy. Species occupancies were consistently affected by stream order, elevation and annual precipitation, while species detection rate was primarily influenced by depth and, to a lesser extent, stream width. The assumption of equilibrium between organisms and their environment is a standard working postulate in SDM’s that is seldom met, particularly for species that are expanding their range like invasive species. Furthermore, for species invading river systems, the dendritic structure of the stream network will constrain the patterns of the expansion. In this thesis, I addressed these issues by describing the distribution of two invasive crayfish in the Sabor river stream network, using a class of geostatistical models developed to deal with SAC in stream networks, known as spatial stream network models (SSNM). Accounting for SAC greatly improved model performance, evidencing that the distribution of these invasive crayfish was more of a product of spatial process than environmental filtering. Biotic interactions are important drivers of species distributions. When native species are displaced from part of their distributional range, they may persist in ecological refuges. These refuges may be patches of habitat that are unsuitable for invasive species or areas where invasive species have not reached due to distance, physical barriers or time lags in the expansion. Identifying the distribution and the environmental drivers of these refuges is of conservation concern. We modelled the distribution of amphibian ecological refuges in the Sabor river catchment, by including as predictor variables the probability of presence of the two invasive crayfish, among other environmental and spatial predictors. We found that the refuges of amphibians are located mainly in the headwaters, and that, under plausible expansion scenarios of the crayfish species, these refuges are likely to contract in the future. Management of stream networks is usually planned at the river basin scale, and as such, it is important to develop frameworks that allow the extrapolation of the community dynamics observed at discrete segments of rivers to a continuous spatial view of the entire river basin. We collected stream fish data on 30 locations on the Sabor river basin, between 2012 and 2019, and used a novel framework to describe and compare the trajectories of the fish communities using their geometric properties in a given dissimilarity space. We computed the mean velocity and the overall directionality of change of the fish community, and used the SSNM framework to relate these metrics to environmental drivers and extrapolate the community dynamics to the entire watershed. We found no evidence of directionality in the change of the Sabor fish communities, supporting the hypothesis that these communities exist in a loose equilibrium state. However, the rate of change was higher in streams draining into the hydroelectric reservoir located near the mouth of the Sabor River. These streams are likely under increased stress from the reservoir, due to alterations of the flow regime and/or expansion of alien species from the reservoir. Overall this thesis advances our understanding of the drivers that govern the distribution of species in stream networks, providing key information for the conservation of these ecosystems. The new set of tools presented here can aid ecologists and conservationists to obtain a more realistic depiction of species distribution and their temporal dynamics at the riverscape scale.
... Milardi et al., 2018). Lowland areas of fragmented rivers are of great concern, since fish communities tend to be there taxonomically homogenized, with native species not contributing much to diversity (Gavioli et al., 2019). ...
... The main objectives of this study were: 1) To estimate the public value of different elements of river ecosystems, including connectivity; and 2) To model conservation priorities based on public values, applying the estimated relative WTP to objective river features. We focused on lower reaches that are the most affected by river fragmentation (Gavioli et al., 2019), and asked university students of different backgrounds (including students of educational sciences that will influence the environmental values of future generations) for the monetary quantities they would pay to improve five river features that provide important river services. Two river services are related to the habitat and overall environmental quality (river connectivity and habitat quality) and three related to the fish community (quantity of fish, number of native fish species and total fish diversity). ...
Article
Full-text available
The social value of natural aquatic ecosystems is very important to set management priorities. River connectivity is essential for the conservation of freshwater ecosystems because barriers alter both abiotic conditions and the biotic communities, compromising biodiversity; however, the appreciation of this river feature has been insufficiently considered in socio-environmental studies that are mainly focused on the acceptance of new dams. Here we used a willingness to pay approach to estimate the value of connectivity, native species, fish diversity (measured as functional diversity or as species richness), fish abundance and environmental quality in three groups of students of different educational background in Asturias (NW of Spain). As in other studies where they are more sensitive to environmental issues, educational sciences students would pay more to conserve and improve river conditions than students of other disciplines. Connectivity was the least valued river feature by students of educational and natural sciences, and the third (before biodiversity and fish abundance) by engineering students. We measured the same features on lowland reaches of four coastal rivers in the Bay of Biscay, and applied declared will amounts to model their appreciation. Differences between the river ranks obtained from functional diversity (that changes with non-native species) and species richness, and small differences between students of different disciplines in the gap between most and least preferred rivers arise from the model. This indicates the importance to involve diverse stakeholder sectors in decisions about rivers. The importance of river connectivity in the conservation of local biodiversity should be explained to general public, perhaps through environmental campaigns.
... The negative impact of exotic fishes is negligible for these freshwater habitats, thereby presuming less moderation in the current scenario (Davies et al. 2005;Gavioli et al. 2019;Panja et al. 2021b). Although, thorough indexing of the invasiveness of the alien fishes up to the trophic level is recommended to uncover the potential threats of invasive species in these freshwater reaches following a recent study (Calizza et al. 2021). ...
Article
Full-text available
Identifying conservation strategy is essential regarding prioritization, planning, and managing biodiversity. The Eastern Himalayan freshwater reaches contain diverse taxa of fish species. Despite having several coarse scales of assessment, the information regarding the fine-scale conservation priorities is scanty. The development of indices from multimeric attributes has been proved efficient, aiding conservation planning, in-depth research, exploitation, policy-making, and public awareness. Therefore, this study aims to provide detailed indexing of conservation values for the freshwater fish species inhabiting the sub-Himalayan Terai–Dooars ecoregion of the Eastern Himalayas. Based on three years of extended sampling in six freshwater reaches, 170 indigenous fish species were identified. Each fish species was assigned a discrete conservation value following their rarity, taxonomic singularity, contribution to β diversity, global threat status, regional importance, and maximal achievable body lengths. Neolissochilus hexagonolepis has the highest onservation value. In contrast, the lowest values were observed for Pethia gelius, Pethia guganio, and Pethia phutunio. The freshwater habitats of upper and lower elevation harbor essential fish species for conservation, driven by precipitation, topographic, and land cover variability. Such results were accomplished through spatial interpolation and prioritization regarding fish conservation, protection, and vulnerability toward the human footprint for this region of the Eastern Himalayas.
... Local contribution to beta diversity approaches can be useful for bioassessment and conservation purposes, as several studies focused on freshwater communities have revealed (e.g. Gavioli et al., 2019;Li et al., 2020). However, it should be noted that sites having high LCBD values are often rather species-poor sites (Heino & Grönroos, 2017). ...
Article
Full-text available
Aim We investigated taxonomic and functional beta diversity of bird communities inhabiting Mediterranean olive groves subject to either intensive or low‐intensity management of the ground cover and located in landscapes with different degrees of complexity. Location Andalusia, southern Spain. Methods We partitioned taxonomic and functional beta diversity into its two additive components, turnover and nestedness. We also explored the contributions of single sites to overall beta diversity (LCBD) and separated the effects of species replacement (turnover) and richness difference (nestedness) in order to identify ecologically unique sites—keystone communities—within the metacommunity. In a further step, we employed abundance‐ and functional‐based indicator species analyses to characterize bird assemblages. Results Taxonomic beta diversity increased with landscape complexity. Although both taxonomic and functional differences among assemblages were driven mainly by species replacement (regardless of management or landscape type), the contribution of trait replacement to the total functional beta diversity was much lower, suggesting that species performing similar functions replace each other between sites. There were no differences in LCBD between management types or categories of landscape complexity, but the contributions of sites to beta diversity decreased as the percentage cover of olive groves increased. Species richness was also important in explaining variation in LCBD as species‐poor sites tended to contribute the most to the local‐to‐regional beta diversity. However, some farms displayed high values of LCBD due to the existence of a high replacement component, indicating that some species recorded in these sites were scarce elsewhere. The indicator species analyses revealed that the woodchat shrike Lanius senator may constitute an excellent indicator of biodiversity in this agro‐forestry system. Main conclusions Our results show that agricultural expansion promotes biotic homogenization and exemplify how the identification of both keystone species and communities can represent a powerful tool for the management of anthropized landscapes.
... Local contribution to beta diversity approaches can be useful for bioassessment and conservation purposes, as several studies focused on freshwater communities have revealed (e.g. Gavioli et al., 2019;Li et al., 2020). However, it should be noted that sites having high LCBD values are often rather species-poor sites (Heino & Grönroos, 2017). ...
Article
Full-text available
Aim: We investigated taxonomic and functional beta diversity of bird communities inhabiting Mediterranean olive groves subject to either intensive or extensive management of the ground cover and located in landscapes with different degrees of complexity. Location: Andalusia, southern Spain. Methods: We partitioned taxonomic and functional beta diversity into its two additive components, turnover and nestedness. We also explored the contributions of single sites to overall beta diversity (LCBD) and separated the effects of species replacement (turnover) and richness difference (nestedness) in order to identify ecologically unique sites -keystone communities- within the metacommunity. In a further step, we employed abundance- and functional-based indicator species analyses to characterize bird assemblages. Results: Taxonomic beta diversity increased with landscape complexity. Although both taxonomic and functional differences among assemblages were driven mainly by species replacement (regardless of management or landscape type), the contribution of trait replacement to the total functional beta diversity was much lower, suggesting that species performing similar functions replace each other between sites. There were no differences in LCBD between management types or categories of landscape complexity, but the contributions of sites to beta diversity decreased as the percentage cover of olive groves increased. Species richness was also important in explaining variation in LCBD as species-poor sites tended to contribute the most to the local-to-regional beta diversity. However, some farms displayed high values of LCBD due to the existence of a high replacement component, indicating that some species recorded in these sites were scarce elsewhere. The indicator species analyses revealed that the woodchat shrike Lanius senator may constitute an excellent indicator of biodiversity in this agro-forestry-system. Main conclusions: Our results show that agricultural expansion promotes biotic homogenization and exemplify how the identification of both keystone species and communities can represent a powerful tool for the management of anthropized landscapes.
... Consistent with theoretical expectations , we find a non-monotonic curvilinear relationship with commonness and rarity (Fig. S6), such that the commonest as well as rare species may not have a strong influence on beta diversity (Brasil et al., 2020). Previous studies have interpreted such covariation as monotonic and linear (Gavioli et al., 2019), even though data are expected to be curvilinear and non-monotonic. ...
Article
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1. Understanding how biodiversity is distributed is increasingly becoming important under ongoing and projected human land use. Measures of beta diversity, and its partitions, can offer insights for conservation and restoration of biodiversity. 2. We ask how different species, functional groups, and land use contribute to beta diversity, and whether invasive species have a negative influence on beta diversity. We address these questions using ant assemblages (Hymenoptera: Formicidae) at 277 sites distributed across five geomorphic land use types in Goa, India. 3. We recorded 68 species (35 genera, 7 subfamilies) of which 5 were invasive. We classified them into eight functional groups. Oecophylla smaragdina—a common tropical arboreal species, and Anoplolepis gracilepis—a globally significant invasive, contributed the most to beta diversity. Large-bodied omnivores which may influence soil functions contributed more to beta diversity than small-bodied predators. Lateritic plateaus contributed most to beta diversity, whereas human-influenced plantations contributed the least. Beta diversity across sites was related to species turnover, whereas nestedness was more prominent for functional groups. This indicates how species replace one another with change in land use, but functional roles are lost despite such turnover. Sites with human land use had higher incidence of invasive species, and invaded sites contributed less to beta diversity than non-invaded sites. 4. Human land use strongly influences diversity and distribution of ant assemblages. Land use may spare local species richness, but not functional groups. A small number of invasive species exert negative influence even in very speciose communities.
... Besides, exotic fishes usually override the critical environmental drivers (relevant to native) as they uniquely rely upon geography and human-mediated dispersal limitations Leprieur et al. 2009). However, evidence regarding the impact of exotic fishes on functional diversity, predation, and trophic overlap with the native fish lack from this region; therefore, the present inferences are solely based on native fish species and presumed to be less moderated by exotics considering the spatial scale of the study (Davies et al. 2005;Gavioli et al. 2019;Milardi et al. 2019). ...
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Forty-seven sample sites were electrofished in 22 streams on the South Carolina coastal plain. Average species numbers adjusted to a constant stream surface area were 12.7, 17.5, 21.4, and 22.0 in first- through fourth-order streams, respectively. Species addition and replacement led to large changes in species composition among stream orders. Relatively small fishes, most of which were generalized insectivores, numerically dominated headwater (first- and second-order) streams. Relatively large fishes, many of which were piscivores or benthic insectivores, were most common in fourth-order streams. Headwater species richness was higher and longitudinal species replacement was greater than often observed in other geographic regions of the United States. A comparative assessment of long-term temperature and precipitation records suggested that high species richness at headwater sites was related to mild climate and lack of steep elevation gradients. The presence of numerous small headwater species created the potential for multiple species replacements as downstream increases in habitat volume permitted the establishment of larger fish with predatory and competitive advantages. Because they support many species uncommon in larger streams, headwater streams in the southeastern coastal plain contribute importantly to biodiversity.