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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.    
  Revised:4D ecember2018 
  Accepted:23Januar y2019
DOI:10.1111/ddi.1290 4
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
©2019TheAuthors.Diversity and Distributions PublishedbyJohnWiley&SonsLtd
Aim: Exoticspeciesareamajorthreattobiodiversityandhavemodifiednativecom-
munities worldwide.Invasionprocesses have beenextensivelystudied,butstudies
Weinvestigate suchpatterns among exoticand nativefishcommunities in upland
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
ronmentalvariablesaffectingthe LCBD,aBoosted RegressionTrees(BRT)method
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
totalsuspendedsolids and totalphosphorus.Communitydispersionamongstream
Main conclusions: Largeriversintheuplandsareimportantinmaintainingnativefish
ers have suffered from biological homogenization. Some rare native species can
showlowcontribution to betadiversity,butstill needconservation actions dueto
Theimportanceofbiodiversityfor ecosystem functioning andresil-
etal., 2012;Hooperet al.,2005;Worm et al., 2006). Nevertheless,
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,
result of threecomponents: alpha(localdiversity),beta(variationof
posed (e.g., Baselga, 2010; Tuomisto, 2010), and recently,Legendre
and De Cáceres(2013) proposed amethod thatnot only estimates
BetaD ive rsit y(S CBD)b yin div idu als pecies. Bot hLCBDandSCB Dcan
for a region a nd have been use d to investigate sp ecies distr ibution
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,
onomic al biodiversi ty, the main shor tcoming of thes e measures is
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
such as speciesrichness (Hooper et al., 2005). Invasionsof exotic
hybridization, competition and indirect effects (Blackburn et al.,
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
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
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
of exotic fis h, and a correspon ding decline of native s pecies, oc-
Here,we(a)investigatedhowspeciesrichness (i.e.,alphadiversity)
andthe uniquenessofcommunity composition (i.e., betadiversity,
tigatedthe relative influenceofmainwater physico‐chemical vari-
munity within stream orders and studied whether wecould relate
native species mightcontribute moretobeta diversitythan exotic
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
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
Mediterranean continentalclimate,withanannualaverageprecipi-
networkconsidered includethePoRiverin allitscourse,theOglio
therighttribu tariesintheEmilia‐Romagnaregion.Asarefe re nceex-
north‐eastofthePoBasin,andtorrentsandriverssouthof the Po
In the upla nd rivers, org anic material orig inating from villa ges
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-
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
&Hillebrand, 2010).Furthermore,the study areawas already ina
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 -
(Emilia‐Romagna Region, 2002, 2005, 2008), the Padova province
(Padova Pr ovince, 2010), the Po Rive r (Po River Water Auth ority,
FIGURE 1 MapofsamplingsitesintheNorthernItaly,altitudinalgradientandLocalContributiontoBetaDiversityforupland(darkgrey
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-
dance, 1–2individualsper site) to5(higherabundance, morethan
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,
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
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-
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
Spatial AnalystTool,the flowdirection and theflow accumulation
based on DEM layer were cal culated. Finally, for th e entire river
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-
stream orderwas thusmanuallychecked andrevised whenneces-
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
orderclass1,41instreamorderclass2 ,55instreamorderclas s3 ,6in
class, 17were included in stream order class 2,53 instream order
2.4 | Statistical analysis
All sta tistical an alyses were pe rformed fo r lowland (20 4 sampling
2.4.1 | Species richness and local contribution to
beta diversity in exotic and native fish species
theLocal ContributiontoBetaDiversity(LCBD)wascalculatedfor
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 ,
sition compared withthe averagecommunity in thedata. From an
ecologicalpoint ofview,the LCBD values represent thedegreeof
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
2.4.2 | Relative influence of main water physico‐
chemical variables on the local contribution to
beta diversity
consideredtobe an efficient methodtodescribeanynonlinearre-
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
perfectrepresentationincase ofstronginteractions between pre-
2.4.3 | Variation in exotic and native community
dispersion among and within stream orders
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”
from individual observations to their group centroid, this test cal-
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-
the dist ance to centroid of wate r physico‐chemic al variables (i.e .,
water physico‐chemical heterogeneity) across sites (Heino et al.,
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
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
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
are reported in Supporting information Appendix S1: Appendix A.
Variation of water physico‐chemical variables alongst reamorder
B. In summary, lowland sites showed the highest anthropogenic
chemicaloxygendemand(COD), biologicaloxygendemand (BOD5)
and total p hosphorus. A lso, electric al conductiv ity, mainly due to
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
higher streamorders(Figure2a,b).Native speciesshowed significant
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
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
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
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
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
LCBDvalues for bothexoticandnative speciesinthelowlandsand
intheuplands, respectively(Figure 4).Asevident inthefittedfunc-
species,a secondimportantfactorwastotalphosphorus inlowland
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)
p<0.014). Based on the pairwise comparisons, within stream
toaltitudinal zones and indrainage canals network. Stream or-
(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,
FIGURE 2 Boxplotsrepresentingthevaluesofexotic(orange)andnative(green)fishspeciesrichnessinthelowlands(a)anduplands(b)
FIGURE 3 BoxplotsrepresentingLocalContributiontoBetaDiversity(LCBD)valuesforexotic(orange)andnative(green)fishspecies
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
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-
p>0.05), whereas in theuplandsites SCBDvalueswere higherfor
diversit y loss. This s tudy investig ated diversit y patterns and t heir
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.
includingwatertemperature,rivermorphology (e.g.,depthand
width)andhabit atdiversity(Allan&C astillo,20 07).Inourstud y,
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
modificationsandflowregulation) couldpartly explain lowna-
tificialdrainage network,butalso pastexoticspeciesinvasions
could have played a central roleinshapingthis distribution. In
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-
river scannotprovid esuitablehabitatsfora llsuchnativespecies
agecanalsnetwork where direct effects (e.g., predationand com-
successful exotic invaders (e.g., Silurus glanis and Cyprinus carpio)
were amplified due to the lower habitat complexity (Castaldelli,
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
regions (Toussaint etal., 2016; Villéger, Blanchet, Beauchard,
Oberdorff, & Brosse, 2011). Upland sites (high stream order
species,suggesting that largeriversathigher elevationscould
munit iesinthelowlands(Milardietal.,2018)andthusunderline
aneed forconservationand possibly restorationof suchsites
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
FIGURE 5 Boxplotsshowingmeandistancetocentroidsalong
FIGURE 6 RelationshipbetweenSpeciesContributiontoBeta
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,
communitytendto changereflectingthisenvironmentalshift,with
for example a loss of clear water species with thewater turbidity
In upland s ites, LCBD was mainl y driven by total ph osphorus,
suggesting a strong role of nutrients on beta diversity patterns.
ness of eutrophication‐tolerant species increased towards higher
Other authors suggested a strong influence of morphological
factors(e.g.,waterdepth,width, flowconditions or substratumty-
were not avail able, and the i nvestigation of t heir role in af fecting
LCBD was not possible. However, supporting the importance of
4.3 | Variation in exotic and native community
dispersion among and within stream orders
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
(exceptfornative communitiesin thelowlands). The absenceof
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
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
degree of co mmunity disp ersions doe s not strongly d epend on
in the lowlands, which showed a weak relationshipwit hwater
physico‐chemicalheterogeneity,perhaps indicating theirsome-
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-
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
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-
toexcludenative speciesfrom afish community.However,theex-
by Heino and Grönroos (2017) sugge sting that specie s with high
SCBD valuesare expectedtohave relatively highlocal abundance
ofSCBD values from species occupancy and abundance (Heino &
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
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
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
some low SCBD values were due to the low sites occupancy of
Oncorhynchus mykiss,inthelowlandsortheEasternmosquitofish,
Gambusia holbrooki,intheuplands.
Taking into account t hese aspects , these analysis outputs re-
field to update the informationavailableand to better understand
conservation (Hermoso, Clavero,&Kennard, 2012) andthelimited
forfutureconservationstrategies found in thispaper: (a) not only
highnative diversity.(b)Inuplandsites,native species showedthe
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
duetotheir risk of localextinctions.Thissuggests tointerpretthe
We thank LL.D. V.E.Manduca and Dr. M.Rizzoli of the Fisheries
Bureau of the Emilia‐Romagna Region for providing the Fish
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
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
Fish data u sed in this stud y are shown in Supp orting info rmation
Marco Milardi‐0001‐6104‐294X
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tohighlightextinctionthreats.Divers Distrib. 2019;25:983–
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... 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. ...
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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. ...
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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 . ...
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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). ...
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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). ...
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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). ...
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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). ...
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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. ...
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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). ...
Underlying spatial and habitat attributes of a river network are crucial to comprehend the bio-spatial arrangements within it, the study of which suffers from a paucity of information. Despite several reports on various piscine assemblages, no study contributes to understanding the characteristic attributes of the freshwater habitats of the sub-Himalayan Terai–Dooars ecoregion. Therefore, this study aims to uncover such underlying features through a precise understanding of the spatial profile of freshwater habitats and additive partitions of piscine beta diversity. A significant spatial association is found in the upper stretches of most of these torrential freshwater reaches confined to the eastward of the River Teesta basin to the tributaries of River Jaldhaka. Such a pattern is aligned with a higher local contribution to beta diversity (LCBD) values. The spatial map of LCBD indicates that the mid-altitude (100 > elevation > 2000 m) region contains unique or rare species assemblages. This fact is further confirmed by the spatial aggregation of characteristically adapted hill stream fish species with higher species contribution to beta diversity (SCBD) values. The results are further explained by relevant climatic, topographic, nutrients (sediments), and habitat attributes of which climate, topographic, substrate, and land cover features are the most contributory factors. Such variables are subjected to severe modulation following increasing anthropogenic pressure and changing climatic conditions, leading to the jeopardy of these freshwater habitats. Therefore, prime importance should be accorded to the ecological restoration value of these spatially structured torrential freshwater habitats for conservation and monitoring in the coming days.
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1. Freshwater catchments can experience nutrient deficits that result in reduced primary and secondary productivity. The most commonly limiting nutrients are nitrogen and phosphorus, either separately or together. This review considers the impact of increasing nutrient limitation in temperate basin stream and river systems, focusing on upland areas that currently or previously supported wild Atlantic salmon (Salmo salar) populations. 2. Anthropogenic changes to land use and increases in river barriers have altered upland nutrient dynamics, with particular impacts on salmon and other migratory fish species which may be net importers of nutrients to upland streams. Declining salmon populations may further reduce nutrient sources, reducing ecosystem and fisheries productivity below desired levels. 3. Experimental manipulations of nutrient levels have examined the impacts of this cultural oligotrophication. There is evidence that growth and biomass of juvenile salmon can be increased via appropriate additions of nutrients, offering potential as a conservation tool. However, further research is required to understand the long-term effects of these additions on salmon populations and stream ecosystems, and to assess the vulnerability of downstream habitats to eutrophication as a result. 4. Although purposeful nutrient addition with the aim of enhancing and conserving salmonid populations may be justified in some cases, it should be undertaken in an adaptive management framework. In addition, nutrient addition should be linked to nutrient retention and processing, and integrated into large-scale habitat restoration and recovery efforts. 5. Both the scientific and the management community should recognize that the ecological costs and benefits associated with adding nutrients to salmon streams may change in a non-stationary world.
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The aim of the study is to present a complete and updated fish inventory of inland waters of the Emilia-Romagna region, Northern Italy, and to highlight the presence of fully exotic fish communities. Overall, based on 208 sampling locations, the observed fish fauna consisted of 45 species, 22 native and 23 exotics. A significant element of the inventory is the identification of xenodiversity hotspots (spatially clustered sites, one lowland and one upland region), where a complete substitution of native species by exotic species was observed (in total seven sites in the lowland and two sites in the upland with no native species presence). These xenodiversity hotspots were found to host specific combinations of exotic species, which may be able to constitute balanced exotic communities. The hotspots of the lowland region are located in the northeast lowland part of the territory, hosting exotic species combinations mainly composed by wels catfish (Silurus glanis Linnaeus, 1758, a large predator), common carp (Cyprinus carpio Linnaeus, 1758, a large benthivore), crucian carp (Carassius spp., a small-bodied generalist) and other less dominant exotic species. The hotspots in the upland region were located in the southwest part of the territory and were dominated by only one exotic species (rainbow trout, Oncorhynchus mykiss (Walbaum, 1792)). A difference between these xenodiversity hotspots is that in the lowland the introductions were mostly unintentional and are not continued, while in the upland the introduction of rainbow trout is intentional and currently carried out by local fishermen.
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Bighead carp (Hypophthalmichthys nobilis) have been introduced throughout Europe, mostly unintentionally, and little attention has been given to their potential for natural reproduction. We investigated the presence of young-of-the-year bighead carp in an irrigation canal network of Northern Italy and the environmental conditions associated with spawning in 2011–2015. The adult bighead carp population of the canal network was composed by large, likely mature, individuals with an average density of 45.2 kg/ha (over 10 fold more than in the main river). The 29 juvenile bighead carp found were 7.4–13.1 cm long (TL) and weighed 9.5–12.7 g. Using otolith-derived spawning dates we estimated that these juveniles were 94–100 days old, placing their fertilization and hatch dates in mid-to-end-June. Using this information in combination with thermal and hydraulic data, we examined the validity of existing models predicting the onset of spawning conditions and the viability of egg pathways to elucidate spawning location of the species. While evidence of reproduction was not found every year, we determined that potentially viable spawning conditions (annual degree-days and temperature thresholds) and pathways of egg drift suitable for hatching are present in short, slow-flowing canals.
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In response to climate change, species distribution shifts resulting from local extinctions, colonizations and variations in population abundances potentially lead to community-level reorganizations. Here, we assess changes over time in stream fish communities, quantify the extent to which these changes are attributable to population declines or increases and identify their main drivers. France. 1980–2012. Stream fish species. We used abundance-monitoring data to quantify changes in composition and uniqueness for 332 stream fish communities between a cold historical period (1980–1993) and a warm contemporary period (2004–2012). Then, we used a model-averaging procedure to test the impacts of factors related to climate, land use and non-native species density and their interacting effects in shaping community reorganization. We observed biotic homogenization over time in stream fish communities, although some communities experienced differentiation. Changes in composition mainly resulted from population declines and were favoured by an increase in temperature seasonality and in non-native species density. Population declines decreased with fragmentation and changes in non-native species density, whereas population increases were negatively driven by changes in precipitation and positively by fragmentation. Our results provide evidence that environmental changes can interact with other factors (e.g., upstream–downstream, fragmentation intensity) to determine community reorganization. In the context of global change, fish assemblage reorganizations mainly result from population declines of species. These reorganizations are spatially structured and driven by both climatic and human-related stressors. Here, we emphasize the need to take into account several components of global change, because the interplay between stressors might play a key role in the ongoing biodiversity changes.
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Species co-occurrence and site-specific characteristics have a great influence on biotic community composition at local scales and thus contribute to large variations at broad spatial scales. In this paper, we studied invertebrate communities in 63 river sites of the Lower Mekong Basin (LMB) sampled over 609 thousand km 2. We identified important macroinvertebrate taxa of the component communities (i.e. annelids, crustaceans, mollusks and insects), and key geo-environmental factors that explained the total variance (BDTotal) of the communities at large spatial scale. We used the " Species Contributions to Beta Diversity " (SCBD) and " Local Contributions to Beta Diversity " (LCBD) approaches to partition total beta diversity (BDTotal), identified the important macroinvertebrate taxa (those with high SCBD indices), and estimated the uniqueness of sites in community composition (LCBD indices). SCBD indices showed which taxa were the most important in structuring the four component communities: there were 29 insect taxa, which mainly characterized the upstream sites, and 18 mollusk, 7 annelid and 6 crustacean taxa, which all represented the downstream sites. We used linear regression models and variation partitioning to investigate the influence of component communities and of geo-environmental factors on LCBD indices. Our results showed great variation in composition within the LMB (BDTotal = 0.80 on a 0-to-1 scale). Five sites of the main channel exhibited significant uniqueness (LCBD indices) in community composition. One of them was a hotspot location occupied by a community with exceptional taxonomic composition, which should be protected. Four other sites were degraded by human activity and in need of restoration. Multiple regressions indicated that the global LCBD indices are better explained by the environmental factors, i.e. water conductivity, river depth and Secchi depth (adjusted R 2 = 0.26), than by the geographical factors. Among the component communities, mollusks' and insects' LCBDs were the determinants responsible for the variation in the global LCBD indices (adjusted R 2 = 0.843). The uniqueness in community composition of the sites (e.g. LCBDs) that we estimated provides useful ecological information, which could support restoration and conservation planning for the LMB.
Understanding how species are distributed in space and time is a focal element guiding conservational efforts under ongoing climate change and the Holocene extinction. Freshwater habitats are currently one of the most threatened ecosystem types, and studies aiming to unravel factors that govern biodiversity of tropical stream microorganisms are especially scarce. Diatoms play an important role as primary producers in streams and are widely used as ecological indicators. However, relatively little is known about which factors affect diatom communities in the tropics. Here, we studied benthic diatom diversity across 67 tropical streams spanning stream orders 1-5 in Kenya. We examined whether the hypothesis of latitudinal diversity gradient applies for benthic diatoms, i.e. whether tropical streams encompass more species than boreal streams using a comparable boreal dataset. In addition, we studied which environmental, land use and spatial factors control benthic diatom communities using redundancy analysis. We also examined the nestedness and turnover components of beta diversity, factors contributing to diatom species richness, and the uniqueness of the communities across stream orders by using boosted regression trees and local contribution to beta diversity. Finally, we studied whether environmental heterogeneity and beta diversity are related across stream orders and tested their relationship using tests of homogeneity of dispersion and regression analysis. Species richness was not higher in tropical streams than in boreal ones. Tropical diatom communities were controlled jointly by local environmental and spatial factors. Although water chemistry was the most important controlling factor, also physical variables contributed significantly to community variation. Land use had also a significant effect on diatom communities as broad leaved forest streams harboured different diatom communities compared with streams with higher human impact and conductivity, stressing the importance of forests to water quality and diatom biodiversity. Headwater streams encompassed the highest species turnover, whereas nest-edness was higher in higher order streams. Species richness was significantly higher in higher order streams than in headwaters, whereas the uniqueness of the communities peaked in headwaters. Environmental heterogeneity was the highest in headwater streams and was related to high beta diversity, which highlights the importance of habitat heterogeneity to biodiversity. Our results stress the management and conservational importance of headwater streams and tropical montane forests as these environments harbour unique diatom communities important for regional diversity.
Rivers worldwide are impacted by human activities such as habitat degradation, habitat fragmentation, waterway flow regulation, and introduction of exotic species, which are responsible for the reduction or the disappearance of native species in many parts of the world. The Oglio River, a tributary of the Po River in Northern Italy, is a good example of a river with a long history of human alteration and where exotic invasions are present. We used data on water parameters and fish communities along the watercourse to investigate whether low flow conditions, degraded water quality, abundant exotic species, and the presence of migration barriers could be a disadvantage for native species. We used ordination methods (redundancy analysis), variance partitioning analysis, and the threshold indicator taxa analysis to explore changes in community composition and ecofunctional traits along an altitude gradient. We found that exotic species affected native ones more than water quality and hydromorphological parameters. Native species were most abundant in the upper reach of the Oglio River, despite low flow and shallow depth. Moreover, rheophilic and clear water native fish decreased rapidly in the lower reach of the river, where exotic species increased. This distribution could be explained by the presence of barriers in the middle reach, which block exotic species migrating upstream from the highly invaded Po River, and by a lower suitability of the upper reach for some exotic species. Our results provide a general description of the fish fauna of a strongly regulated river and can contribute to develop more effective fish and water management practices.
Aim We aimed to test whether contributions of individual species (SCBD) and contributions of single sites (LCBD) to overall beta diversity can be predicted by species metrics and species characteristics and also by community metrics and ecological variables, respectively. Location A mainland‐island landscape in the southern Brazilian Atlantic Forest domain. Methods Dung beetles were sampled along 100 sampling units within four large forest fragments. We partitioned beta diversity into SCBD and LCBD aiming to explore their relationships to species and site characteristics, respectively. We then used a combination of multivariate methods and beta regression analyses to examine patterns in SCBD and LCBD. The occupancy, total abundance, niche position, niche breadth and biological traits of species were used as predictor variables for SCBD values. Community metrics, environmental and spatial variables, and temporal patterns in the beta diversity components of turnover and nestedness were used as predictor variables for LCDB values. Results We found that SCBD was strongly related to various species characteristics, such as occupancy, abundance and niche position, but was not related to biological traits of species and niche breadth. In particular, occupancy and its quadratic term showed a very strong unimodal relationship with SCBD, suggesting that intermediate species in terms of site occupancy contribute most to beta diversity. LCBD was mostly explained by variation in species richness, with a negative relationship being detected. Litter height and large‐scale spatial variables were also important in explaining variation in LCBD. Main conclusions SCBD and LCBD were highly predictably related to species occupancy and species richness, respectively. Environmental conditions and large‐scale spatial variables also correlated with LCBD values. Understanding the determinants of SCBD and LCBD may thus hold a key to various general ecological, bioassessment and conservation issues. Protecting sites with high LCBD values may be a suitable approach to practical biodiversity conservation.
The freshwater populations of native fish species (Ns) have reached critical levels in many parts of the world due to combined habitat deterioration by human interventions and exotic fish species (Es) invasions. These alarming conditions require combined and well-designed interventions for restoring environmental quality and restricting Es invasion. The aim of the study is to propose a method to design spatially explicit priorities of intervention for the recovery of Ns populations in highly impacted freshwater systems by exotic multi-species invasion and water quality (WQ) degradation. WQ and Es are used as Ns descriptors, which require intervention. The method uses gradient analysis (ordination method of Canonical Correspondence Analysis) for assessing the weights of Ns descriptors' effects, which are further used to develop weighted severity indices; the severity index of WQ (Swq) and Es invasion (Se), respectively. Swq and Se are further merged to one combined total severity index St. The proposed method provides a) a ranking of the sites, based on the values of S t , which denotes the priority for combined intervention in space and can be visualized in maps, b) a ranking of the most important Ns descriptors for each site to perform site-specific interventions, and c) Es rankings based on their potential threat on Ns for species-specific interventions. WQ, Es and Ns data from 208 sampling sites located in the Emilia-Romagna Region (Northern Italy) were used as a case study for the presentation of the proposed method. The application of the method showed that the north and northwestern lowland areas of Emilia-Romagna region presented the higher priority for intervention since the Ns of these areas are the most impacted from combined Es invasions and WQ degradation. Specific Es belonging to cyprinids, which are mostly responsible for the decline of aquatic vegetation and the increase of water turbidity, and a top Es predator (Wels catfish) were mostly present in these areas. Additionally, the most important WQ stressors of Ns were found to be COD, BOD and temperature that are all connected to oxygen depletion. The aforementioned conditions in the areas described by high priority for intervention can be used as a basis for the development of specific Ns conservation practices targeting the containment of the most harmful Es, the restoration of aquatic vegetation and the improvement of oxygen conditions.
While the significance of anthropogenic pressures in shaping species distributions and abundances is undeniable, some ambiguity still remains on their relative magnitude and interplay with natural environmental factors. In our study, we examined 91 late-invasion-stage river locations in Northern Italy using ordination methods and variance partitioning (partial-CCA), as well as an assessment of environmental thresholds (TITAN), to attempt to disentangle the effects of eutrophication and exotic species on native species. We found that exotic species, jointly with water quality (primarily eutrophication) and geomorphology, are the main drivers of the distribution of native species and that native species suffer more joint effects than exotic species. We also found that water temperature clearly separates species distributions and that some native species, like Italian bleak (Alburnus alborella) and Italian rudd (Scardinius hesperidicus), seem to be the most resilient to exotic fish species. We also analyzed the dataset for nestedness (BINMATNEST) to identify priority targets of conservation. As a result, we confirmed that altitude correlated negatively with eutrophication and nestedness of exotic species and positively with native species. Overall, our analysis was able to detect the effects of species invasions even at a late invasion stage, although reciprocal effects seemed comparable at this stage. Exotic species have pushed most native species on the edge of local extinction in several sites and displaced most of them on the rim of their natural distribution. Any potential site- and species-specific conservation action aimed at improving this situation could benefit from a carefully considered prioritization to yield the highest results-per-effort and success rate. However, we encourage future research to update the information available before singling out specific sites for conservation or outlining conservation actions.
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.