Data from: Diversity patterns of native and exotic fish species suggest homogenization processes, but partly fail to highlight extinction threats

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. 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. 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 with 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.