Testing the native invasion hypothesis to explain anthropogenic influence on stream fish assemblages

In communities or regions where non-native fish species still do not predominate, changes in the assemblage composition are driven by loss, gain or substitution by native species only. We investigated the native invasion hypothesis in small streams, in which human modifications may influence fish assemblage composition by boosting the expansion and establishment of widespread species, as well as of species commonly found in large streams and rivers. Fish community data from 54 lowland streams from South Brazil were used to investigate this hypothesis. We found a positive relationship between cropland cover at the catchment scale and the dominance of fish species that commonly inhabit large streams or rivers (inferred on museum records). We also observed a weak and negative relationship between site elevation and the percentage of widely distributed species in fish assemblages. Our results partly support the hypothesis of native invasion in lowland streams, but the low explanatory power of the models suggests that it is less pronounced compared to highland streams. Our results contribute to understand inconsistencies among studies on the effects of land use on stream fish assemblages using traditional metrics (alpha and beta diversities). For instance, land use can initially increase fish species richness in small streams by favouring the occurrence and establishment of fish species common to rivers. In this sense, alternative metrics that consider specific changes in native species distribution, such as proliferation of common species, should be used to better assess the mechanisms that drive changes in communities of aquatic ecosystems.