Worldwide freshwater fish homogenization is driven by a few widespread non-native species

Introduction of non-native species have changed the composition of freshwater fish assemblages throughout the world and hence the dissimilarity between them, either toward homogenization (i.e. decrease in dissimilarity) or differentiation (i.e. increase in dissimilarity). However, there is still no assessment of individual contributions of non-native species to this overall trend at the global scale. Here, we disentangle individual non-native species effect from the global effect of the whole introduced species pool at the biogeographic realm scale and test which determinant can explain the effect of non-native species on changes in assemblage dissimilarity. Our results show that the contribution of introduced species on changes in dissimilarity is highly variable and all directions of changes are observed through the introduction process, i.e. either toward homogenization, differentiation or no change. Overall, only a few widespread species contribute to the worldwide homogenization pattern, whereas most of introduced species slightly contribute to the global change in dissimilarity. The effect of species on change in dissimilarity was influenced by the introduction pressure but also by whether introduced species were translocated (i.e. introduced to other basins within their biogeographic realm) or exotic (i.e. introduced from other biogeographic realms). Homogenization is strongly determined by the species translocated within a realm and only by few widespread exotic species whereas the majority of exotics contribute to a differentiation effect. Nevertheless, under future intensified human pressure, the exotic species spread across realms is predicted to increase and their differentiation effect might turn towards homogenization, and might trigger the global homogenization trend.