In recent years, there has been growing concern about how species invasions and extinctions could change the distinctiveness of formerly disparate fauna and flora, a process called biotic homogenization. In the present study, a null model of biotic of homogenization was developed and applied to the European freshwater fish fauna. We found that non-native fish species led to the greatest homogenization in south-western Europe and greatest differentiation in north-eastern Europe. Comparing these observed patterns to those expected by our null model empirically demonstrated that biotic homogenization is a non-random ecological pattern, providing evidence for previous assumptions. The place of origin of non-native species was also considered by distinguishing between exotic (originating from outside Europe) and translocated species (originating from within Europe). We showed that exotic and translocated species generated distinct geographical patterns of biotic homogenization across Europe because of their contrasting effects on the changes in community similarity among river basins. Translocated species promoted homogenization among basins, whereas exotic species tended to decrease their compositional similarity. Quantifying the individual effect of exotic and translocated species is therefore an absolute prerequisite to accurately assess the spatial dynamics of biotic homogenization.
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