Historical assemblage distinctiveness and the introduction of widespread non-native species explain worldwide changes in freshwater fish taxonomic dissimilarity

Aim Taxonomic dissimilarity between assemblages can result from two processes - the replacement of species (turnover) and differences in richness - but it remains unclear how anthropogenic drivers (introductions and extirpations) affect these processes. Here, we investigate how historical patterns and anthropogenic drivers shape the changes in dissimilarity and its turnover component in freshwater fish assemblages both in historical (i.e. pre-industrial) times and at the present day. Location World-wide. Methods We used a global database of fish assemblages in 1054 river basins throughout the world to quantify the change in turnover through its contribution to taxonomic dissimilarity between the two periods. We then used random forests to disentangle the relative roles of historical distinctiveness, species introductions and species extirpations on changes in dissimilarity and on the contribution of its turnover component. Results The pairs of basins exhibited a decrease in the contribution of turnover to taxonomic dissimilarity of 82% and 61% in homogenized and differentiated pairs, respectively. The historical number of shared species and its change after species introductions and extirpations accounted for more than 60% of the change in dissimilarity and in the turnover contribution in the six biogeographical realms. Main conclusions The historically high distinctiveness between pairs of basins, due to a low number of shared species, has promoted high levels of taxonomic dissimilarity. The world-wide changes in taxonomic dissimilarity among fish assemblages are to a large extent coupled to a decrease in the contribution of taxonomic turnover to taxonomic dissimilarity. Hence, the dissimilarity between assemblages declines with decreasing species replacement. These changes are mainly explained by introductions of a small set of widespread non-native species, which promote an increase in the number of shared species. Alongside historical distinctiveness, species introduction processes can determine whether assemblages become more homogenized or more differentiated. Our results suggest that taxonomic differentiation can quickly turn to homogenization as the number of species introductions increase