Abstract
Aim Human‐induced loss of native species and introduction of non‐native species have altered richness and composition of species assemblages world‐wide. During the past 15 years many studies have focused on changes in taxonomic similarity and identified numerous yet often contrasting reasons for these changes. This study aims to quantify taxonomic changes in freshwater fish assemblages between the mid‐19th century and today, while explicitly separating its different components and drivers. Location Geographic Europe, 251 river basins > 2500 km². Methods Pairwise catchment comparisons of historic and contemporary fish species inventories, with and without migratory fish, using Jaccard similarity; quantification of relative species turnover using a newly developed Reshuffling Index; determination of the threshold (here: number of catchments occupied) that displays how widespread a species must be to cause homogenization. Results The European freshwater fish fauna changed profoundly since the mid‐19th century. All river catchments exhibited an average net gain of 5.7 species, leading to an overall increase in faunal similarity across Europe of 3.1% (4.6% if migratory species are excluded). However, species turnover was much higher than indicated by the net gain. On average, 20% of the historic assemblages became reshuffled. The native catchment range size of an introduced species primarily determined its impact on taxonomic similarity change, irrespective whether it is translocated within or introduced from outside Europe and whether the species is of fisheries importance or not. Main conclusions The concurrent use of multiple indices allowed disentangling the main components and drivers of taxonomic change. It became evident that prevention of intended or unintended species introduction will not lower the rate of taxonomic homogenization per se. However, most species actually caused taxonomic differentiation despite their range gain. All introduced species still considerably contributed to taxonomic change with potential negative effects on ecosystem functions.