Fine-scale analysis of heavily invaded Italian freshwater fish assemblages

Inland waters are highly vulnerable to the introduction and spread of non-native species, due to heavy human use of aquatic ecosystems and the natural linkages among streams and lakes. This is particularly noticeable in freshwater fish communities. To better evaluate how these communities are affected by non-native species introductions, we conducted a fine scale analysis of the changes in Italian freshwater fish assemblages after species introduction. For this analysis, we collected information on fish species present in 44 basins. The present Italian freshwater fish fauna is composed of 48 native and 41 established introduced species, while a further 15 introduced species were reported but not yet considered naturalized. The changes of the fish assemblages mostly took place in the last two centuries and have increased recently, with nearly 60% of the species introduced in the last three decades. The number of species introduced per basin ranged from 0 to 35 (mean 10.85 ± 7.77 species/basin), and in ten basins the number of species introduced is now equal to the number of native species or even higher. In the past, introduced species mainly originated from America, but in last decades, an increase of introductions from other parts of Europe and Asia was recorded. Our results show that basins already rich in native species tend to get even richer as a consequence of the establishment of introduced species. This confirms the trend toward a biotic homogenization of ecosystems even at a local scale, due to an increase in the human-mediated spread of generalist species.