Fish communities in Italian sub-alpine lakes: Non-native species and anthropogenic pressures increase community dissimilarities

In European lakes, anthropogenic pressures have increased significantly since the 1950s, facilitating colonisation by non-native species and increasing the potential for further invasions. Here, we determined the effects of anthropogenic pressures (i.e., habitat alterations and introduction of non-native species) on the fish communities of Italian sub-alpine lakes. We hypothesised that established non-native species would have more competitive traits against anthropogenic stressors, such as habitat alteration, than native species. Thus, we expected that non-native species would dominate lake communities and reduce native species occurrence and abundance depending on the degree of anthropogenic alterations. Overall, we predicted that the increase in anthropogenic pressures after the 1950s had led to homogenisation of the fish communities of the lakes in the region. We tested these hypotheses using data on 15 sub-alpine lakes, covering a broad geographical and morphological gradient, and compared the 2007-2014 fish community composition (sampled according to the CEN protocol plus point-abundance electrofishing) with variables of lake habitat and anthropogenic pressures (based on the Lake Habitat Survey, a method to evaluate the hydromorphological conditions of lakes according to the European Water Framework Directive) and fish communities before 1950, the latter based on bibliographic information. Following our hypothesis, non-native species showed higher prevalence of traits that increase their competitiveness against anthropogenic alterations (e.g., tolerance to pollution). In addition to lake morphology, the community composition of non-native fish determined as abundance (NPUE) and biomass (BPUE) was positively related to anthropogenic pressures. Since the 1950s, 19 non-native species have colonised the Italian sub-alpine lakes, and the occurrence of native species has decreased by ~27%. However, contrary to our expectation, these changes have increased the β-diversity of the fish communities in the lakes.