Non-native species in Italian freshwater habitats: a macroecological assessment of invasion drivers

The relative role of propagule pressure, abiotic and biotic variables as determinants of non-native species occurrence differs among studies, hindering the synthesis of emergent patterns in invasion ecology and preventing generalisation for conservation actions. In order to produce a broad and general assessment of the occurrence of alien species in aquatic habitats, we proposed a macroecological approach to assess the drivers of occurrence of alien species in all biota (microorganisms, plants and animals) across several natural habitats in freshwater ecosystems in Italy, and we generalised the results of the analysis to provide a risk map of the occurrence of alien species. We determined that abiotic climatic variables were good predictors of alien species occurrence. Indeed, these variables, together with propagule pressure, expressed as the proximity to major inhabited areas, and differences in the receiving community, expressed as the native species richness, played a crucial role in determining the number of alien species. Furthermore, we found evidence of an influence of body size in determining the occurrence of the non-native species. By using the predictions of our model, we addressed the probability of the occurrence of alien species in freshwater habitats across the whole country and highlighted areas at higher risk.