The relative role of propagule pressure, abiotic and biotic variables as determinants of alien species occurrence differs among studies. This limits the synthesis of emergent patterns in invasion ecology and generalisations for conservation actions. In order to produce a broad and general assessment of the invasion pattern in aquatic habitats we here propose a macroecological approach to assess the drivers of occurrence of alien species within the whole biota (microorganisms, plants and animals) across several natural freshwater ecosystems of Italy. We simultaneously tested three groups of variables (propagule pressure, abiotic and biotic characteristics), selected as putative predictors of invasibility of a site. Propagule pressure, expressed as proximity to larger inhabited areas, and differences in the native species richness of the receiving community, had a significant role in determining the number of alien species occurrences. Furthermore, body size influenced the occurrence and colonization processes of alien species. Finally, climatic characteristics were relevant in determining the chances that a site was invaded, confirming the role of these abiotic filters in the invasion process.
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