Functional similarity shapes global patterns of fish naturalization and invasion richness.

Freshwater ecosystems are among the most threatened by biological invasions, but it remains unclear why some exotics are more likely to be successful (known as naturalization success) and why some communities tend to harbor more exotics than others (measured as invasion richness). Here, using trait-based analysis, we addressed the knowledge gaps based on data from 1,691 fish communities worldwide. We found a latitudinal pattern of naturalization success, showing that successful exotic species were functionally more similar to native species at high latitudes than at low latitudes. Furthermore, the latitudinal gradient of invasion success was driven by contemporary climate, with higher exotic-native functional similarity in stressful climate conditions (i.e., high latitudes), while the opposite was observed in benign climates (i.e., low latitudes). Functional similarity was further positively associated with invasion richness, suggesting that preadaptations to novel environments (i.e., high similarity) may facilitate invasion richness. Moreover, invasion richness increased with increasing basin area and human pressure, but the effect of native diversity on richness was variable. Overall, the results highlight the joint influence of ecological processes (i.e., environmental filtering and niche differentiation) and human activities on fish invasions, with the relative role of ecological processes being well-elucidated through functional similarity.