Effects of non-native species on phylogenetic dispersion of freshwater fish communities in North America

Aim: Most studies focusing on terrestrial ecosystems have shown that non-native species are more likely to invade phylogenetically clustered communities, that the introduction of non-native species increases phylogenetic clustering in the recipient communities, and that the latter is more conspicuous for recipient communities at a larger spatial scale. However, whether these patterns hold for aquatic ecosystems is rarely tested. Here, we investigate the phylogenetic structure of native and non-native freshwater fish assemblages at a continental spatial extent. Location: The continental United States and Canada. Time period: Current. Methods: We collated native and naturalized non-native species lists of freshwater fishes for each watershed at local and regional spatial scales. Two phylogenetic metrics (MPD_ses and MNND_ses) representing different evolutionary depths were used to quantify the phylogenetic dispersion of each watershed fish assemblage and were related to temperature and precipitation using correlation and regression analyses. Values of the metrics were compared between the two spatial scales. Results: Our study found that (1) phylogenetically more diverse fish communities tended to be invaded by fewer non-native fish species, and that non-native species more frequently invaded communities with lower phylogenetic dispersion (i.e. higher phylogenetic clustering), (2) introduction of non-native fishes reduced phylogenetic clustering in some watershed but increased clustering in others, (3) introduction of non-native species caused phylogenetic dispersion to increase in watersheds with low phylogenetic diversity and to decrease in watersheds with high phylogenetic diversity and (4) phylogenetic clustering for fish assemblages at the larger spatial scale was smaller than that at the smaller spatial scale. Main conclusions: Support for Darwin's naturalization and preadaptation hypotheses in freshwater fish assemblages depends on the phylogenetic diversity of assemblages within the invaded watersheds. Our results are contrary to previous studies that found that the introduction of non-native species increases phylogenetic clustering in the recipient communities more strongly at a larger spatial scale.