Fluvial network topology shapes communities of native and non-native amphipods

Habitat connectivity crucially influences dispersal of organisms. It is especially seen as an important driver of the spatial structuring of biological communities in ecosystems that have intrinsic and general connectivity patterns, such as the universal dendritic structure of fluvial networks. These networks not only define dispersal of native species, but also represent corridors of biological invasions, making understanding network topology effects on invasion dynamics and subsequent diversity patterns of high interest. We studied amphipod community diversity and structure in the upper 27,882-km(2) drainage basin of the river Rhine in Central Europe, focusing on differences between native and non-native species. Overall, species richness increased along the network from headwaters to the outlet nodes. We found, however, contrasting patterns of native and non-native amphipod richness along the network, with headwater nodes representing refugia for native species and more downstream nodes being hotspots of biological invasions. Importantly, while species turnover (beta-diversity) of native species increased with distance between nodes in the network, this was not the case for non-native species, indicating a much lower dispersal limitation of the latter. Finally, the overall amphipod community structure closely mirrored the topological modularity of the network, highlighting the network's imprint on community structure. Our results underpin the importance of connectivity for community formation and the significance of rivers for biological invasions and suggest that empirically observed matches of diversity patterns in rivers predicted by null models are the long-term outcome of species invasions and species sorting.