The longest fragment drives fish beta diversity in fragmented river networks: Implications for river management and conservation.

Connectivity plays a crucial role in maintaining the structural and functional attributes of river networks. Therefore, the loss of connectivity (fragmentation) alters the functioning and diversity patterns of the biota at local and regional scales. The global hydropower boom is one of the main drivers of river network fragmentation, with significant effects on the diversity of riverine biota. Analyses of beta diversity of fish assemblages in rivers with different degrees of fragmentation can give new insights into mechanisms that contribute to the responses of these assemblages to fragmentation. Here, fish beta diversity within six river networks of central Chile with different levels of fragmentation was studied to assess the responses of fish assemblages to fragmentation. A hypothesis of a significant effect of fragmentation on the beta diversity of native and non-native fish in riffles and pools was tested. This effect is expected to be modulated by both changes in environmental heterogeneity and direct obstruction of natural dispersal routes. Beta diversity based on variation of assemblage structure and environmental heterogeneity showed significant differences among river networks. Fish beta diversity showed a clear response to fragmentation in recently fragmented rivers. Specifically, the beta diversity of native fishes in pools and non-native fishes in riffles decreased with increase of the ratio between the longest non-fragmented sections of the river network to the total length of the network. These effects of fragmentation on fish assemblages were modulated by the biological features of each species, and open-water species were most severely affected. These results have significant implications for planning of the placement of new barriers in river networks subjected to hydropower boom. Planning of the placement of new barriers should consider the maintenance of long, connected sections within river networks in order to minimise the effects of fragmentation on fish biodiversity.