Responses of species and phylogenetic diversity of fish communities in the Lancang River to hydropower development and exotic invasions

Multiple anthropogenic disturbances have been modifying the structure and diversity of local assemblages worldwide through both introductions of nonnative species and disappearance of native species. Most previous studies have concentrated on the effects of human disturbances on species diversity, but their effects on phylogenetic diversity have been relatively seldom documented. Here, we measured the temporal changes in species and phylogenetic alpha diversity in 12 mainstream regions of the Lancang River (LCR) in the context of hydropower development and widespread exotic invasions. We found that disturbances by dams and widespread introductions of exotic fishes have induced a marked loss of native species (average 36.8 species disappeared per region, accounting for 47.2% of the historical native species) in the downstream regions over time, whereas few native species were disappeared (average 4 species disappeared, accounting for 23.8% of the historical native species) in the upstream regions. In terms of phylogenetic diversity, however, the average taxonomic distinctness (Δ+, a taxonomic proxy for phylogenetic diversity) of the entire assemblages significantly increased (an increase of 4.4% compared to historical Δ+) in all regions, while native Δ+ insignificantly decreased in downstream regions, and the variation in taxonomic distinctness (Λ+, a proxy for variation in phylogenetic diversity) of the entire assemblages significantly increased (24.6%) in downstream regions, while native Λ+ significantly decreased (12.0%) in all regions. Such changes in phylogenetic diversity were triggered by invasions by distantly related exotic species and disappearances of congeneric endemic species. Across the LCR, the relationships between species richness, Δ+, and Λ+ and altitude all became obscure due to the combined effect of introductions and disappearances. In dammed regions, the disappearance rate and rate of change in the taxonomic distinctness of native assemblages showed negative correlations with the distances of cascade dams. Our study showed that intensive hydropower development and exotic invasions have significantly affected both the taxonomic and phylogenetic diversity of fish assemblages in the LCR, especially in its downstream region. However, the response of phylogenetic diversity to human disturbance was inconsistent with that of species diversity, implying that examining the response of phylogenetic diversity could provide additional information for assessing the impacts of human domination.