Evidence that dams promote biotic differentiation of zooplankton communities in two Brazilian reservoirs

Human activities may change beta diversity—the spatial variation in species composition—in different ways. Positive and negative trends in beta diversity are referred as biotic differentiation and homogenization, respectively. In this context, river damming is likely to be a major cause of changes in beta diversity over time. Here, we evaluated the impact of damming on zooplankton beta diversity in two Brazilian reservoirs. We predicted that damming would cause biotic differentiation due to the creation of areas with different hydrological conditions, which would allow the colonization and population growth of species belonging to different zooplankton groups. Our results for the total zooplankton community were consistent with the hypothesis of biotic differentiation, either due to the increased mean beta diversity or due to the tendency of increasing beta diversity over time after damming. An indicator species analysis also showed that a large proportion of taxa that can be categorized as euplanktonic were mainly indicators of the period after damming, whereas the opposite was true for testate amoebae. Increased beta diversity should be interpreted as an impact of damming. However, we speculate that, under a process of water quality deterioration, biotic homogenization is likely to occur, reversing the patterns we observed.