Effects of low-head dams on fish assemblages in subtropical streams: Context dependence on species category and data type: Low-head dams affect stream fish assemblages

Anthropogenic disturbances may cause cosmopolitan species to replace endemic species, which will alter both the within-community diversities and between-community similarities of stream fish assemblages. In this study, we used data collected from headwater streams within the Xin'an basin, China, to evaluate the effects of low-head dams on the alpha diversity and community similarity of fish assemblages. Our aims were to determine whether the changes in fish diversities and similarities related to dam-associated disturbance are dependent on the species category (i.e., indigenous vs. native-invasive species) or data type (i.e., occurrence-based vs. abundance-based indices). We found that low-head dams significantly decreased the alpha diversity of the indigenous species in the impoundments but increased that of the native-invasive species. However, the magnitude of this change was weakened if the two categories of fishes were not distinguished. Additionally, low-head dams significantly decreased the occurrence-based similarities of the indigenous fishes but increased those of the native-invasive fishes. Despite the positive correlation between the occurrence-based and abundance-based indices, the former significantly overestimated the community similarities. Although most pairs of communities showed the same direction of changes for the two indices, some cases presented contrasting outcomes, including "perceived homogenization" (i.e., occurrence-based differentiation but abundance-based homogenization) and "perceived differentiation." Our results suggest the importance of distinguishing indigenous and native-invasive species and considering both occurrence-based and abundance-based indices in future research on how anthropogenic activities affect fish diversities and similarities in streams.