Low-head dams induce biotic homogenization/differentiation of fish assemblages in subtropical streams

Human-mediated introduction of cosmopolitan species and extirpation of endemic species modify community similarity, resulting in community-distinctiveness decrease or increase. Data from four basins in the Wannan Mountains, China, was used to evaluate the effects of low-head dams on fish assemblage homogeneity/heterogeneity. We aimed to examine whether the changes in fish assemblage similarity differed between taxonomic and functional metrics, and whether the outcome of homogenization/differentiation depended on incidence-based or abundance-based approaches. Taxonomic differentiation exceeded functional differentiation for incidence-based approaches and was accompanied by weak functional homogenization for abundance-based approaches. The extent of taxonomic differentiation using incidence-based approaches was higher than that using abundance-based ones, suggesting that the former significantly overestimated the degree of taxonomic differentiation; however, an opposite trend was verified for functional aspects, suggesting that the latter could detect nuanced patterns of fish functional homogenization/differentiation. Additionally, partial pairs exhibited considerable variation in patterns of perceived homogenization and differentiation for taxonomic and functional aspects. In conclusion, this study demonstrates the strong dependence on data type of perceived patterns in homogenization/differentiation and highlights the importance of abundance-based approaches in structuring homogenization/differentiation of fish assemblages at small spatial scales; otherwise, the qualitative and quantitative conclusions may be erroneous or contradictory. Keywords: Low-head dam, Stream fish, Biotic homogenization and differentiation, Taxonomic and functional diversities