Effects of low‐head dams on fish assemblages in subtropical streams: Context dependence on species category and data type

Abstract

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 damassociated 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 occurrencebased 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.

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

“…Previous studies have shown that incidence-based dissimilarity may play a crucial role when communities differ mostly in species compositions and are geographically far apart on global or regional scales (Legendre, 2014). However, abundance-based approaches may exhibit higher sensitivity in structuring homogeneous or heterogeneous patterns when species composition may differ primarily in species abundance due to weightings with differentiated abundance in small spatial extents (Cassey et al, 2008;Liu et al, 2019;Dai et al, 2020). Indeed, the population size (i.e., abundance) of the same species may differ substantially across locations, and the number of individuals is not equally distributed across different species in the same locations.…”

“…Indeed, the population size (i.e., abundance) of the same species may differ substantially across locations, and the number of individuals is not equally distributed across different species in the same locations. Therefore, changes in population abundance can also affect the degree to which communities differ in composition, and a growing body of evidence has shown that the choice of similarity index can influence the magnitude of biological similarity computed (Cassey et al, 2008;Liu et al, 2019). For instance, incidence-based approaches may underestimate the functions of dominant species in biotic homogenization if species abundance data are overlooked (La Liu et al, 2019).…”

“…Therefore, changes in population abundance can also affect the degree to which communities differ in composition, and a growing body of evidence has shown that the choice of similarity index can influence the magnitude of biological similarity computed (Cassey et al, 2008;Liu et al, 2019). For instance, incidence-based approaches may underestimate the functions of dominant species in biotic homogenization if species abundance data are overlooked (La Liu et al, 2019). Furthermore, in some case, incidence-based and abundance-based indices also may result in contrasting outcomes (i.e., perceived homogenization and perceived homogenization) depending on how species abundance changes drastically enough relative to changes in species composition over time (Cassey et al, 2008;Liu et al, 2019).…”

“…For instance, incidence-based approaches may underestimate the functions of dominant species in biotic homogenization if species abundance data are overlooked (La Liu et al, 2019). Furthermore, in some case, incidence-based and abundance-based indices also may result in contrasting outcomes (i.e., perceived homogenization and perceived homogenization) depending on how species abundance changes drastically enough relative to changes in species composition over time (Cassey et al, 2008;Liu et al, 2019). In this context, a considerable number of ecologists have recently questioned the accuracy of incidence-based data (Barwell et al, 2015;Liu et al, 2019;Dai et al, 2020), especially when stochastic differences in rare species are artifacts of undersampling.…”

“…Furthermore, in some case, incidence-based and abundance-based indices also may result in contrasting outcomes (i.e., perceived homogenization and perceived homogenization) depending on how species abundance changes drastically enough relative to changes in species composition over time (Cassey et al, 2008;Liu et al, 2019). In this context, a considerable number of ecologists have recently questioned the accuracy of incidence-based data (Barwell et al, 2015;Liu et al, 2019;Dai et al, 2020), especially when stochastic differences in rare species are artifacts of undersampling. Unfortunately, the quantification of biotic homogenization or differentiation based on abundance data has received less attention in real ecological assemblages (but see Cassey et al, 2008;Liu et al, 2019;Dai et al, 2020).…”