Fish Community Homogenization in a Tropical Insular River: Water Pollution and Biological Invasions As Principal Contributors to Patterns of Multidimensional Β-Diversity

Insular river ecosystems are highly vulnerable to anthropogenic disturbances, yet the combined effects of human pressures on fish biodiversity remains poorly understood. Based on spring and autumn 2024 field surveys, we examined taxonomic, phylogenetic, and functional β‑diversity of fish assemblages in the Nandu River, China’s longest insular river. We quantified the contributions of abiotic and biotic factors, including geo‑climatic, local environmental, spatial, and biological invasion factors to community patterns. Results showed a clear longitudinal homogenization trend from upstream to downstream, dominated by species turnover, consistent with community reshaping associated with anthropogenic stressors. Tributaries exhibited taxonomic and phylogenetic homogenization but functional differentiation, revealing adaptive patterns in relation to local stressors. Local environmental factors were the primary contributors to total β‑diversity and its turnover component, with water pollution indicators (COD, total phosphorus) showing the strongest associations. For the nestedness component, biological invasions were the dominant contributors to taxonomic and phylogenetic β‑diversity, whereas functional β‑diversity was primarily associated with local environmental factors. Overall, environmental filtering (mainly contributed by water pollution) outweighed dispersal limitation in structuring fish communities, while biological invasions predominantly affected nestedness patterns. Our findings highlight the contributions of water environmental degradation and biological invasions. Additionally, the multidimensional β-diversity framework provides support for local watershed management and offers a transferable ecological assessment method for biodiversity conservation in human-disturbed island river systems globally.