Nonlinear dynamics of fish beta diversity and the disproportionate influence of non-native species in cascade-dammed...

Cascade dam development is transforming freshwater ecosystems worldwide, yet the long-term responses of fish beta diversity to progressive hydrological disturbances remain poorly understood. We analyzed data from the upper reaches of the Yellow River on the Qinghai-Tibet Plateau spanning five decades (pre-1975 to 2023), across a 1983-km river corridor influenced by sequential cascade dams. We quantified temporal shifts in taxonomic, phylogenetic, and functional beta diversity and determined the contributions of species extinctions and introductions to multidimensional beta-diversity change. Although damming is generally expected to promote biotic homogenization by simplifying habitats and favoring widespread generalist or non-native species, thereby making communities more similar, our results revealed pervasive biotic differentiation, with communities becoming increasingly distinct across all diversity dimensions. Beta diversity followed a nonlinear trajectory, increasing sharply during early dam construction and stabilizing in later stages. Both native species extinctions and non-native introductions contributed to these patterns, but their relative importance shifted over time, with introductions showing a greater contribution at later stages. Non-native species, particularly those introduced from outside the Yellow River basin, exerted disproportionate effects on phylogenetic and functional beta diversity owing to their distinct evolutionary histories and ecological traits. Environmental predictors such as reservoir age, cumulative upstream reservoir capacity, and individual reservoir capacity were strongly associated with patterns in beta diversity, reflecting the cumulative and spatially structured effects of long-term hydrological modification. Our findings provide quantitative, multidimensional evidence that non-native species introductions can outweigh native species extinctions in shaping beta diversity under sequential disturbance. We highlight the importance of proactive management of non-native species and long-term, multidimensional biodiversity monitoring to detect postdisturbance dynamics and guide timely conservation interventions. Collectively, our results offer a framework for identifying and anticipating the cumulative ecological consequences of cascade dam development and other chronic ecosystem modifications in the Anthropocene.