Environmental Stress Drives Biotic Homogenization and Food-Web Destabilization Across River Basins with Contrasting Pollution

Escalating anthropogenic stressors, particularly intensive land use and water pollution, are accelerating the degradation of riverine ecosystems worldwide. However, the mechanisms by which these pressures undermine ecological stability remain poorly understood. Here, we combined environmental DNA (eDNA) metabarcoding across six trophic groups with a meta-food-web framework to assess multitrophic biodiversity and food-web structure in two Chinese river basins representing contrasting degradation regimes, which together span a gradient of anthropogenic impacts commonly observed in river systems worldwide. Our results revealed pervasive biotic homogenization in the high-stress basin across all trophic levels, along with a structurally simplified food-web characterized by increased modularity but markedly reduced nestedness and stability. Stress responses were stage-dependent: in the lower-stress basin, diversity and network properties tracked gradients of land-use intensity and water pollutants, whereas in the high-stress basin, further increases in stress produced muted structural changes, consistent with a saturation-like degraded state. Food-web destabilization was primarily driven by indirect effects of environmental stress mediated through changes in biodiversity and network topology. Specifically, diversity loss was the dominant cause of reduced stability, while increased predator niche overlap was linked to the erosion of network complexity, particularly through declines in species richness and nestedness. These findings demonstrate that environmental stressors destabilize riverine ecosystems by degrading both the multitrophic biodiversity and the integrity of ecological interactions. Conservation strategies must therefore move beyond species-centric approaches to protect multitrophic networks and support freshwater ecosystem resilience under accelerating global change.