Scale-Dependent Biodiversity Responses to Drying and Land Use in River Metacommunities Facing Global Change.

Global change is increasing river drying and transforming surrounding landscapes through human land use, with important consequences for freshwater biodiversity. Yet how river drying and land use jointly influence biodiversity across spatial scales remains poorly understood. Using macroinvertebrate data from 49 riverine metacommunities in southeastern France, we examined how river drying and land use affect biodiversity across spatial scales and the metacommunity properties underlying these patterns. We estimated species-area relationship (SAR), with intercept and slope of SAR curve representing patterns analogous to alpha- and beta-diversity, respectively, and fitted structural equation models to assess how flow intermittence, land use (% of non-natural surface), and network size affected the metacommunity properties underlying these patterns, including intraspecific aggregation, species abundance distribution, total abundance, and regional species pool. Flow intermittence increased intraspecific aggregation and reduced evenness, leading to a lower SAR intercept and a steeper slope, reflecting lower local diversity but greater compositional differentiation among non-perennial rivers, and revealing strong scale-dependent responses of biodiversity to drying. Trait analyses further showed that flow intermittence favored aerial passive dispersers while reducing the relative abundance of aquatic active dispersers, suggesting that altered dispersal dynamics in non-perennial river networks contribute to biodiversity organization. In addition, land use primarily influenced biodiversity through nonlinear effects on the metacommunity species pool, with intermediate levels of land use associated with the highest regional diversity. Moreover, land use weakened the positive effect of flow intermittence on aggregation, indicating that anthropogenic pressures can homogenize metacommunities in drying regimes. Our findings demonstrate that flow intermittence strongly influences biodiversity across spatial scales through different mechanisms rather than uniformly reducing it. As climate change is forecast to increase the proportion of non-perennial rivers worldwide, our results highlight the importance of incorporating metacommunity processes and multi-scale diversity patterns into the conservation and management of these ecosystems.