Biodiversity temporal trends are reshaping food web structure and redundancy in riverine ecosystems

Biodiversity temporal changes are expected to profoundly impact ecosystem functioning and stability, yet large scale empirical evidence remains limited. Bridging this gap requires aligning biodiversity temporal trends with food web theory and Biodiversity Ecosystem Functioning research, both of which provide strong predictions about the ecological consequences of biodiversity loss. Most insights on those consequences currently rely on spatial comparisons of ecological communities, assuming they reflect temporal trends, a space-for-time substitution. Here, we analyze biodiversity and food web time series from over 400 riverine fish communities across France (1995-2018) to evaluate how biodiversity temporal trends reshape food web structure and community functioning. We examine relationships among species richness, community biomass, and three key food web metrics: connectance, weighted average trophic level, and trophic pathway redundancy. We also test the space-for-time hypothesis using spatial gradients and a theoretical food web model. Our structural equation models and robustness analysis reveal that declines in species richness strongly correlate with reduced community biomass, with top trophic-level species playing a disproportionate role in food web changes. Declining biomass is associated with the loss of top trophic levels and decreased connectance, while temporal changes in species richness negatively correlates with changes in connectance. Both biomass and species richness declines reduce trophic pathway redundancy, suggesting that biodiversity loss weakens community robustness to future perturbations. Our findings align with food web theory and Biodiversity-Ecosystem Functioning research, support the validity of space-for-time approaches for basic food web metrics, and demonstrate the relevance of food web metrics as indicators of ecosystem function and fragility.