Food Web Structure Mediates Positive and Negative Effects of Diversity on Ecosystem Functioning in a Large Floodplain River

AbstractThe biodiversity-ecosystem functioning (BEF) and the food web structure (FWS) theories are cornerstones of contemporary ecology. However, while several theoretical hypotheses predict a link between BEF and FWS, the integration of both frameworks has only recently been considered. In this study, we applied structural equation models to evaluate 73 sink food webs of predatory fish from the Paraná River, encompassing a wide gradient of community richness. Our analysis revealed a well-supported causal model where species richness drives food web structure, increasing link density, modularity, intermodular connections, and weak interactions while decreasing nestedness. Both link density and modularity were positively associated with standing biomass, suggesting that communities with multiple energy pathways and strong complementarity effects tend to support higher biomass. Surprisingly, while species richness had the largest overall effect on biomass, this effect was indirect, mediated through three positive pathways and one negative pathway. These findings highlight the complex associations between BEF and FWS, suggesting that anthropogenic impacts on modularity, such as community functional homogenization, could shift positive BEF effects to negative, with cascading consequences for entire ecosystems.