Abstract
Aim We examined the diversity–biomass relationship in stream fish communities and quantified direct and indirect effects of abiotic variables on this relationship. Location France. Time period 1992–2012. Major taxa studied Stream fishes. Methods We analysed the relationship between biodiversity (species richness and functional diversity) and fish community biomass at 1,357 stream sites distributed throughout France. We used piecewise path analysis to quantify effects of abiotic and biodiversity variables on biomass and assess relative contributions of native and non‐native species on the diversity–biomass relationship. Results Biodiversity showed a direct positive relationship with biomass after controlling for sampling effort, and direct effects of biodiversity variables on community biomass exceeded those of climate and physical habitat variables. Our analysis indicates that positive effects of species richness on biomass exceeded those of functional diversity. Indirect effects of abiotic variables mediated by biodiversity metrics indicated that biomass increased in warmer, lowland habitats. Human impact had no effect on native biodiversity but had a positive effect on non‐native biodiversity. Main conclusions We provide evidence that direct effects of biodiversity on community biomass outweigh those of abiotic variables in riverine fish communities, but resource partitioning alone is unlikely to drive the effects of biodiversity on biomass in this system. Quantification of the relative roles of anthropogenic impacts, biodiversity and environmental context in the regulation of ecosystem functioning will be necessary to understand the potential consequences of ongoing global change.