Biodiversity effects on ecosystem functioning are driven by resource quality and contamination

Biodiversity–ecosystem functioning relationships are increasingly explained by species' traits and resource quality differences rather than richness alone, yet cross‐trophic effects and environmental context remain poorly resolved. We experimentally tested how basal resource diversity and chemical stress interact to shape ecosystem functioning in a detritus‐based freshwater system. We manipulated leaf litter diversity (alder, hazel and beech) and selectively contaminated alder with the insecticide imidacloprid, quantifying microbially mediated decomposition, detritivore leaf processing efficiency and detritivore growth. Leaf litter identity was the primary driver of ecosystem functioning. Alder, a nutrient‐rich species, disproportionately enhanced process rates, highlighting the importance of species identity and resource quality. Leaf mixtures combined species differing in resource quality and generated non‐additive effects, with complementarity and dominance enhancing detritivore‐mediated processes, particularly leaf processing efficiency. In contrast, microbial decomposition showed weak or negative diversity effects, suggesting constraints on resource use. Imidacloprid reduced detritivore growth without altering leaf processing efficiency, due to direct toxicity after consumption or indirect effects of imidacloprid on nutritional quality. Contaminated alder acted as an ecological trap, as detritivores preferentially consumed a high‐quality resource despite the negative consequences associated with the imidacloprid treatment, suppressing positive biodiversity effects. Our results demonstrate that leaf species identity, likely reflecting differences in resource quality, and consumer behaviour jointly regulate the transfer of biodiversity effects across trophic levels, while environmental stress can disrupt these mechanisms independently of richness. Read the free Plain Language Summary for this article on the Journal blog.