Hundreds of experiments conducted over the last decades demonstrate a positive relationship between species diversity and ecosystem functions. Following good experimental practice, most of these studies have manipulated species richness artificially by assembling communities randomly. Yet, natural communities along ecological gradients often show niche-based responses to selection gradients and species extinction order is generally not a random process. Such responses are commonly trait-mediated and the effects of communities on ecosystems' functions also depend on species traits. In an effort to disentangle the relationship of trait mean value and diversity with ecosystem functioning, we revisited a community assembly mesocosm experiment that simulated habitat heterogeneity and a typical gradient of productivity to test how body size diversity and composition of cladocerans responded to such gradients and whether and how such trait responses impacted top–down control of unicellular algae, a key ecosystem function in aquatic systems. Nutrient addition lead to an increase in average body size (CWMBS), which ultimately resulted in an increased zooplankton resource use efficiency (RUEZP). This increase in CWMBS acted as a buffer for ecosystems functions despite the decline in species richness under high phosphorus levels. Habitat heterogeneity modified the response of both richness and size diversity (SD) along the nutrient gradient, suggesting that habitat structure provided by aquatic plants can modify zooplankton diversity under eutrophic conditions. We also show that CWMBS and SD are both important and largely independent determinants of variation in RUEZP, whereas the explanatory power of species richness was mostly shared with SD. Overall, our findings illustrate the potential for a key trait, such as body size, to predict top–down control of algae through selection effect mediated by differences in trait mean, as well as complementarity associated with trait diversity among coexisting species.
