The link between biodiversity and ecosystem functioning is now well established, but the challenge remains to develop a mechanistic understanding of observed effects. Predator-prey interactions provide an opportunity to examine the role of resource partitioning, thought to be a principal mediator of biodiversity-function relationships. To date, interactions between multiple predators and their prey have typically been investigated in simplified agricultural systems with limited scope for resource partitioning. Thus there remains a dearth of studies examining the functional consequences of predator richness in diverse food webs. Here, we manipulated a species-rich intertidal food web, crossing predator diversity with total predator density, to simultaneously examine the independent and interactive effects of diversity and density on the efficiency of secondary resource capture. The effect of predator diversity was only detectable at high predator densities where competitive interactions between individual predators were magnified; the rate of resource capture within the species mixture more than doubled that of the best-performing single species. Direct observation of species-specific resource use in monoculture, as quantified by patterns of prey consumption, provided clear evidence that species occupied distinct functional niches, suggesting a mechanistic explanation of the observed diversity effect.