1. Food‐web complexity‐stability relations are central to ecology, and many empirical studies show greater food‐web complexity leads to lower population stability. Here, predator population variability decreased with increasing prey diversity in aquatic microcosm experiments, an example of greater food‐web complexity leading to greater population stability. 2. Prey diversity as well as different sets of prey species within each level of prey diversity produced differences in predator population dynamics, demonstrating the importance of both prey composition and prey diversity in determining predator population stability. 3. Prey diversity can affect predator population dynamics through at least three groups of mechanisms: prey reliability, prey biomass, and prey composition mechanisms. The results suggest that greater prey reliability at higher prey diversities enhances predator stability and provide support for MacArthur (1955).