We tested whether standing autotrophic (unicellular algal) biomass is sensitive to variation in initial consumer (nondecomposer, heterotrophic protistan) diversity in experimental microbial microcosms. Our results showed a strong, negative relationship between autotrophic biomass and consumer species richness. Additional microcosm experiments showed that this relationship was due largely to the direct and indirect trophic effects of consumers on autotrophs and decomposers. Collectively, these experiments suggest that standing autotrophic biomass is a function of community composition, trophic structure, and consumer diversity, the determining factors being the number of trophic levels, the discreteness of trophic levels, and the ecological equivalency of species within trophic levels. Our results suggest that ecosystem response to variation in consumer diversity can range from abrupt changes between “green” (high autotrophic biomass) and “barren” (low autotrophic biomass) states to relatively smooth trans...