The conventional wisdom of most ecologists is that herbivores are generally incapable of strongly affecting plant populations in natural communities. Thus, ecologists have largely focused on the role of competition for limited resources but have ignored herbivory as a primary factor determining plant success. Here, we present a quantitative review of herbivore manipulations and find that herbivores do exert important effects on plant biomass—equally as important as those of plant competition. This result should alter the way plant communities are investigated. Furthermore, we find that the effects of invertebrate herbivores are significantly stronger than those of vertebrates; this is in contrast to widely held views. Quantitative syntheses of accumulated studies, such as the one presented here, can provide surprising answers to a broad scope of biological questions. This is especially important in fields lacking a strong theoretical basis, in which generalities are born from empiricism rather than deductive theorizing.