The mechanisms of species coexistence and diversity constitute the foundation of community ecology, yet the identity of these mechanisms remain as important questions in ecology. Current theory indicates that coexistence occurs as species face multiple limiting factors, such as finite resources or consumers, and trade off in their ability to cope with these limitations. Therefore, consumers should promote resource diversity, as in the case of herbivores and plants. However, in practice, herbivores do not always affect plant diversity as predicted. I examine potential mechanisms by which herbivores may, and may not, affect plant diversity using empirical approaches in the context of ecological restoration and theoretical modeling. I show that herbivores may increase plant diversity by limiting even brief periods of dominance, though preferentially consuming those species that are dominant in terms of competitive ability or biomass is not a requirement or assurance of this outcome. Moreover, I show that the proximal mechanisms by which herbivores increase diversity, specifically competition for light, are still not well understood. Thus, more research is necessary to determine those mechanisms as well as the full range of conditions in which consumers may increase diversity or allow coexistence.