People used to think herbivores increase plant diversity by avoiding dominance. Dominant species tend to exclude subordinate species as competitive exclusion. However, the effects on plant diversity caused by variation in dominance could be beneficial or negative. Herbivores do increase bio-diversity by consuming dominant plant species, but they can also prefer eating subordinate species according to plants’ palatability and quality. In addition to the preference of herbivores, herbivores' effects on plant diversity are also influenced by other factors, defense trade-off theory, the predator-prey interaction , and inner traits of the environment and herbivores.