The consumption of living plant tissue by animals. Plants and the animals that consume them constitute roughly one-half of the scientifically described species. Herbivorous species occur in most of the major taxonomic groups of animals, including the vertebrates (such as grazing fish, tortoises, geese, and hoofed mammals), echinoderms (sea urchins), mollusks (snails and slugs), nematodes (roundworms), and arthropods (including crabs, lobsters, copepods, amphipods, and isopods; mites; and especially insects, such as beetles, caterpillars of moths and butterflies, larvae of many flies, sawflies, grasshoppers, aphids and their relatives, thrips, and stick insects). Herbivorous insects alone may account for one-quarter of all species. The fraction of all biomass produced by plants that is eaten by herbivores varies widely among plants and ecosystems, ranging from less than 1% to nearly 90%. Thus, in terms of both the number of species involved and the role that herbivory plays in the flow of energy and nutrients in ecosystems, herbivory is a key ecological interaction between species.