Grazing

Grazing is the consumption by animals of herbaceous vegetation at its place of growth, whether in pasture or on rangeland. In its noun form, grazing refers to the land where this takes place. The term shares etymological roots with grass; technically, grazing is a subset of herbivory and is distinct from browsing, which involves woody or brushy plants rather than grasses and forbs. In common usage, however, this distinction is often overlooked, with grazing employed as a synonym for herbivory, perhaps because many animals are both grazers and browsers. Grazing animals are diverse, including both wild and domesticated species and several taxa, including birds such as geese, insects such as grasshoppers, and mammals ranging from mice to kangaroos to elk. Here again, common usage often deviates from strict definitions, applying the term more narrowly to domesticated livestock, including some species that are technically browsers, such as goats.

Grazing animals and grasses coevolved, each adapted to the other. The animals developed digestive systems capable of converting herbaceous plant material into energy, and the grasses developed the capacity to withstand periodic defoliation and even benefit from it. Grasses' growth points are generally at or near ground level, beyond the reach of the animals' bite; removal of older leaves and stems can enhance grass growth (by allowing greater sunlight to reach growth points and because younger leaves are more efficient); animals can spread and fertilize grass seeds, and their hooves can increase seed-soil contact for germination. New growth on recently grazed grasses can in turn attract grazing animals. In some systems, grazing can benefit grasses indirectly by altering competition with other kinds of plants. On the other hand, overgrazing can occur, such that these symbiotic interactions break down. Compared to predator–prey relations, ecological theory regarding plant–herbivore interactions is poorly developed.

Whereas unfenced animals can vacate denuded areas, fenced animals may consume beyond the land's capacity.

Grazing animals also coevolved with humans; their interactions can be broadly classified as hunting, pastoralism, and ranching. The domestication of certain grazing species between 10,000 and 4,500 years ago, marking the transition from hunting to pastoralism, dramatically augmented human capacities for settlement and agriculture. In landscapes where crop agriculture was marginal, domesticated grazing animals made human inhabitation possible where it otherwise might not have been. The domesticated animals gained enhanced food provision (to varying degrees) and protection from nonhuman predators, while humans gained a reliable source of [Page 807]traction, transportation, manure for fuel or fertilizer, milk, blood, meat, skins, wool, and so on. The evolutionary sequence of plant and animal domestication is a subject of debate, but it is clear that they were complementary in their overall effect for both humans and the plant and animal species involved.

The relationship of grazing animals to grazed plants is among the most complex in ecology, notwithstanding its apparent simplicity and an enormous body of research on the subject. Although patterns are evident at numerous scales, exceptions are abundant and relationships across scales are exceedingly complex. One prominent expert, S.J. McNaughton, cautions that “no straightforward generalizations are possible regarding the immediate effects of herbivores on plant growth and resource allocation. Consequences of tissue damage are under the complex control of plant genetics, intensity and frequency of herbivore effects, plant developmental stage at the time of herbivore impact, plant tissues that are affected, and the modifying effects of such other environmental factors as light, nutrients, temperature, and water.”

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