Overgrazing

Effects of overgrazing occur where there are more grazing animals than the land and vegetation can support. Overgrazing has negatively affected many regions globally, including large portions of sub-Saharan Africa, the southwestern United States, and the Indian subcontinent in Asia. Areas that have been severely damaged by overgrazing may experience desertification, invasion by nonnative plant species, and an increase in endangered plant and animal species.

Background

Herbivores are animals that feed on plant material, and grazers are herbivores that feed primarily on grasses. Common examples are horses, cattle, antelope, and bison. Overgrazing occurs when grazer populations exceed the carrying capacity of a specified area. Carrying capacity is the number of individual organisms the resources of a given area can support without the area being damaged; carrying capacity varies from region to region. In overgrazing conditions, there are insufficient grasses and forbs to support the animal population in question. Depending on the grazer’s strategy, emigration or starvation will follow. Grasslands can handle normal grazing; only overgrazing adversely affects them.

Overgrazing can result either from the presence of too many domestic grazers pastured in a specific area or the presence of too many wild herbivores on the rangeland. The latter can happen when natural events, such as fire, or human developments, such as farming or suburbanization, crowd wildlife into a more restricted range than the organisms had occupied under undisturbed conditions. In many regions of the world, species of wild herbivores that once experienced seasonal migrations over a wide geographic area have become confined to a much smaller range, resulting in more stress on forage resources. As part of rangeland management, national parks and wildlife refuges must thin herds of grazers periodically to preserve the forage and ensure the long-term survival of the various plant and animal species in an ecosystem.

Grasses’ Defenses Against Grazing

Grasslands and grazers coevolved; thus, grasses can withstand grazing within the ecosystem’s carrying capacity. All plants have a site of new cell growth, called the meristem, where growth in height and girth occur. In most plants, the meristem is located at the top of the plant (the apical meristem). On grasses, the meristem is located at the junction of the shoot and root, close to the ground. If a plant’s meristem is removed, the plant dies. If grasses had an apical meristem, grazers—and lawn mowers—would kill grasses. Grasses survive mowing and grazing because they do not have an apical meristem. With a few exceptions, such as sheep, grazers do not disturb the meristem, and sheep do so only during overgrazing conditions. At proper levels of grazing, grazing actually stimulates grass to grow in height in an attempt to produce a flowering head for reproduction. Grazing also stimulates grass growth by removing older plant tissue at the top that is functioning at a lower photosynthetic rate.

Grazers

Mammalian grazers have high, crowned teeth with a great area for grinding to facilitate opening of plants’ cell walls in an attempt to release nutrients. The cell wall is composed of cellulose, which is very difficult for grazers to digest. Two major digestive-system grazing strategies have evolved. Ruminants, such as cows and sheep, evolved stomachs with four chambers to allow regurgitation in order to chew food twice to maximize cellulose breakdown. In addition, intestinal bacteria digest the cellulose, releasing fatty acids that nourish the ruminants. Other grazers, such as rabbits and horses, house bacteria in the cecum, a pouch at the junction of the small and large intestines. These bacteria ferment the ingested plant material. The fermented products of the bacteria nourish these grazers.

Global Causes of Overgrazing

Overgrazing by livestock has been caused by a variety of factors. In pastoral societies—in which wealth is traditionally measured in terms of how many cattle, sheep, or other grazer individuals are owned—the rangeland is held in common; thus, the commons may become overgrazed. This happens because no single individual feels responsible for the land and instead worries only about increasing the size of his or her herd. Prior to industrialization many pastoral societies were nomadic and followed seasonal migration patterns, which helped prevent overgrazing. However, as world population has grown and the amount of common land available for grazing has shrunk, few truly nomadic societies remain. Estimates indicate that approximately 6 percent of the world’s population is pastoralist, but many formerly nomadic groups—such as the Tuareg in North Africa, Maasai of Kenya, and Tsaatan (also known as Dukha) of Mongolia—now reside in fixed settlements year-round. The inability to move their herds as they once did can result in overgrazing in the areas around their villages.

Although overgrazing is often associated with pastoral nomads, overgrazing can and does occur where raising livestock is a purely commercial enterprise. The western United States, for example, experienced severe overgrazing on public lands in the nineteenth and early twentieth centuries when investors overstocked the range in hopes of making a quick profit in the cattle industry. Similarly, private landowners occasionally attempt to raise more grazers than their pasturage can support, but may not realize they have overstocked until the land begins to show negative effects, such as or desertification.

Effects of Overgrazing

Overgrazing can lead to a number of basic ecological problems. Overgrazed regions can experience desertification, for example. Many of the grasslands of the world occur in drier regions, such as the American Southwest and northern Africa, where annual rainfall is not enough to support forests. Desertification is the intensification and expansion of deserts at the expense of neighboring grasslands. When overgrazing occurs along desert perimeters, the plant removal leads to decreased shading. This decreased shading increases the local air temperature. When the temperature increases, the air may no longer be able to cool down enough to release moisture in the form of dew. Dew is the primary source of precipitation in deserts, so without it, desert conditions intensify. Even a slight decrease in desert precipitation is serious. The result is hot and dry conditions, which lead to further plant loss and, potentially, to monocultures.

The overgrazing of grasslands, combined with the existence of nonnative species in an ecosystem, can result in the endangerment of species of native grasses and the creation of monocultures in regions where certain species have been removed. A monoculture is an that has lost diversity; that is, one species becomes dominant and crowds out everything else. Because of the globalization of agriculture and trade, nonnative invasive species have become a problem in grasslands worldwide. For example, at one time, cattle in the American Southwest fed exclusively on native grasses. Then nonnative grass species arrived in the New World via the guts of cows shipped from Europe. The nonnative grasses began to compete with the native grasses. European grass species have seeds with prickles and burs; southwestern native grasses do not, making them softer and more desirable to the cattle. Hence, European grasses experienced little, if any, grazing, while the much more palatable southwestern native grasses experienced cattle grazing to the point of overgrazing. The result was drastic decline or loss of native species. Animals dependent on native grassland species must then emigrate or risk extinction. For example, many ecologists conjecture that the Coachella Valley kit fox in California is threatened because of the loss of grassland habitat upon which it is dependent.

Solutions

Desertification may be irreversible, but the elimination of grazing along desert perimeters can at least help to prevent further desertification. One method to reestablish native grass species involves controlled-burn programs. Nonnative grassland species do not appear to be as fire-resistant as native grass species are. Therefore, controlled-burn programs have been used in some overgrazed grassland areas in an attempt to reestablish native grass species and eliminate nonnatives. If successful, such programs will increase the of the area and will improve the health of the ecosystem.

Bibliography

Barnes, Robert F., et al., eds. Forages, Volume One: An Introduction to Grassland Agriculture. New York: Wiley-Blackwell, 2003.

Brown, Lester R. “Eroding Soils and Shrinking Cropland.” In Plan B: Rescuing a Planet Under Stress and a Civilization in Trouble. New York: Norton, 2003.

Collins, Scott L., and Linda L. Wallace, eds. Fire in North American Tallgrass Prairies. Norman: University of Oklahoma Press, 1990.

DeBano, Leonard F., Daniel G. Neary, and Peter F. Ffolliott. Fire’s Effects on Ecosystems. New York: J. Wiley, 1998.

Ehlert, Krista. "The Lasting Effects of Overgrazing on Rangeland Ecosystems." South Dakota State University, 8 Apr. 2022, extension.sdstate.edu/lasting-effects-overgrazing-rangeland-ecosystems. Accessed 28 Dec. 2024.

Gerrish, Jim. Management-Intensive Grazing: The Grassroots of Grass Farming. Purvis, Miss.: Green Park Press, 2004.

Gilles, Jere Lee, and Keith Jamtgaard. “Overgrazing in Pastoral Areas: The Commons Reconsidered.” Sociologica Ruralis 21 (2008): 129-141.

Gordon, I. J., and H. H. T. Prins, eds. The Ecology of Browsing and Grazing. Berlin: Springer, 2008.

Holechek, Jerry L., Rex D. Pieper, and Carlton H. Herbel. Range Management: Principles and Practices. 5th ed. New York: Prentice-Hall, 2004.

Humphrey, L. R. The Evolving Science of Grassland Improvement. New York: Cambridge University Press, 1997.

Mysterud, Atle. “The Concept of Overgrazing and Its Role in Management of Large Herbivores.” Wildlife Biology 12, no. 2 (2006): 129–141.

Nation, Allan. Paddock Shift: Changing Views on Grassland Farming. Jackson, Miss.: Green Park Press, 1997.

Pollan, Michael. “Grass: Thirteen Ways of Looking at a Pasture.” In The Omnivore’s Dilemma: A Natural History of Four Meals. New York: Penguin Press, 2006.

Stanley, Jim. “Overgrazing.” In Hill Country Landowner’s Guide. College Station: Texas A&M University Press, 2009.

Vallentine, John F. Grazing Management. 2d ed. San Diego, Calif.: Academic Press, 2001.