Overgrazing

OVERGRAZING IS A term without a precise scientific meaning. Yet, it is widely used and its use has enormous implications for the management of live stock and wildlife and for the livelihoods of individuals and societies throughout the world. It is called one of the most destructive human practices on earth. It is said to be harmful to vegetation and wildlife and to cause soil erosion and desertification. But without a precise definition, these claims have no real meaning, nor can they be proved or disproved.

Range management is a profession that uses range science and practical experience to maintain and improve grazing system components (plants, animals, soil, water) and the production of goods and services from rangelands in combinations needed by society. Range management defines grazing as the consumption of standing forage (edible grasses and forbs) by livestock or wildlife, and browsing as the consumption of edible leaves and twigs from woody plants (trees and shrubs) by large-hoofed animals. Forage plants coevolved with grazers and have developed the capacity to recover from grazing. But this capacity depends on how much tissue is lost, when it is lost in the life cycle, and how frequently it is lost. From the individual plant's point of view, overgrazing could be defined as exceeding its capacity in any of these ways. However, this is not a definition that can be easily generalized.

Range management does not offer a scientifically validated and agreed upon definition of overgrazing. This is because there are many different grazing systems, different ways to measure the effects of grazing that are difficult to measure and calculate in practice (examples are carrying capacity, utilization, and range condition and trend), and different temporal and spatial scales over which the effects could be measured. In addition, since the 1980s, nonequilibrium models of ecology have been replacing the equilibrium models on which the range succession model, which has guided range science since the 1950s, is based. The range succession model posited a predictable, linear relationship between grazing and vegetation change and the existence of an optimal level of grazing that could balance desired outcomes. Nonequilibrium models, on the other hand, emphasize variability and unpredictability, making it even more difficult to define overgrazing.

As it is commonly used, the term overgrazing indicates damage or harm to vegetation caused by grazing or browsing. For example, wildlife biologists debate whether the decline of willow and aspen in [Page 1315]Yellowstone National Park is due to overgrazing by elk. Most often the term refers to harm to vegetation caused by domestic livestock grazing. It is also frequently used to explain environmental change in regions where ranching or “pastoralism” is practiced. Because the term overgrazing does not have a precise definition, its common usage is problematic for several reasons. First, harm is often in the eye of the beholder. For example, rangelands in arid regions can appear to be overgrazed from the perspective of an observer from a wetter climate. Thus, eastern visitors to the western United States might see western rangelands as overgrazed. Or European visitors to Africa might enthuse over herds of nondomestic ungulates grazing the African savannah, but deplore the depredations of native livestock. Second, perceived harm is often attributed to livestock grazing without actual evidence of a causal relationship.

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