Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature reserves. It can also be caused by immobile, travel restricted populations of native or non-native wild animals.
Overgrazing reduces the usefulness, productivity and biodiversity of the land and is one cause of desertification and erosion. Overgrazing is also seen as a cause of the spread of invasive species of non-native plants and of weeds. Degrading land, emissions from animal agriculture and reducing the biomass in a ecosystem contribute directly to climate change.
Overgrazing can be reversed or prevented by removing grazers in order to give plants time to recover between grazing events. Successful planned grazing strategies have been support in the American bison of the Great Plains, or migratory Wildebeests of the African savannas, or by holistic planned grazing.
Overgrazing typically increases soil erosion.
With continued overutilization of land for grazing, there is an increase in degradation. This leads to poor soil conditions that only xeric and early successional species can tolerate.
Native plant grass species, both individual bunch grasses and in grasslands, are especially vulnerable. For example, excessive browsing by white-tailed deer can lead to the growth of less preferred species of grasses and ferns or non-native plant species  that can potentially displace native, woody plants, decreasing the biodiversity.
Overgrazing is used as an example in the economic concept now known as the Tragedy of the Commons devised in a 1968 paper by Garrett Hardin. This cited the work of a Victorian economist who used as an example the over-grazing of common land. Hardin's example could only apply to unregulated use of land regarded as a common resource.
Normally, rights of use of Common land in England and Wales were, and still are, closely regulated, and available only to "commoners". If excessive use was made of common land, for example in overgrazing, a common would be "stinted", that is, a limit would be put on the number of animals each commoner was allowed to graze. These regulations were responsive to demographic and economic pressure; thus rather than let a common become degraded, access was restricted even further. This important part of actual historic practice was absent from the economic model of Hardin. In reality the use of common land in England and Wales was a triumph of conserving a scarce resource using agreed custom and practice.
There have been overgrazing consequences in the region Sahel region. The violent herder–farmer conflicts in Nigeria, Mali, Sudan and other countries in the Sahel region have been exacerbated by land degradation and overgrazing. See 2010 Sahel famine.
Various countries in Sub-Sahara Africa are affected by overgrazing and resulting ecological effects. In Namibia, overgrazing is considered the main cause of woody plant encroachment at the expenses of grasses on a land area of up to 45 million hectares.
In many arid zones in Australia, overgrazing by sheep and cattle during the nineteenth century, as pastoralism was introduced by European settlers, caused many long-lived species of trees and shrubs to give way to short-lived annual plants and weed species. Introduced feral rabbits, cats and foxes exacerbated the threat to both flora and fauna. Many bird species have become extinct or endangered, and many of the medium-sized desert mammals are now completely extinct or only exist on a few islands of Australia.
Overgrazing can also occur with native species. In the Australian Capital Territory, the local government in 2013 authorised a cull of 1455 kangaroos due to overgrazing. Maisie Carr (1912-1988), Ecologist and Botanist, undertook significant research and studies in overgrazing and established consequences on the surrounding land in Australia.
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- ^ Garrett Hardin, "The Tragedy of the Commons", Science, Vol. 162, No. 3859 (December 13, 1968), pp. 1243-1248. Also available here and here.
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