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Overgrazing occurs when plants are exposed to intensivegrazing for extended periods of time, or without sufficientrecovery periods.[1] It can be caused by eitherlivestock in poorly managedagricultural applications,game reserves, ornature reserves. It can also be caused by immobile, travel restricted populations ofnative ornon-nativewild animals.
Overgrazing reduces the usefulness,productivity andbiodiversity of the land and is one cause ofdesertification anderosion. Overgrazing is also seen as a cause of the spread ofinvasive species ofnon-native plants and ofweeds. Degrading land,emissions from animal agriculture and reducing the biomass in a ecosystem contribute directly to climate change[2] between grazing events.
Overgrazing typically increasessoil erosion.[3]
With continued overutilization of land for grazing, there is an increase in degradation. This leads to poor soil conditions that onlyxeric and earlysuccessional species can tolerate.[4] Ameta-analysis of 148 studies found that the value of most ecosystem functions declines with increasing grazing intensity and that increasing aridity weakens positive impacts of light grazing.[5]
Native plantgrass species, both individualbunch grasses and ingrasslands, are especially vulnerable. For example, excessive browsing bywhite-tailed deer can lead to the growth of less preferred species of grasses and ferns or non-native plant species[6] that can potentially displace native, woody plants, decreasing the biodiversity.[7][8]
Turning to the aquatic environment, Ling et al. (2015)[9] have documented the phenomenon of catastrophicsea urchin overgrazing and its role inmarine ecosystem regime shifts. Their study underscores the urgent need for effective management and conservation strategies to mitigate the profound ecological impacts of overgrazing, highlighting the issue's global scope. Similarly, on the Mongolian steppes, Liu et al. (2013)[10] found that approximately 60% of vegetation decline could be attributed to climate factors, with the rest significantly influenced by increased goat density due to overgrazing. This points to a complex interplay betweenclimate change and grazing practices in ecosystem degradation.
Further expanding our understanding, Stevens et al. (2016)[11] investigatedwoody encroachment in South Africansavannahs over a 70-year period, identifying overgrazing, global changes, and the ecological effects ofmegafauna extinction as key factors. Their findings shed light on the multifaceted drivers behind changes in savannah ecosystems. Echoing this theme of alternative strategies to combat overgrazing, Kriegisch et al. (2019)[12] demonstrated how drift-kelp availability could reduce the foraging movement of overgrazing sea urchins, suggesting that alternative food sources may significantly influence grazing behaviors and aid in managing marine ecosystem pressures.
In a similar vein, the research by Cai et al. (2020)[13] presents a stark example of the terrestrial impact of overgrazing, showing how the fertile island effect collapses under extreme conditions in shrub-encroachedgrasslands. This case study emphasizes the critical need for sustainable grazing practices to protect soil health and maintain ecosystem functionality, further illustrating the wide-reaching consequences of overgrazing across diverse habitats.
Overgrazing is used as an example in the economic concept now known as theTragedy of the Commons devised in a 1968 paper byGarrett Hardin.[14] 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.[15] 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 theSahel region. The violentherder–farmer conflicts in Nigeria,Mali,Sudan and other countries in the Sahel region have been exacerbated by land degradation and overgrazing.[16][17] See2010 Sahel famine.
Various countries inSub-Sahara Africa are affected by overgrazing and resulting ecological effects.InNamibia, overgrazing is considered the main cause ofwoody plant encroachment at the expenses of grasses on a land area of up to 45 million hectares.
In many arid zones inAustralia, overgrazing by sheep and cattle during the 19th century, aspastoralism was introduced by European settlers, caused many long-lived species of trees and shrubs to give way to short-livedannual plants andweed species. Introducedferal rabbits,cats andfoxes exacerbated the threat to bothflora 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 fewislands of Australia.[18]
Overgrazing can also occur with native species. In theAustralian Capital Territory, the local government in 2013 authorised acull of 1455kangaroos due to overgrazing.[19]Maisie Carr (1912-1988), Ecologist and Botanist, undertook significant research and studies in overgrazing and established consequences on the surrounding land in Australia.
In theCaribbean region, overgrazing is a threat to vegetation areas where there is livestock farming, which is an important source of livelihood and food security for many people. a combination of small scale livestock farming with small ruminants, and mixed farming is practised. However, livestock consume vegetation faster than it can be renewed and this leads to land degradation, loss of vegetative areas, and soil erosion resulting in poor quality feed and reduced livestock yields and income. Also, these grazing lands are critical in controlling carbon dioxide and mitigating risks against severe weather such as floods and droughts. Overgrazing weakens ecological conservation.[20]
InNew Zealand, overgrazing is a massive threat to the native species of flora and fauna, especially the native bushes which are often overlooked by invasive species looking for homes.[citation needed] Mustalids, rabbits, hares and possums often eat the plants that hold soil together. This makes the ground very unstable and crumbly. If soil is unstable, it is prone to collapse in extreme weather events such as floods and heavy rain. This is detrimental to farmers of crops and animals alike. 40% of the native species of New Zealand have been wiped out by pests including humans.[21]
