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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 agriculture applications, , or . It can also be caused by immobile, travel restricted populations of native or non-native Wildlife.
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 invasive plant and of . Degrading land, emissions from animal agriculture and reducing the biomass in a ecosystem contribute directly to climate change between grazing events. Successful planned grazing strategies have been in support of the American bison of the Great Plains, or migratory wildebeest of the African ,In balance with, and accompanied by, prides of keystone predators. or by holistic planned grazing. "Holistic Land Management: Key to Global Stability" by Terry Waghorn. Forbes. 20 December 2012.
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. A meta-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.
Native plant grass species, both individual and in , 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 speciesCôté, S. D., Rooney, T. P., Tremblay, J. P., Dussault, C., & Waller, D. M. (2004). "Ecological impacts of deer overabundance". Annu. Rev. Ecol. Evol. Syst., 35, 113-147. that can potentially displace native, woody plants, decreasing the biodiversity.Baiser, B., Julie Lockwood, La Puma, D., & Aronson, M. F. (2008). "A perfect storm: two ecosystem engineers interact to degrade deciduous forests of New Jersey". Biological Invasions, 10(6), 785-795.Horsley, S. B., Stout, S. L., & DeCalesta, D. S. (2003). White‐tailed deer impact on the vegetation dynamics of a northern hardwood forest. Ecological applications, 13(1), 98-118.
Turning to the aquatic environment, Ling et al. (2015) have documented the phenomenon of catastrophic sea urchin overgrazing and its role in marine 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) 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 between climate change and grazing practices in ecosystem degradation.
Further expanding our understanding, Stevens et al. (2016) investigated woody encroachment in South African Savanna over a 70-year period, identifying overgrazing, global changes, and the ecological effects of megafauna 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) 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) presents a stark example of the terrestrial impact of overgrazing, showing how the fertile island effect collapses under extreme conditions in shrub-encroached Grassland. 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.
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.Susan Jane Buck Cox (Spring 1985). "No Tragedy on the Commons". Journal of Environmental Ethics, Vol. 7. In reality the use of common land in England and Wales was a triumph of conserving a scarce resource using agreed custom and practice.
Overgrazing can also occur with native species. In the Australian Capital Territory, the local government in 2013 authorised a Culling 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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