Pastoral agriculture occupies around 20% of the land surface of the world and is the predominant form of land use in some of its more fragile ecosystems, particularly in nontemperate regions. In the past 20 years, understanding of the key processes that influence plant ruminant relationships in grazed ecosystems has increased greatly, but their development into management systems to manage the impacts of grazing, trampling, and excretal return of nutrients has been slow. The reasons for this are complex, but there is an urgent need not only to develop management systems to protect pastoral resources against uncontrolled increases in stocking density in the context of potential climate change, but also to ensure that the appropriate stocking density and mix of livestock species is used to meet the objectives of the system. Ruminant livestock have the potential to increase as well as decrease ecosystem services.
In temperate regions, intensively managed systems have been developed that use simple grass and grass-legume pastures and where only one livestock species is present. These pastures can withstand high grazing pressures without reducing their productivity. In these pastures, uncertainties of weather or variation in soil quality can be buffered by the use of fertilizers and supplementary feeding. Such systems have low plant diversity and, particularly in Europe, this has led to the need to develop more extensive forms of management, sometimes using combinations of livestock species, to meet multiple objectives, including biodiversity objectives, from pastoral resources. This trend is likely to continue and will require a greater understanding of grazing behavior at larger spatial scales than currently exists.
In Mediterranean regions of the Old World, high stocking densities of particularly sheep and goats have existed for several thousand years. Such ecosystems are often considered degraded, and are believed to provide a sufficient range of ecosystem services, but there is a counterargument that they have reached a sustainable equilibrium. In other parts of the Mediterranean Old World, reductions in grazing livestock numbers have occurred because of social changes and this has led to scrub encroachment and increased summer fire risk. These issues will require resolution for other Mediterranean climatic regions in the world in the future.
In the semiarid and arid regions, pressures on land for the growing of crops, reduction in the prevalence of systems where livestock are moved from site to site as in Africa and Asia, and economic pressures elsewhere in the world have led to increases in grazing pressures of many pastoral resources. In combination with the stochastic nature of rainfall, the greater grazing pressures associated with the socioeconomic changes noted earlier are likely to cause a greater incidence of discontinuous shifts in plant species composition, which often leads to a reduction in the value of the resource for livestock. Issues of stability and resilience of these ecosystems to the impacts of livestock are central issues that have yet to be fully understood.
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