Livestock Production Systems

systems because of their dependence upon natural vegetation which is controlled by weather changes. Grazing systems tend to be closed systems, where animal waste products (manure) are used by the system.

In arid rangelands, livestock mobility is key to successful maintenance of vibrant livestock and wildlife populations. These regions have tended to be the most controversial areas of livestock use. However, a series of studies does show that the extent of environmental degradation has been exaggerated. Three different works demonstrate this point. R. Mearns (unpublished data) concluded that abiotic factors such as rainfall, rather than livestock density, determine long-term primary plant production and vegetation cover. Tucker et al. (1991) demonstrated the resilience of arid lands using satellite imagery. Their work showed movement of the southern belt of the Sahara depends on rainfall and that the southern boundary of the Sahara is moving north. This movement is occurring after the long droughts which occurred in the 1970s and is contrary to how we normally view the resource base in this region. de Haan et al. (1997) demonstrated that, in the Sahel, livestock productivity in terms of meat production per hectare and per head has increased over the past 30 years. This type of long-term trend would be difficult to obtain if pastoralists constantly degrade the environment they utilize.

Semiarid and subhumid rangelands are more static systems than arid zone rangelands. Due to the higher rainfall levels, these areas contain larger amounts of plant biomass and have considerable biodiversity. These areas also receive heavier livestock grazing and allow opportunities for mixed farming systems to expand. As a result of growing human population pressures, these lands and the biodiversity they contain may be of most concern. For example, data from Mali showed that land degradation in the 600 to 800 mm rainfall area was significantly greater than in the 350 to 450 mm rainfall area. However, such a trend is by no means a global phenomenon. As Milchunas and Lauenroth (1993) demonstrate, in the semiarid and subhumid regions, moderate grazing had no impact on biomass production, species composition, and root development.

Livestock grazing in tropical rain forests (the humid zone) has more than anything else typified the negative effects of livestock development on the environment. Table 2 provides estimates of the main causes of deforestation (Bruenig, 1991). In the humid region, data on biodiversity losses in the rain forest are dramatic. Since 1950, the world has lost about 200 million ha of tropical forest, with the resulting loss of unique plant and animal species. Forested areas of Central America have declined from 29 to 19 million ha since 1950, although after 1990 the rate of deforestation in Central America decreases. In the 1980s rain forest in Central America disappeared at the rate of 30,000 ha/year; this had declined to 320,000 ha

Table 2 Estimated Causes of Deforestation (percent of total deforestation)

Region

Crops

Livestock

Forest Exploitation

South America

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