About 60 percent of the world's pasture land (about 2.2 million km2), just less than half the world's usable surface is covered by grazing systems. Distributed between arid, semi arid and sub humid, humid, temperate and tropical highlands zones, this supports about 360 million cattle (half of which are in the humid savannas), and over 600 million sheep and goats, mostly in the arid rangelands. The distribution of livestock over the different ecological zones is provided in Annex Table 2.
Grazing systems supply about 9 percent of the world's production of beef and about 30 percent of the world's production of sheep and goat meat. For an estimated 100 million people in arid areas, and probably a similar number in other zones, grazing livestock is the only possible source of livelihood.
Grazing can be visualized as beautiful cows in lush pastures in north-western Europe or New Zealand - livestock in harmony with nature. Indeed, livestock can improve soil and vegetation cover and plant and animal biodiversity, as described in this chapter's case studies of widely different conditions in Kenya, the western United States and Guinea. By removing biomass, which otherwise might provide the fuel for bush fires, by controlling shrub growth and by dispersing seeds through their hoofs and manure, grazing animals can improve plant species composition. In addition, trampling can stimulate grass tillering, improve seed germination and break-up hard soil crusts.
However, many people associate grazing animals with overgrazing, soil degradation and deforestation. To them livestock keeping in arid regions of the tropics provokes images of clouds of dust, bleached cow skeletons and an advancing desert. The two most quoted sources are the Global Assessment of Soil Degradation (Oldeman et al., 1991), which estimates that 680 million hectares of rangeland have become degraded since 1945, and Dregne et al., (1991) who argue that 73 percent of the world's 4.5 billion hectares of rangeland is moderately or severely degraded. In humid areas, livestock are associated with ranch encroachment and deforestation of tropical rainforests and competition with wildlife.
Prolonged heavy grazing undoubtedly contributes to the disappearance of palatable species and the subsequent dominance by other, less palatable, herbaceous plants or bushes. Such loss of plant and, in consequence, animal biodiversity can require a long regenerative cycle (30 years in savannas, 100 years in rainforests). Excessive livestock grazing also causes soil compaction and erosion, decreased soil fertility and water infiltration, and a loss in organic matter content and water storage capacity. On the other hand, total absence of grazing also reduces biodiversity because a thick canopy of shrubs and trees develops which intercepts light and moisture and results in over-protected plant communities which are susceptible to natural disasters.
The environmental challenge is thus to identify the policies, institutions and technologies which will enhance the positive and mitigate the negative effects of grazing. Environmental challenges, issues and options differ significantly according to climate and land capabilities. Livestock-environment interactions are therefore described separately for the arid, semi-arid and sub-humid, humid rainforest, and temperate and tropical highlands grazing systems respectively. As will be seen, that differentiation is particularly important for the arid eco-systems. As aridity increases, so does variability of rainfall, to the extent that the periodicity of rain becomes the single most important factor affecting the state of the natural resource base. Classical concepts of vegetation succession and climax vegetation do not apply in such environments and new concepts are required. These are detailed below.
Next section Grazing systems of the arid areas