The pressure of a high animal population on limited grazing areas has led to progressive deterioration of native grasslands in developed and developing countries. As grazing pressures rise, the better forage plants lose vigour, give up the struggle and disappear, being replaced by inferior species (often poisonous or unpalatable) resulting in the production of less and less food for man. As grazing pressure increases, the reserve forage needed to carry animals through severe droughts and floods disappears. Overgrazing also disrupts cycles that assure relatively even distribution of water and nutrients, which prevent erosion, especially on semi-arid grasslands that easily degrade into deserts.
The recurrent tendency of stock-keepers is to overgraze their lands with excessively large herds. Traditional efforts to solve the problem of stock overgrazing have been: to induce the stockkeeping peoples to give up their animals and become farmers; to force stock reduction; to establish block systems, forcing the pastoralists to fallow a part of their range each year; or to establish fenced ranches. None of these solutions has had notable success.
About 50 per cent of land cover in Australia has been changed by complete clearing, thinning of vegetation, overgrazing, changed fire regimes and other habitat modifications. In New Zealand, large areas of forest had been cleared and virtually all grazeable land converted to pasture by 1920 to provide wool, meat and dairy exports. Today, around 50 per cent of land in both countries is used for grazing (Commonwealth of Australia 1996, New Zealand Ministry for the Environment 1997).
Overgrazing is a principal factor in the deterioration of rangeland. In general, overgrazing reduces grazing duration during the growing season as well as during the dry season, often forcing settled farmers to buy food supplements. For nomads, overgrazing causes a decrease in the milk production of their animals, which forces them to increase the number of animals, further increasing the pressures on the range. In the long run, both farmers and nomads may decide to leave the region or settle around deep boreholes. A special disease may appear among cattle - botulism in the Sahelian areas is one example. Finally, in areas where water erosion occurs and where wells exist, sand dunes may fill up the wells.
In Southern Africa, escalating land degradation over the past decade has been caused by increased livestock. Overgrazing causes more than half the soil degradation in the sub-region. In Namibia, livestock production subsidies actually encourage farmers to raise more livestock than if they had to meet the full costs themselves (Byers 1997). With new economic policy changes under way in the region of the Southern African Development Community (SADC), including the removal of such subsidies, stocking rates are expected to decline over the next decade.