Some 20 million African pastoralists commit the majority of their time to and derive most of their income from domestic livestock-keeping. In response to the arid environments they inhabit, pastoral communities have adopted a mobile system of livestock-keeping, based on mixed herds of sheep, goats, cattle, camels and donkeys. Of necessity they move when pasture in an area becomes depleted or soiled. Only in this way can animal production be maintained and pastoral households sustain themselves. Extensive livestock production systems of this type are frequently the best way much of this land can be fully utilised.
Such is the vagary of rainfall throughout Africa's rangelands that almost all pastoral communities face cycles of good and hardship years. During good years herders increase and diversify their herds, whilst consecutive hardship years or `pastoral drought,' human and livestock disease, or livestock theft may result in large livestock losses and the consequent temporary collapse of household food production. Seldom, however, do environmental factors alone conspire to overwhelm the pastoral production of entire ethnic groups, since pastoralists have `drought responses,' including mobile and adaptive grazing strategies, livestock and cereal exchanges, the establishment of diverse herds, herd splitting, and non-pastoral activities (agriculture, wage labour etc.).
Underpinning these systems is often a network of affinal and `stock-friendships,' in which each herding family is involved in the reciprocal giving and receiving of livestock. In these ways, pastoral households are able to minimise the impact of crisis years, rebuild and maintain viable herds. Thus, pastoralists through the centuries have maintained effective control of vast tracts of rangelands, currently estimated at more than 500 million hectares in sub-Saharan Africa (World Bank 1989).