These two interactions are critical to the process of desertification. It is proposed that (1) was the trigger for the contemporary epoch of desertification in Sahelian Africa and that (2) is the only way this problem can be managed in the future. Desertification is a man-made process. Therefore the solution can only be effected through the human component. Intervention at any lower level in this system will be of no consequence.

   Interaction(1) has been identified by many authors as the primary cause of desertification in Sahelian Africa and elsewhere, (Noy-Meir,1974b; Sinclair and Fryxell,1985; Lamprey,1983). The interaction in Sahelian Africa included the reduction in the amount of the poor (but reliable) resources available to the nomadic pastoralists by the extension of cultivation into the dry-season savannah grasslands, and the encouragement of the transition from a nomadic to a sedentary system of grazing. The latter was most important because contact with non-arid and urban systems, medical care, veterinary services, and even supplementary feeding for livestock, became available for the first time.

   These factors removed existing controls on both human and livestock populations and both dramatically increased, exceeding the carrying capacity of the system. The 'crash', the beginning of the landscape degradation process, was exacerbated by the additional stress of drought. But drought was not the primary causal agent. The primary causal agent was the greatly increased impact of herbivores, interaction (5), on the vegetation and soils, i.e. 'overgrazing'. This overgrazing was precipitated by (i) an absolute increase in the population of pastoralists( ≈ 3% per year) and domestic stock, too many in time, and (ii) by the transition to sedentary grazing, too many in space.

   In the history of mankind, nomadic pastoralism was once the pursuit of an appreciable proportion of the world's population, particularly within the savannah biome (Hadley, 1985). Today they are a minority, enjoying little social prestige or political power (Spooner and Mann, 1982; Sandford,1983). In Fig. 4.5 the interaction (2) is also strong. The future of pastoralists, pastoralism (and arid ecosystems?) is being determined by socio-political forces from outside the arid lands. Interaction (2) is currently a positive feedback. The plight of the pastoralists in the 1969--75 drought has increased the flow of aid and other support to maintain the existing levels of human population which nullified the negative feedback of diminished food supply. The result of this has been that after the 1969--75 epoch was an even more severe one in 1980--84 (Breman and de Wit, 1983; Sinclair and Fryxell, 1985). The long-term destructive effect of this positive feedback has been recognized, and has led to recommendations that aid be tied to land-management change to reinstate some system control (e.g. Breman and de Wit, 1984; Sinclair and Fryxell,1985).