4.4.1 THE SETTING AND THE SYSTEM

It has been the extensive land degradation, particularly of Sahelian Africa, that has been brought to the attention of the world in the 1960s and again in the early 1980s. Each time the plight of the land and its users was highlighted only because of drought. Indeed, drought is blamed by some not as the catalyst of desertification but as its cause (e.g. Sandford,1983). The nature of this linkage is convincingly disputed by Sinclair and Fryxell (1985), Courel et al. (1984), Rasool(1984) and Gornitz and NASA (1985). The droughts, the prolonged periods of below-average rainfall, served to collapse systems within which there had been slowly building substantial stress as the result of overuse during the between-drought years.

   This case study, the description of the processes and effects of desertification on land use in Africa, is primarily based on the writings of Sinclair and Fryxell(1985) and Lamprey (1983). Both authors reject the 'drought hypothesis', i.e. that the two epochs of desertification observed since 1960 are caused by drought; rather they propose an alternative 'overgrazing' hypothesis.

   The Sahel of Africa, its name derived from an Arabic word meaning 'the shore', is the southern boundary of the Sahara desert. It is a fringing belt of semi-arid landscape, 5000 km long, stretching from the Atlantic Ocean to the Red Sea. In ecological terms it is a transition zone between the hyper-arid Sahara desert in the north and the humid savannahs in the south. Precipitation varies between 100 and 200 mm in the more arid northern grasslands and 400-600 mm in the southern savannahs. The Sahel is approximately 500 km wide with a rainfall gradient of approximately 1 mm/km (Le Houerou,1980).

   Rainfall in the Sahel results from a continental-scaled weather pattern, the inter-tropical convergence zone (ITCZ), which produces a rainfall belt that moves from south to north. This moving rainfall swathe is followed by migratory insects, birds, and the large mammalian herbivores (Sinclair and Fryxell,1985). The most spectacular of these migrations are those of the wildebeest (Connochaetes taurinus) in the Serengeti of Tanzania and Kenya, and the kob(Kobus kob) of Sudan. In both cases more than 1 million animals move from the higher rainfall (600+ mm/year) savannahs, tall perennial grasslands of relatively low quality in the dry season (Jan.-Mar.), to the lower rainfall (< 400 mm/year) but high-quality grasslands in the wet season. These grasslands of the more arid northern edge of the Sahel support a very high animal biomass (density) for a short time while the grass remains green, i.e. resources for the herbivores are rich and plentiful.

   Other smaller herbivores that do not migrate en masse have adjusted to regular, seasonally limited supplies of quality food and water by a range of adaptations that include dietary changes (grazing-browsing), or small-scale migration from patch to patch of temporary water and food generated by the spatially patterned rainfalls.

   The natural nomadic movement of the wildebeest and kob provides for periods in both wet and dry seasons when there is no, or very little, grazing. During these periods the perennial grasses reproduce and build up reserves to maintain vigour, essential for the persistence of these grasslands under grazing (Belsky,1986; McNaughton,1984,1985; Sinclair and Fryxell,1985).

   A second conclusion is that this nomadic strategy allows a larger population of herbivores to exist than would be possible under a sedentary strategy. Under a sedentary system herbivore populations would be forced to exist on abundant but low-quality grasses for most of the year, and this would reduce growth and reproductive rates. The alternative nomadic strategy provides the opportunity for the harvesting of high-quality forage for a sufficiently long period to improve reproductive success and build reserves to survive on the poor resources of the savannahs during the long dry season (Sinclair et al., 1985).