Traditionally, migratory pastoralists lived with their cattle in balance with the vegetation. From the 1950's, many people settled in the south of the Sahel in villages around watering points, resulting in overgrazing in these areas. Improved medical and veterinary aid led to a rapid increase in the populations of people and livestock. Some of the land previously available for grazing was converted to growing cash crops, increasing the density of livestock in the remaining grazing lands.
1. Open Acacia scrub with perennial grasses
2. Desertification (mosaic of bare ground and unpalatable shrubs)
In the southern Sahel, a rapid increase in the populations of people and livestock has resulted in overgrazing. Constant intensive grazing has destroyed the rootstock of palatable perennial shrubs, giving way to short-lived, shallow rooted annuals. Subsequently, the annuals were grazed out, leaving a landscape of bare soil and shallow rooted unpalatable shrubs. Much of the topsoil with its nutrients, was blown or washed away, leaving bare rock. Silt, which settled in drainage areas, baked hard after rain. Roots could not penetrate this hard layer and no germination could occur. The grasslands have been replaced by desert. A continuous drought has accompanied this shift in vegetation.
Did the shift in vegetation type trigger the prolonged drought, or did the drought contribute to the shift? Coupled biosphere-atmosphere simulations (Wang and Eltahir, 2000a) have shown that a warming of 2.5oC(*) sea surface temperature (SST) is sufficient to trigger a shift from a self-sustaining wet climate equilibrium to a self-sustaining dry climate equilibrium. A 20% reduction in vegetation cover (i.e. 1% per year for the 20 years preceding the drought onset) is enough to maintain this shift, in the form of a multi-decadel drought. The most likely scenario for triggering the Sahel drought includes regional changes in land cover and changes in the patterns of global and regional sea surface temperature, which was seen around the time of the onset of the drought. The impact of human activities on the landscape was not included in this study.
Fernandez et al. (2002) produced a model for cropping (subsistence and cash) and livestock farming in Western Niger, in the southern Sahel. Here the soils are sandy, low in organic matter and deficient in phosphorus and nitrogen. Productivity is limited by soil fertility, which is related to a combination of fallowing (for non-manured croplands) and herbage intake by livestock. Some key thresholds in the model include:
1. For unmanured cropland, soil fertility can be maintained when 3/8 of the arable land is left fallow.
2. Soil fertility is affected by the ratio of total herbage intake by livestock to total palatable herbage available during the wet season. The threshold for sustainability is set at 1/3 of the mass of palatable herbage at the end of the growing season, to allow for continual growth of annuals during the wet season and to account for the limits of grazing efficiency.
3. Economic sustainability was measured as a minimum threshold for the basic needs of household members.
* 2.5oC is the figure used in the simulations as a surrogate for a documented warming in the southern hemisphere oceans in association with shift of the inter-hemispheric distributions of the SST and surface pressure around the time of the onset of the drought.
State 2: During the famines, emergency drought assistance provided food aid for starving people. Some attempts were made to restore the grasslands in bare areas by reducing livestock numbers, but this has had little effect. The bare areas have remained in this state for more than 20 years.