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Desertification reduces soil fertility, particularly base cation content, organic matter content, pore space, and water-retention capacity. Desertification also reduces vegetative productivity, leading to long-term declines in agricultural yields, livestock yields, plant standing biomass, and plant biodiversity. These changes reduce the ability of the land to support people, often sparking an exodus of rural people to urban areas. Breaking the strong connection of people to the land produces profound changes in social structure, cultural identity, and political stability.
In Niger, on farmed land where organic carbon in the top 10 cm of the soil has fallen from 0.3 to 0.2% 4 years after coming out of fallow, millet yields fell from 280 to 75 kg ha-1 (Bationo et al., 1993). Modeling of the 4,000 km2 Mgeni River watershed in South Africa showed that conversion of more than one-quarter of the watershed from forest and rangeland to agriculture and exotic tree plantations since the area was colonized would double mean annual runoff in urban areas and other areas of reduced land cover (Schulze, 2000).
In the Senegal Sahel, the densities of trees with a height of >3 m declined from 10 trees ha-1 in 1954 to 7.8 trees ha-1 in 1989; the species richness of trees and shrubs fell from 16 species per 4 km-2 around 1945 to about 11 per 4 km-2 in 1993 (Gonzalez, 1997, 2001). These changes have caused a 25-30 km shift of the Sahel, Sudan, and Guinean vegetation zones in half a century, proceeding at an average rate of 500-600 m yr-1. Arid Sahel species expanded in the northeast, tracking a concomitant retraction of mesic Sudan and Guinean species toward areas of higher rainfall and lower temperature to the southwest.
In the Senegal Sahel, human carrying capacity in 1993 stood at approximately 13 people km-2 at observed patterns of resource use, compared to an actual 1988 rural population density of 45 people km-2 (Gonzalez, 1997, 2001). This means that people with no other alternatives need to cut into their natural resource capital to survive. Such changes across Africa have pushed a rural exodus that may have displaced 3% of the population of Africa since the 1960s (Westing, 1994).
Desertification also will cause conversion of perennial grasslands to savannas dominated by annual grasses. Such changes have occurred in the Kalahari Gemsbok National Park in South Africa, where Landsat imagery showed increases in exposed soil surface (Palmer and van Rooyen, 1998). Such declines often are irreversible (Schlesinger et al., 1990).
The tragic death of as many as 250,000 people in the Sahel drought of 1968-1973 (UNCOD, 1977) demonstrates the vulnerability of humans to desertification. As desertification proceeds, agricultural and livestock yields decline, reducing people's options for survival. Furthermore, not only do local people lose the vital ecosystem services that dead trees and shrubs had provided; the loss of firewood, traditional medicine species, and emergency food species render them more vulnerable to future environmental change.
Adaptations by farmers and herders in Africa to climate change and desertification have involved diversification and intensification of resource use (Davies, 1996; Downing et al., 1997). Resourceful diversification responses by women in Bambara and Fulbe households in Mali (Adams et al., 1998) reflect the importance of women in guiding adaptation strategies across Africa. In southern Kenya, Maasai herders have adopted farming as a supplement to or replacement for livestock herding (Campbell, 1999). In Kano, Nigeria, peri-urban vegetable gardening has expanded (Adams and Mortimore, 1997), revealing a common diversification trend in small cities across west Africa. In northern Cameroon, Fulbe herders have increased the number of herd displacements between pasture areas and even resorted to long-distance migration, sometimes introducing significant changes to their way of life (Pamo, 1998).
In the future, seasonal climate forecasting (NOAA, 1999; Stern and Easterling, 1999) may assist farmers and herders to know times of higher probability of success of resource diversification or intensification. Seasonal forecasts for Africa currently exhibit moderate skill levels (Thiaw et al., 1999) but skill levels and user communications are not yet high enough to permit users to confidently implement field applications (UNSO, 1999; Broad and Agrawala, 2000). Neither trade nor technology will likely avert the widespread nutritional and economic effects of desertification through the 2020s (Scherr, 1999).
Other adaptations to desertification involve more efficient management of resources. In Niger, farmers with access to credit will adopt low-cost, appropriate technologies for wind erosion control, including windbreaks, mulching, ridging, and rock bunds (Baidu-Forson and Napier, 1998). Across Africa, farmers traditionally have adapted to harsh environmental conditions by promoting natural regeneration of local trees and shrubs. Natural regeneration is a practice whereby farmers and herders seek to reconstitute vegetative cover by setting aside parcels of land or by selecting valued trees in their fields, pruning them, straightening them, and raising them to maturity. The Sereer in Senegal (Lericollais, 1973) and the Mossi in Burkina Faso (Kessler, 1992) have achieved doubling of tree densities in certain semi-arid areas with Acacia albida and Butyrospermum parkii, respectively.
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