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Other than pigs, domestic livestock in Africa are concentrated in the arid and semi-arid zones. This is because the more humid areas were historically prone to livestock diseases such as nagana (a trypanosome carried by the tsetse fly—Ford and Katondo, 1977), typically support grasses of low digestibility (Scholes, 1990, 1993), and often are densely settled by crop agriculturalists. The overwhelming majority of these animals feed predominantly off natural grasslands and savannas, although crop residues are an important supplement during the dry season. Many urban and rural families also keep poultry.
Domestic livestock play a central role in many African cultures. Cattle and camels, in particular, have an importance that goes beyond the production of meat. Their value is based on the full set of services they supply (milk, meat, blood, hides, draft power), their asset value as a form of savings, and their cultural symbolism. It would be difficult and damaging for these cultures to abandon pastoralism in the event that it becomes climatically, environmentally, or economically unviable.
Although classical concepts of animal carrying capacity may not be very useful as local management tools in the context of African semi-arid systems with high interannual variability (Behnke et al., 1993), they remain valid as indicators of animal production when they are applied over decadal periods and large areas. Many researchers have demonstrated a strong link between long-term, large-area herbivore biomass (LHB, kg km-2) in African wildlife and pastoral systems and mean annual precipitatiation at one site (see also Le Houerou, 1998, for a similar analysis in west Africa and the Mediterranean basin). Thus, in broad terms, changes in range-fed livestock numbers in any African region will be directly proportional to changes in annual precipitation. Given that several GCMs predict a decrease in MAP on the order of 10-20% in the main semi-arid zones of Africa, there is a real possibility that climate change will have a negative impact on pastoral livelihoods. The following additional factors must be considered.
The causal chain between rainfall and animal numbers passes through grass production, which also is approximately linearly related to rainfall (Breman, 1975; Le Houerou and Hoste, 1977; Rutherford, 1995). The slope of this relationship, which can be expressed as WUE, is a function of soil nutrient availability (De Ridder et al., 1982; Scholes, 1993). WUE also is a function of CO2 concentration in the atmosphere (Mooney et al., 1999), especially in semi-arid regions. Because the CO2 concentration will rise in the future, its positive impact on WUE (which is on the order of 20-30% for doubled CO2, even in C4-dominated grasslands such as these) will help to offset reduction in rainfall of the same magnitude. Simulations of grassland production in southern Africa indicate an almost exact balancing of these two effects for that region (Scholes et al., 2000).
About 80% of the grazing lands of Africa are in savannas, a vegetation formation that consists of a mix of trees and grasses. Grass production in savannas is strongly depressed by tree cover (see the many references reviewed by Scholes and Archer, 1997). Because domestic livestock, with the exception of goats, predominantly eat the grass in these systems, future changes in tree cover are an important issue from the point of view of carbon sequestration and livestock production. Tree biomass ultimately is related to climate and soil type, but the mechanism appears to be via fire frequency and intensity. If fire frequency and intensity were to decrease—as a result of climate changes (for instance, an increase in dry-season rainfall) or, more likely, changes in land management—woody cover would increase. This has been demonstrated in numerous fire experiments in Africa (Trapnell et al., 1976; Booysen and Tainton, 1984). Given the vast area of the savannas, the carbon sequestration potential is substantial (Scholes and van der Merwe, 1996). In addition, emissions of tropospheric ozone (O3) precursors would decrease if savanna burning were reduced. The disadvantage would be a more than proportional decrease in livestock carrying capacity, as a result of the nonlinear suppressive effect of trees on grass production.
The bioclimatic limit of savannas in southern Africa is related to winter temperatures. An increase in temperature of 1-2°C—well within the range predicted for next century—would make the montane grasslands (highveld) of southern Africa susceptible to invasion by savanna trees (Ellery et al., 1990).
In moister regions, animal productivity is limited not by the gross availability of fodder but by its protein (nitrogen) content (Ellery et al., 1996). Increasing the CO2 concentration or the rainfall will not increase the protein availability; thus, livestock in these regions are likely to be less responsive to the direct effects of atmospheric and climate change. Under elevated CO2, the carbon-to-nitrogen ratio of forage will decrease, but this will not necessarily lead to decreased forage palatability despite the dilution of protein (Mooney et al., 1999). This may be because in grasses, the bulk of the excess carbon is stored in the form of starch, which is readily digestible. Widespread use of protein and micronutrient feed supplements and new technology for the control of veterinary diseases will have a greater impact on livestock numbers and productivity, especially in the "miombo" region of south central Africa.
Domestic livestock, like other animals, have a climate envelope in which they perform optimally. The limits of the envelope are quite broad and can be extended by selecting for heat or cold tolerance, feed supplementation, or providing physical shelter for the animals. African cattle are mostly from the Bos indicus line, which is more heat-tolerant than the European line of Bos taurus. In extremely hot areas (mean daily warm-season temperatures greater than body temperature), even the Bos indicus breeds are beyond their thermal optimum (Robertshaw and Finch, 1976). Meat and milk production declines, largely because the animals remain in the shade instead of foraging. There is limited potential for extending this limit through breeding. Adaptation would require substitution by a species such as the oryx, which is physiologically equipped for high temperatures and low water supply.
In the higher altitude and higher latitude regions of Africa, livestock (typically sheep) currently are exposed to winter temperatures below their optimum. Mortality often results when cold periods coincide with wet periods, if the animals have not been herded to shelter. These episodes are likely to decrease in frequency and extent in the future.
Livestock distribution and productivity could be indirectly influenced via changes in the distribution of vector-borne livestock diseases, such as nagana (trypanosomiasis) and the tick-borne East Coast Fever and Corridor Disease (Hulme, 1996). Simulations of changes in the distribution of tsetse fly (Glossina spp.) indicate that with warming it could extend its range southward in Zimbabwe and Mozambique, westward in Angola, and northeast in Tanzania, although in all these simulations there were substantial reductions in the prevalence of tsetse in some current areas of distribution. The tick Riphicephalus appendiculatus was predicted to decrease its range in southern and eastern Africa and increase its range in the central and western part of southern Africa (Hulme, 1996).
One land-use model (IMAGE 2.0—Alcamo, 1994) projects that large parts of Africa will be transformed to pastoral systems during the 21st century. The model logic that leads to this conclusion is that increasing urbanization and a rising standard of living typically are associated with a change in dietary preference toward meat. The area that currently is used for meat production therefore would need to expand, assuming that meat demand was not met by import or by increased productivity of existing herds. These are reasonable but untested assumptions, and their consequences have major implications for biodiversity conservation and atmospheric composition. The areas indicated as being converted to pastures (largely the subhumid tropics) already support cattle to some degree. Increased cattle production would require widespread tree clearing (leading to conversion of a carbon sink into a carbon source), eradication of key cattle diseases, and the use of protein and micronutrient feed supplements. The quantity of fuel consumed by savanna fires would decrease (because it would be grazed), reducing the release of pyrogenic methane (CH4) and O3 precursors, but the production of methane from enteric fermentation would increase. Because methane production per unit of grass consumed is higher for enteric fermentation than for savanna fires (Scholes et al., 1996), the result is likely to be a net increase in radiative forcing.
Food security in Africa already is affected by extreme events, particularly droughts and floods (e.g., Kadomura, 1994; Scoones et al., 1996). The ENSO floods in 1998 in east Africa resulted in human suffering and deaths, as well as extensive damage to infrastructure and crops in Kenya (Magadza, 2000). Floods in Mozambique in 2000 and in Kenya in 1997-1998 sparked major emergency relief as hundreds of people lost their lives and thousands were displaced from their homes (Brickett et al., 1999; Ngecu and Mathu, 1999; see also <www.reliefweb.int>). The cost in Kenya alone was estimated at US$1 billion (Ngecu and Mathu, 1999). Droughts in 1991-1992 and 1997-1998 affected livelihoods and economies and heightened renewed interest in the impacts of climatic hazards (e.g., Kadomura, 1994; Campbell, 1999). For example, the impacts of the 1991-1992 drought in Zimbabwe are estimated to have been 9% of GDP (Benson and Clay, 1998).
Such climatic episodes can serve as an analog of climate change. Irrespective of whether climate change will cause more frequent or more intense extreme events, it is apparent that many aspects of African economies are still sensitive to climatic hazards. At the local level, some coping strategies are less reliable (Jallow, 1995)—for instance, Campbell (1999) notes that plants and trees used as food by pastoralists in southern Kenya declined between 1986 and 1996. National governments often struggle to provide food security during times of crisis (Ayalew, 1997; Gundry et al., 1999). For national and international agencies, the cost of climatic hazards—impacts, recovery, and rehabilitation—may result in a shift in expenditure from reducing vulnerability to simply coping with immediate threats (e.g., Dilley and Heyman, 1995).
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