The major livestock products in this system are milk for local consumption with excess being sold to neighbors and very little processed to butter or sour milk (Wilson, 1995). Sale of livestock is a recent post-colonial phenomenon comprised mainly of small ruminants that are normally slaughtered except in times of drought, when cattle are sold to destock the herd. Wildlife are important as a source of bush meat, especially in Central and West Africa (Asibey and Child, 1990; Ntiamoa-Baidu, 1997; Thibault and Blaney, 2003) and as a source of income through tourism, especially in East and Southern Africa (Humavindu and Barnes, 2003; Reilly et al., 2003; Phutego and Chanda, 2004).

Wildlife competes with livestock for range resources in these systems (Prins, 1992; Skonhoft, 1998; Skonhoft and Solstad, 1998). The advent of colonialization and the subsequent creation of independent states have instituted formal laws that control the use of range, usually by reserving large tracts of land for wildlife to the detriment of pastoralists and their livestock (Prins, 1992; Blench, 2001). Policies allowing for flexible land tenure systems and diversification of pastoral livelihoods would help the sustainability of this system.

It is generally agreed that this system of livestock production today faces challenges from increasing population pressure that impede the movement of trekking livestock in pursuit of feed and water, the expansion of cropping land into pastoral lands and the need for increased productivity to supply goods and services to growing populations. While earlier perceptions of policy makers and external donors was that the system is inefficient, current knowledge has shown that the flexible opportunistic management strategies used by pastoralists are sensible, highly productive and environmentally sustainable (Behnke et al., 1993; Reid and Ellis, 1995; Scoones, 1995; Swift, 1996). The challenge for AKST is to bring new technologies such as satellite imagery and quantitative modeling processes to provide further insights into productivity patterns of the system and offer policy options that ensure that the system can continue to contribute to the overarching goals of this assessment.

Agropastoral system. This system is found in the semiarid, subhumid and humid tropics and in tropical highland areas (Sere and Steinfeld, 1996). Livestock are dependent on natural forage and cropping is important but there is low integration with livestock. Livestock migration at certain times of the year is common (Devendra et al., 2005). The major livestock species are cattle, goats, sheep, poultry and, where religious and cultural beliefs allow, pigs. Wildlife is abundant in this system, sometimes leading to conflicts with people and livestock (Prins, 1992; Barnes et al., 1996; Skonhoft, 1998; Blom et al., 2004; Bassett, 2005; Ogutu et al., 2005).

Livestock productivity is higher than in the pastoral system but still insufficient to meet the needs of the growing population in SSA. The main products are meat, milk, skins, manure and draft power plus sociocultural services. In areas close to urban centers, meat, milk and skins are processed for sale to urban dwellers. This is particularly well developed in densely inhabited areas of East and southern Africa for meat where cold storage facilities allow for longer term storage. In other countries sales at specific religious periods

ensures sustainable incomes to livestock owners (e.g., Ethiopia and Nigeria). Drought is a major threat in this system as it results in crop failure and massive sales of livestock (asset attrition). The challenge for AKST is developing reliable early warning systems to avert catastrophic effects of droughts and designing livestock management systems that alleviate shortages during dry season grazing.

The dominant source of feed is the range and its management has been a top priority in terms of legislation and policies in East Africa and Southern Africa. The conventional wisdom has been that agropastoral systems of SSA are overstocked and policies have targeted population reduction (Hardin, 1968; Behnke et al., 1993). The concept assumes that a rangeland has a stable state vegetation mix which is destabilized by grazing and as long as the destabilization is not excessive the range will return to its steady state vegetation. If grazing is excessive then the range loses some of its vegetation species and performs below potential reflected in reduced animal productivity. This view is countered by the assertion that in dry environments the longrun primary productivity of the range is influenced more by rainfall (and other abiotic factors) than by intensity of grazing by livestock or wildlife (Ellis and Swift, 1988; Scoones, 1989, 1992; Behnke et al., 1993).

This dynamic has led to the notion of nonequilibrium ecosystems that are better managed through flexible and opportunistic strategies that allow overstocking during wet seasons and destocking during dry seasons, or the provision of externally sourced supplementary feed during these periods, when massive stock losses occur through death due to starvation (Behnke et al., 1993). AKST has contributed to the changing perception of rangeland management in dry areas of SSA. There are divergent views on this as other researchers have found the impact of livestock to be critical (Briske et al., 2003). However new range management strategies that integrate local knowledge and involve active participation of local communities could be the answer to the issue of whether these systems are sustainable in perpetuity and at what stocking rate they would collapse, if ever. The use of AKST from all sources in the evolution of such systems might be beneficial in the long term.

Mixed crop-based systems. These systems are the most important livestock production systems in SSA in terms of animal to people ratio and animal productivity per unit of land (Sere and Steinfeld, 1996) and form the backbone of smallholder agriculture (Devendra et al., 2005). The systems predominate in humid and sub-humid agroecological zones but they are also found in arid and semiarid tropics and the tropical highlands of East and West Africa. The systems can combine livestock with either annual or perennial crops though the latter is limited in SSA and they exist both in irrigated and rain-fed areas. Ruminant animals graze native pastures and use crop residues as additional feed sources after harvest, whereas nonruminants depend on crop byproducts and household kitchen wastes.

 The main livestock species kept in these systems are cattle, sheep, goats, donkeys, poultry and pigs. The integration of livestock and crop production is an integral component of these systems and allows for efficient use of labor and other resources (Wilson et al., 1983; Devendra et al.,