Africa is the earth's second largest continent, comprising 20% of its surface. Largely tropical, Africa extends as well into temperate zones to 37°N and 35°S. Eastern and southern Africa display steep elevation gradients due to the prevalence there of volcanic orogeny and rifting (29). Local landscapes are distinguished by substantial geological heterogeneity, dissected land forms, and resultant steep gradients of precipitation and vegetation. The consequent pronounced fragmentation of habitats and sharp juxtaposition of distinct vegetation types, combined with climatic oscillations in geological time, contributed to major adaptive radiations of the mammalian fauna (102, 120).
Early zoological expeditions recorded that habitat fragmentation and wide spatial variation of animal densities and diversities were distinctive features of African ecosystems (92, 138, 162, 226). Those early records provided the bases of natural history information on animal distributions, habitat preferences, feeding habits, and general ecology; scientific research followed only much later (201). Modern scientific study of African savanna-grassland mammals began in the 1950s (23, 24, 107, 108, 148, 149, 197, 203, 204, 210, 230), long after the distributions and densities of the major game animals had been affected by growing human populations, colonial land and hunting policies, and virulent exotic diseases that affected the animals both directly and indirectly (57). The mammalian fauna has been increasingly isolated and fragmented within game reserves of varying size, habitat diversity, and animal species diversity; the ability to sustain it in the absence of active management is increasingly questioned (112, 187). For species with population sizes greater than 100 individuals, game reserve area (A) and faunal species richness (S) are related by S = 2.62 A 0.18 (47). But habitat diversity also plays a major role in controlling species richness that tends to override simple species-area curves (224).
The most evident of the biologically interesting features of African ecosystems are the high densities and diversities of large (>5 kg) mammalian herbivores and carnivores (101). Excluding primates, more than 90 large herbivore species exist on the continent (120). At least 10 species occur in most African game parks, and more than 20 are found in such large and diverse areas as Serengeti in Tanzania and Kenya, and Kruger in South Africa (35). The literature on behavior and ecology, often directed toward defining appropriate c conservation policies, has been ably reviewed elsewhere (38, 51, 113).
This review concentrates on three questions about the large mammalian herbivores of African ecosystems:
1. What are the mechanisms of resource partitioning in the diverse fauna?
2. What factors determine the densities and distributions of the large mammals?
3. What do answers to the first two questions reveal about community and food web organization in general?
Since the savanna-grasslands of northwestern Africa lack the topographical and climatic heterogeneity of the east and south (29), the fauna there has always been comparatively depauperate in historical times, and it is increasingly so recently (170). Therefore, we concentrate on eastern and southern Africa.