Part III
The distinguishing feature of Africa’s large mammal fauna is its diversity of grazing ruminants. Relative security from predation contributes importantly to niche separation among herbivore species of similar size, coupled with distinctions in grass height grazed. Grazers present on other continents prior to the late Pleistocene extinctions tended to be very large and mostly non-ruminants. Although some deer consume much grass, none is specialised in dentition and digestive anatomy for an exclusively grass diet. This is perhaps because the C3 grasses prevalent in higher northern latitudes are more readily digested than the C4 grasses prevalent through tropical and subtropical Africa, especially during the season of plant dormancy. Grasses growing under higher rainfall elsewhere in the tropics seemingly require hindgut fermentation to handle their high fibre contents. The prevalence of volcanic soil substrates under moderately low rainfall regimes in Africa contributes to the high local abundance levels attained by some of the grazing ruminants. Equids are widely distributed, but do not reach the biomass densities shown by grazing ruminants like buffalo and wildebeest under favourable conditions. Browsers, apart from elephants with their exceptionally broad dietary range, remain much less abundant than similar-sized grazers.
The local abundance of grazers is enhanced further by their concentrations around remaining sources of surface water during the dry season. Migratory populations concentrate additionally in nutritious but ephemeral grasslands during the wet season. Locally intense grazing restricts the spread of fires. The outcome is a mosaic interspersion of cropped, burnt and ungrazed areas. Regions remote from surface water burn frequently while those in close proximity to water can become grazed down to bared soil, especially in drought years, leading to starvation-induced mortality among grazers.
Elephants make a major contribution to the openness of the tree canopy in Africa’s savannas, in interaction with fire and smaller browsers. They induce mortality among quite tall trees by toppling and debarking and uprooting or severely breaking regenerating saplings and shrubs. Nevertheless, in some situations the damage they impose keeps woody plants at shrub height, enhancing food availability for them and smaller browsers. The latter restrict the woody plant cover through the defoliation that they impose locally on seedlings, counteracting bush encroachment. Nevertheless, all herbivores, even elephants, are selective among tree species and size classes in the damage they impose. Certain woody species may gain a selective advantage as a consequence, while others get excluded. Thus, compositional changes may result rather than a general opening of the woody canopy, especially in broad-leaved miombo woodlands. Areas further than 10 km from water sources can provide a refuge for vulnerable tree species because elephants are water-dependent and feed on woody plant parts mostly during the dry season. Trees with deep roots are resistant to felling, but not necessarily to bark damage. The grazing and browsing impacts of various herbivores contribute in addition to geology, water redistribution and fire spread to the spatial heterogeneity that is a striking feature of Africa’s savannas.
The form of the dynamics generated by rainfall variation disrupts equilibrium tendencies within savannas. If wet-season conditions persisted for multiple years rather than months, savannas would become transformed into woodland or forest, with far fewer large herbivores. If dry-season conditions precluded plant growth for long enough, savannas would come to resemble deserts. Animals must cope with contrasting aridity not only seasonally, but also over successive years in response to multi-year fluctuations in rainfall. Primary production can be halved in drought years, correspondingly reducing the ecosystem capacity to support herbivores. Megaherbivores cruise through the dry years with little reduction in their abundance, unless drought conditions last several years or animals become crowded around few remaining water sources. Grazing ruminants can incur substantial population collapses when the grass cover becomes depleted within range of water sources. Browsers may have the effects of rainfall failure alleviated by the capability of savanna trees to regenerate their foliage from stored water reserves. Carnivores constrict the spatial distribution of their prey rather than local abundance in relatively secure habitats. Migrants largely escape this limitation.
Africa’s large herbivore diversity originated in the later Miocene when grazing ruminants diversified and the earliest hominins made their appearance. Large carnivores were also more diverse then. The large Miocene relicts faded out during the course of the Pliocene and Pleistocene, while a pulse of extinctions took place among some of the largest grazers during the late Pleistocene and subsequent transition into the Holocene.
This represents the ecosystem context that our hominin predecessors entered when they colonised spreading savannas back in the Miocene. While landscapes became acutely depauperate of plant resources during dry seasons, they were locally packed with herbivores representing a wide range in body sizes. How did early humans use the opportunities provided to exploit animal rather than plant resources to make a living under conditions that became increasingly arid as the Pleistocene advanced? This is the story to be taken up in the following chapters forming the last section of this book.
These are the notable points to carry forward from Part III:
1.Africa’s large herbivore fauna is unrivalled in its diversity of medium–large grazers, especially ruminants.
2.These grazers can attain exceptionally high population levels through exploiting relatively nutritious, volcanically enriched savanna grasslands.
3.Grazers concentrate in the vicinity of remaining sources of surface water during the dry season.
4.Drought-related die-offs among large herbivores can be substantial in drought years.
5.Conditions were prevalently drier and hence more widely fertile during the course of the Pleistocene than experienced during the current interglacial interlude.
6.The herbivores that became extinct during the most recent glacial extremes in aridity were all grazers and among the largest species of their type.
Figure III.3
Migratory wildebeest concentration in Serengeti NP.