Part III
Africa’s large herbivore assemblage is the product of the environments that also nurtured human origins and these animals contributed to evolutionary transitions in our hominin lineage. Around 90 species of large herbivore can be tallied on the continent, around half of them associated with the savanna biome (Figure III.1; the rest live in forests or deserts). All of them are ungulates, representing the orders Artiodactyla (with even-toes) and Perissodactyla (with odd-toes), except for the African elephant (Proboscidea). They span a range in body mass from ~5000-kg elephants down to 5-kg dikdiks. Over half of the savanna inhabitants obtain their diet primarily from grasses, while the remainder feed mostly on the leaves of trees, shrubs and other plant types. In doing so they must also adjust to the seasonal variation in wetness versus dryness that is the defining feature of savanna environments, along with midday heat loads and seasonally restricted sources of drinking water. At the same time their survival is threatened by numerous large carnivores. Grazing and browsing have ramifying effects on the vegetation cover, modify the spread of fires, and accelerate the recycling of the mineral nutrients back to the soil. However, the diversity of large herbivores that we see today was exceeded back in time during the period when humans evolved.
Figure III.1
Diversity of extant large herbivores in Africa in various tribes represented by grazers, browsers and mixed feeders. The number of species within each family or tribe in each feeding category, across all habitats, is in parentheses.
Several of these animal species have been the focus of my research. During my doctoral study of white rhinos in the Hluhluwe-iMfolozi Park, I wandered along the pathways that these mega-grazers followed, inspected what grasses they ate, unravelled their social relationships, tracked their journeys to water and back, and observed how they interacted with other animals, during daylight and darkness (Figure III.2A). My observations led me to recognise the distinctive features that rhinos shared with other ‘megaherbivores’ (animals weighing more than 1000 kg once adult), such as elephants and hippos.1 Their very large body size helps them resist environmental variation, but makes them vulnerable to human overkill. The conservation problem underlying my study was the transforming influence of white rhinos on the vegetation cover, affecting habitat conditions for other species.
Figure III.2
(A) Observing white rhinos on foot in Mfolozi Game Reserve; (B) watching kudus in Kruger National Park from a Land Rover; (C) recording what an impala in Nylsvley Nature Reserve ate, using a keyboard coupled to a tape recorder; (D) following after a foraging kudu at Nylsvley.
The limited (3.5-year) duration of this study left unanswered the fundamental question of how large herbivores might achieve some form of balance with the plant cover. An opportunity to address it was provided by my post-doctoral study on the population dynamics of kudus in Kruger National Park (NP), undertaken ensconced mostly in a 4×4 vehicle (Figure III.2B). Each kudu could be recognised from variation in stripe patterns, photographically recorded, allowing me to document population changes by registering individual births and deaths. This study revealed how sensitively recruitment and mortality responded to annual variation in rainfall.2
To explore in more detail what governed the plant species that kudus and other browsers chose to eat, I followed habituated young animals in the Nylsvley Nature Reserve, recording every plant they consumed from sunrise to sunset each day (Figure III.2C,D). This revealed how plant secondary metabolites, particularly condensed tannins, influenced diet selection and hence the potential impact of browsing in suppressing bush encroachment.3,4
Subsequent studies by my students explored aspects of the ecology of black as well as white rhinos, sable and roan antelope (Hippotragus niger and H. equinus, respectively), wildebeest and zebra, buffalo, eland, gemsbok (Oryx gazella) and elephant, the latter mainly through what they do to trees. Field observations became incorporated into computer models linking diet selection to population dynamics.5,6
This Part III of the book will cover features of the ecology of Africa’s large herbivores in the context of savanna vegetation and underlying physical features typical of Africa, as documented in Parts I and II. This provides the foundation for identifying how these animals interact with ecosystem processes such as fire spread and nutrient cycling and contribute to spatiotemporal heterogeneity. The time horizon is then extended back into the past to review how large herbivore assemblages and associated carnivores have changed since the Miocene epoch when Africa’s modern fauna along with savanna-inhabiting hominins originated.
References
1.Owen-Smith, RN. (1988) Megaherbivores: The Influence of Very Large Body Size on Ecology. Cambridge University Press, Cambridge.
2.Owen-Smith, N. (1990) Demography of a large herbivore, the greater kudu Tragelaphus strepsiceros, in relation to rainfall. The Journal of Animal Ecology 59:893–913.
3.Owen-Smith, N; Cooper, SM. (1987) Palatability of woody plants to browsing ruminants in a South African savanna. Ecology 68:319–331.
4.Owen-Smith, N. (1994) Foraging responses of kudus to seasonal changes in food resources: elasticity in constraints. Ecology 75:1050–1062.
5.Owen-Smith, RN. (2002) Adaptive Herbivore Ecology: From Resources to Populations in Variable Environments. Cambridge University Press, Cambridge.
6.Owen-Smith, N. (2009) Dynamics of Large Herbivore Populations in Changing Environments: Towards Appropriate Models. John Wiley & Sons, Chichester.