Habitat and diet selection by the African elephant at the landscape level : a functional integration of multi-scale foraging process
Abstract Understanding the role of elephant in the structure and functioning of African savanna ecosystems requires a mechanistic understanding of their habitat and diet selection at the landscape scale. To this end a functional approach based on optimal foraging theory was devised. Given a short foraging time per unit nutrient, a low level of cell wall digestion, and a limited ability to recycle microbial protein, it was hypothesised that elephants take advantage of a high passage rate of ingesta and process a large amount of food with cell solubles rich in energy and nutrients per unit time to meet their nutritional demands. Accordingly, elephants were predicted to select habitats and diets that maximize their rate of intake of digestible energy and nutrients relative to what is available in the landscape. However, because safety, distance from surface water and shade also potentially influence foraging decisions by elephants, habitat and diet selection were predicted to be the result of a trade-off between the rate of intake of energy and nutrients and these non-dietary factors. To test this prediction, a mechanistic ingestion model that was developed specifically for elephants was used to estimate the spatio-temporal pattern of the rate of protein intake achieved by elephants inhabiting a medium-sized reserve in a semi-arid savanna environment. The rate of protein intake was assumed to be a proxy measure of the rate of intake of digestible energy and nutrients. The response of elephants to the estimated pattern of intake was as per prediction, with both habitats and food types being selected in accordance with the rate maximizing premise. The mechanistic approach to foraging used in the study provided possible functional explanations for several well known characteristics of the feeding behaviour of elephants. Differences in diet selection between bulls and cows were potentially explained in terms of sex related differences in the rate of protein intake across food types that were largely due to adult bulls harvesting heavier trunkloads than members of family units. Sexual segregation in habitat selection was potentially explained in terms of (1) sex related differences in the rate of protein intake across food types, and (2) the tendency of the short-term rate of protein intake to be a more important explanatory variable than cost distance from water for the spatial distribution of family units, with the converse being true for the spatial distribution of bulls. Seasonal change in the diet of elephants was well explained by temporal variation in the rate of protein intake across food types. Distance from water was shown to have a strong influence on the distribution of elephants even in a medium-sized reserve that was relatively well supplied with surface water. The influence of surface water differed between the sexes and was strongly dependent on the spatio-temporal pattern of the rate of protein intake. The study showed elephants to be primarily grazers, only switching to browse from woody plants when herbaceous forage of adequate quantity and quality is unavailable. This finding was used to construct an alternative hypothesis for the “elephant problem” that explains elephant-induced woodland loss in terms of (1) a man-induced shift in the diet of elephants from a historic diet of grass to a modern diet primarily composed of browse, and (2) a break down of the natural controls of elephant populations. Implications for the management of elephant-vegetation systems are discussed, with proposed foci for management being the restoration of the historic diet and distribution of elephants by altering the boundaries of protected areas to incorporate key grass resources and restricting surface water to historic sites.