Foraging behaviour and spatial dynamics of Serengeti herbivores
This thesis is an investigation of the foraging ecology of Thomson's gazelles ('Gazella thomsoni thomsoni' Gûnter) and wildebeest ('Connochaetes taurinus albojubatus' Burchell) with respect to nutritional constraints and interspecific interactions in Serengeti National Park, Tanzania. In controlled feeding trials, I measured daily digestible energy intake rates constrained by cropping rate (cropping constraint) and gut fill (digestion constraint). Measures of the gazelle functional response and 'ad libitum ' daily intake combined with maturational declines in grass digestibility suggested that maximum daily energy intake should occur on 25 g/m2 biomass swards with cropping rate constraining intake on sparser swards and digestion rate constraining intake on denser swards. Using energy model predictions, I examined gazelle distributions relative to sward energy density in Serengeti. Patterns were consistent with the digestion and combined constraint model at small spatial-temporal scales. Cropping model predictions were inversely related to observed animal densities. Nevertheless, the high availability of optimal biomass swards in the gazelle's wet season range obscured energy selection patterns at all but the smallest spatial and temporal scales. I estimated digestion and cropping constraints for wildebeest and analyzed their distributions relative to grass and water from an existing data set. Their patterns indicated selection for swards of moderate height and greenness year-round, but at smaller spatial-temporal scales, selection was for sward greenness. This indicates that wildebeest move globally to short swards and locally to swards of exceptional quality. Such local patch selection by wildebeest may facilitate energy gain by Thomson's gazelles. Density of female gazelles with respect to wildebeest aggregations suggested they exploit immature forage remaining in wildebeest exploited patches. The proposed facilitation hypothesis was consistent with the recent gazelle population trajectory. A general application of the energy constraint model relative to ruminant body mass indicated a pattern of predictable habitat selection based on the allometry of energy intake constraints. Small ruminants with small guts but high metabolic demands should select sparse grass swards whereas large ruminants should select dense swards. This prediction was supported by Serengeti data. A distillation of results suggests a strong role for digestion constraints in regulating ruminant energy gain and patch selection and species assembly.