Water availability and its influence on the structure and dynamics of a savannah large mammal community

Abstract

The Amboseli ecosystem is in a semi-arid savannah environment in which water availability is highly seasonal, a feature which has an important bearing on the structure and efficiency of the large mammal community.

Data are presented to show that the seasonal movements, a wet season dispersal and dry season concentration of water-dependent species (obligate drinkers) can be related to the seasonality of rainfall and water availability, but that similar patterns do not prevail in the water-independent species.

Evidence suggests that water availability is a crucial parameter in calculating the carrying capacity of a range, and that the duration of ephemeral supplies, which reflect rainfall seasonality, evapotranspiration, and soil water storage capacity, is equally important. A simplified model is presented which calculates the carrying capacity of water-bound communities–taking into account these two limiting factors.

There appears to be a physiological barrier to a heavy utilization of the low water content grasses beyond reach of the water-dependent species, since most water-independent species select high water content browse in order to maintain a positive water balance. Significantly, while the water-independent community is almost exclusively composed of browsers, most water-dependent species are grazers. The biomass density of large mammals beyond 15 km from water, a radius which encloses 99-5% of the biomass of water-dependent species, is extremely low compared to areas within 10 km of water.

As a consequence of the physiological barrier to a heavy utilization of low water content forage in water-deficient areas, the food chain efficiencies are appreciably lower than those with permanent water sources. Livestock development programmes have long appreciated this constraint and employed water development schemes to increase range-carrying capacity.

Seasonality in the movement patterns of Amboseli is compared to other large mammal communities and management implications are discussed.