DO ELEPHANTS OVER-UTILIZE MOPANE WOODLANDS IN NORTHERN BOTSWANA

Evidence from southern African semi-arid savannas suggests that high e lephant densities in nature reserves cause the over-utilization of woo dlands. Northern Botswana, with its prolific elephant population, is e xpected to follow such a pattern unless the densities of elephants whi ch could be sustained by indigenous woodlands are determined and maint ained at carrying capacity. Above ground biomass production was estima ted for mopane (Colophospermum mopane) woodlands, a principal food sou rce for elephants, which grows over much of northern Botswana. Densiti es of trees and shrubs, dimensions of plants and elephant densities we re recorded within stratified plots situated according to a regional r ainfall gradient. Mean estimates of above ground biomass (foliage and twigs) were 9.41 and 7.83 t ha(-1) for shrubs and trees respectively. A logistic model described the maximum levels of biomass removal from plants before over-utilization of mopane occurred. Variables incorpora ted in the model included above ground biomass of mopane shrubs and tr ees, growth rates of plants and expected off-take by elephants. The mo del predicted a complete biomass regain within 10 y if no elephant bro wsing occurs. Intensive elephant browsing in woodlands containing 15 e lephants km(-2), however, can suppress biomass production if growth ra tes of plants fall below 70% of the maximum annual rate. Nonetheless, there was no substantial evidence to suggest that elephants will reduc e the biomass of mopane woodlands in northern Botswana below a sustain able level if their numbers are allowed to increase considerably beyon d the current estimate.