Mean annual rainfall < 400mm

Population unlikely to be viable in the long term

Mean annual rainfall 400-500mm

Population will decline in periods when cumulative rainfall deviations from the mean are in deficit mode

Reliability of perennial water

Regardless of available grazing, habitat cannot be used if surface water is unavailable

Grass sward and cover conditions

Range must contain suitable grass species within preferred physiognomic savannah structure

Habitat suitability reduced by changes in grass sward composition, loss of shade and trampling effects

Human encroachment on range

Disturbance and competition with cattle effectively reduces available range

Veterinary fences

Prevent dispersal and remove essential linkages with larger Botswana species populations

Excessive burning results in shortage of dry season grazing and limits population performance

Level of illegal hunting

Illegal hunting reduces population growth rate and becomes unsustainable at levels exceeding 10%

Level of legal hunting

Trophy quotas greater than 2% of population result in loss of all potential breeding males

Disease

Proximate mortality factor actng preferentially on animals which are already in poor condition

Late dry season rainfall

Absence of dry season rainfall likely to increase mortality and produce short term population fluctuations