Roan, sable and tsessebe were relatively abundant and widely distributed in north-eastern Namibia at the turn of the century. None of these species occurred naturally in areas where the mean rainfall was lower than 400mm. Today, the population levels of all three species are a matter for concern: at best, the number of roan in Namibia is about 800, sable 1,200 and tsessebe 350. More than half of the animals in each of these species populations are on commercial farms and, of these, more than half are in areas with a mean annual rainfall below 400mm - which does not bode well for their long term survival. In the areas which should contain substantial, viable populations all three species, the numbers are low and the populations are dispersed in small isolated groups. Various explanations can be put forward for the parlous state of roan, sable and tsessebe in areas where they should occur. As with buffalo (see Buffalo Management Plan), their abundance is largely controlled by rainfall and this has been below average since the mid-1980s. However, a more subtle effect of rainfall appears to operate on these species - their numbers correlate with the cumulative deficits and surpluses above and below the mean rainfall, particularly towards the margins of the range of rainfall which are acceptable to the species. These cumulated deficits and surpluses produce long term variations in habitats, rendering them unfavourable for roan, sable and tsessebe for extended periods.