The Effects of Rainfall

Several theories for he Decline of Roan, Sable and Tsessebe have been put forward. Many of the areas in which it is being attempted to conserve roan, sable and tsessebe lie below the lower rainfall limit which the subcontinental data indicates are acceptable for the species. In semi-arid ecosystems, variations in annual rainfall may have profound effects on the performance of these three species. Rainfall in late dry season appears to be critical, affecting tseesebe condition, survival rate, late stage of pregnancy and early stage of lactation (Dunham and Robertson 2001, Dunham et al 2003). Erb (1993) modelled the roan population in the Waterberg Plateau Park in Namibia incorporating the late dry season rainfall and found that the relationship between early season rainfall and calving success was important for roan antelope in the Waterberg. Roan start to calve as early as August when the veld is still dry. Good rains in September/October result in an early green flush which provides lactating cows with the necessary dietary protein to meet a demand which escalates during late pregnancy and early lactation.Sable in Zimbabwe were found likely to die from parasites soon after onset of rains if weakened by stress during the dry season (Grobler 1981).

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Rain surplus / deficit

The long term surplus or deficit in rainfall seems to be the main determinant of the vegetation structure, particularly of grasslands, in any given locality. A prolonged drying out process affects water tables and the catenas in vegetation from river banks to the upper reaches of catchments. Species such as tsessebe find their preferred habitats shrinking to narrow bands close to rivers. Sable and roan are be affected by changes in species composition in grass swards and shifts between perennial and annual grasses.

Dunham and Robertson (2001) found that the following formula adequately described the rise and fall of tsessebe numbers in Kruger National Park:

Adult tsessebe survival is a function of the amount of dry season rainfall, the integral of the annual rainfall (the accumulated deficit/surplus) and the numbers of tsessebe actually present at the time.

Effects of these variables on a tsessebe population

• Rainfall in the late dry season appears to be critical, affecting the animals' ability to maintain condition and, hence, survival. The effect on females during the late stage of pregnancy and the early stage of lactation is particularly pronounced.
• The long term surplus and deficit in annual rainfall appears to act on habitats. A sequence of years whith rainfall below average may result in unfavourable physiognomic changes in habitats.
• Population density, would act negatively on population increase. The higher the number of animals, the greater would be the degree of intra-specific competition for food and the amount of energy expended by adult males on maintaining territories. With the low numbers of roan, sable and tsessebe in Namibia, this effect is unlikely to be marked.