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).
Back to top
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
- 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
- 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.