*The individual populations of roan on commercial farms are
all very small numbering from less than 10 to 60 animals.
Data Quality and Interpretation
The data available for roan, sable and tsessebe are not very
valuable for comparative purposes or for detecting trends:
- the same areas have not been surveyed consistently
from year to year (this effect is particularly prevalent
in the Caprivi) and
- different survey techniques have been used on different
surveys. The surveys which have been carried out include
- waterhole counts,
- total counts from fixed wing aircraft,
- total counts from helicopters and
- sample surveys based on line transects with calibrated
At present there is no acceptable alternative to the standard
transect survey method or the random block count method.
Tthe population estimates for all areas (with the exception
of roan in the Waterberg from 1975-1990) are too erratic to
justify any attempts to fit modelled data to observed numbers.
It is necessary to be conscious at all times that many of
the observed swings in population numbers may be no more than
artifices arising from irregular and incomplete surveys.
- the accumulated rainfall surplus reached a peak in the
five years between 1975 and 1980. It was also during this
time that the populations of all three species appeared
to be booming.
- After 1980 the surplus began to decrease although it
did not change to a deficit in all areas simultaneously.12
After 1994 all areas went into a rainfall deficit mode13
and this appears, on most of the figures, to coincide
with population 'slumps' for all species populations.
- Therelationship with the cumulative rainfall surpluses
and deficits becomes increasingly critical as the lower
rainfall limits of the species range are approached. In
the Eastern Caprivi, where annual rainfall is normally
well above 500mm, the effects appear less striking. In
Kruger National Park, the range preferred by roan and
tsessebe is in a marginal rainfall area and the rainfall
deficit/surplus relationship is strong. The decline of
roan and tsessebe since the 1970s in north-western Zimbabwe
may be due to the same phenomenon.
- There is little to be gleaned from the late dry season
rainfall data, mainly because the small ups and downs
in population numbers from year to year which this factor
would influence are obscured by the poor population estimates.