Variations in annual rainfall may have profound effects
on the performance of reedbuck, waterbuck, lechwe and puku.
Rainfall is the principal determinant of numbers of common
ungulates in southern African savannas (Mills et al 1995).
There is a strong relationship between the survival of tsessebe
and the accumulated surplus or deficit in rainfall over a
long period (Dunham & Robertson 2001, Dunham et al 2003).
A strong correlation between adult and juvenile mortality
and the rainfall in the late dry season was found. Rainfall
in late dry season appears to be critical, affecting the animals'
condition, survival rate, late stage of pregnancy and early
stage of lactation.
The long term surplus or deficit in rainfall may be the main
determinant of the condition of the floodplain grasslands
in the Caprivi, which is the main habitat for all of these
species. A prolonged drying out process would affect water
tables and the catenas in vegetation from river banks to the
upper reaches of catchments. Reedbuck, waterbuck, lechwe and
puku would find their preferred habitats shrinking to narrow
bands close to rivers. Martin (2004) examined the extent to
which the given regime of adult female and juvenile mortality,
which was derived for average conditions, would have to change
in order to throw the 'generic' population into a decline
Joubert and Mostert (1975) 'felt' that there were no more
than 50 reedbuck in the country in 1975. This was at a time
when the cumulative rainfall deviations were in a deficit
mode. Whilst placing no weight on the survey estimates for
17), the species was recorded in modest numbers from 1978
to 1994. During this time the cumulative surplus/deficit in
rainfall was positive. From 1995-2002 the long term rainfall
regime swung into a deficit mode coinciding with concerns
expressed about the status of reedbuck.
Estimates of lechwe numbers (Figure
18) were consistently higher than 4,000 up until 1995,
coinciding with a peak in the cumulative rainfall surpluses.
The drop from 13,000 estimated in 1980 to 4,000 estimated
in 1986 appears to track the long term rainfall regime fairly
closely. It seems that after 1986, the lechwe numbers were
unable to bounce back immediately to follow the secondary
peak in cumulative rainfall surpluses which occurred around
1990. After 1996, the long term rainfall moved into a deficit
mode and the lechwe population appears to have crashed. In
1998, the estimate was only 119 animals on a comprehensive
survey of the Caprivi.
19 lechwe numbers are shown in relation to the cumulative
surplus/deficit rainfall record for Andara beginning in 1915.
It is of interest to note that in 81 years only one complete
cycle of surpluses and deficits has occurred (1914-1977).
A second cycle is underway at present but it would appear
to be less than half-way through its time. Early narrative
accounts of lechwe (Shortridge 1934) suggest that lechwe populations
were high in the 1930s - which coincided with a positive half-cycle
in the rainfall lasting from 1914-1944. Child and von Richter
(1969) remark that in the late 1960s lechwe were in decline
in northern Botswana and the Caprivi. Joubert and Mostert
(1975) estimated that there were fewer than 100 lechwe in
Namibia in 1975. This was during a negative rainfall cycle
lasting from 1945-1977. The early part of the survey record
for lechwe shown in Fig.18 coincided with a rainfall surplus
period from 1978-1984 and, during this time, lechwe were fairly
abundant. Having now entered a deficit mode, it would seem
that lechwe populations are once more in a parlous state.
If, as it seems, this long term rainfall regime is significant
in regulating lechwe populations, then there is little that
can be done to assist the species in its natural range. The
best strategy would be to protect a small nucleus of the animals
so that when the rainfall moves once more into a surplus mode,
the population can begin to increase.