Hierarchy of factors limiting roan, sable, tsessebe
Figure12: Roan, sable tsessebe potential range and
Figure 10: The location of veterianry control fences
Within the hierarchy of
limiting factors the primary limitation for roan, sable
and tsessebe populations is likely to be a deficit
in the accumulated rainfall.
- All management efforts directed at secondary factors are
unlikely to surmount this fundamentally negative effect.
- a surplus in the accumulated rainfall need not necessarily
produce a linear increase in population growth rates - it
should rather be seen as the removal of a primary limiting
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- Competition with other species: Management interventions
aimed at reducing competition with other species do not
appear compatible with the general aim of increasing biological
diversity. This simply results in an ongoing need for such
interventions which, when they are withdrawn, result in
the situation reverting to the status quo. However, the
specific case of elephants may be an exception. The
negative influence of large numbers of elephants on the
habitats required by the three species is likely to
be the most severe limiting factor after rainfall.
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- Human settlements: Within the Caprivi and to the
west of Khaudum Game Reserve the ad hoc location and spread
of human communities and their cattle is resulting in loss
of wildlife range and direct competition for grazing resources.
Situations exist in the Caprivi where, to maintain linkages,
it would be highly desirable to establish or secure roan,
sable and tsessebe populations in certain communal lands
but ad hoc settlement is proving to be a debarring constraint.
It is unlikely that populations of roan, sable and tsessebe
can be established in areas where human densities exceed
10 persons/km2 (Figure
12).Unplanned human settlement is likely to be a direct
threat to the long term survival of the species. (see
limiting factors for buffalo)
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fences are an important limiting factor in their
influence on movements of roan, sable and tsessebe between
Botswana and Namibia. Many populations are becoming isolated
as a result of the placement of fences.
With the low densities of the populations of all three species
in the Caprivi and Bushmanland linkages with the Botswana
population are highly desirable.
- Roan Sable and
Tsessebe are water dependent and stay close to surface
water. In the Caprivi, this means they are tied to
the large rivers for a large part of every year.
- The subpopulations in the west of the Caprivi (Mahango
and the western "Core Area") are effectively isolated
from the remainder of the Caprivi by the arid terrain
in the central part of the Caprivi Game Reserve and also
effectively isolated from Botswana by the veterinary
10) along the international boundary.
- If settlement and subsistence agriculture continue
to develop in the vicinity of the Kwando River in Namibia,
the subpopulations in Mudumu and the western "Core Area"
of Caprivi Game Reserve will become isolated and will
be linked only through Botswana.
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Artificial water: More habitats could be made
available for roan, sable and tsessebe by the supply of
artificial water particularly in the Caprivi Strip and
three species are waterdependent and seldom move further
than a few kilometres from surface water. In the Caprivi,
this means they are tied to the large rivers for a large
part of every year. This limits the ability of populations
in the eastern and western ends of the Caprivi Game Reserve
to maintain contact and, in conjunction with the veterinary
fences along the Botswana border and a hostile environment
in Angola, could result in the total isolation
of various subpopulations.
The development of game water supplies in the large Kalahari
Sands area of the Caprivi Game Reserve would not be simple:
- the average depth of water below the surface as varying
from as much as 300 metres in the west of the Caprivi
Strip to 35 metres in the east (Mendelsohn and Roberts
1997, page 39).
- most of the boreholes which have been sunk in the
area are non-functional or
- provide only small quantities of water. (However,
this latter feature might prove valuable to roan, sable
and tsessebe: if large amounts of water were available
it is likely that the water points would be captured
by elephants and buffalo herds.)
Garstang (1982) observes that tsessebe have 'aesthetic
preferences' for certain types of surface water and will
generally avoid drinking at concrete water troughs.
Fire: The existing fire regimes in the Caprivi
limiting factors for buffalo) and the resulting loss
of grazing are likely to nullify the marginal gains which
roan, sable and tsessebe might get from careful use of fire
to return areas to grassland, where bush encroachment has
occurred through excessive cattle grazing. Unlike roan and
sable tsessebe are
attracted to burns- although they will seldom move more
than short distances in order to feed on a burn (Joubert
and Bronkhorst 1977).
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Disease: Roan, sable and tsessebe may be susceptible
to various diseases of which anthrax is likely to be the
most serious (Pienaar 1961). De Vos and Imes (1976) document
a rare skin disease contracted by sable in a holding facility
in Kruger National Park. However, there is no evidence in
the literature that disease has ever been a significant
limiting factor for these species and, if it were, there
is little in the way of management measures available to
mitigate the effects (de Vos et al 1973). Together, predation
and disease tend to be secondary factors acting on undernourished
animals. Disease may differentially affect juveniles but
the resultant mortality is likely to cause population fluctuations
rather than any long term alterations to basic population
growth rates (Sinclair 1974b).