Primary factors

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 factor.
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Secondary factors

1. 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 This simply results in an ongoing need for such interventions which, when they are withdrawn, result in the situation reverting to the status quo.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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2. 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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3. Veterinary 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.
• 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 fence (Figure 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.
With the low densities of the populations of all three species in the Caprivi and Bushmanland linkages with the Botswana population are highly desirable.
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4. 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 Tsumkwe. All 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.
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5. Fire: The existing fire regimes in the Caprivi (see 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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6. 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).