Rainfall and the decline of a rare antelope, the tsessebe (Damaliscus lunatus lunatus), in Kruger National Park, South Africa Original Research Article

The number of tsessebes in Kruger National Park, South Africa, increased during 1977–1985, but declined after 1986. We used tsessebe number and age structure to determine year-to-year variation in adult survival. Adult survival rate was positively correlated with dry-season rainfall (a measure of grass productivity during the dry season) and with the cumulative rainfall surplus. Juvenile survival rate (as indexed by the juvenile:female ratio) was also correlated with the cumulative rainfall surplus. Tsessebe feed in broad, grass-covered drainage lines within Colophospermum mopane shrubland on basaltic soils. Here drainage is slow and the cumulative rainfall surplus or deficit, relative to the mean annual rainfall, is an index of dry-season soil moisture, which, in turn, determines dry-season grass productivity and hence the food supply for tsessebe. Adult survival rate was density-dependent, indicating that there was intraspecific competition for food. When the relationships between survival rates and rainfall were used in a model of tsessebe population dynamics to predict juvenile and adult survival rates from the recorded annual and dry-season rainfalls, the modelled population changes were similar to those observed. We conclude that changes in tsessebe numbers probably resulted from rainfall-induced changes in food availability during the dry season, which caused adult survival to decline after 1986. The principal management implication is that the tsessebe decline can be reversed only by several successive years of above-average annual rainfall. The importance of green grass during the dry season as a key resource is emphasised, not only for tsessebe, but also for other African antelopes that graze selectively.