An investigation of topo-moisture gradients in the eastern Karoo, South Africa, and the identification of factors responsible for species turnover

Using the methods of direct gradient analysis, we investigated some of the factors responsible for species turnover in the dwarf shrubland/grassland interface of the eastern half of the Nama-Karoo biome, South Africa. The analysis suggested that taxa are distributed along topographic/moisture gradients, with two broad syntaxonomic classes differentiated from one another by substratum. If the patterns of species turnover on the two major substrata (sandstone and dolerite) differ significantly, taxa may be responding to environmental stresses other than elevation and rainfall. The null hypothesis being that if the two substrata parallel one another along a topographic/moisture gradient, the species replacement rates should be similar. The null hypothesis was rejected as species replacement rates on sandstone at high rainfall and high elevation are higher than on dolerite at comparable environmental conditions. The differences are attributable to species-poor degraded sites in topo-moisture class C (1200-1300 m elevation and 300-350mm median annual rainfall). The topo-moisture class has a high coefficient of variation in mean annual rainfall. Topo-moisture class C on sandstone pediments supports a degraded flora. The high entropy of the samples suggests that the flora is well-adapted to coping with the climatic uncertainty of this region. There does not appear to be an encroaching karoo front, but rather a flora of generalist species which are successful at coping with the high coefficient of variation in precipitation. It is suggested that if environmental conditions change due to global warming, the intruding front will only extend into areas of increased climatic uncertainty. Pastoral management strategies which increase environmental uncertainty should also be discouraged