Floristic diversity in the Cape Flora of South Africa

Comprising a land area of ca 90000 km(2), less than 4% of the total land area for the Southern African subcontinent, the Cape Floristic Province is one of the world's richest areas in terms of botanical diversity for its size. An estimated 8650 species of vascular plants occur in this area, about 65% of which are endemic. This is about 42% of the estimated total for all of southern Africa. The number of species packed into so small an area is remarkable for the temperate zone, and compares closely with species totals for areas of comparable size in the wet tropics. The Cape Region consists of a mosaic of sandstone and shale substrates that give rise to soils of quite different types, and in addition local areas of limestone add to the edaphic diversity. Climates across the region are extremely variable, and the predominant orographic rainfall pattern ranges from 2000 mm locally to less than 100 mm, often with extremely steep gradients, the result of a mountainous landscape. The edaphic diversity resulting from a mosaic of different soils is compounded by sharp local gradients in precipitation that creates an unusual number of local habitats. A feature of the some of the soils in the Region is low nutrient levels and many of the plants on such soils have low seed dispersal capabilities, a factor important in explaining the high levels of local endemism. Species richness in the Cape Region is hypothesized to have resulted from the presence of a complex mosaic of diverse habitats and steep ecological gradients against a background of relatively stable climate and geology after the mediterranean climate was established there sometime after the beginning of the Pliocene. A local or ecological mode of speciation may have been more important under these conditions than allopatric speciation.