Aim To evaluate the role of habitat heterogeneity on species richness and turnover in the mega species-rich Cape Floristic Region (Cape), the mediterranean region of southern Africa.
Location The Cape and Drakensberg regions of southern Africa.
Methods Bioclimatic data were used to estimate habitat amount and habitat heterogeneity in the Cape and Drakensberg regions; these data were then used to explain the patterns of species diversity in the Pentaschistis clade (Poaceae) in these two regions. Habitat variables were used to create ‘bioclimatic units’ to characterize 1′× 1′ cells in southern Africa and to describe the niches of species. Using these bioclimatic units, the niche and range sizes of species in the two regions were compared. A phylogram was used to test for age and lineage effects.
Results Pentaschistis species diversity and turnover are higher in the Cape than the Drakensberg. There is no significant difference in the habitat amount and heterogeneity between the two regions. Species occupy the same range of estimated niche sizes, yet there are significantly more range-restricted Pentaschistis species in the Cape.
Main conclusions The roles of age- and lineage-related effects are rejected; biodiversity differences lie in the regions. Current macrohabitat does not explain the differences in biodiversity between the two regions. The larger number of range-restricted species in the Cape cannot be explained by dispersal mechanism or the occupation of range-restricted habitats. Species of Pentaschistis and other Cape clades share characteristics associated with species from historically climatically stable areas, and palaeoclimatic and palaeontological evidence indicates the Cape climate has been more stable than the Drakensberg climate throughout the Pleistocene. We conclude that the corresponding lack of extinction might have allowed an accumulation of species in the Cape. Similar climatic and biological evidence for the south-west Australian Floristic and Mediterranean regions indicate that the same mechanism might explain the high species richness of these mediterranean regions.