Biodiversity, habitat and range-size aspects of a flora from a winter-rainfall desert in north-western Namaqualand, South Africa

We examined patterns of richness and endemism in relation to phylogeny (family membership), growth form and habitat in the flora of an arid (< 60 mm annual rainfall), sandy coastal belt between Port Nolloth and Alexander Bay on the north-west coast of Namaqualand, South Africa. This region forms part of the species-rich Succulent Karoo biome. In an area of 750 km(2), 300 plant species were recorded with 24% endemnicity. This level of diversity and endemism is exceptionally high for a desert region. Plant distribution is strongly influenced by habitat type. Diversity, especially of endemic species, was concentrated on rocky or unusual (lag-gravel pavements) habitats. Endemics were not random phylogenetic nor biological assemblages. Dwarf leaf-succulent shrubs, especially Mesembryanthemaceae, were significantly over-represented amongst the endemic component. The results are discussed in terms of a model for plant diversification in an arid environment with relatively predictable sources of moisture.