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 (fam ily membership), growth form and habitat in the flora of an arid (< 60 mm a nnual rainfall), sandy coastal belt between Port Nolloth and Alexander Bay on the north-west coast of Namaqualand, South Africa. This region forms par t of the species-rich Succulent Karoo biome. In an area of 750 km(2), 300 p lant species were recorded with 24% endemnicity. This level of diversity an d endemism is exceptionally high for a desert region. Plant distribution is strongly influenced by habitat type. Diversity, especially of endemic spec ies, was concentrated on rocky or unusual (lag-gravel pavements) habitats. Endemics were not random phylogenetic nor biological assemblages. Dwarf lea f-succulent shrubs, especially Mesembryanthemaceae, were significantly over -represented amongst the endemic component. The results are discussed in te rms of a model for plant diversification in an arid environment with relati vely predictable sources of moisture.