Variation in species richness among the offshore islands of the southwestern Cape

Abstract

Fifteen islands lying off the southwestern Cape coast between the estuary of the Olifants River and Cape Agulhas were faunistically and floristically surveyed in the last few years. The statistical relationships between species richness, island area and distance from the mainland were examined for all visible taxa taken together and for the main taxonomic subsets. Distance from the mainland which never exceeds 9 km has no apparent effect on island species richness. In the species/area regression analysis the slopes have values varying between 0,49 for spiders and 0,77 for plants and the overall slope is 0,80. These values are apparently the highest ever reported in island biogeographic studies. The results are believed to be due to the small size (1 - 222 ha) of the islands studied coupled with a very high incidence of ecological disturbance on the smaller islands.