ISLAND BIOGEOGRAPHY - EFFECT OF GEOGRAPHICAL ISOLATION ON SPECIES COMPOSITION

Island biogeography theory attempts to explain and predict among-islan d variation in species richness. However, two islands with the same nu mber of species may still differ from each other considerably in their species composition. In this study we test the hypothesis that among- island variation in species composition is predictable and can be rela ted to the corresponding differences in distance to the mainland. We f ocus on woody plants inhabiting islands in the Clarks Hill Lake, a res ervoir completed in 1954 on the Savannah River, between Georgia and So uth Carolina, USA. Two groups of islands were sampled: islands that we re logged prior to the filling of the reservoir and islands that were not logged. Each island was surveyed for the presence of all tree and shrub species, and its distance from the mainland was determined. In b oth groups of islands, the degree to which two islands are similar in their species composition was negatively and significantly correlated with their difference in distance to the mainland. Species richness, h owever, was correlated with distance to the mainland only on logged is lands. We conclude that geographic isolation may affect species compos ition on islands, and that such an effect may occur even in the absenc e of a corresponding effect on species richness.