Island biogeography: effect of geographical isolation on species composition

Island biogeography theory attempts to explain and predict among—island variation in species richness. However, two islands with the same number 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 related to the corresponding differences in distance to the mainland. We focus on woody plants inhabiting islands in the Clarks Hill Lake, a reservoir completed in 1954 on the Savannah River, between Georgia and South Carolina, USA. Two groups of islands were sampled: islands that wee 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 both 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, however, was correlated with distance to the mainland only on logged islands. We conclude that geographic isolation may affect species composition on islands, and that such an effect may occur even in the absence of a corresponding effect on species richness.