Vascular Plant Species Diversity on Two Barrier Islands in Southwest Florida

Abstract

The equilibrium theory of island biogeography was examined in relation to the indigenous vascular floras of two barrier islands of different size and distance from the southwest Florida mainland. On Cayo-Costa Island, which has a land area of 5.6 km 2 and is located 12.8 km from the mainland, 261 species were identified. On Sanibel Island, which has a land area of 43.6 km 2 and is located 3.2 km from the mainland, an inventory in the mid-1950's identified 266 species. The similar number of species, despite the differences in island area and distance from the mainland, was attributed to the incompleteness of the inventory of Sanibel in the 1950's. An inventory of Sanibel from 1965 to 1985 identified 432 native species. This almost two-fold difference in the number of species on Sanibel as compared with Cayo-Costa is more consistent with equilibrium theory which predicts a significantly greater number of species on a larger island located closer to the mainland. However, comparison of the species-area relations of Cayo-Costa and Sanibel with other studies of island floras suggests that Sanibel should have had even more species. Urban development and the spread of alien plant species following the construction of a causeway linking Sanibel to the mainland in 1963 may have reduced the number of native plant species on Sanibel.