Biogeography of the Puerto Rican Bank: Species‐Turnover on a Small Cay, Cayo Ahogado

The equilibrium species—number of plants on Cayo Ahogado, a small sand cay east of Puerto Rico, is 11.2 if all species are considered, and 6.8 if only established species are counted. Establishment is defined in terms of reproductive success and presence on the island for at least two consecutive visits. There were only five species in the permanent flora. Species—turnover rates of plants were high, migration being on the average 14.4 species/year (all species), or 1.6 species/year (established species only). Extinction rates were 17.2 and 0.8 species/year for total species and established species respectively. Immigration is chiefly by flotation, particularly of propagules released by flood waters in Puerto Rico; extinction is caused by failure to find suitable habitat, wave erosion, and hurricanes. Nine individuals of three species of lizard arrived, probably by flotsam as time of arrival correlated with flooding in Puerto Rico, and persisted for varying times up to 5 months. No reproduction occurred. Species turnover of invertebrates was high. Eighty—one species (77 terrestrial, 1 marine water strider, and 3 semiterrestrial crustacea) were recorded from the cay. Equilibrium number was 9.8 for established species and 14.2 for total species. Direct flight was probably the most imporrant mode of immigration for the entire fauna because of the large number of strong fliers among the ephemerals. If only established species are considered, flotsam transport becomes important. Human transport and passive dispersal by wind were relatively insignificant. Trophic structure of the fauna remained relatively constant although taxonomic structure varied. It is postulated that species—numbers on small cays are determined in part by the type of trophic structure which can be built upon available energy reserves, and that establishment of immigrants is influenced by whether they fit into the trophic scheme.