The insular biogeography of small Bahamian cays

1 The species occurring on 128 small cays in the central Exumas and 54 small cays near Andros, Bahamas, were surveyed. 2 Step-wise multiple linear regressions selected the log transformations of vegetated area or total area as the variables explaining the most variation in species number at Andros. In the Exumas, the ratio of vegetated area to total area, which is an expression of both insular area and exposure, was the most frequent first-selected variable. 3 In both archipelagoes, cays with vegetation were significantly larger, higher and closer to a mainland island than cays naturally devoid of vegetation. 4 High degrees of nestedness were observed in both archipelagoes, both at the level of the entire species assemblage and at the level of individual species. Ninety per cent of the species that did not display significant nested patterns were rare species, occurring on less than or equal to 3 cays each. 5 Step-wise multiple logistic regressions revealed that the incidence of most species was positively related to some measure of area (14 of 14 species in the Exumas, 8 of 12 species on Andros) and negatively related to distance (12 of 14 species in the Exumas, 3 of 12 species on Andros). Correct classification was > 75% for all but one species. 6 Species turnover was measured for 77 cays in the Exumas, which were surveyed annually over a 4-year period. Turnover was very low, and most of the observed turnover was due to immigrants that never became established. Contributing factors to the observed low turnover included the absence of annuals and the physical stability of these predominantly rocky cays. 7 Propagules of Sesuvium portulacastrum were introduced to 10 cays naturally lacking vegetation. After 40 months, S. portulacastrum was still present on nine of 10 cays and had increased in vegetated area from approximately 1 m2 up to 650 m2, indicating that non-vegetated cays were physically able to support plant life, at least in the short term. 8 A 'small island effect' (i.e. a weak species-area relationship on very small islands) was apparent at Andros and probably resulted from smaller cays being washed over by waves.