The roles of island area per se and habitat diversity in the species±area relationships of four Lesser Antillean faunal groups

1. We analysed the relationships between species richness, island area, and habitat diversity for birds, bats, butter ̄ies, and reptiles and amphibians on 19 islands in the Lesser Antilles. Habitat diversity was quanti®ed by Simpson's index based on the total areas of ®ve vegetation types on each island. Island area varied over two orders of magnitude (13±1510 km) and habitat diversity varied between 1 and 3 ́7 equivalents of equally abundant habitat types. 2. Because the Lesser Antilles consist of an inner arc of high, volcanic islands and an outer arc of low-lying islands formed of uplifted marine sediments, correlations between area and elevation (r=0 ́32) and between area and habitat diversity (r=0 ́40) were weak. Habitat diversity was, however, strongly correlated with maximum island elevation (r=0 ́85). 3. Simple correlations of species richness with island area were signi®cant for all four faunal groups, and simple correlations of species richness with elevation and habitat were signi®cant for all groups except bats. In multiple regressions of species richness on area and habitat diversity together, area was a signi®cant e€ect for birds and bats, and habitat diversity was a signi®cant e€ect for birds, butter ̄ies, and reptiles and amphibians. 4. These results suggest that the four Lesser Antillean taxonomic groups di€er in their responses to area and habitat diversity. For butter ̄ies and for reptiles and amphibians, the relationship of species richness to area is probably a fortuitous consequence of a relationship between habitat diversity and area. Bird species richness responds independently to both habitat diversity and area, and bat species richness is in ̄uenced by area but not by habitat diversity. 5. We suggest that this variation is related to di€erences in several biological traits of the di€erent faunal groups. Strong habitat-diversity e€ects are likely in taxa with high degrees of habitat specialization, populations large enough to have a low probability of stochastic extinction, life-cycles that include a resistant resting stage that reduces vulnerability to catastrophic extinction, or a combination of these traits. In contrast, strong area e€ects are likely in taxa with weak habitat specialization, low population density, or both. 6. At least in Lesser Antillean birds, it is unlikely that immigration depends on island size. Therefore, the species±area relationship for birds is probably generated by island-size-dependent extinction. Among the four taxonomic groups we studied, only butter ̄ies are likely to show a `rescue e€ect' stemming from frequent between-island movement of individuals, as only butter ̄ies exhibited low levels of endemism and lacked a unique area e€ect for species richness.