AVIFAUNAL DIVERSITY AND ENDEMISM ON TROPICAL INDIAN-OCEAN ISLANDS

Species diversity and endemism of resident breeding birds (excluding pelagic and introduced species) on fourteen tropical Indian Ocean archipelagos were related to seven geographic variables using stepwise linear regression analysis. The total number of species within an archipelago and the numbers of continental species (species found in source areas) and Indian Ocean endemics (species restricted to the study area but found within more than one archipelago) were related positively to the number of large islands and total land area. The number and proportion of continental species were predicted also by proximity to source areas and decreasing elevation. The number and proportion of endemic species (those found only within a single archipelago) were related positively to elevation. Relationships of species numbers and proportions in each species category to geographic variables were generally similar to those found in the known avifauna of the tropical Pacific region, but three differences between the two regions were evident. The avifauna of the Indian Ocean region (1) was much less diverse overall, (2) had proportionately more continental and fewer endemic species, and (3) had several families that were much more poorly represented despite their diversity in source areas. These differences may be interpreted by recognizing the importance of island isolation and area in contributing to both speciation and diversity. Increasing isolation decreases local (within-archipelago) diversity through lower colonization rates, but may at least partially compensate for this decrease on a regional (among-archipelago) scale through increased rates of speciation. Increasing area increases both local and regional diversity by providing space for more populations and by decreasing extinction rates.