ON THE TEMPORAL DEVELOPMENT OF NESTED SUBSET PATTERNS OF SPECIES COMPOSITION

Certain assemblages of insular vertebrates exhibit "nested subsets," where small biotas contain a nonrandom subset of the species in richer ones. As species are added to and removed from such communities via colonization and extinction, changes in the degree of nestedness can be expected. Such temporal changes were examined by comparing modern distributions of plants, arthropods, and various vertebrate groups with available historical records. The time scales for these comparisons ranged from post-glacial time periods to months and days. Nested subset patterns were found for various taxonomic groups, being especially marked in fragmented biotas thought to be shaped by local extinctions. However, several biotas assembled by colonization and/or dispersal also exhibited nested patterns. Comparisons of the same communities at different time periods generally showed little change in degree of "nestedness". The prevalence of nested subset patterns in assemblages shaped by both extinction and colonization indicates that this type of structure may be a more general property of communities, rather than a specific attribute of islands undergoing biotic relaxation.