THE INFLUENCE OF COLONIZATION IN NESTED SPECIES SUBSETS

Biotic communities inhabiting collections of insular habitat patches o ften exhibit compositional patterns described as ''nested subsets''. I n nested biotas, the assemblages of species in relatively depauperate sites comprise successive subsets of species in relatively richer site s. In theory, nestedness may result from selective extinction, selecti ve colonization, or other mechanisms, such as nested habitats. Allopat ric speciation is expected to reduce nestedness. Previous studies, bas ed largely on comparisons between land-bridge and oceanic archipelagos , have emphasized the role of selective extinction. However, colonizat ion could also be important in generating strong patterns of nestednes s. We apply a recently published index of nestedness to more than 50 i sland biogeographic data sets, and examine the roles of colonization, extinction, endemism, and, to a limited extent, habitat variability on the degree on nestedness. Most data sets exhibit a significant degree of nestedness, although there is no general tendency for land-bridge biotas to appear more nested than oceanic ones. Endemic species are sh own to generally reduce nestedness. Comparisons between groups of non- endemic species differing in overwater or inter-patch dispersal abilit y indicate that superior dispersers generally exhibit a greater degree of nestedness than poorer dispersers, a result opposite that expected if colonization were a less predictable process than extinction. Thes e results suggest that frequent colonization is likely to enhance nest edness, thereby increasing the compositional overlap among insular bio tas. The prevalence of selective extinction in natural communities rem ains in question. The importance of colonization in generating and mai ntaining nested subsets suggests that (1) minimum critical areas will be difficult to determine from pat terns of species distributions on i slands; (2) multiple conservation sites are likely to be required to p reserve communities in subdivided landscapes, and (3) management of di spersal processes may be as important to preserving species and commun ities as is minimizing extinctions.