Investigating causality of nestedness of insular communities: Selective immigrations or extinctions?

Most archipelagos are comprised of a nested set of communities, with species on depauperate islands representing proper subsets of those on richer islands. The causality for this common intra-archipelago pattern, however, remains elusive. Here, I present a Monte Carlo approach for investigating whether nestedness results from selective extinctions, selective immigrations, or both. Results for mammal communities of three nearshore archipelagos and two montane forest archipelagos suggested that nestedness may result from both forces. Nestedness was significantly associated with area (extinctions) for all archipelagos studied, and with isolation (immigrations) for islands of Lake Huron and the American Southwest. The degree of nestedness is influenced by the degree of variations among species and islands. In addition, our ability to assess causality of nestedness is influenced by our ability to calculate biologically relevant measures of isolation, and by the potentially confounding effects of selective immigrations and extinctions on community nestedness.