The Principle of Nested Subsets and Its Implications for Biological Conservation

Abstract

Abstract: The nested subset hypothesis states that the species comprising a depauperate insular biota are a proper subset of those in richer biotas, and that an archipelago of such biotas, ranked by species richness, presents a nested series. The pattern characterizes the distributions of mammals in three different archipelagos, and it appears more strongly developed among faunas in the process of, relaxation (landbridge islands) than in those derived by overwater colonization (oceanic islands). The generality of the nested subset pattern and factors that may produce it are evaluated using distributions of land birds in the New Zealand region. Monte Carlo simulations show that species composition of these insular avifaunas is highly nonrandom, exhibiting significantly nested structure. However, avifaunas on nine oceanic islands lack the structure typical of all islands together or of the 22 landbridge islands. Thus, distinctive patterns of species composition, as well as species number, may distinguish landbridge and oceanic islands.Because they are isolated fragments of once‐continuous distributions, landbridge islands and their biotas have important implications for long‐term biological conservation in preserves. Real or virtual islands that have undergone faunal relaxation support only a fraction of the species expected in equivalent mainland areas. Furthermore, the species inhabiting fragments are not a random collection of those in the source pool, but are rather nested subsets of the species in richer, more intact biotas. The species preserved in such fragments tend to be the most abundant, generalist species that are least in need of special protection.