THE RELATIONSHIP BETWEEN NESTED SUBSETS, HABITAT SUBDIVISION, AND SPECIES-DIVERSITY

Biotic assemblages are said to be nested when the species making up re latively species-poor biotas comprise subsets of the species present a t richer sites. Because species number and site area are often correla ted, previous studies have suggested that nestedness may be relevant t o questions of how habitat subdivision affects species diversity, part icularly with respect to the question of whether a single large, conti guous patch of habitat will generally contain more species than collec tions of smaller patches having the same total combined area. However, inferences from analyses of nestedness are complicated by (1) variabi lity in degrees of nestedness measured in natural communities, (2) var iability in species-area relationships, and (3) the fact that nestedne ss statistics do not account for the size of habitat patches, only in the degree of overlap among sites with different numbers of species. B y comparing various indices of nestedness with a ''saturation index'' that more directly measures the effect of habitat subdivision, it is s hown that the first two of these factors are not as important as the t hird. Whether a single large site or several smaller ones having the s ame total combined area maximizes species diversity is dependent on (1 ) overlap in species composition among sites and (2) the number of spe cies per unit area in the different sites. Because nestedness indices do not account for species number at a site, they cannot accurately pr edict how habitat subdivision affects species diversity patterns. Stil l, nestedness analyses are important in that they indicate the degree to which rare species tend to be found in the largest, or the most spe cies-rich, sites, patterns not revealed by the saturation index. Both types of analysis are important in order to obtain a more complete pic ture of how species richness and compositional patterns are influenced by habitat subdivision.