Assessing nestedness for insular bird assemblages: contrasting island area and isolation for multiple dimensions of biodiversity

The Theory of Island Biogeography has focused on the effects of island area and isolation as determinants of the taxonomic diversity found in these ecosystems. However, species are not independent from each other in their evolutionary and ecological relatedness. Thus, it is essential to use an approach that simultaneously considers the multiple dimensions of biodiversity to understand the patterns of diversity in insular assemblages. We tested the effects of island area and isolation from the mainland in taxonomic, phylogenetic and functional nestedness among nine insular bird assemblages in southern Brazil. We used a framework designed for dealing with nested structures of communities, in which species-poor sites are expected to be subsets of the richest ones. We compiled literature data on bird species composition for each island, and assessed the phylogenetic and functional nestedness through a phylogenetic tree and a dendrogram of species similarities, respectively. We found that, for these three components of biodiversity, island area is the main driver of nestedness among assemblages, indicating that smaller islands are not only inhabited by subsets of species, but they also present a subset of evolutionary lineages and functional diversities. Results on the influence of isolation on community structure were all non-significant. Island area is thought to be a proxy for habitat diversity, thus it is likely that a subset of diversity at smaller islands arises as consequence of their low diversity of habitats, which in turn should reflect a lower number of bird species and low phylogenetic and functional diversities.