Species pool effects on niche-based island biogeography : insights from the bird communities in Melanesian Archipelagos

A fundamental goal of biogeography is to determine how communities of species assemble. Islands offer an excellent opportunity to study community assembly because they have discrete boundaries, their species compliment can be readily quantified, and biogeographic processes operating at broad spatial scales can be separated from the effects of local interactions. Indeed, islands have contributed extensively to the development of biogeographical theory. Historically, the focus of island biogeography studies has been to predict the species richness of their communities. Yet, species richness provides no information about the ecological differences among the species that comprise a community, and thus cannot explain why different types of species do, or do not, appear in island communities. The niche of any particular species represents all of the abiotic and interspecific interactions that determine the survival of that species in a given location. Thus, a niche-based description of island communities differs from a richness-only description as it can link biogeography to the ecological interactions that ultimately drive successful colonisation and evolution on an island. However, island isolation and island area - the key predictors of species immigration and extinction, respectively - and the influence of the composition of the regional species pool have rarely been investigated for their effect on niche differences among species occupying an island. Understanding how the regional species pool, isolation and area influence niche-based diversity on islands provides an important link between biogeography and the ecological interactions that ultimately determine the composition of island communities. I used data on the island birds across Melanesian archipelagos to explore the role of biogeography in driving the niche-based structure of island communities. Specifically, I studied how the composition of the regional species pool, island isolation and island area influence: i) variation in a key functional trait (body size), ii) occupancy and saturation of niches, and iii) the phylogenetic structure of island communities. To do so, I studied three Melanesian archipelagos; the Bismarck, Solomon and Vanuatu islands, each of which represent a separate biogeographic region with a different degree of isolation from the primary source of avian colonists, mainland New Guinea. As such, the influence of oceanic dispersal barriers on the composition of the avifauna of each archipelago can be clearly distinguished. Using this model system, I demonstrate that the effects of island isolation and island area extend beyond determining the number of species on an island to influencing maximum body size, niche occupancy and the phylogenetic structure of island communities. Further, the research I describe in this thesis demonstrates that the influence of island isolation and area on the ecological and evolutionary differences among species that comprise each island community is mediated by the diversity of the regional species pool. Thus, the species observed in island communities, and the niches they occupy, may not be a product of local ecological interactions. Rather, immigration from the regional species pool may determine the persistence of a species on an island.Doctor of Philosoph