Abstract

Aim For seabirds, food supplies and nest sites are largely driven by oceanographic gradients and island habitats, respectively. Research into seabirds' ecological roles in insular ecosystems is crucial to understanding processes that structure seabird nesting assemblages. We examined the influence of island physiography and oceanographic factors on the spatial variation in alpha- and beta-diversity of nesting seabird assemblages. Location South-eastern Pacific Ocean. Taxon Birds. Methods We compiled data from 53 seabirds breeding on 41 coastal and oceanic islands using different sources: our field records, online databases, environmental reports and literature. We used generalized linear models (GLM) to describe the effect of island physiography (area, elevation and isolation) and oceanographic factors (surface temperature, salinity and primary productivity) on seabird species richness (alpha-diversity). We applied multivariate GLM to test the effects of physiographic and oceanographic predictors on species composition (beta-diversity). We used Jaccard dissimilarities on species occurrences per island to calculate beta-diversity partitioned into turnover and nestedness. Polynomial models allowed us to model these metrics against geographical and environmental gradients and so analyse patterns in seabird beta-diversity across spatial scales. Results Species richness was highest in Galapagos, Pitcairn and Rapa Nui. Changes in seabird alpha-diversity across islands were determined by island area and distance to South America but not by oceanographic variables. Physiographic and oceanographic factors were significant in determining beta-diversity. Changes in beta-diversity were mostly due to species replacement (beta-turnover) across three major island Systems (Galapagos Archipelago, Chilean coastal islands and oceanic islands of the south-eastern Pacific). The contribution of beta-nestedness was restricted to small scales (within archipelagos). Main conclusions Physiographic and oceanographic factors explain species diversity of seabird assemblages on islands of the south-eastern Pacific. Oceanographic variables did not affect species richness but significantly influenced species composition. Change in species composition reflects gradients across three marine biogeographical realms: Temperate South, Eastern Indo-Pacific and Tropical Eastern Pacific. The low degree of species nestedness may reflect multiple evolutionary origins.