The role of island physiography and oceanographic factors in shaping species richness and turnover of nesting seabird assemblages on islands across the southeastern Pacific

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 α and β-diversity of nesting seabird assemblages.
Location Southeastern 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 (α-diversity). We
applied multivariate GLM to test the effects of physiographic and
oceanographic predictors on species composition (β-diversity). We used
Jaccard dissimilarities on species occurrences per island to calculate
β-diversity partitioned into turnover and nestedness. Polynomial models
allowed us to model these metrics against geographical and environmental
gradients and so analyze patterns in seabird β-diversity across spatial
scales. Results  Species richness was highest in Galápagos,
Pitcairn, and Rapa Nui. Changes in seabird α-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 β-diversity. Changes in β-diversity were mostly
due to species replacement (β-turnover) across three major island Systems
(Galápagos Archipelago, Chilean coastal islands, and oceanic islands of
the southeastern Pacific). The contribution of β-nestedness was restricted
to small scales (within archipelagos). Main conclusions Physiographic and
oceanographic factors explain species diversity of seabird assemblages on
islands of the southeastern 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.