Marine subsidies and island biogeography are equally important in shaping temperate island bird communities

Background/Question/Methods

Dispersal limitation, environmental filtering, and biotic interactions are the main mechanisms driving species’ distributions across landscapes. These mechanisms are deeply intertwined with processes underlying island species immigration and extinction rates–rates that are central to predicting how species assemble according to the classical theory of island biogeography. Island isolation and size influence immigration and extinction rates, which vary with species' dispersal ability, responses to environmental conditions, and biotic interactions. Recent work has suggested that these rates also vary in response to nutrient inputs from the ocean. Particularly on small islands, marine nutrient inputs can increase terrestrial productivity. This increase corresponds with higher resource availability, minimizing environmental filtering. Alternatively, with an increase in resources, certain traits may allow some species to outcompete others and become dominant, altering community composition. In this study, we fit a spatially explicit joint species distribution model to evaluate the effects of island biogeography, marine subsidies, species traits, and species interactions on the community composition of terrestrial breeding birds on 89 temperate islands. Along with providing estimates for relative contributions, this approach provides species-specific responses to environmental parameters, to one another, and gives an estimate for the proportion of variation due to species’ biological traits.

Results/Conclusions

Physical characteristics related to island biogeography explained 23.5% of variation in species distributions, while marine influences explained 24.8%. We also found variation in species-specific responses to both island characteristics and marine influences, but no significant effect of any one biological trait we examined. Interestingly, we detected more negative associations between species at plot-level than at island-level. This implies a role for local competition; however, on a closer look, we found that none of the species that show preferences for marine inputs simultaneously have negative associations with one another. Overall, we found evidence for equally important roles of marine subsidy and island biogeography in structuring island bird communities, suggesting that disentangling the effects of island biogeography, marine inputs, and biotic interactions is a useful next step in understanding species distributions on islands.