Ecological effects of marine subsidies to temperate islands

Sometimes referred to as "nature's laboratories", islands have long been used to test fundamental ecological theories. One highly influential theory, MacArthur and Wilson's (1967) Theory of Island Biogeography, provided an early framework for predicting species diversity according to island size and distance from mainland. Since then, further studies have considered the additional effects of climate, habitat diversity, in situ evolution, and invasive species, among others. Although islands are generally well-studied, most work has considered islands as entirely isolated entities. In 2001, the Subsidized Island Biogeography Hypothesis was proposed. This theory considers islands as meta-ecosystems; their ecologies may be fundamentally impacted by inputs from the surrounding marine habitat. Marine inputs can increase the quantity of resources available on islands, which may lead to an increase in productivity. This increased productivity affects the diversity and composition of island communities. Depending on the direct and indirect recipients of marine inputs, island trophic dynamics might also be affected. In this thesis, I study the effects of marine inputs on island species diversity, community composition, and food webs on 99 islands on the Central Coast of British Columbia, Canada. Specifically, in Chapter 2, I evaluate the effects of marine subsidies on terrestrial breeding bird species richness and total density. I find higher species richness on both larger islands and those receiving fewer marine-derived nutrients. Meanwhile, bird densities are higher on smaller islands and those with more marine-derived nitrogen in the soil. To evaluate the mechanisms behind these patterns, in Chapter 3, I dig deeper into species-level responses to marine inputs and island characteristics. I find that bird species distributions are just as well explained by marine influences as classically studied island biogeography parameters. Finally, to better understand ecosystem level effects of marine subsidies, I evaluate nutrient enrichment in six trophic levels across island food webs, and trace marine-derived nutrients from the soil into upper-level consumers. Overall, in this thesis, I connect fundamental ecological theories about species distributions on islands with theories about marine subsidies and demonstrate the value of considering island ecosystems in a meta-ecosystem framework rather than as isolated entities.