The Effects of Seabirds, Rats, and Ecosystem Restoration on Invertebrate Food Webs

Burrowing seabirds that nest on islands transfer nutrients from the sea, disturb the soil through burrowing, damage tree foliage when landing, and thereby modify the surface litter. One of the greatest effects seabirds may have on their recipient ecosystems may be via the nutrient subsidies they transfer onto islands from the sea. How these nutrients effect their recipient ecosystems however, depends on many factors such as water availability. However, seabirds are in decline worldwide, as are their community- and ecosystem-level impacts, primarily due to invasive predatory mammals. Seabird islands are vulnerable to the invasion of predatory mammals such as rats, which can have lasting effects even after these pests are eradicated. Once these islands are restored and seabirds start to return the ecosystems can recover quickly, returning to a pre-disturbance state within as little as 20 years. However, legacy effects of the invasive mammals may occur meaning ecosystems may revert to alternate stable states. The direct and indirect effects of seabirds, their decline and recolonisation on ecosystems are inherently complex. I employed network analysis of invertebrate food webs, as a means of simplifying ecological complexity, to better understand the effects seabirds, their loss, and recolonization, may have on island invertebrate communities. I found that on rat-invaded islands the invertebrate food webs were smaller and less complex than on their seabird-dominated counterparts, likely due to the suppression of seabird derived nutrients and consequent effects on trophic cascades. There was also an interplay between nutrient subsidies and water availability, where invertebrate food webs were larger and more complex as litter water increased and soil C: N slightly decreased. When comparing a restored island to invaded islands and those never invaded I found that the restored island supported some areas that were virtually indistinguishable from an invaded island and it demonstrated strong environmental gradients ...