Background/Question/Methods Seabirds are indicators of ocean and island ecosystem health. Seabirds around the world face two different risk suites attributable to global change: 1) island habitat loss, via land use change or invasive species, and 2) overharvest, pollution and climate change. Seabirds escaping the first set of threats will still inevitably be threatened by the second. We compared island ecosystems around the world, in the presence and the absence of seabirds, to elucidate the importance of seabirds to island ecosystem function in a changing world. Results/Conclusions The loss of seabirds may irrevocably transform the island ecosystem, unless active restoration takes place. Nutrient input and disturbance regimes change dramatically in the absence of seabirds, shifting the island to an alternate state. Changes to plant species composition and vegetation structure, in particular, may create a feedback loop making the island less attractive to seabirds. Climate change can only exacerbate these feedbacks. Seabirds are truly keystone species on many of the world's islands, and their conservation depends on restoration at the population, community, and ecosystem level.