This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Protecting seabirds and restoring natural vegetation could give low-lying tropical atoll islands a fighting chance against the threat of sea-level rise, a new study reveals. As Earth's atmosphere continues to warm, sea levels are projected to rise, threatening to swamp "tropical paradise" atoll islands, such as those found in the Indian and Pacific oceans.
Publishing in the journal Science Advances, an international team of researchers, led by marine scientists at Lancaster University, shows how seabird nutrient flows affect atoll islands and the coral reefs that surround them—influencing their ability to adapt to rising sea levels. Lead author and Distinguished Professor Nick Graham of Lancaster University said, "Replacing old, abandoned coconut palm tree plantations with natural vegetation, and restoring seabird populations by eradicating invasive rats, could be key to the survival of these tiny low-lying islands. "Atoll islands are not static; they are naturally active systems that can change and adapt to their environmental conditions if they are allowed to do so.
"Our findings show that a combination of a healthy population of resident seabirds, protected from invasive species such as rats, as well as native forests can help atolls keep pace with the sea-level rises projected to happen by the end of the century under 2°C warming." The researchers combined ecological data sets for 28 Indian and Pacific Ocean atoll islands across the Seychelles and the Chagos Archipelago in the Indian Ocean, and Tetiaroa atoll in French Polynesia, in the Pacific. By looking at seabird census data, the researchers estimated the amount of "energy" seabirds are bringing from the open ocean back to each of the small atoll islands and how the nutrients—deposited in seabirds' droppings—influence the islands and their surrounding marine ecosystems. They found that the presence of seabirds and native vegetation has a positive effect on three key processes that support atoll island resilience to sea-level rise: Previous studies have shown that seabirds play a vital role in recirculating nutrients from the open ocean where they feed to the islands where they roost, breed and nest.
However, many tropical islands have now lost their seabird populations due to a combination of lost roosting habitats and predators in the form of invasive rats, which arrived as stowaways on boats in previous centuries, decimating bird populations. Many atolls have also seen their native vegetation stripped away and land converted to development or crops, with coconut plantations now estimated to dominate around 60% of Pacific atoll forests. Reef growth is key to reducing wave-driven erosion of island coastlines.
The researchers found that corals grow much faster on reefs around islands with healthy seabird populations compared to bird-free islands. These nutrient-enhanced corals also calcified faster, forming more hard reef substrate. Using these data, the researchers calculated the "reef accretion potential" for the islands, revealing how conservation measures, such as rat eradication, might influence the overall reef growth.
They found that reef growth rates on lagoonal and ocean-facing reefs around rat-free islands, and therefore with abundant seabird populations, were 1.8 and 2.5 times greater, respectively, than those on reefs around rat-infested islands. Casey Benkwitt of The Nature Conservancy in California said, "The reef accretion rates for rat-free, bird-populated lagoonal reefs would be sufficient to closely keep up with predicted sea-level rises up to the end of the century under a 2° C warming scenario. Importantly, this will help reduce wave energy reaching the islands.
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