Reefs grow up to 2.5 times faster beside islands cleared of rats

Coral reefs next to seabird-rich tropical islands that have no rats grew up to 2.5 times faster than reefs beside rat-infested islands, according to a study of 28 atoll islands in the Seychelles and Chagos Archipelago plus Tetiaroa Atoll in French Polynesia. Nick Graham, a Distinguished Professor at Lancaster University, led the work, published in Science Advances on October 14, 2026. The researchers argue that seabird droppings, carried in from the open ocean, set off a chain of benefits that could help low-lying islands keep up with rising seas.

Droppings, reefs and the 2.5 times figure

Seabirds feed at sea and roost on land, so their droppings move energy and nutrients from the open ocean onto islands and onto the reefs around them. The team used seabird census data to estimate how much of that fertiliser each island receives. Many tropical islands have lost their seabirds to a shortage of roosting habitat and to invasive rats, which arrived as stowaways on boats, and without the birds that fertiliser stops arriving, the Lancaster University release explains.

Around rat-free islands with seabirds, reefs grew 1.8 times faster on the lagoon side and 2.5 times faster on ocean-facing reefs than around rat-infested islands. Corals also calcified faster, building more of the hard reef framework. Casey Benkwitt of The Nature Conservancy, a co-author, said that reef accretion under rat-free, bird-populated conditions would be sufficient to closely keep up with predicted sea-level rises.

The sample spans two oceans, which is part of its weight. Twenty-eight islands lie in the Seychelles and Chagos Archipelago in the Indian Ocean, and the remaining site is Tetiaroa Atoll in the Pacific. The paper, titled “Seabirds and native vegetation drive ecosystem processes that support atoll island resilience,” frames three protective processes that seabirds and native plants strengthen together: coral reef growth, sediment production and soil stabilisation. Each works on a different layer of the island, from the living reef offshore to the roots under the beach.

Sediment that rebuilds the island

Reefs matter to atoll islands because they manufacture the sand and gravel that the islands are made of. Lagoonal and ocean-facing reefs near seabird-populated islands produced 2.3 to 2.8 times more sediment. For an island of roughly 14 hectares, Chris Perry of the University of Exeter estimated about 170 extra cubic metres of sediment a year. “This is a sizeable uplift in the sediment production potential of these systems,” Perry said.

The authors link that sediment supply to the central question for atoll nations: whether a ring of sand barely above the waterline can grow as fast as the sea rises. The release says that, with rats removed and seabirds back, reef growth could roughly keep pace with sea-level rise through 2100 under a 2 degree Celsius warming scenario. It is a conditional result, resting on what the sampled islands show rather than a promise for every atoll.

Roots, coconuts and the vegetation question

The third process the study measured sits on land. Fine root length was 4.6 times higher in areas with high native vegetation than in areas with little, and seabirds were also linked to stronger root systems. Roots bind loose sand and slow erosion, so native forest and seabirds appear to reinforce each other in holding an island together.

That raises an uncomfortable fact about the Pacific, where coconut plantations are estimated to cover about 60 percent of atoll forests. Graham pointed to replacing old, abandoned coconut palm plantations with natural vegetation as one measure. “Atoll islands are not static, they are naturally active systems,” he said, framing islands as landforms that can adapt if their ecology is intact.

Restoration as a low-engineering option

The practical message is about priorities. Removing rats and replacing abandoned plantations are conservation actions, and the study recasts them as coastal defence. Graham’s phrase about islands being active systems carries the argument: an atoll that keeps producing sediment and holding it in place with deep roots is doing the work that seawalls would otherwise have to imitate.

Yadvinder Malhi, Director of the Leverhulme Centre for Nature Recovery at the University of Oxford, described restoring and boosting the wildness of these islands through seabirds and native vegetation as a way to build resilience. Jennifer Appoo of the University of the Seychelles added that island restoration benefits more than biodiversity, and that it may be especially valuable where engineering solutions are unfeasible, such as sparsely inhabited atolls in the Seychelles.

The study was funded by the Bertarelli Foundation and is a case study, so its numbers describe the islands sampled. The full analysis is in the Science Advances paper. Lancaster has covered the same research thread before, including an earlier Seychelles study of seabird nutrients on reefs around rat-free islands, a story on seabirds as helpers for reefs and work showing restored rat-free islands could support far more breeding seabirds.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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