Protecting seabirds and restoring natural vegetation could give low-lying tropical atoll islands a fighting chance against the threat of sea-level rises, 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 today in the journal Science Advances, an international team of researchers, led by marine scientists at Lancaster University, show 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, 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.”

Three protective processes

The researchers combined ecological datasets 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 boosting effect on three key processes that support atoll island resilience to sea level rise:

· coral reef growth

· sediment production

· and soil stabilisation

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.

Coral reef growth

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 reefs around rat-infested islands.

Co-author Dr 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.

“In contrast, reef growth rates without seabird populations are predicted to have limited ability to keep up with projected sea-level rises.”

Sediment production

Reefs fertilised by seabird nutrients also enhanced another key process on the surrounding reefs: the production of sediments that can build or maintain islands. Sediment production potential due to greater parrotfish biomass and other organisms that produce sediments was substantially higher for islands with seabirds, with an estimated 2.3 to 2.8 times higher sediment production rates on lagoonal and ocean-facing reefs around seabird populated islands.

Professor Chris Perry, from the University of Exeter and a co-author on the study, said: “For a small ~14 hectare island, this translates to ~170 cubic metres more sediment being produced on the surrounding reefs every year. This is a sizeable uplift in the sediment production potential of these systems, with potentially substantial benefits to island maintenance and growth.”

Soil stabilisation

The researchers also looked at the role of native plants and trees in stabilising atoll island soils and sediment.

Native plants and trees, which have long and dense root systems, help to lock sediment in place and prevent it being eroded. This contrasts with the coconut trees historically cultivated on many atoll islands, which have a much less effective root system for binding and holding soils and sediment.

The team estimated fine root length to be 4.6 times higher in areas of high native vegetation compared to areas with low amounts of native vegetation, significantly increasing soil binding potential and demonstrating the importance of restoring native vegetation on atolls.

The study also shows when seabirds are present, the root systems of these native plants are also enhanced.

“The study shows beautifully how atolls are an intimately interconnected island-reef system, a geo-ecological system. Restoring and boosting the wildness of these islands through seabirds and native vegetation helps these islands to adapt to global change, said Professor Yadvinder Malhi Director of Leverhulme Centre for Nature Recovery at the University of Oxford, a co-author of the study. “These islands provide a model example of how restoration and rewilding can help ecosystems adapt to climate change, giving insights that can be applied across the natural world.”

The findings suggest that rewilding of atolls can help their climate resilience.

Dr Jennifer Appoo, of the University of the Seychelles, also a co-author on the study, said: “Our results show new evidence that island restoration is not only beneficial to biodiversity conservation but can enhance the resilience of low-lying atoll islands in the face of sea-level rise, particularly where costly engineering solutions are unfeasible or inappropriate, as can be the case in countries with uninhabited or sparsely inhabited atolls, such as Seychelles.”

The results of the study, which was supported by the Bertarelli Foundation, are outlined in the paper ‘Seabirds and native vegetation drive ecosystem processes that support atoll island resilience’.