Restoring native vegetation, protecting seabird populations and removing invasive rats could help vulnerable atoll islands maintain their natural defences against rising sea levels according to an international study published in Science Advances.
The research led by marine ecologist Nick Graham of Lancaster University in the United Kingdom, examined ecosystem resilience across 28 atoll islands in the Pacific and Indian oceans. The findings suggest that healthy interactions between seabirds land vegetation and coral reefs can strengthen the natural processes that help islands build and stabilise their land.
Atolls are low lying islands formed from coral reef systems and are particularly vulnerable to sea level rise, coastal erosion and extreme weather. However, researchers found that these ecosystems can remain dynamic with natural processes helping reefs and islands accumulate material and respond to changing environmental conditions.
A key part of this process involves seabirds, which feed in the open ocean and return to islands to nest. Their nutrient rich droppings, known as guano transfer marine nutrients onto land, supporting plant growth and improving the productivity of surrounding coral reef ecosystems.
More productive reefs can support organisms that contribute to reef building. Parrotfish, for example, feed on algae and other material on coral surfaces and produce sand sized particles that can help replenish beaches. Corals calcifying algae and shell forming organisms also contribute to the reef framework, helping reefs grow and buffer islands against waves.
Native vegetation plays another important role. Plants with extensive, interconnected root systems help stabilise the shallow, porous soils found on atolls, reducing erosion and providing habitat for nesting seabirds.
In contrast, coconut plantations can offer less effective soil stabilisation because coconut palms generally have less extensive root systems than many native plants. Replacing some plantations with native forests and scrub could therefore improve both habitat quality and the stability of island soils.
Invasive rats also disrupt these natural systems by preying on seabird eggs and chicks, reducing bird populations and weakening the transfer of nutrients from the ocean to land. Removing rats can help seabird colonies recover allowing nutrients to circulate more effectively through the ecosystem.
The study used an approach known as ecosystem energetics which examines how energy and nutrients move between organisms and habitats. Researchers combined measurements of seabird populations native vegetation, coral communities and reef organisms to understand how these components interact.
The findings showed that reef accretion potential the capacity of reefs to build upward through the accumulation of calcium carbonate and other material was twice as high on lagoon facing and ocean facing reefs around rat free islands with abundant seabirds as around islands affected by rats.
Co-author Casey Benkwitt, a restoration scientist with The Nature Conservancy said the estimated reef building potential could be sufficient to closely match projected sea level rise through the end of the century under a global warming scenario of about 2°C.
Reefs that lack substantial seabird populations may be less likely to maintain this pace the researchers noted.
The study included islands in the Seychelles and Chagos archipelagos in the Indian Ocean, as well as Tetiaroa in French Polynesia in the Pacific. The research builds on wider evidence that many coral reefs have historically kept pace with rising seas, although growth rates vary considerably between locations.
However, the researchers cautioned that restoring ecosystems is not a guaranteed solution for atoll survival. The study did not directly compare individual islands before and after restoration and differences in seabird abundance and rat infestation were assessed across existing island conditions.
Furthermore, reef accretion does not necessarily mean an island will grow vertically. Changing waves winds and ocean currents can move sediment between locations, influencing whether material remains where it is needed to maintain beaches and island surfaces.
The findings also apply most directly to largely uninhabited or sparsely populated islands where native ecosystems have room to recover. On densely developed atolls ecological restoration may strengthen natural protection but cannot replace the need for appropriate coastal engineering, infrastructure planning and adaptation measures.
Ecologist Hillary Young, commenting on the research in Science Advances highlighted the importance of understanding the connections between seabirds, native forests, coral reefs and island formation rather than managing land and marine environments separately.
The study reinforces the role of nature based solutions in climate adaptation. Restoring native habitats protecting seabirds, controlling invasive species and conserving coral reefs could help strengthen the natural resilience of vulnerable islands.
Nevertheless, researchers emphasised that rapid reductions in greenhouse gas emissions remain essential to limiting long term sea level rise and protecting atoll communities.