The protection of seabirds and the recovery of native vegetation can help low-lying coral islands resist sea-level rise more effectively. An international study concluded that nutrients carried by birds to land contribute to reef growth, while native plants help stabilize soils and reduce erosion.
Published in the scientific journal Science Advances, the study analyzed ecological data from 28 islands in the Indian and Pacific Oceans, including the Seychelles, the Chagos Archipelago, and Tetiaroa Atoll in French Polynesia. The investigation, led by Lancaster University in the United Kingdom, included the participation of Sebastian Steibl, from the University of Auckland in New Zealand.
The researchers found that reefs around islands without invasive rats, where seabird populations tend to be more abundant, exhibited higher growth rates. Native vegetation also revealed an important role in sediment retention, helping to maintain land stability.
“The ecological health of coral reefs and island vegetation is linked to the resilience of atolls to sea-level rise,” explains Sebastian Steibl.
The nutrients from birds favor the reefs
Seabirds feed at sea and transport nutrients to the islands where they rest, nest, and reproduce. These nutrients, deposited through their droppings, enrich terrestrial and marine ecosystems, promoting the growth of corals and other organisms that contribute to sediment formation.
The study found that the growth rates of lagoonal reefs and ocean-facing reefs were, respectively, 1.8 and 2.5 times higher around islands without invasive rats, compared with islands affected by these rodents.
The reefs fertilized by the nutrients from birds also showed greater potential for sediment production, essential for forming and maintaining the islands. The estimated production was between 2.3 and 2.8 times higher on reefs around islands with seabird populations.
The growth of reefs is particularly important because it can reduce coastal erosion caused by wave energy. By contributing to the formation of calcareous structures and to sediment production, reefs help maintain the materials that make up these islands.
Casey Benkwitt, a coauthor of the study and a researcher with The Nature Conservancy, in California, notes that lagoonal reefs with abundant bird populations on islands without rats could grow at a pace close to what is needed to keep up with the sea-level rise projected by the end of the century under a 2 °C warming scenario.
Conversely, reefs without seabird populations are expected to have a more limited capacity to keep pace with that rise.
Native vegetation helps fix the soils
The investigation also identified significant differences between native vegetation and the coconut plantations that dominate many of these islands. Native plants and trees tend to have longer and denser root systems, which help hold sediments in place and reduce erosion.
The researchers estimated that the length of fine roots was 4.6 times higher in areas with greater native vegetation coverage, compared with zones where it was scarce. This difference increases the soil-binding capacity.
The presence of seabirds also appears to favor the development of the root systems of native plants, reinforcing the link between terrestrial and marine ecosystems.
The restoration of these habitats could involve replacing abandoned old coconut plantations with native vegetation and eradicating invasive species, such as rats, which contributed to the decline of many seabird populations. These rodents arrived on several islands via ships over the past centuries.
Nick Graham, the study’s principal author and a professor at Lancaster University, believes that the recovery of these ecosystems could be decisive for the future of the atolls.
“Our results show that a combination of healthy seabird populations, protected from invasive species such as rats, and native forests can help atolls keep pace with the sea-level rise projected by the end of the century under a 2 °C warming scenario,” he says.
A natural protection, but not a guaranteed solution
More than 700,000 people live on atolls in the Indo-Pacific region, where livelihoods, local knowledge, and cultural traditions are deeply tied to the land. These islands are also important biodiversity refuges, hosting about 31 million seabirds, as well as several endemic land bird species.
The researchers emphasize that protecting seabirds, reefs, and native vegetation can strengthen the natural capacity of islands to respond to sea-level rise. However, the study does not demonstrate that these measures alone are sufficient to prevent land loss.
The results point rather to the importance of restoring the ecological processes that help atolls grow, retain sediments, and resist erosion, in a context where climate change poses an increasing threat to these communities and ecosystems.
