Rewilding could give atolls a fighting chance

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 warms, sea levels rise, threatening to swamp ‘tropical paradise’ atoll islands, such as those in the Indian and Pacific Oceans, say an international team of researchers, including the University of Auckland’s Dr Sebastian Steibl.

Published in the journal Science Advances, the research shows how seabird nutrient flows affect atoll islands and the coral reefs that surround them – influencing their ability to adapt to rising sea levels caused by climate change.

“The ecological health of coral reefs and island vegetation are linked to how resilient atoll islands will be in the face of sea-level rise,” says Steibl.

“Over 700,000 people inhabit atolls across the Indo-Pacific,” Steibl says. “For them, their cultural heritage, cultural ties, local knowledge systems, and livelihoods are inextricably linked to the atolls that they have been inhabiting for millennia.”

Atoll islands can be hotspots of tropical biodiversity, providing homes for 31 million seabirds and many unique native land birds, he says.

Lead author of the research paper, Distinguished Professor Nick Graham of Lancaster University, UK, says: “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 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 bring from the open ocean back to each of the small atoll islands and how the nutrients - deposited in seabirds’ droppings - influence the islands and surrounding marine ecosystems.

They found the presence of seabirds and native vegetation boosts three key processes that support atoll island resilience to sea level rise:

• coral reef growth
• sediment production
• and soil stabilisation

Previous studies have shown 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 lost roosting habitats and predators, such as invasive rats which arrived as stowaways on boats in previous centuries.

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 percent 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 this data, the researchers calculated how conservation measures, such as rat eradication, might influence overall reef growth.

Growth rates on reefs in lagoons around rat-free islands, which had abundant seabird populations, were 1.8 times greater than reefs around rat-infested islands. Ocean-facing reefs grew 2.5 times faster around rat-free islands.

Co-author Dr Casey Benkwitt of The Nature Conservancy in California, says reef growth rates for rat-free lagoon reefs with healthy bird numbers are likely to be able to keep up with predicted sea-level rises until the end of the century, under a 2° C warming scenario. Importantly, this would help reduce wave energy reaching the islands, she says.

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

Reefs fertilised by seabird droppings enhanced another key process: the production of sediments that can build or maintain islands. Sediment production was about 2.3 to 2.8 times higher on reefs around islands with seabirds.

Higher numbers of parrotfish and other organisms that produce sediments were found on reefs around islands with seabirds. The researchers looked at the role of native plants and trees in stabilising atoll island soils and sediment.

Native plants and trees with long, 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 less effective root systems 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 few native plants. These roots significantly increased soil binding.

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

The research was supported by the Bertarelli Foundation.