Rat-free atolls with seabirds have reefs growing up to 2.5 times faster

The study was published in the journal Science Advances by an international team of researchers led by marine scientists at Lancaster University.

The researchers combined ecological data sets 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.

Using seabird census data, the researchers estimated the amount of 'energy' seabirds bring from the open ocean back to each atoll island and how the nutrients deposited in their droppings influence the islands and surrounding marine ecosystems.

Lead author Nick Graham of Lancaster University said that replacing old, abandoned coconut palm 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."

Corals grow much faster on reefs around islands with healthy seabird populations than on reefs around bird-free islands, and these nutrient-enhanced corals also calcified faster, forming more hard reef substrate.

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 those on reefs around rat-infested islands.

Sediment production potential, due to greater parrotfish biomass and other sediment-producing organisms, 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.

Coconut plantations are now estimated to dominate around 60% of Pacific atoll forests.

Many tropical islands have 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.

Reef growth is key to reducing wave-driven erosion of island coastlines.

Using the data, the researchers calculated the 'reef accretion potential' for the islands, revealing how conservation measures such as rat eradication might influence overall reef growth.

The Pacific part of the 28-island dataset comes from one atoll, Tetiaroa in French Polynesia; the other locations named are the Seychelles and the Chagos Archipelago in the Indian Ocean.