Published: Sat, July 14, 2018
Research | By Jody Lindsey

Rat-free islands vital to keeping coral reefs healthy

This large supply of nitrogen made its way into the sea, to the benefit of a wide array of marine creatures, including macroalgae, filter-feeding sponges, turf algae, and fish on adjacent coral reefs.

With potential benefits for not only island ecosystems but also nearby coral reefs, eradicating island rat populations to restore seabird-sourced nutrients should be a priority, the authors conclude.

The research found that the total amount of fish was around 50 percent higher on islands with more birds, while seabirds near coral communities were healthier due to a stream of nutrients and the abundance of fish, which ate dead algae and coral growing on the reefs.

An extraordinary set of remote tropical islands in the central Indian Ocean, the Chagos islands provided a ideal "laboratory" setting as some of the islands are rat-free, while others are infested with black rats - thought to have been introduced in the late 1700s and early 1800s.

As the state of the world's coral reefs appear to be getting worse with every passing year, an worldwide team of scientists studying the reefs of tropical islands have come up with an unusual idea for how to help them: Kill all the nearby rats.

The new study, published today in the journal Nature, examined tropical ecosystems in the northern atolls of the Chagos Archipelago to uncover how rats have impacted surrounding reefs.

There were also significantly more fish on rat-free reefs than on those around "ratty islands".

Lead scientist Professor Nick Graham, from the University of Lancaster, said: "Seabirds are crucial to these kinds of islands because they are able to fly to highly productive areas of open ocean to feed".

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Researchers said that rat control should be considered a conservation priority to protect these coral reefs.

In the study, researchers compared six rat-infested islands in the Indian Ocean with six nearby islands that are rat-free. Soil and plants on the rat-free islands had more of a nitrogen isotope that's common in marine food sources, which birds commonly consume and then poop out on islands.

The findings suggest that the nutrient-rich guano from birds can seep beyond the islands, and into the sea and the surrounding corrals.

Professor Graham said: "We also found that fish on the reefs adjacent to islands with seabirds were growing faster and were larger for their age than the fish on reefs next to rat-infested islands".

Invasive rats that decimate the population of island seabirds have unwanted consequences on the health of coral reefs.

"These results show how conservation can sometimes be a bloody business, where doing right by the ecosystem means there is a time to kill". Near the islands where there weren't any rats, grazing and bioerosion of the coral reefs were 3.2 and 3.8 times higher, respectively.

"The results of this study are clear".

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