The rodents have ravaged the seabird populations on 90 percent of the world’s island archipelagoes, said Nick Graham, a marine ecologist at Lancaster University in England, who led a study comparing rat-infested islands with ones that are rat-free.
“Where there are rats, the skies are empty, the islands are very quiet. If you go to an island with no rats, the sky is full of seabirds, it’s very noisy, very pungent. You can smell the guano — or bird poo — in the air,” Graham told Reuters.
And it is the guano — or lack of it — that is affecting the coral.
The large bird populations on the rat-free islands produce guano that enriches the soil with nitrogen that makes its way into the sea, benefiting the coral and other organisms including fish.
The study, conducted in the Chagos Archipelago in the middle of the Indian Ocean, found the mass of fish was 50 percent greater around rat-free islands.
“Fish are unique in coral reefs, providing important functions that help reefs stay healthy. They clear away algae following disturbances, so new corals can settle and grow back on the reef,” Graham said. “They also bio-erode dead reef substrate to provide a solid surface for new corals to grow on. These two processes were up to four times greater adjacent to islands with seabirds compared to islands with rats.”
The study, published in the journal Nature, calls for rat control to be an urgent conservation priority on remote tropical islands.
“Eradicating rats is ‘low-hanging fruit,’ ” Graham said. “It’s something we know how to do, it’s not hugely costly, and once rats are removed and seabirds return we know that it will bolster productivity and the functioning of these ecosystems.”
The latest report by the United Nations Intergovernmental Panel on Climate Change, released this month, said coral reefs would be all but wiped out if global temperature cannot be kept to no more than 2.7 degrees Fahrenheit above preindustrial levels.