How Rats Remake Coral Reefs

By eating seabirds, the rodents weaken the flow of nutrients into the oceans, endangering coral reefs and the fish that live there.

Yellow and blue fusiliers shoal over a reef in the Chagos Archipelago.
Yellow and blue fusiliers shoal over a reef in the Chagos Archipelago. (Nick Graham)

In the middle of the Indian Ocean, on a small island in the Chagos Archipelago, black rats scurry about in search of food. Meanwhile, 250 meters offshore, in the coral reef that circles the island, jewel damselfish graze on patches of algae. These two creatures will never meet, but their lives are nonetheless connected. Through a complicated chain of events, the rats are suppressing the growth of the fish, and endangering the fate of the reefs in which they live.

In the late 18th and early 19th centuries, humans inadvertently carried black rats to two-thirds of the Chagos islands. Wherever the rodents landed, they started devouring the local seabirds like terns and noddies, preying upon eggs, chicks, and even adults. Today, rat-free islands have 760 times more seabirds on them than rat-infested ones. You can see, hear, and smell the difference. “Islands with no rats are noisy, their skies are full of birds, and they absolutely stink of guano,” says Nick Graham from Lancaster University. “But if you step foot on an island with rats, the skies are empty, it’s quiet, and it doesn’t smell. The difference is unbelievable.”

The seabird slaughter has less obvious consequences, too. Seabirds are conveyor belts of nutrients. By feeding in rich oceanic waters far from shore and then pooping on land, they effectively fertilize islands with valuable nitrogen. Indeed, Graham’s team found that the soils on six rat-free islands had around four times as much nitrogen as those of six rat-infested ones. They found similar differences in the leaves of a local shrub, which accumulated more nitrogen when it grew on islands unaffected by rats.

These differences don’t stop at the shoreline. Nitrogen leaches from the island soils into the surrounding water, spurring the growth of marine life. Graham’s team found that sponges and seaweed growing 100 meters off the shore of rat-free islands contained more nitrogen than those growing around rat-infested islands. Even damselfish, living 250 meters away and three meters down, built up more nitrogen around rat-free islands, and grew faster as a result.

It’s not just the damselfish, either. By snorkeling around the islands, the team found that fish on the surrounding reefs were 50 percent more abundant around islands with no rats. “It was mind-boggling to see just how strong the differences are, across the board, for everything we looked at,” says Graham.

In 2016, the Chagos Archipelago was hit by a heatwave that killed off 75 percent of its corals. Everything that Graham found suggests that the survivors will fare much worse around rat-infested islands than around rat-free ones. That’s partly because seabird guano is also rich in phosphorus, which helps corals to grow and resist high temperatures. It’s also because the nitrogen-rich waters are better at sustaining parrotfish, which graze away the algae that compete with corals for space. Parrotfish also munch on the corals themselves, but that’s important for clearing away dead sections and making room for new growth. Without these services, reefs might struggle to recover.

“This is a lovely study [that fills] a huge gap in our knowledge,” says Christa Mulder from the University of Alaska at Fairbanks. In 2011, when she co-wrote a textbook on island seabirds, everything scientists knew about the birds’ influence on aquatic habitats fit into a single chapter, which mostly looked at lakes and wetlands. “There was no information at all on their impacts on coral reefs.”

Likewise, most studies of invasive rats have focused on their effects on islands themselves or their shorelines. “The depth of this research shows just how much ecosystems impacted by invasive rats have to lose,” says Holly Jones from Northern Illinois University, who studies island ecology. Reefs that encircle rat-infested islands may be less resilient in the face of climate change, and the people who depend on those reefs for their livelihoods, via tourism or fishing, could also suffer.

“I think that this study would increase the pressure to rid islands of rats and other invasive predators,” Mulder adds. These animals have decimated seabird populations in around 90 percent of the world’s islands. But rats have already been eradicated from around 580 islands worldwide, including some very large ones. South Georgia Island, which spans more than 420 square miles, was declared rat-free in May after three years of work. New Zealand has committed to wiping out the rodents from the country by 2050. These measures have been driven by the need to protect the islands’ birds, but Graham’s study shows that when birds return, coral reefs also benefit.

“We really struggle to find good management solutions for coral reefs, so this is low-hanging fruit,” says Graham. “We know how to eradicate rats.” So far, only three islands in the Chagos Archipelago have been successfully de-ratted, and Graham estimates that it would cost just $2 million to do the other 37. “Considering how much money is put into other conservation efforts, that’s not a huge amount,” he says.