Land-dwelling rats are upending life for coral reef fish

When rats invade tropical islands, they can trigger a chain reaction that reverberates all the way to coral reefs, researchers say

(Video: Illustration by Emily Sabens/The Washington Post; Rachel L. Gunn; iStock)
7 min

The jewel damselfish spends its life underwater, tending to a small patch of algae, oblivious to anything outside its coral reef home.

But the tiny fish, like a marionette, is being pulled by a distant force it does not even know exists.

A rat invasion on isolated islands in the Indian Ocean has led to a remarkable ecological cascade that is upending life not just for the tiny fish, but for species across the coral reefs where it lives.

“It’s disrupting the natural order,” said Rachel L. Gunn, a coral reef fish ecologist and postdoctoral fellow at Germany’s University of Tuebingen.

Around the world, coral reefs are under unprecedented threat. Plastic litter and other pollution are choking fish. Overfishing is shrinking the population of sharks and other endangered animals.

And most alarmingly of all, acidifying waters and rising ocean temperatures are conspiring to dissolve and bleach the vibrant corals that sustain these ecosystems. A leading scientific body, the U.N. Intergovernmental Panel on Climate Change, warns if Earth warms much more than 2 degrees Celsius, coral reefs will virtually disappear.

Now add rats to that mess.

In a new paper, Gunn and other researchers found that rats’ presence on islands altered the behavior of damselfish by changing a key flow of nutrients. The study not only adds to a body of research into a startling connection between land and sea, but it underscores a need to consider extraordinary measures to rid islands of nonnative rats.

“It just makes it really hit home how important these linkages between islands and reefs are,” said Casey Benkwitt, an ecologist at Lancaster University in England who co-wrote the paper with Gunn.

‘Like no place else’

Wherever humans have gone, rats have followed.

Thought to originally be from Asia, the black rat hitched a ride with traders and conquerors to make its way to Europe. For centuries, rats were blamed for spreading the bubonic plague and triggering the Black Death — though modern researchers question the connection.

But Rattus rattus is responsible for another type of destruction.

Stowing away in the ships of European colonizers, the rodents spread to Australia, the Americas and nearly every other corner of the globe — including, in the 1700s, to the Chagos Archipelago, a group of islands previously uninhabited by either people or rodents.

Even today, setting foot on the remote tropical atolls more than 2,000 miles east of Tanzania is like stepping back in time, Benkwitt said. The islands, still disputed between the United Kingdom and Mauritius, sit in the middle of one of the world’s largest marine protected areas. Unfamiliar with humans, the red-footed booby and other native birds don’t fly away around researchers.

“It’s like no place else I’ve ever been,” Benkwitt said. It’s “almost prehistoric because there are just these seabirds everywhere.”

At least, that is, on islands without rats.

Normally, flocks of frigatebirds, terns and noddies feed out in the open ocean and return to shore to deposit guano. “That’s a fancy word for seabird poop,” Gunn said.

In reefs, feces is gold. Guano that washes into the water provides a crucial source of nutrients across coral ecosystems, including for the damselfish — who, by the way, is no damsel.

The speckled fish is only a few inches long, but it aggressively defends its territory. Each fish “farms” a small patch of turf algae, plucking out unwanted organisms like a gardener weeding a flower bed. They also shoo away other animals, including much larger creatures.

“These damselfish will go for divers,” Gunn said. “They will get in your face and they’ll try and get you out of their territory.” That hostility is key for guarding its algal food, but presents a challenge for researchers. Gunn had to build scaffolding out of plastic pipe to mount a camera and film damselfish behavior remotely.

‘A shot in the dark’

Rat-infested islands — or “ratty” islands, as the researchers call them — have far fewer birds, as the invasive rodents eat the eggs and offspring of the unsuspecting avians. The density of seabirds is more than 700 times higher on rat-free islands.

The “cacophony of noise” and an “ammonia smell in the air” make it obvious when an island has avoided a rodent invasion, said Nick Graham, a professor of marine ecology at Lancaster University who co-wrote the paper. On rat-infested atolls, “it’s quiet, the skies are empty and you don’t get that pungent smell.”

Underwater, the difference is more subtle between reefs around ratty islands compared to those around bird-filled ones that have been spared from rats. “It’s quite hard to jump in a reef and tell the difference,” Graham said.

But under closer examination, the contrast is clear.

Bathed in smelly guano, algae around bird-filled islands is more nutritious, previous research has shown. Both coral and fish appear to grow faster around rat-free islands. Scientists have found the fertilizing effect of guano, too, from the Scattered Islands near Madagascar to Fiji in the South Pacific.

And with less nutritious algae to defend, damselfish around the ratty islands are more sluggish and less aggressive than their better-nourished counterparts, according to a study published in January in the journal Nature Ecology and Evolution. The fish around rat-infested islands also had to farm bigger territories to get all the nutrients they need.

“In hindsight, it made total sense,” Graham said of the connection between rats and reefs, which he began researching after a conversation with a bird biologist aboard a boat and culminated in a 2018 paper. “When we first went out to do the actual research, it was really a bit of a shot in the dark.”

Bird poop may even help coral against its most existential threat: climate change. In 2015 and 2016, a marine heat wave roiled reefs from the Chagos Archipelago to Australia’s Great Barrier Reef. Yet coral around bird-filled islands had more of the calcifying algae thought to help with recovery.

Eradication efforts

The world has a big rodent problem. Rats have reached 90 percent of islands, according to one estimate, helping drive the great auk, a flightless North Atlantic bird, to extinction in the 19th century while decimating the populations of scores of other seabirds worldwide. According to the International Union for Conservation of Nature, nearly 1 in 3 seabird species today is threatened — in part because of the rats.

The discovery of the importance of bird poop for coral reefs is helping renew efforts to eradicate nonnative rats.

After centuries of devastation, authorities declared the island of South Georgia in the South Atlantic rat-free after an aggressive campaign of planting poisoned bait. New Zealand hopes to rid its shores of rats by the middle of the century. Geneticists are even looking into ways of editing the DNA of rodents to render females infertile.

But rats are persistent. Efforts to exterminate them on Eagle Island in the northern portion of the Chagos Archipelago, for instance, failed in 2006. Invasive rodents even survived nuclear bombs tested by the United States in the Marshall Islands.

Even so, a charity called the Chagos Conservation Trust is aiming to “rewild” 30 islands in the archipelago by clearing them of rats, by distributing bait by hand and even perhaps by drone. The trust hopes to start the work within three years, trust director Sarah Puntan-Galea wrote in an email.

For Benkwitt, the research into the devastation wrought by rats provides a ray of hope. “I try to be an optimist, which is sometimes hard when you study coral reefs,” she said. Rats are yet another problem for coral ecosystems near the brink.

“But it also means the flip side of that is true,” she added. Ridding islands of rats can help both birds and reefs. “So there’s a hopeful way to look at that linkage.”

This article is part of Animalia, a column exploring the strange and fascinating world of animals and the ways in which we appreciate, imperil and depend on them.