When the Killing’s Done, Island Wildlife Roars Back

New research shows that eradicating invasive species lets unique and imperiled wildlife recover.

The world’s only oceanic hummingbird, the Juan Fernández firecrown. (Photo: Island Conservation)

Mar 21, 2016· 3 MIN READ
Richard Conniff is the author of House of Lost Worlds: Dinosaurs, Dynasties, and the Story of Life on Earth and other books.

For conservation biologist Holly Jones, one of the best experiences of her work on island wildlife was the night she went out hunting for an endangered lizard called the tuatara on Stephens Island in New Zealand. The place was cacophonous with seabirds, which also happened to be attracted to her headlamp. At one point, she found herself sitting in the dark with birds in her lap, at her shoulders, and flapping endlessly around her head. It was like Hitchcock’s The Birds, she said, except that she was ecstatic to be part of this island explosion of life.

Stephens Island happens to be the site of one of the most notorious episodes in the history of humanity’s enraptured—but rocky—affair with islands. In 1894, a crew of lighthouse keepers arrived there, bringing a cat named Tibbles with them. The cat was soon coming back to the lighthouse with small, flightless birds in its teeth. One of them turned out to be a new species, the Stephens Island wren. Within a year or two, a rapidly expanding community of cats had driven it to extinction. By 1897, there were so many cats killing so many birds that a lighthouse keeper urged the authorities “to employ some means to destroy them.” It took another 27 years, but the successful effort to eradicate the cats was the chief reason such an abundance of seabirds survived to greet Jones that night on Stephens Island.

What happened there is now standard conservation practice around the world to protect the incredible diversity of species on islands. Jones, who teaches at Northern Illinois University, is the lead author on a study published Monday in Proceedings of the National Academy of Sciences that looks at the long-term effects of eradicating cats, rats, goats, pigs, and other invasive mammals from islands. On the 181 islands where biologists have conducted follow-up studies, Jones and her coauthors found that eradication turns out to be one of the most effective strategies “for protecting the world’s most threatened species.”

That may seem improbable. Worldwide, we now spend about $22 billion a year on biological conservation. In that context, the tens of millions spent annually on island conservation is a pittance. (Both amounts are trivial in the context of, say, the $600 billion the United States alone spends every year on the military.) Islands are also comparatively small, occupying just 5.5 percent of the planet’s land surface area. But isolation has made them a natural experiment station for biological variation and evolution. So they are home to 15 percent of all terrestrial species—and 37 percent of all critically endangered species on the authoritative Red List maintained by the International Union for Conservation of Nature.

Anacapa, in the Channel Islands off southern California, is a typical success story. The buildup to the eradication of invasive black rats was long and controversial. Outraged animal rights groups filed a lawsuit to stop the killing, and Jones once fielded a call from a protester urging that the rats be relocated instead. (“I was like, ‘OK, what’s your address?’ ”) The protesters were perhaps understandably concerned that no harm should come to any sentient being. They just didn’t understand that this was, in effect, a decision to consign entire species to extinction.

The project was also complicated and expensive, at a total cost of $1.8 million, because Anacapa is home to a deer mouse subspecies found nowhere else in the world and vulnerable to the same poison being used against the rats. So the killing, in 2001–02, had to take place in stages, with the deer mice protected on a separate islet and in captivity, until they could be reintroduced after the rats and the poison were gone.

The main goal was to protect the Scripps’s murrelet, said Nick D. Holmes, a coauthor of the new study and director of science at Island Conservation, a nonprofit working on such projects worldwide. These birds breed in caves around the perimeter of the island, and the rats were hitting them hard. “But almost immediately after extermination, researchers began to see eggs hatching,” Holmes said, and the young birds were no longer being eaten alive in their nests. The population has tripled since then, and 10 years later, the ashy storm petrel, a bird not previously known to have lived on the island, also began breeding there.

Overall, the new study reports, island restorations are now known to have benefited 236 species, a quarter of them threatened with extinction. Removal of invasive animals has so far enabled four species—the island fox, Seychelles magpie, Cook’s petrel, and black-vented shearwater—to be downgraded to a less risky category on the Red List. After a project on Hauturu Island in New Zealand, a type of petrel that had been thought for 150 years to be extinct reappeared and began breeding again.

The study also notes a few negative effects of eradications, including temporary population declines, because some raptors and gulls accidentally died after consuming poisoned carcasses. In four species, the reduction in population is likely to be permanent. On Australia’s Macquarie Island, for instance, removal of invasive rabbits has forced the brown skua, a seabird, to switch back to natural but less abundant food sources.

The results, said Holmes, are “reason to celebrate and to be optimistic. It shows that this type of conservation intervention makes a difference, and it illuminates that there is more work to do.” The study counts 804 islands still eligible for eradication of invasive species. Among them are the Juan Fernández Islands, off the coast of Chile, best known as the setting for Daniel Defoe’s novel Robinson Crusoe. “This is an incredible place,” said Holmes, but it’s also a complicated project because of the varied topography and the social issues involved in working with the human inhabitants.

The islands are home to the world’s only oceanic hummingbird—and to a menagerie of goats, cats, rats, and rabbits. The invaders will have to go, of course. But the new study demonstrates that the islands’ rich native diversity will then have an excellent chance to flourish again in its once-and-future isolation.