In 2002 conservationists used helicopters to bomb Anacapa Island, off the coast of California, with the rodent-killing poison brodifacoum. They managed to wipe out their targetâ€”an invasive species of black rats that had been living on the island for more than a centuryâ€”but they also knocked out a native population of deer mice and killed some rodent-eating raptors, like the peregrine falcon.
It sounds like a conservation effort gone awry, but the effort, recounted at yesterday's Wildlife Conservation Societyâ€“sponsored State of the Wild conference, is considered a success. That's because the conservationists had trapped enough of the native deer mice to preserve their ability to thrive when they were released back into the wild six months after the poisoning event. The rats that had plagued the island and prevented seabirds from roosting were gone; the mice were back, and there was no irreparable damage to the ecosystem.
The Anacapa effort is just one of a growing number of attempts to roll back the invasion of various types of mammals on the islands of the world: rats, this time from Campbell Island near New Zealand, as well as 160,000 goats from Isabella Island in the Galapagos through 2006, to name just a few. There have been 160 "eradications" of pigs, goats or sheep, 75 of feral cats, and 332 of rats and other rodents, according to Josh Donlan, director of Advanced Conservation Strategies, which carries out such efforts. "People usually start to get uncomfortable here," Donlan says. "This is pretty aggressive conservation."
It started with ornithologist Ken Stager's visit to Clipperton Island in the Pacific in the late 1950s. He found a community of 55 feral pigs where he had hoped to find nesting grounds of masked and brown boobies. "Luckily, Ken Stager had a shotgun with him," Donlan says. He killed all the pigs and, by 2003, 150 masked boobies had grown to 112,000. "Today it's the largest and most important boobie nesting site in the Pacific. The question is: How do we scale Stager's success?"
Biologists refined the formula into something a bit more scientific in New Zealand in the 1970s. Faced with the eradication of native birds on small islands below the ravening maws of invading rats, they baited traps with rodenticide-laden food and proved that rats could be eliminated from these small islands. Over the intervening decades, the methods have evolved. Helicopters now drop poison across islands as large as 13,000 hectares (about 32,100 acres), or ferry in goat hunters.
These successes have inspired grandiose visions of new targets, such as the beavers introduced in the 1950s to the southern tip of South America. They have spread to cover 16 million hectares (39.5 million acres) of land and left a swath of destruction "which you can basically see from an airliner," Donlan says. "Given the success of the Galapagos and New Zealand, we feel confident that it's possible to remove beavers from such a large area."