To help a tiny, rare seabird with a declining population, the U.S. Fish and Wildlife Service is studying a plan to kill off non-native house mice with poison on the Farallon Islands, a marine sanctuary off the coast of Northern California.
But even the idea that a poison would be used in the pristine environment has drawn criticism from some conservation groups that argue the aerial drop of thousands of rodenticide pellets would instead harm many creatures throughout the food chain.
The possible use of mouse poison on these rugged islands about 28 miles west of the Golden Gate Bridge demonstrates the balancing act faced by wildlife managers as they seek to eradicate one species to benefit another.
At risk is the small Ashy Storm-petrel, a seabird with a global population estimated at no more than 10,000. Half of the Storm-petrel's global population uses the Farallon Islands, known to researchers as California's Galapagos Islands, to breed.
More than 400 species of birds have been identified on the Farallones, and the refuge is home to a menagerie of whales, great white sharks, seals and sea lions.
Yet at certain times of year the islands' ground appears to move, as thousands of mice scurry across it. Some researchers estimate that at times there are as many as 500 mice per acre.
"We know this is a massive infestation of mice, and rodenticide is the only thing proven to be the most effective," said Doug Cordell, a spokesman for the service. "But we're not going to do anything that carries more risk than benefit to the ecosystem of the Farallones, that's the bottom line. We haven't settled on an option yet, including the option of doing nothing."
The mice, believed to have been passengers on 19th century sailing vessels that stopped at the islands, alone do nothing to the Ashy Storm-petrel.
Instead, their numbers draw the attention of hungry migratory burrowing owls. Those owls feast on the mice until the population dwindles, then turn to Storm-petrels, their chicks and eggs.
The Ashy Storm-petrel is not currently listed as an endangered species, but Cordell said it is considered endangered by international conservation groups and in the U.S. is considered a species of concern.
Pesticides have been used to protect birds from rodents on islands before, with mixed results.
In 2002, the National Parks Service used a rodenticide called brodifacoum, which is being considered for use on the Farallones, on Anacapa Island, one of the Channel Islands off the coast of Santa Barbara.
The project, the first of its kind at the time, was done to protect the diminutive Xantus's Murrelets from non-native black rats.
While there was some collateral damage to other species, a study published in the journal Marine Ornithology found that nesting and breeding efforts by Xantus's Murrelets improved greatly after the black rat was killed off.
In October 2008, about 48 metric tons of the rodenticide was dropped over Rat Island in the Aleutians as part of a habitat restoration project led by fish and wildlife.
The poison was believed to be the cause of death of 46 bald eagles and hundreds of gulls, according to a 2010 study of the carcasses by the Ornithological Council.
"This is one of those complex issues, and right now there doesn't appear to be any clear easy answers," said Maggie Sergio, director of advocacy for WildCare, an animal rescue organization opposed to the use of the poison. The group said it had gathered more than 2,000 signatures for a petition opposing the proposal.
"We need to continue looking for alternatives and do something else other than drop tonnage of rat poison over pristine wilderness. There's got to be a better way," she said.
Still, not all conservation groups are opposed in principal to the idea of using rodenticide to save the struggling Storm-petrel.
Graham Chisholm, executive director of Audubon California, said the scientific review process is in its early stages, and that he believes rodenticide will only be used after much research and consideration.
"Clearly something needs to be done," Chisholm said. "The threat of house mice on the Ashy Storm-petrel breeding colony has been documented and presents a threat to birds' long-term survival."