Federal wildlife regulators are considering carpet-bombing the Farallon Islands next fall with potent pesticides aimed at eradicating hordes of house mice, an invasive population grown so large officials say it has radically altered the islands' ecology and now threatens rare seabirds.

But animal welfare groups contend the lethal chemicals could ruin the prized sanctuary and drift throughout the food web, killing not just the rodents, but birds, reptiles and even microscopic crustaceans.

A few dozen people from both sides of the issue discussed their views Thursday night at a public meeting at San Francisco's Fort Mason, where the U.S. Fish and Wildlife Service was expected to gather input as it works this year to finish a set of possible solutions to the mouse problem. While the agency emphasizes that an aerial drop of rodenticide in 2012 is only a proposal at this point, officials acknowledge there are few other viable options.

That's not a suitable response, according to at least 1,500 people who have signed a petition circulated by WildCare, a 50-year-old Marin County animal rehabilitation center that has made pesticide reduction its mission in the past several years.

"These are man-made problems," said Maggie Sergio director of advocacy for the nonprofit. "Is the aerial dumping of tons of poison over a pristine wilderness area really the answer? We don't think so."

Risks to habitat

The controversy roiling around the Farallon Islands National Wildlife Refuge - a federally managed island complex about 27 miles west of the Golden Gate - underscores not only the risks associated with applying lethal compounds to a large swath of delicate habitat, but the bigger quandaries posed by nonnative species.

A few lowly house mice were likely brought to the Farallones in the 1800s as stowaways aboard fur trading ships. With few predators, the population ballooned over the last century, now peaking at roughly 60,000 annually. Those plentiful, bite-size morsels drew the attention of burrowing owls, another nonnative species. Once the owls chew through most of the rodents each year, they turn their attention to the eggs and chicks of the Ashy Storm-petrel, a small gray seabird that breeds and nests in the Farallones and just one other spot on the globe.

Exterminating the mice and deterring the owls, the U.S. Fish and Wildlife Service said, will reduce mortality among a species whose Farallones population tumbled 40 percent over a particularly bad 20-year period. The petrel is listed as a "species of concern" in California and "endangered" by the International Union for Conservation of Nature.

Meanwhile, nearly 500 mice ply each of the 120 acres of the South Farallon Islands, the biggest landmasses in the array. Biologists like to say that during the mice's yearly boom, "the grass moves."

"People are rightly concerned about the incidental impacts of this rodenticide, and so are we," said Doug Cordell, spokesman with the U.S. Fish and Wildlife Service. "But in this kind of project, you have to eradicate every mouse. You can't leave a single one, or the problem comes back."

Problem with pesticide

But the probable tactic - airplane- or helicopter-dropped pellets soaked with brodifacoum - has its own set of problems, Sergio said. An anticoagulant, brodifacoum triggers a massive bleed out from the mouth, eyes, rectum and ears. Though the petrels, which consume fish and mollusks, wouldn't eat the tainted mice, other birds like gulls might. What's more, Sergio said, the pellets could decay into the soil and water, harming protected salamanders as well as krill, the tiny invertebrates that lie at the base of the food chain.

In a similar but larger project in the Aleutians in 2008, the pesticide managed to work its way to the top of that ecosystem.

More than 40 bald eagles were killed after the contractor, Island Conservation, dumped 46 tons of pesticides onto Rat Island that fall in an effort to eliminate the namesake vermin. In total, 420 birds were killed, including gulls, ducks, teals, cormorants, murres and ptarmigans, according to a report on the incident by the Ornithological Council, a nonprofit policy group. Island Conservation is consulting on the Farallones project.

Aside from the risk of unintended consequences, WildCare and other allies like the Marin Humane Society and anti-pesticide groups question the broader wisdom of saving one protected bird over another.

Owls, petrels protected

Though they are not indigenous to the Farallones, California's dwindling burrowing owls, like the Ashy Storm-petrel, are also listed as a "species of special concern."

"We've pushed them off the mainland to the Farallones because we've encroached on their habitat," Sergio said. "Now they're out there trying to survive, which is what nature does, and we want to remove them."

Ellie Cohen, president of the Point Reyes Bird Observatory, said man-made problems can be corrected and removing the owls might be the answer.

"Animals are dying now, animals will die in the future," Cohen said. "It's not about bad or good, it's about trade-offs."