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The largest seabird colony in the continental United States is facing a major threat. But it’s not predator versus bird—it’s conservationist versus conservationist.

The Farallon Islands—about 30 miles west of San Francisco and collectively known as Farallon National Wildlife Refuge—host more than a quarter of a million seabirds of at least 12 species, including, according to recent estimates, 2,000 Ashy Storm-Petrels (possibly half the global population) and some 1,400 Leach’s Storm-Petrels. The Farallons are also home to 500 mice per acre, “one of the most dense infestations of rodents anywhere in the world,” according to U.S. Fish and Wildlife Service spokesman Doug Cordell. (The mice were first transported to the islands via visiting ships.) The rodents’ populations follow an annual cycle, reaching “plague-like densities” each year before dwindling back down due to lack of food, .

Williams writes: 

"The mice are shredding the native ecosystem. They spread seeds of invasive plants, prey on the pollinators of native plants, are thought to infect shore habitat of seals and sea lions with dangerous pathogens, eat the camel crickets, and compete with (and probably eat) the arboreal salamanders. And by attracting burrowing owls to the islands, these mice threaten the ashy stormpetrel with extinction. The owls feast on mice, but that’s not a good thing, even for the owls. Now, instead of resting briefly and continuing south on their fall migration, some owls remain on the Farallones into winter, unable to tear themselves away from the mouse smorgasbord. With the first rains of late fall, seeds germinate and the mice are without a main source of food — so they start to eat each other. Then all save a few (although enough to regenerate plague-like densities the following year) starve to death.

With the mice gone, the owls switch to a diet of ashy storm-petrels. Then, because there aren’t enough petrels to sustain them, some owls sicken and die. Owls that leave face a late and dangerous migration. Mice were wreaking such ecological havoc that as early as 2004, the U.S. Fish and Wildlife Service, which manages national wildlife refuges and is legally mandated to restore and protect native ecosystems thereon, resolved to rid the South Farallones of the invasive aliens."

While there seems to be little disagreement that there is a problem, there is considerable disagreement about just how to solve it. The tried-and-true method is using a highly toxic rodenticide called brodifacoum, which causes massive internal hemorrhaging in any animal that eats it. It can also kill whatever eats the poisoned rats. In 2008, when the USFWS used brodifacoum to eradicate rats from the Aleutian island of Hawadax, it killed 46 Bald Eagles and 320 Glaucous-winged Gulls in the process, though both species have since recovered. In the Farallons, the primary victim of secondary poisoning would likely be Western Gulls.

The USFWS is now considering a plan to dust the Farallons with brodifacoum-laced grain to eradicate the mice once and for all, a plan supported by most bird and environmental advocacy groups. Golden Gate ����APP, which generally opposes the use of rodenticides, supports it in this case too, , “If action is not taken to rid the island of mice soon, all of the Ashy Storm-Petrels and probably Leach’s Storm-Petrels could be lost.”

But some, including the Ocean Foundation, the Animal Legal Defense Fund, and a determined activist named Maggie Sergio, are doing everything they can to stop it. They argue that many of the long-term effects—like how the rodenticide breaks down in the soil, and the possible impact on other wildlife on the islands and in the surrounding waters—remain unknown. Sergio, who has penned the use of brodifacoum, is finding an audience for her arguments: At this point, more than 32,000 people have added their names to demanding the project be canceled.

Whether the petitioners will prevail remains to be seen. The USFWS still needs to produce the final draft of an environmental impact statement, nine years in the making, on the plan—and the USFWS’s Cordell says the service won’t be able to do that until it receives another round of funding. Right now, there is not even an estimate on when that money might come through, but when it does, the final EIS, and a final plan for the Farallons, won’t be far behind.

Read “” in Outdoor America.

Correction: This article has been updated to more accurately reflect the problem the mice pose to the island's habitat.