FARALLON NATIONAL WILDLIFE REFUGE, Calif. — On the craggy, guano-covered Farallon Islands, tiny brown blurs of fur dart furtively across the ground between thousands of holes.
This chain of small rocky islands that jut sharply out of the Pacific Ocean 27 miles west of San Francisco is known as "California's Galapagos" for its abundance of squawking seabirds, barking sea lions and great white sharks.
Yet amid this native menagerie lives an intruder: brown house mice brought by human vessels from another time. And the rodents have now colonized here in a density unseen anywhere else in the world, researchers said.
The mice pose a serious threat to a number of plants and animals that occur here naturally. However, just how to rid the islands of the pests — by dropping poison pellets, or by other methods — has proven to be a lightning rod issue for the U.S. Fish & Wildlife Service, resulting in impassioned opposition. The service has delayed releasing its plans as viable alternatives are sought.
The mice attract hungry migratory burrowing owls, which make homes in the islands roughhewn cliffs. Once the owls are here, the predators also eat a rare gray seabird called the Ashy Storm-petrel, which breeds on the islands and whose numbers are in decline.
While the cute Storm-petrel is the face of the service's campaign to eradicate the Farallon mice, there are other slithery, skittering creatures that may benefit from mouse eradication. The rodents eat food shared by the Farallon arboreal salamander, a moist, bug-eyed, spotted creature unique to the island. The mice also feed on a species of cave cricket found only on the Farallones.
But critics worry that introducing poison to these islands will harm too many other, interconnected species — like the owls and other birds that may eat the poisoned mice, living packed together like sardines in a can.
Still, many bird experts support plans to eradicate the mice to help the Storm-petrel, saying the risk is worth the reward of returning this unique place to its natural state.
"The common theme is that these non-native species fundamentally throw off the ecological balances of these special places. The Farallones are the most significant breeding ground for seabirds in the lower 48," said George Wallace, vice president for oceans and islands for the American Bird Conservancy in Washington D.C.
"This is critically important work and fundamental to restoring the natural balance of these islands."
With large waves crashing on rocks and the sour smell of bird guano in the air, the service's manager of the islands, Gerry McChesney, held up a bucket full of blood-caked remains of Storm-petrels. The bird's numbers have dwindled to no more than 10,000 globally.
"(Ashy Storm-petrels) tend to have low reproductive success to begin with so ... if you have extra adult mortality, that can have big impacts on its population," said McChesney.
One of the leading plans being studied is the use of a mouse poison called "brodifacoum," which kills the mice by causing internal bleeding.
Sale of products made with brodifacoum was banned this year by the U.S. Environmental Protection Agency because of their toxicity to humans and wildlife, but it has been used on other islands for similar projects with impressive long-term results despite some immediate collateral damage. The agent has a "very high" toxicity to mammals and birds, according to the EPA.
Other options include doing nothing, sterilizing the mice with another agent or introducing a fatal mouse disease. A complete study of viable plans is expected in the spring, and officials said there was no way action would be taken that would do more harm than good to the beloved islands.
There is precedent for using brodifacoum. It was used on Anacapa Island off the coast of Santa Barbara, Calif. by the National Parks Service in 2002. While it resulted in some collateral damage, the poison successfully removed non-native black rats, which were damaging a small bird called the Xantus's Murrelet. That species has since made a comeback, according to studies.
In 2008 dozens of bald eagles and hundreds of gulls were killed when the poison was used to rid Rat Island in the Aleutians of its namesake species.
No matter what decision is made, any plan would have to kill all of the tens of thousands of mice to be effective, a difficult proposition.
"All it takes is one pregnant female to survive. With the mouse population estimated at 40,000, getting every single mouse to consume the poison pellets will not happen," said Maggie Sergio, director of advocacy for WildCare, a group opposed to the use of mouse poison.
"So the question needs to be asked; if you cannot guarantee every single rodent will be eradicated and we know there will be a devastating impact to other species and the environment, why do this?"
Pete Warzybok, one of five Point Reyes Bird Observatory biologists currently living in an old house on the island, said the fate of the Storm-petrel and other species is a good reason why. Studies have shown a 40 percent decline in Storm-petrels on the Farallones over the past 20-years, and a corresponding increase in burrowing owls.
Warzybok said factors such as climate change and other predators are also at play in the Storm-petrel's decline, but that eradicating mice is a concrete step that can help that and other species survive.
"You have to manage for what you can control," said Warzybok.