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Can Dumping Poison on the Farallon Islands Save It From Mice Overpopulation?

USFWS plans to shower world’s most mice-infested archipelago with rodenticide-infused food pellets.

About 30 miles off the coast of San Francisco, the Farallon Islands are craggy, other-worldly outposts, a seasonal home to a variety of marine life such as elephant seals and sea lions, dozens of species of shore and seabirds, and a few government researchers. The archipelago, which is protected as congressionally designated wilderness area, serves as one of the last refuges for many of California’s native nesting birds. Unfortunately, the Farallones also happen to have one of the largest mice populations of any island chain in the world, and the rodents are wreaking havoc on the islands’ ecosystem. The rodents, mostly Eurasian house mice, have reached numbers exceeding 60,000. Southeast Farallon Island — the largest island in the chain and only island where scientists conduct research on the ground — has become so overwhelmed by mice in the past few years the ground is said to “move with mice” during peak breeding season.

Farallon IslandsPhoto by Erik Oberg, Island ConservationOn Southeast Farallon Island, the ground is said to “move with mice” during peak breading season.

Researchers who stay on the island have told horror stories about going on bird counts at night and having the ground crawling with so many mice that one researcher stopped counting nest sites and started breaking mice necks with his bare hands. 

Last year, the US Fish and Wildlife Services (USFWS), which oversees the islands, and a handful of conservation nonprofits, came up with a plan to get rid of the pesky rodents called the South Farallon Islands Invasive House Mouse Eradication Project. The plan, which reviews 49 suggested ways of getting rid of the pest, concludes that the only way to get rid of the mice is by poisoning them.

How mice came to overrun the islands is something of a mystery. It is speculated that mice came with nineteenth century marine mammal hunters who inadvertently carried rodents aboard their boats. Looking for fatty seal meat and pelts from fur seals, many Americans and Russians took part in the trade. Towards the middle of the century, the islands also fell prey to egg collectors, who ended up decimating bird populations by the beginning of the twentieth century.

President Theodore Roosevelt signed an Executive Order in 1909 to protect the northern Farallon islands from these threats, which was extended to the southern patch of islands in 1969. With environmental protections in place, the mice population grew.

While an official decision has yet to be reached in terms of how to handle the rodent infestation, surprisingly, many conservation groups are in support of eradication by dropping poisoned food pellets on the islands through a process referred to as “aerial broadcasting.” The pellets will be made of either Brodifacum-25 or Diphacinone-50, both anticoagulant poisons that cause mice to bleed to death. The hope is that the pellets will kill the mice and help the ecosystem rebound.

One reason that conservation groups want the mice gone is concern for native species. “We are concerned about the declining population of the ashy storm-petrel which nests on the island,” says Bob Lewis, president of the Farallon Islands Foundation, a nonprofit focused on conservation and restoration of island habitats.  “[We] believe that the mice that are present are at least partly responsible for the poor reproductive success of the storm-petrel.”

The ashy storm-petrel, a pigeon-sized light-grey bird with a white stripe across its wings, is listed as endangered by the International Union for the Conservation of Nature. Petrel numbers are also under threat from burrowing owls, which have come to the island in increasing numbers to feed on the mice. When mice are scarce (which may happen in off-breeding seasons), the owls end up eating the ashy storm-petrel chicks. 

“We would like to see the mice removed,” says Lewis. “We believe it will minimize the time burrowing owls remain on the island, and will also minimize predation on ashy storm-petrels.  Ultimately, we hope removal of the mice will result in an increase of the storm-petrel population.”

Rodent infestations aren’t unique to the Farallones.  More than 80 percent of island chains across the world are plagued by invasive rodents, which prey on local birds and animals, transmit disease and affect other native wildlife by competing for food. The use of poisoned pellets to keep rodent populations at bay on islands has gained traction during the past several years.

Last July, South Georgia Island in the South Atlantic Ocean dealt with its rat population by dumping 200 tons of poison pellets on the islands by helicopter. According to Island Conservation, a nonprofit that advocates removing invasive species from islands, South Georgia Island — which will see the completion of the project in 2015 — is just one of more than 400 islands that have been cleared of invasive rodents.

Not all conservation groups, however, are onboard with the USFWS eradication project on the Farallones. Those opposed to the project say that the poisoned pellets would also kill birds and other animals via secondary poisoning, especially raptors like peregrine falcons, which generally feed on the mice. (Read about how rat poisons like these harm raptors here.) They point out that the poisoned pellets might also affect seagulls. 

To minimize the impact on gulls, the USFWS tested gull hazing — spooking the gulls away from the area — on the islands in 2012. Gerry McChesney, the agency’s National Wildlife Refuge manager, says there are several colorful ways to do this, including setting off pyrotechnics, using loudspeakers to broadcast raptor calls near gull roosting sites, implementing gull effigies (gull carcasses hanging by a pole), and flashing laser beams at them at night.

The USFWS also plans to avoid owl deaths by trapping and removing owls from the island before the rodenticide is released.

But none of these safety measures sit well with some animal rights groups, which say that using rodent poison is bound to harm other species. (According to a report by the nonprofit group Ornithological Council, a 2008 aerial drop of poisoned food pellets on Rat Island in the Aleutians killed 420 birds including gulls, ducks, teal, cormorants, murres, and bald eagles.)

“The introduction of brodifacoum or diphacinone into the delicate ecosystem of the Farallon Islands can only lead to significant environmental damage, death, and destruction. It cannot be the only alternative,” states Bay-Area based wildlife rehabilitation center WildCare in its petition against the eradication project.  The group would like to see a solution that is “non-toxic and environmentally sustainable.”

Before the poisoned pellet proposal came to the forefront, several alternatives were considered, including using bait stations to trap the mice, sterilization, and introducing other predators like feral cats. One alternative included simply leaving the mice alone. 

As the Environment Impact Statement for the USFWS plan continues to wind its way through the public review process, the fate of the Farallon Islands rodent population remains uncertain. “[It’s] going on quite a bit longer than what we had originally foreseen,” McChesney says. For now, at least, WildCare and other organizations against the eradication plan can breath a sigh of relief, as the project’s earliest possible start time has been pushed back to Fall 2015. 

Carly Nairn
Carly Nairn is a freelance audio and print journalist. She has created work for The San Francisco Bay Guardian, J. The Jewish Weekly, and KQED, among others. She is a graduate of UC Berkeley’s School of Journalism, and currently lives in San Francisco.

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