Mice Make Trouble in the Farallones

The Farallon Islands have been crawling with house mice for years. They may have stowed away on boats and ridden out to the islands as early as the 1800s. Mice can be annoying or -- if they're your pets -- cute, but in the Farallones they’re causing problems on a life-and-death scale.

“People on the island talk about how the ground moves because there's so many mice,” explains Brad Keitt, the Director of Conservation for Island Conservation. The non-native mice attract burrowing owls, which would normally stop by the island for a meal, then head out. But the mouse bounty has caused them to extend their stay. And when the mouse population falls in the winter, the owls switch to eating birds, including the ashy storm-petrel, an endangered species that only nests on islands off the coast of California.

Santa Cruz-based Island Conservation, whose mission is to stop extinctions by eradicating invasive species on islands, is working with the U.S. Fish and Wildlife Service and PRBO Conservation Science to rid the South Farallon Islands of the rodents. But as anyone who’s had a mouse infestation knows, that’s easier said than done.

"You have to get rid of every single mouse," says U.S Fish and Wildlife Service spokesman Doug Cordell. "Any method you choose has to be 100% effective, but it can't cause significant harm to other species."

The Fish and Wildlife Service is considering its options. One of those is to drop rodenticide-pellets on the islands. That idea has drawn vocal criticism from San Rafael wildlife organization WildCare.

“The pellets are only for mice, but anything else could eat these things,” says Maggie Sergio, the director of wildlife advocacy at Wildcare. Birds that eat seafood probably wouldn’t be attracted to them, but birds like gulls might. Sergio’s concerned about secondary poisoning, too: Hawks and owls could be affected by eating the poisoned mice.

Sergio acknowledges it’s a difficult decision: Is it worth possibly poisoning other animals to save the ashy storm-petrel? What about the burrowing owls? (Burrowing owls, though not native to the Farallones, naturally live in California, and they’re a “species of special concern [pdf]” due to habitat loss on the mainland.)

Last week the Fish and Wildlife Service held a public meeting at Fort Mason to gather feedback on the project. And they're accepting public comment until June 10 (the deadline's been extended). At this stage, nothing's set in stone, says Cordell.

"It's a tough call. We know we have a real problem out there. The question is what to do about it." He emphasizes that using a rodenticide is just one of the options he expects will be put forward in an environmental impact statement due this fall, but that he's open to hearing other suggestions. Doing nothing may be an option as well, he says.

The effects of invasive species on any ecosystem range from annoying to catastrophic. On islands, it’s always a bit more interesting. Islands usually have fewer species living on them, and those species are often not well-adapted to mainland dangers. The dodo comes to mind. (This fat flightless pigeon living on an island without humans was no match for hungry sailors.) As humans have become more mobile, we’ve also gotten better at distributing plants and animals to previously isolated places all over the world, to the detriment of native species.

And that’s one thing everyone agrees on: This is a man-made problem. What they haven’t figured out yet is how--or if--to fix it.