Farallon Islands mouse poisoning plan divides conservation community
Golden hills and colorful leaves may define autumn for much of Northern California, but on the South Farallon Islands it is the mice — tens of thousands of them.
Since they arrived on the island refuge west of San Francisco Bay as shipboard stowaways in the19th century, the invasive house mice have overwhelmed its steep, granite outcrops each fall.
Their numbers peak with the availability of food in late summer and fall, and then plummet into inescapable suffering and collapse.
At their height, the tiny rodents are “plague-like” — so numerous “the ground is seething” at night, according to ornithologist Peter Pyle, a research scientist who spent 20 fall seasons at the Farallon National Wildlife Refuge field station earlier in his career.
Official estimates put the peak population around 500 mice per acre, or 60,000 across the 120 acres that make up the southern islands. They are omnivorous, and ravenous, consuming plants and seeds, insects, amphibians, seabirds and even marine life.
When the food runs out, they turn on each other, die of starvation or cold, or succumb in flooded burrows.
This stark cycle of life and death, however, is not primarily what concerns the U.S. Fish & Wildlife Service, the federal agency in charge of the refuge, which is the largest seabird breeding colony in the lower 48 states.
Rather, it is their impact on other species and the ecological balance writ large that has divided conservation interests across the region, especially over the agency’s plan to poison the mice.
Saving the petrels
Using a strategy 17 years in the making, the agency hopes to shower the southern islands with 2,880 pounds of grain pellets laced with rodenticide in hopes of wiping out nonnative mice when they are at their lowest numbers at the end of 2022.
The model has been used around the globe to restore native ecosystems knocked off-kilter by invasive rats and mice.
The hope is the absence of mice will shore up populations of native cricket and salamander species found nowhere else on Earth, as well as save a rare island plant called maritime goldfield, which is threatened by invasive grasses spread by the mice.
But the main goal is to save a diminutive, smoke-gray bird called the ashy storm-petrel, a Fish & Wildlife “Bird of Conservation Concern” listed as endangered by the International Union for Conservation of Nature.
The stormy-petrel is considered at risk of extinction on the Farallones, where nearly 60% of the world’s declining breeding population once nested.
Many of the birds are eaten by burrowing owls that stop on the islands mid-migration to feast on the abundant mice.
Instead of moving on, a handful of owls stay through the winter, and once the rodent population crashes, they turn to the ashy storm-petrels.
Though the burrowing owls are few in number — conservatively 8 to 12 — they consume large quantities of petrels, which offer minimal nutrition, biologists say.
Some of the owls, also a California Bird Species of Concern, actually die of starvation, but not until after they have plundered the storm-petrel colony.
If the owls have no reason to stop — i.e., no mice — the hope is they’ll all move on and leave the petrels alone, proponents of the eradication plan say.
The world’s storm-petrel population, estimated at 8,000, is projected to drop more than 60% over the next two decades, said Pete Warzybok, who has spent more time on the island than anyone else as Farallones Program Leader with Point Blue Conservation Science.
Even the most ardent proponents admit that poisoning the rodents is not ideal. And they acknowledge there will be unintended deaths among other animals.
But the plan to save the storm-petrels and restore a healthier, more natural ecology is modeled on hundreds of similar operations they say have proven successful around the world.
“If we did not feel we could do this successfully, with minimal, non-target mortality, we would not do this project,” Refuge Manager Gerry McChesney said.
“The Farallones, it’s a very special place, and this is a project that we have been researching and planning for a decade and a half,” he said. “We’ve gone about it very carefully and methodically ... we didn’t want to leave any stone unturned.”
But some guardians of the coastal environment have been fighting the plan for years and will again when it comes before the California Coastal Commission on Thursday.
Two years ago, the federal agency withdrew a version of the plan after questions from commissioners. The plan has been updated to address concerns, and commission staff now recommends approval.