California Considers Dropping 3,000 Pounds of Poison on Infested Islands

Non-native house mice have overrun the Farallons and they’re threatening several types of birds, insects, and amphibians. Some wildlife biologists believe the best way to eradicate these mice is to drop 3,000 pounds of rat poison on the national wildlife refuge.

“This course of action was a difficult decision for us,” said Gerry McChesney, the manager of the Farallon Islands National Wildlife Refuge for the USFWS. “But we have come to the conclusion…that this project is necessary and the right thing to do to stop the ecosystem carnage done by mice.”

After years of debate, the California Coastal Commission (CCC) finally voted 5-3 in December to move forward with the poison airdrop. The decision was not made lightly, and it came amidst vocal opposition from the public. But scientists believe it’s the best decision, and the situation offers several lessons for hunters and anglers debating invasive species in their own communities.

The Facts President Theodore Roosevelt established the Farallon Islands National Wildlife Refuge in 1909 as a preserve and breeding ground for marine birds. It’s located about 30 miles west of San Francisco, and every year, 350,000 breeding birds of 13 different species descend on the islands to mate and raise their young.

One of these species is a threatened bird called the ashy storm petrel. The ashy storm petrel is found almost entirely on the California Coast, and half of the world’s population of 5,000 to 10,000 breeding pairs mate on the Farallon Islands. Those pairs lay a single egg and spend an enormous amount of time (for a bird) raising young, which slows their reproductive rate and makes them susceptible to invasive species.

Unfortunately, that’s exactly the problem they’re facing on the Farallons. The mouse population has grown to as many as 1,270 individuals per acre more than a century after escaping naval ships. Researchers who regularly work on the island say the ground appears to “move” with mice.

The mice don’t usually predate directly on the storm petrel, but they do encourage burrowing owls to overstay their welcome. The owls visit the islands every year, but instead of moving on prior to the storm petrel’s arrival, they stay to feast on the mice population.

Here’s the problem: the mice population goes through boom-and-bust cycles, so when many of the mice disappear, the owls start targeting storm petrels. Only about eight owls stay on the island, but those eight can take a huge toll on the storm petrel population.

Point Blue Conservation Science, whose scientists live on the islands year-round, report that preliminary modeling shows a 2.7% per year decline, equating to an overall decline of 32%, in ashy storm petrel capture rates on the South Farallon Islands since 2007. In the next 20 years, the total population could be reduced by another 63%.

“More is known about that island and its ecosystem than pretty much any place an eradication has been proposed,” Brad Keitt told MeatEater. “The impact on the [storm petrel] population is clearly documented and established through modeling and expert analysis.”

Keitt is the oceans and islands director for the American Bird Conservancy (ABC). ABC supports the USFWS plan to eradicate the mice from the Farallons, which is significant since both supporters and opponents of the plan cite dangers to birds as justification for their position. Keitt told us the ABC believes the poison airdrop is “the only solution to achieve the project goals.”

The petrels aren’t the only threatened species affected by the mice. The rodents feed on the Farallon camel cricket, and they compete for food with the Farallon arboreal salamander. Neither species exists anywhere else in the world.

The Debate Most folks agree that something needs to be done about the mice. None of the CCC commissioners who voted against the plan contradicted the seriousness of the problem, and scientists have known for years that island populations are at the greatest risk of extinction. Fun fact: of the nearly 250 recorded animal extinctions since 1500, 80% have been on islands.

But the idea of dropping 3,000 pounds of mouse poison (a rodenticide known as “Brodifacoum-25D Conservation”) on a famous and sensitive wildlife refuge is a bridge too far for some. The commissioners who rejected the USFWS proposal expressed indignation that an aerial poison drop is the best solution.

“We’re looking at one approach: nuke ‘em, and that’s it,” said Commissioner Robert Uranga.

“I don’t have a sense that it’s going to have a happy ending,” said Commissioner Carol Groom, who also voted against the plan.

Even the commissioners who voted for the proposal were careful to express their distaste for doing so. Commissioner Katie Rice described the poison as “horrible” and said she was voting yes with a “heavy heart.”

Ultimately, however, a majority of commissioners were convinced that the USFWS’s solution is the best available. The poison will be dropped in bait pellets from airplanes fitted with special directional dispersal buckets that will be able to aim the pellet drop and avoid marine areas. McChesney explained that the USFWS selected aerial dispersal because much of the landscape is too rugged to reach.

Several commissioners pressed McChesney about other options, but as he explained to Commissioner Sara Anubzadeh, the other 48 solutions the USFWS considered are either untried (like mouse contraception) or would require unacceptable disturbance to the island.

“If [alternative solutions] are done throughout the entire year, we don’t know what the impact on all those birds is going to be,” he explained. “You’re probably going to end up impacting the ecosystem more than you’re going to help it.”

The rodenticide solution may be unpalatable to the public, but it’s proven to be effective on other islands. Rats were removed from California’s Anacapa Island in 2003 using aerially dispersed rodenticide, and 10 years later, the sea birds have made a “profound” recovery, according to the National Park Service. The technique has also been used in three national wildlife refuges in the Pacific, two islands off the coast of Mexico, and many islands off the main landmasses of New Zealand.

Despite these assurances, some commissioners were still understandably worried about the impact to non-target species. Birds could eat poisoned mice and become sick themselves, and some of the bait will end up in the ocean.

The pellets that end up in the water will dissolve within an hour. Gregg Howald, an island conservation expert advising the USFWS, told the commission that “there’s no evidence of any impact on any degree of scale” to marine life. Birds might be affected by the poison, but the USFWS is taking many precautions to avoid any significant impact. They’ll be dropping the pellets to avoid sensitive breeding seasons, hazing gulls and other birds from poisoned areas, and capturing and holding predatory birds until the danger has passed.

Bottom line? While the poison will doubtless impact some non-target species, the one-time nature of the rodenticide drop ensures that these impacts will not be long term or population wide. The benefit of eradicating the mice, on the other hand, will be enormous.

The Takeaway for Hunters and Anglers House mice on the Farallons aren’t likely to affect any game species, but the public debate surrounding this issue can inform other debates about invasive species that impact the critters we love to pursue.

First, it’s crucial that science overcomes optics. Public opinion should be taken seriously, and in this instance especially, opposition is understandable. But it shouldn’t trump true scientific consensus, and that was a real concern in this case.

The members of the CCC were well-versed in the scientific realities surrounding this proposal, but many still expressed concern about how the public would perceive their decision.

Commissioner Hart worried that the public would be wary of contaminated fish even though the threat to marine life is virtually nil. “One of the things that concerns me is, so often with the public that’s eating fish, they know that this has happened, and it will impact their attitudes if nothing else,” she said.

Commissioner Aminzadeh, who voted for the proposal only after assurances were made that the commission would have a chance to review a more detailed plan, also referenced the “public outcry” in her questions to the USFWS.

Commissioner Rice acknowledged the opposition from the public but ultimately decided to go with the scientists. “I would hate to see us get in the way of allowing this island to heal and getting rid of these mice,” Rice said.

The PR battle surrounding the Farallons is complicated by the fact that the situation is, well, complicated. But the CCC’s vote demonstrates that, given enough time, good solutions to real problems can overcome the perceptions of vocal but misguided opposition.

“It’s not an easy soundbite,” Keitt said. “It can’t fit in a tweet. But if you look at the data, the story is compelling. You have to weigh the long-term benefits with the short-term impacts.”

That PR battle was won, in part, because the proposal to eradicate the mice wasn’t justified by a desire to purify the Farallons. As Keitt explained, this proposal passed muster because it identified a real, persistent problem and offered a workable, permanent solution.

“This project is not a result of a knee-jerk reaction to the claim that all invasive species are bad, and we have to go back to pristine ecosystems with no invasive species,” he said. “This is based on a clear set of data showing that if we do not remove mice from the Farallons, native species that only occur in the California current will be on a trajectory towards extinction.”

From feral hogs in the South to feral cats in Hawaii to zebra mussels in Michigan, invasive species are always on the radar for hunters and anglers. As the Farallon mice eradication project demonstrates, it’s possible to reach a science-based solution that protects native species—not to return to a “pure” state of nature, but to properly care for the animals we’ve put in harm’s way.

The CCC’s vote was a crucial first step, but this debate isn’t quite over. The USFWS still must finalize a plan, and the regional director of the U.S. Fish and Wildlife Service will have to review it. If all goes well, the program could be up and running in 2024.