The decision on whether California will allow the dropping of 1.5 tons of rat poison on islands near San Francisco has been postponed after debate about the effort to kill mice to save a bird pitted conservation scientists against each other.
At a Wednesday meeting in San Luis Obispo, the California Coastal Commission directed U.S. Fish and Wildlife Services to withdraw its application and reapply again, with a plan to return to the commission with additional information about alternatives to the proposal to drop a rodenticide called brodifacoum on the Farallon Islands National Wildlife Refuge 27 miles west of San Francisco.
“We are afraid of this poison. We have seen what it’s done,” Commissioner Dayna Bochco said at the meeting.
The proposal from Fish and Wildlife Services comes after years of research about how to best eradicate the invasive house mice that have taken over the 12-acre island, with a density that exceeds more than 490 rodents per acre at their annual peak.
It’s among the highest recorded for any island, according to a commission staff report, prompting Commissioner Donne Brownsey to describe it as “like a horror movie.”
The mice are causing havoc on an ecosystem that’s been the focus of intensive restoration efforts.
Set in the middle of the Greater Farallones National Marine Sanctuary, the islands contain the largest seabird breeding colony in the United States including several rare species such as the ashy storm petrel and the endemic Farallon arboreal salamander.
Petrels only lay one egg a year and mice eat them. The bird’s population is expected to decline by 63 percent in the next 20 years if the mice are not eradicated, according to Lauren Garske-Garcia, a staff member of the Coastal Commission who recommended approval of the plan.
Some conservation groups are concerned about the effect of the poison on non-target species, including the potential of it traveling to the mainland where rodenticide has been found in thousands of wild dying and dead animals up and down the state.
“It’s like dropping a nuclear bomb on this island, as far as I can see,” said Commissioner Roberto Uranga, who questioned whether it was acceptable to drop a poison that has a lifespan of about 120 days, according to research provided by Fish and Wildlife Services.
Supporters of the plan say they’ve researched all the options and that this is the best one to support full restoration of marine wildlife, including native vegetation on the island. Mice currently eat the seeds of those plants.
Long-term benefits of removing the mice outweigh the short term risks to the islands, supporters say.
The proposed plan is to drop the poison by helicopter in the fall, when the number of birds are a minimum. Then, Fish and Wildlife Services would perform gull hazing to prevent seagulls from becoming exposed to the bait.
The alternative that commissioners expressed the most interest in is contraception, but some Fish and Wildlife Services officials warned that not enough research has been done and the Farallon Islands are not an appropriate place to experiment.
Conservation groups are not unified in their positions on the plan to poison the mice.
A coalition of local organizations including Hands across the Sand, Raptors are the Solution and the Sierra Club all signed a letter in opposition to the plan, which they fear would bring risk not limited to the island environment because non-target animals would ingest poisoned mice or eat the bait and die on the mainland.
The letter, which was read at Wednesday’s meeting, called the rodenticide a “highly toxic and system poison known to have adverse effects across the ecosystem,” and argued that the “health of the food chain across the coast is not worth the risk.”
The American Bird Conservancy shares concerns with other wildlife advocates about the use of rodenticides in urban interface areas, but supports the plan to eradicate the mice on the island.
Hannah Nevins, director of the conservancy’s seabird program, said ashy storm petrel, a small bird that fits in the palm of your hand, has a lifespan of about 28 years. She said she witnessed 40 petrel carcasses below the nest of an owl, which migrate to the island because of the mice.
“This is a significant impact to a species that only lays one egg a year and has a hard time recovering from invasive predators,” Nevins said at Wednesday’s meeting. Using bait, she said, is a “globally significant and recognized action.”
At the meeting, the Coastal Commission staff member who wrote the report recommending approval of Fish and Wildlife Services’ plan acknowledged the difficulty of the decision.
“This sounds like it’s a really difficult situation and there’s a natural emotional response that we feel when we hear ‘dropping rat poison on an island’,” Garske-Garcia said. “I do believe they’ve looked at this in a thorough and comprehensive way. In this situation, it is the solution to take care of the mice.”
The impact of a five-week rat dropping program on an island is far more acceptable than chronic use of bait on the mainland because it allows a boundary and a targeted approach.
Once mice eat the bait, Garske-Garcia said, they retreat into their burrows when dying. A few days to a week later, the mouse carcass breaks down into bacteria and the poison will not persist in the environment.
Garske-Garcia said killing the mice slowly with contraception will be difficult. Mice are sexually mature in three weeks, so killing them all would be like a game of “whack-a-mole,” she said.
“It’s important that we do that in a strong hit and walk away. We need to get all of the mice and be done,” Garske-Garcia said.
No date has been set for a future decision by the Coastal Commission.