Forty-three kilometers (27 miles) off the coast of San Francisco lies the rugged, isolated Farallon Islands. The environmentally sensitive habitat is home to the largest seabird breeding colony in the contiguous US; every year, 350,000 birds from 13 different species come to rest on the islands, as do rare species such as the Farallon arboreal salamander and the endemic Farallon camel cricket. Another unwelcome critter calls this remote island chain home: mice – and the US government has a plan to remove the nuisances once and for all.

A proposed plan put forward by the US Fish and Wildlife Service calls for the dumping of nearly 1,360 kilograms (3,000 pounds) of an anticoagulant rodent poison called Brodifacoum-25D, eliciting concern from California organizations about the potential for unintentional poisoning of other species, as well as impacts to local water quality and oceanic runoff that could make its way into human food chains.

Mice were introduced to the Farallones two centuries ago and have since wreaked “long-term ecological damage” as they eat native species, compete with local wildlife, and even prey on baby birds during the breeding season. Their population is among the highest of any island in the world with more than 490 individuals covering one acre during peak season. Proponents of the plan say that their eradication will restore local animal and plant populations, but it does not come without cost.

“Implementation of the proposed project will lead to non-target wildlife species mortality. Nevertheless, while this effect is unavoidable it would not be significant in the context of species populations in the region,” writes the US Fish and Wildlife Service in its 33-page report, noting that the benefits of removing the mice will outweigh any cost associated with unintentional deaths in non-targeted species. The USFWS says that effects on marine resources would be restorative, with “temporary adverse effects minimized and monitored.”

Others aren’t so convinced. Certification paperwork of the rodenticide reads that “predatory and scavenging mammals and birds might be poisoned if they feed upon animals that have eaten bait” – a cost opponents say isn’t worth the potential benefits. As of June 27, dozens of opposition comments were recorded, including those from The Ocean Foundation and the Counties of Marin and Santa Cruz. An independent assessment conducted by Coastwalk California notes the high failure rate of eradicating rodents from island ecosystems, particularly mice.

“In short, if a creature is populating an island to the point that no habitat remains for anything else, then, yes, some form of control may be justified, but the method applied must be one that does not destroy the very ecosystem we are trying to preserve,” the organization writes, adding that an analysis of data provided by the USFWS suggests that “indirectly, the mice are attracting an average of six burrowing owls a year, which occasionally kill the chicks of the ashy storm-petrel, USFWS has failed to demonstrate that the mice are having a negative impact on the Farallon Islands.”

Greater Farallones National Marine Sanctuary responded directly to the plan, noting that depositing any material and other matter is “prohibited and thus is unlawful for any person.” The agency goes on to request a more detailed analysis into the effects of the plan, including an outline of disposal methods for mice carcasses and an assessment of potential environmental contaminants.

“Undoubtedly the poison will travel up the food chain; not only killing the intended mice, but also the entire predator/carnivore community living with the coastal zone. This is exactly how the food web is destroyed for generations,” wrote self-identified wildlife biologist Kim Fitts in a comment. Another likened the plan to “using a bomb to kill a mosquito.”

A hearing on the proposal is set for July 10.