Ship-brought mice hit 1,270/hectare off SF, reshaping seabird ecosystem

Accidentally introduced to the Farallon Islands by ships in the 19th century, invasive house mice have reached alarmingly high densities of up to 1,270 individuals per hectare. This densification has caused significant disruptions to the delicate Farallon Islands ecosystem, situated around 30 miles off San Francisco's coast. A recent study published in the journal Ecosphere, alongside conservation reports from Point Blue Conservation Science and the San Francisco Bay Wildlife Society, highlights how these house mice (Mus musculus) have reshaped the food web on the South Farallon Islands.

The Farallon Islands National Wildlife Refuge houses the largest seabird nesting colony in the contiguous United States, hosting over 300,000 breeding birds from 13 species. However, the unintended arrival of house mice during maritime trade times has led to a long-lasting ecological challenge that continues to affect native wildlife.

The high mouse populations contribute to indirect predation - as mouse numbers surge in autumn, reaching up to 1,270 per hectare, they provide an abundant food source for burrowing owls (Athene cunicularia), which migrate to the islands during this season.

However, when mouse numbers decline during winter, the owls shift their prey to ash-colored storm-petrels (Hydrobates homochroa), a globally vulnerable seabird that breeds on the islands. With only one egg laid per year, the Ashy storm-petrels are particularly susceptible to owl predation. Moreover, the invasive mice also consume large quantities of native plant seeds and compete with endemic salamanders for food, potentially hindering the growth of native vegetation and the spread of invasive species such as maritime goldfields. This can compromise essential soil protection and nesting habitats within the ecosystem.

In response to this environmental threat, the U.S. Fish and Wildlife Service, along with Point Blue Conservation Science and other conservation organizations, initiated the South Farallon Islands Invasive House Mouse Eradication Project. This project involves spreading rodenticide bait via helicopter during late autumn, a period when mouse numbers start to naturally decline and seabird populations are at their least.

The rodenticide used in the project, Brodifacoum-25D Conservation, has been utilized in similar rodent removal efforts worldwide, resulting in native ecosystems and seabird population recoveries in many cases.

The eradication project underwent extensive environmental review, and in December 2021, the California Coastal Commission approved the plan, finding that the long-term ecological benefits outweighed short-term risks. Measures to protect other wildlife during the operation include the temporary relocation of burrowing owls and the use of various deterrents, such as pyrotechnics, sound-based deterrents, and trained falcons, to keep Western Gulls away from the island during bait application.

The project aims to ultimately interrupt the cycle of exploding mouse populations attracting owls that subsequently prey on storm-petrels, and also enhance native seed dispersal and the recovery of native invertebrate and reptile populations.