The U.S. Fish and Wildlife Service and its partner, Point Blue Conservation Science, are proposing to restore the native ecosystem of the South Farallon Islands by removing non-native, invasive house mice and eliminating their negative impacts on the wildlife and wild habitats of the islands. The ecosystem is severely out of balance, putting the survival of threatened species and the unique habitat at risk, and the presence of invasive house mice have harmful direct and indirect impacts on the islands’ breeding seabirds. Removing invasive house mice will allow the ecosystem of the globally important Farallon Islands National Wildlife Refuge to recover from past human impacts, restoring the island to a more natural state that will also build resilience to the impacts of climate change.

The U.S. Fish and Wildlife Service, which manages the land as the Farallon Islands National Wildlife Refuge, is the lead agency on the project. The Service’s partner on the effort is Point Blue Conservation Science, a non-profit organization dedicated to conserving birds, other wildlife, and ecosystems through innovative scientific research and outreach. Point Blue is the globally recognized expert on the wildlife of the South Farallon Islands, which it has been studying on-site since 1968.

The Farallon Islands comprise three groups of small islands located nearly 30 miles west of San Francisco, California. Southeast Farallon Island, part of the South Farallon Island group, is the largest island at 70 acres.

• The Farallon Islands (or Farallones) host a unique ecosystem. Surrounded by the productive ocean waters of the Gulf of the Farallones, the islands host the largest seabird nesting colony in the contiguous United States, with 350,000 breeding birds of 13 species. The rare ashy storm-petrel, a species found almost entirely on the California coast, has its largest colony here. Thousands of seals and sea lions of five species also utilize the islands for breeding and resting, and the endemic Farallon arboreal salamander and Farallon camel cricket occur nowhere else in the world. In addition, the Faralloneshost a unique plant community dominated by the maritime goldfield (also called Farallon weed), a species found only on seabird nesting islands in California and Oregon.

In 1909, President Theodore Roosevelt established the Farallon National Wildlife Refuge as a preserve and breeding ground for marine birds. The Refuge originally encompassed only the North and Middle Farallon Islands and Noonday Rock. In 1969 the Refuge was expanded to include the South Farallon Islands. The Farallon Islands National Wildlife Refuge, as it was renamed in 2017, is one of 562 National Wildlife Refuges managed by the U.S. Fish and Wildlife Service across the U.S.

The islands are surrounded by a marine area that is designated a California State Marine Reserve (established in 2010) managed by the California Department of Fish and Wildlife and within the Greater Farallones National Marine Sanctuary (established in 1981) managed by the National Oceanic and Atmospheric Administration.

A small, rotating contingent of researchers from Point Blue Conservation Science resides on Southeast Farallon Island year-round to monitor wildlife, take care of the facilities, and safeguard the islands. Refuge Managers from the U.S. Fish and Wildlife Service and other researchers also occasionally visit the islands. The refuge is closed to the public, but boat tours take passengers close enough to enjoy spectacular island landscapes, thousands of seabirds, seals, sea lions, and other marine life.

Of the nearly 250 recorded animal extinctions since 1500, 80% have been on islands. When causes could be determined, non-native, invasive species were responsible for 54% of these island extinctions.

Island species are especially vulnerable to extinction because of their small populations and limited habitat. Many island species have also evolved in isolated environments with limited competition and an absence of native predators. These island species are adapted to ecosystems that are comparatively safer than most continental ecosystems, making them vulnerable to introduced, invasive species. Non-native species often quickly alter and damage the ecosystems to which they are introduced, both directly and indirectly, and cause native species to decline or even become extinct.

Non-native house mice were introduced, probably accidentally, to the South Farallon Islands by human visitors well before the Service’s acquisition of the islands in 1969. Among these early visitors were seal hunters, commercial egg collectors, lighthouse keepers, and the U.S. Navy. Many of the seals and seabirds have recovered significantly from the harvesting that occurred in the 19th century and the large-scale human disturbance that finally ended with the establishment of the Refuge. But the invasive house mice remain as a damaging legacy.

The invasive house mouse population fluctuates seasonally and annually, but a recent sample of mouse density on Southeast Farallon Island suggests a density of up to 1,270 mice per acre at certain times of the year. The population of mice on the Farallon Islands can reach plague levels and is among the highest mouse densities reported on any island worldwide. The density is such that, according to researchers on the island, the ground appears to “move” with mice.

Invasive, non-native house mice have direct and indirect harmful impacts on the islands’ breeding seabirds, especially ashy storm–petrels, but also on Leach’s storm-petrels, as well as on native salamanders, crickets and other invertebrates, and native plants.

Ashy storm-petrels are harmed indirectly by the mice, which attract large numbers of burrowing owls that stop at the islands to rest on their fall migration and find a plentiful food source when invasive mice are at a seasonal population peak. So instead of continuing on their migration, several owls remain on the islands through the winter to feed on the mice. This keeps them on the island long past the natural time for them to move on. But the mouse population crashes each winter, forcing the owls to seek other prey. The owls then feed on the rare ashy storm-petrels, reducing their population. The owls also feed extensively on rare Farallon camel crickets. Removing house mice from the South Farallon Islands would break this chain of events. With no mice to feed on, the visiting owls would continue on their southward migration instead of feeding on rare Farallon birds and crickets.

Invasive mice also feed extensively on native invertebrates, reducing their populations, and compete for food with endemic Farallon arboreal salamanders. In addition, mice feed heavily on native plant seeds such as the maritime goldfield, reducing their populations in favor of more hardy non-native plants.

The Service conducted a thorough, science based Environmental Impact assessment over a number of years and came up with three alternatives to consider:

1. Alternative A: Taking No Action, which would allow mice to remain on the South Farallon Islands and continue to negatively impact the islands’ storm-petrels and other native and endemic species.
2. Alternative B (the Service’s Preferred Alternative): Eradicate invasive house mice from the South Farallon Islands by the aerial broadcast of rodent bait containing Brodifacoum-25D Conservation.
3. Alternative C: Eradicate invasive house mice from the South Farallon Islands by the aerial broadcast of rodent bait containing Diphacinone-50 Conservation.

1. The complete removal of invasive house mice from the South Farallon Islands using the best available methods;
2. Minimize and mitigate any negative impact to the native species and other natural and cultural resources of the islands;
3. Ensure human safety is preserved during project implementation;
4. Ensure that benefits of mouse removal outweigh any incidental negative effects from project implementation; and
5. Prevent the future reinvasion of house mice through implementation of a biosecurity plan.

Science and history tell us that we need to act now to address invasive species on island ecosystems, where 75% of bird, amphibian, mammal and reptile extinctions have occurred. Many of those extinctions have been caused by invasive species. We need to act before the island reaches its tipping point that could result in the demise of the ashy and Leach’s storm-petrels, Farallon arboreal salamander, Farallon camel cricket, or other species. We need to act to remove the invasive house mouse before it is too late.

1. To increase the population sizes of ashy and Leach’s storm-petrels;
2. To restore native ecosystem functions altered by invasive house mice;
3. To increase the abundance and recruitment of native vegetation;
4. To increase the productivity and abundance of endemic Farallon arboreal salamanders and endemic Farallon camel crickets and other native invertebrates;
5. To improve the wilderness character of the Farallon Islands; and
6. To improve species and ecosystem adaptability and resilience in light of projected future climate change.

The U.S. Fish and Wildlife Service reviewed all options to remove invasive house mice from the island and prepared an environmental Impact Statement that reviewed scientific literature, input from a broad range of experts, and public input. In total, 49 different removal methods were assessed, including a sustained population control program, the use of bait stations, fertility control, trapping, introduction of a biological control agent such as snakes or cats, and burrowing owl relocation. After reviewing all options in coordination with a variety of federal, state and conservation partners, the U.S. Fish and Wildlife Service conducted several pre-eradication studies, reviewed lessons learned from past projects, and consulted with collaborating agencies. After a thorough assessment grounded in a careful, rigorous and transparent process and science-based decision making, all but two potential action alternatives were dismissed.

There are no approved contraceptive products for mice, and the Farallon Islands are too important to use an untested solution. If there was a contraceptive bait approved for use, fertility control is still not an appropriate solution for restoring the Farallon Islands for multiple reasons, including:

• Contraceptives only cause temporary infertility and must be introduced into the ecosystem regularly and permanently to continue controlling the population
• Any contraceptive would need to be developed and approved for use in a bait form so it could be available to all mice around the island, and this type of product has not been developed
• If a solid bait were developed, it would need to be delivered by aerial broadcast to reach the inaccessible parts of the islands. The disturbance caused to Farallon seabirds and marine mammals by multiple drops per year would have catastrophic impacts.

NEPA

The U.S. Fish & Wildlife Service was required to evaluate the environmental effects of this proposed project according to the guidelines for implementing the National Environmental Policy Act (NEPA). The multi-year NEPA process included notifying other government agencies, private stakeholders, and the public of the proposed project and giving the opportunity for all to provide comments into the scope of the issues to be addressed in the Environmental Impact Statement (EIS) and, later, to provide comments on the draft EIS.

The Service has now completed its analyses and Final EIS. The Notice of Availability for the Final EIS was published in the Federal Register on March 15, 2019. The Final EIS is available from regulations.gov, Docket No. FWS–R8–NWRS–2013–0036.

1. Seabirds: Ashy and Leach’s storm-petrels are expected to benefit as a consequence of reduced predation and improved survivorship. In particular, eradicating house mice is expected to reduce the numbers of overwintering burrowing owls and resulting owl predation on storm-petrels.
2. Salamanders: The endemic Farallon arboreal salamander is anticipated to benefit from the removal of a likely competitor for invertebrate prey and a potential predator of salamander eggs and juveniles.
3. Invertebrates: Native invertebrates of the South Farallon Islands, including the endemic Farallon camel cricket, are expected to benefit from reduced predation pressure from invasive mice and other predators attracted to the islands by the mice, such as burrowing owls.
4. Plants: Native plants stand to benefit as a consequence of reduced seed and seedling predation.

The ashy storm-petrel is a small, grayish-brown nocturnal seabird. Its breeding range is limited to rocks and small islands off the coast of California, and northern Baja California, Mexico. Some experts estimate that there are only about 5,000-10,000 breeding ashy storm-petrels. Nearly half the world population breeds on the South Farallon Islands. The remainder nest on the Channel Islands off southern California and other small rocks and islands along the coast. They spend most of their time far at sea where they feed on small fish and invertebrates. Because of their secretive nature, they are rarely seen on land. However, in the fall months they congregate in large flocks at sea which are a favorite sight of ocean birdwatching trips.

Because of its small world population and the numerous threats it faces, the species is listed as Endangered by the International Union for Conservation of Nature (IUCN). The U.S. Fish & Wildlife Service has identified the ashy storm-petrel as a Bird Species of Conservation Concern and it is listed as a Bird Species of Conservation Concern by the California Department of Fish and Wildlife. Taking measures to reduce risks to the ashy storm-petrel population, such as mouse eradication on the South Farallon Islands, is expected to help keep the rare, threatened bird off the federal Endangered Species list.

Yes, as visiting migrants. However, it is not natural for the owls to remain on the islands for extended periods of time, such as through the winter, when they prey on storm-petrels. The presence of mice entices them to stay longer than they naturally would.

The total amount of rodent bait needed would be about 2,917 lb (1,323 kg), or 1.45 tons. Because the bait contains only a small fraction (0.0025 percent) of rodenticide, this amount of rodent bait contains only 1.16 oz (33g) of brodifacoum, in total. The remainder of the bait is made of cereal grains.

The Service is confident that once mice are removed, most if not all burrowing owls landing on the South Farallones in the fall will continue on their migratory path and find more suitable wintering areas on the mainland, having found insufficient food resources on the islands.

Predatory bird populations on the South Farallon Islands have been monitored by biologists from Point Blue Conservation Science since 1968. This long-term data shows that most birds of prey that do not eat mice, such as Sharp-shinned Hawks and Cooper’s Hawks, do not over-winter on the Refuge even though they are occasionally present briefly during the fall migration period. In contrast, the mouse-eating Burrowing Owls have a much higher tendency to spend the winter on the South Farallones.

Gulls
Gulls, especially western gulls, would normally be at risk of either consuming bait pellets directly or consuming mice that have eaten bait, and could experience negative impacts from the rodenticide. However, based on extensive research and field tests, the Service is confident that gulls can be kept away from an impacted area during a mouse eradication. Specifically, the Service would conduct a gull hazing program to remove gulls from the island during the operation and for the short duration of bait availability. In addition, the Service would remove any mouse carcasses detected, to further reduce the risk to gulls and other scavengers.

Raptors
Raptors, such as burrowing owls and peregrine falcons, could also be at risk if they fed on exposed mice, gulls or invertebrates. Thus, efforts would be made to capture, hold or translocate any raptors present on the islands for the duration of the eradication project.

Salamanders
Salamanders could be exposed to rodenticide by consuming insects that have eaten bait pellets. However, the threat to salamanders is expected to be low. Nonetheless, because of the uniqueness of the island population, a limited number of salamanders would be captured and held for the duration of the project as a precautionary measure.

Only the land surface of the islands, and not the marine environment, would be targeted for bait application. Measures to reduce the risk of bait entering the marine environment would include GPS precision guided flight paths and the use of a precision application bait bucket. However, a small amount of bait could still inadvertently enter the marine environment immediately adjacent to the islands. Because bait pellets would disintegrate quickly upon entering the water and the rodenticide is not soluble in water, risks to marine wildlife and water quality would be very low, based on other eradication projects. Very few of the fish and marine invertebrate species living in the waters around the Farallon Islands would be expected to consume an encountered bait pellet. Studies have shown that most invertebrates are not impacted by rodenticides such as brodifacoum. Given these factors, the risk of exposure to rodenticide by marine fish and invertebrates would be limited to a small number of individuals, with no significant impacts to populations.

Extensive research and field tests have been conducted on the islands, accompanied by lab testing and modeling by the Service and its partners, to ensure minimal risk to native wildlife. Efforts to minimize impacts to island resources include timing of implementation to avoid sensitive breeding periods and times when migratory wildlife are most abundant. The most acceptable time period for the eradication would be between early October, when seabird and pinniped breeding seasons have largely concluded, and the end of December before the first northern elephant seal pups are born. In addition, other actions to be implemented to minimizing risk include a hazing plan to protect gulls and other birds from exposure to potential risks, capture and hold or relocation of predatory birds, the use of bait stations in certain sensitive areas, and the use of specialized equipment and techniques to minimize the risk of bait drift into the marine environment. Additionally, the physical and chemical properties of the bait formulation, low water solubility of the rodenticide and strong chemical affinity of brodifacoum to the grain matrix significantly reduce the chance of rodenticide contaminating aquatic or marine environments.

Since the Service began active stewardship of the South Farallon Islands in 1969, several restoration and enhancement measures have been implemented, including:

• Closing the Refuge to public access to protect wildlife and habitats.
• Designating all but Southeast Farallon Island as wilderness.
• Closing off sensitive areas to human access (including biologists).
• Removal of feral rabbits and cats from the islands in the 1970s, with the last of these invasive animals removed in 1974.
• Controlling introduced (non-native) flora.
• Installation of boardwalks to prevent trampling of sensitive habitat and limiting most human activities to only a few established trails.
• Minimizing night lighting and screening it from view so that nocturnal species are not disturbed.
• Removing unneeded structures to maximize natural habitat available to wildlife.
• Limiting the number of people allowed on the island at one time.
• Construction of the “Murre Ledge” to shield an expanding Common Murre colony from human disturbance, using materials from removed building foundations.
• Construction of artificial habitats for crevice-nesting species, including ashy storm-petrels, using the remains of removed building foundations.
• Rebuilding and maintaining rock walls that are used by crevice-nesting seabirds, including ashy storm-petrels.

Yes, invasive rodent removals have been successfully completed on nearly 700 islands worldwide, including on California’s Anacapa Island in the Channel Islands National Park, three National Wildlife Refuges in the Pacific, two islands off the coast of Mexico, many islands off the main islands of New Zealand, and recently, multiple islands in the Galápagos Archipelago. Land managers have successfully eradicated house mice from more than 60 islands worldwide. Nearly all of these successful projects utilized techniques like that proposed for the South Farallon Islands house mouse eradication. Learn more about successful island restorations here.

Additional information can be found at the Fish and Wildlife Service’s refuge page here.

For further information, contact Farallon Islands National Wildlife Refuge manager Gerry McChesney at Gerry_mcchesney@fws.gov.