Photo: View from Farallon Islands from above. Credit: USFWS
Updated January 4, 2016.
Located 30 miles off the San Francisco coast, the Farallon Islands are home to an extraordinarily diverse assemblage of animal life. Hundreds of thousands of seabirds nest on the islands each year, creating a cacophony of sound. Sea life thrives in the waters around the Farallones, including gray whales, sea turtles and dolphins.
But amid the rocks lurks an adorable intruder: the house mouse.
The nonnative mouse has reportedly attracted large numbers of migratory burrowing owls, which have had detrimental effects on an endangered seabird known as the ashy storm-petrel. Because mouse populations naturally decline in the winter, the owls switch to feeding on the seabirds.
Between 1972 and 1992, storm-petrel populations on the Farallon Islands dropped by nearly 40 percent.
“If burrowing owl predation continues at current levels, the storm-petrel population will decline by 27 percent every ten years,” said Nadav Nur, ecologist at Point Blue Conservation Science.
There are roughly 6,000 ashy storm-petrels remaining worldwide, half of which are found on the Farallones. Unchecked owl predation could have serious consequences for the species, researchers say.
Further complicating matters is the fact that house mice are voracious omnivores. Not only do they prey on the Farallon camel cricket and other rare island invertebrates, they also spread the seeds of invasive plants in their droppings.
Despite their small size, the mice have become a big problem. A 2010 survey estimated the mouse population on the Farallon Islands was close to 200,000 individuals, the highest reported density for any island in the world.
“The mouse population on the Farallones is off the charts,” said Gerry McChesney, Refuge Manager for the U.S. Fish and Wildlife Service (USFWS). “In the fall, when their population is at its peak, the ground looks like it’s moving.”
In 2013, the USFWS became embroiled in controversy after it proposed a plan to eliminate mice from the South Farallon Islands. Although a number of alternatives are currently under consideration, the use of the rodenticide brodifacoum has attracted the most attention.
The U.S. Environmental Protection Agency banned the consumer sale of products containing brodifacoum in 2013, in part due to its “very high” toxicity to non-target mammals and birds. Several investigations, including a five-year study conducted by the California Department of Fish and Wildlife, have reported high incidence of brodifacoum poisoning in non-target birds and mammals.
USFWS officials acknowledge that several bird species on the Farallones would be at risk of eating the poisoned mice, including gulls and migratory burrowing owls. However, they are also quick to point out that options exist for minimizing the risk to non-target wildlife, including trapping and removing raptors from the island and installing devices to deter gulls.
“None of the project partners would be willing to proceed with an attempt to eradicate invasive mice from the Farallon Islands if the results were likely to lead to significant harm to any native species,” said Dan Grout, Project Manager for Island Conservation, a non-profit working closely with USFWS.
Though there have been a number of effective eradications on islands, unsuccessful projects often cast a long shadow.
In 2008, Island Conservation spearheaded a rodent eradication project on the aptly named Rat Island in Alaska. The aerial distribution of 46 tons of brodifacoum did eliminate rats from the island, but it also resulted in the deaths of over 420 birds, including 46 bald eagles and 320 glaucous-winged gulls.
Of the 82 mouse eradication projects to date worldwide, just over half have been successful. But proponents argue that the success rate of large-scale eradication projects has drastically improved in recent years. Since 2007, 10 of 11 eradications performed have successfully eliminated mice from islands.
For now, USFWS researchers are proceeding cautiously.
“We have to look at every project with a fresh perspective,” said McChesney. “We can’t assume that because another project went well, the same will be true for our project. Every island is different.”
The proposed Farallon Islands restoration project remains in limbo, as researchers evaluate potential risks and alternatives.
More information on the effort to restore the Farallon Islands can be found here.
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