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A surplus of mice on the Farallon Islands have caused banded burrowing owls to stay year round instead of migrating,…

The Farallon Islands comprise three groups of small islands located nearly 30 miles west of The City. They host a unique ecosystem, with significant populations of nesting seabirds, marine mammals, and native salamanders, insects and plants.

However, researchers say the ecosystem is severely unbalanced, putting threatened species at risk. The main threat to the islands is house mice.

The restoration project Restore the Farallones, a partnership between the U.S. Fish and Wildlife Service and Point Blue Conservation Science, has been working to restore the islands’ ecosystem for over 50 years. Wildlife and conservation experts met Wednesday to discuss the next steps in restoring the Farallon Islands, after the U.S Fish and Wildlife Service submitted a new application on the project, which must be approved by the California Coastal Commission. The CCC, which had questions about the project in the wake of a previous report from the researchers, is expected to discuss the topic at its June meeting.

For years, Point Blue researchers have been the only humans allowed to live on the islands. Experts at Wednesday’s meeting, including one of the island’s inhabitants, Point Blue’s Farallon Program Manager Pete Warzybok, spoke about impacts the mice have on native plants and other wildlife and advocated for eradication.

“A one-time application of rodenticide is the only proven tested solution that can safely achieve 100% eradication, and restore this key ecological site,” said Zach Warnow, director of communications for Point Blue.

Warzybok said the mice eat native plant species preferentially over invasive plant species, and spread the seeds of invasive plant species, thereby altering the island’s vegetation. He also said they prey on the invertebrate population, competing with native salamanders for resources. They also consume salamanders and salamander eggs.

Additionally, the mice attract burrowing owls. Though the owls are a natural part of the ecosystem, they typically migrate. But seeing the number of mice available for them to feed on incentivizes them to stay, Warzybok said. That leads to them preying on a bird species called storm-petrels.

“Unfortunately, what happens in the winter, when it starts to rain and starts to get colder, the mice go through a seasonal decline, and their mouse population crashes leaving the owls without much food,” Warzybok said. “It’s at this point that they have to switch to feeding on something else and they start capturing storm- petrels.”

If nothing is done, the storm-petrel population is expected to decline by 60% over the next 20 years, which is significant, Warzybok said, given that the islands are home to half of the world’s storm-petrel population.

Additionally, Warzybok said, removing mice will help to “buffer the Farallon ecosystem against the impacts of climate change, warming, air temperatures [and] changing rainfall patterns.”

“By removing the impacts caused by the mice, we’re giving the entire Farallon ecosystem a greater capacity to withstand those changes and to adapt to long term climate impacts,” Warzybok said.

mbeck@sfexaminer.com