October 19, 2011

The teeming mice of the Farallon Islands


The final solution, everyone admits, will undoubtedly involve human intervention in a problem caused by humans.


The Farallon Islands are crawling with nonnative house mice, which could be seen in broad daylight darting and scampering in and out of burrows, on crags amid the cliffs and, as if in mocking defiance, around the 124-year-old Victorian house where scientists study the island ecosystem.

The mice are one of the last remaining introduced species left on the islands – and their population has grown to “plague-like” proportions, according to biologists, who are hatching a scheme to kill off the wily rodents, which devour insects and spiders and attract owls, which also chow on seabird chicks….

The mice population has ballooned over the last century. The 60,000 or so mice – about 500 mice per acre – now make up what is believed to be the highest density of rodents on any island in the world.

The teeming hordes devour the island’s insects, the same food that the endemic Farallon arboreal salamander needs to survive. They also attract owls.

“The burrowing owls show up in the fall when the mice population is at its peak, which is now,” said Gerry McChesney, the manager of the Farallon National Wildlife Refuge for the Fish and Wildlife Service. “They find this smorgasbord of mice, but then the mice population crashes in the winter, right when a lot of breeding seabirds arrive on the island.”

The owls, in turn, begin eating the birds, particularly the Ashy storm petrel, a small gray seabird that breeds and nests in the Farallones, which are home to half of the world’s population of the species. These birds, which are listed as a “species of concern” in California, have yet to recover after losing 40 percent of their population in a 20-year period ending in 1992. Only 10,000 to 15,000 are left in the world.