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Mice Capades

Potential use of rodenticides on the mice of the Farallon Islands divides environmentalists

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From 1980 through 2003, Peter Pyle worked at the Farallon Islands off the coast of Marin. The veteran bird researcher counted seabirds, observed them feeding their young during nesting time, and many times witnessed and recorded great white sharks attacking and killing pinnipeds in the ocean waters surrounding the legendary islands. Pyle worked for the Point Reyes Bird Observatory at the time, and with a rotating team of assistant scientists, he lived in a small house on Southeast Farallon Island, the 357-foot-high crag visible to landlubbers from 30 miles away but off-limits to the general public. They hiked about the rocky shores, received grocery deliveries twice a month, and often fished for lingcod from a small skiff in the hours before dinnertime.

Today, Pyle, now working with the Institute for Bird Populations in Point Reyes, remembers his many seasons at the islands with a strange blend of sweet nostalgia and dread that makes the skin crawl—for the islands, now as then, are crawling with house mice. The animals are non-native, introduced accidentally more than a century ago by boaters, and every summer and fall their population explodes to grotesque numbers on two of the islands—namely, Southeast Farallon Island and an abutting crag called West End that becomes separated from the bigger island at high tide.

"They're just crawling around everywhere," says Pyle, who was working with the Point Reyes Bird Observatory during his years of island research. "It's like some invasion-of-the-rats movie."

HOME SWEET HOME The Farallon Islands are home to scientists and researchers—and up to 100,000 mice. - PETE WARZYBOK/POINT BLUE
  • Pete Warzybok/Point Blue
  • HOME SWEET HOME The Farallon Islands are home to scientists and researchers—and up to 100,000 mice.

The resident scientists, he says, sometimes kept a small compost heap in back of the house where hundreds of mice could be seen at a glance. Walking about on the rocky landscape, mice peeked out from nearly every crack and burrow. Nights in the old Victorian house were especially unsettling, he recalls. The rodents swarmed though the old dwelling. They skittered about on the counters, knocked over dishes, defecated on the dinner table and tousled sleepers' hair. Many individuals, Pyle says, have made attempts at controlling the animals using snap traps. Killing 50 a night can be easy, but it's a futile effort on an island whose mouse population in high season may reach 60,000 to 100,000.

The main problem associated with the Farallon Islands' mice is a complex of ecological imbalances. For one, the mice prey on two native species that live nowhere else: the camel cricket and the arboreal salamander. The rodents' presence has also attracted a population of burrowing owls, predators that previously only used the island for brief migratory stopovers but who now, due to the abundance of mice, remain for long periods.

When the mouse population suddenly plummets early each winter, the owls abruptly find themselves with almost nothing to eat. This turns their attention to native birds, in particular the ashy storm-petrel, a rare species that nests on the islands every winter and spring. The owls, according to experts, are slowly whittling away the petrels' population. But the owls prefer mice, and if only the rodents could be eliminated, the owls, too, might go away.

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