(WILDLIFE CONSERVATION) FARALLON ISLANDS, CA — Off the coast of San Francisco on the Farallon Islands lives the Ashy Storm Petrel, a scarce seabird found only along the California Current. But the sudden infestation of the house mouse now threatens the entire island’s ecosystem. The mice eat and reproduce all day. The owls feast on the mice well into the winter then turning to the petrel after the mice population recedes. The problem is the petrel don’t provide enough food for the owls who then starve to death. Read on for more details on this food-chain calamity and possible solutions to the epidemic. — Global Animal
New York Times, John Upton
Winter after winter, burrowing owls have been overstaying their traditional migratory visit to the storm-swept Farallon Islands, dining on the petrels that have returned to breed. The attacks have taken a heavy toll on the petrels, a native bird that breeds slowly on the small, rocky islands.
The stress on the ashy storm petrels, which nest only on the Farallones and several other West Coast islands, is one of the many reverberations caused by a tiny island invader with a voracious appetite: the house mouse.
The tiny rodents, whose numbers soar and plummet every year as the seasons change, have thrown the island food web into chaos by chewing plants down to their roots, eating salamanders and insects, attacking chicks and bird eggs and attracting owls that assault nesting birds.
Efforts to address the centuries-old mouse problem are part of the island restoration work that has been going on for decades. The owls, which once were gone by late fall, now stay on at the Farallones well into winter, according to researchers, feasting on a buffet of mice before turning to the petrels after the rodent population dies back.
The owls are native to California, and during southerly fall migrations small numbers of them have long stumbled upon the isolated granite outcrop, a wildlife refuge 27 miles west of the Golden Gate and open only to researchers. Exotic migratory birds are also frequently spotted, but they normally flap away after resting for a day or two.
“The owls that are out on the islands have overshot the coast,” said Dan Grout, a scientist at Island Conservation, a nonprofit group, who has studied the mouse plague for the federal government.
Mr. Grout found that in the fall, an average of 500 tiny mice dart among nooks, rocks and holes on every acre of the islands. That puts the intensity of the mouse plague on par with outbreaks at Australian grain farms that are considered the most severe in the world.
Most vegetation on the outcrop is low-lying, putting much of its wildlife and already limited food supply within easy reach of the rodents. “They eat, eat, eat, eat, eat all day and all night,” Mr. Grout said.
The mice are so hungry that they cannibalize each other when they are caught together in his traps, Mr. Grout said recently as he scanned an island field infested by the rodents.
In winter, the mice, which are thought to have been introduced to the islands as stowaways in centuries past on the ships of hunters and egg traders, run out of food and their population collapses.
The burrowing owls, suddenly lacking a smorgasbord of mouse meat, resort to hunting petrels. But the petrels do not provide enough food, and some owls starve to death before the spring, too emaciated to brave a trip back to the mainland in the rough winter conditions.
The havoc that the mice play on the Farallones’ food web is a sore spot among the small group of researchers allowed on the refuge. They live together for weeks or months at a time, in a 19th-century house that must be repeatedly mouseproofed.
Since the islands became a national wildlife refuge in 1969, the researchers have watched some of the islands’ bird, seal and sea lion populations rebound after centuries of overhunting. Hares and cats were eradicated in the 1970s — and the researchers say mice are now the biggest roadblock to ecological recovery.
To get rid of the mice, the federal Fish and Wildlife Service is considering blanketing the islands with brodifacoum, a rat poison so dangerous to other wildlife that the Environmental Protection Agency has proposed banning it as a consumer product. The poison is an anticoagulant; its victims bleed to death. If it is used, gulls, falcons and other wildlife would eat poison pellets and poisoned mice, eventually suffering the same fate as the mice.
Such an extreme measure is needed, the island researchers argue, because if even two mice survive, they could breed and the plague would return.
“We’re supportive of the eradication goal,” said Melissa Pitkin, an island researcher who works for the Point Reyes Bird Observatory, but only if an environmental study by the federal government — set to be released next fall — “shows that the benefits outweigh the costs.”
More New York Times: http://www.nytimes.com/2011/10/23/us/voracious-mice-scramble-food-chain