Lions push bighorn onto an island

  • Paoha Island in Mono Lake

    Map by Diane Sylvain
  • ON THE EDGE: Sierra Nevada bighorn sheep

    John Wehausen photo
 

Sierra Nevada bighorn sheep, the latest addition to the federal endangered species list, may survive extinction with the help of Mono Lake, itself threatened for decades by the diversion of its feeder streams to Los Angeles (HCN, 12/8/97).

Mono Lake can help to save bighorn by providing what one expert calls "an imaginary zoo" - in this case, an island in the lake. Biologists plan to capture a few of the remaining sheep to protect them from numerous mountain lions, then put them on an island where the bighorn have roamed for centuries.

With the lake level rising as a result of court orders regulating the Los Angeles Department of Water and Power diversions, and drowning a temporary land bridge, Paoha Island will offer a natural pen where the bighorn can raise lambs without fear of lions, said Steve Torres, a bighorn sheep expert with the California Department of Fish and Game.

"What the sheep need to survive is an imaginary zoo. Paoha looks ideally suitable. We'd be foolish not to try this," Torres said.

The Sierra bighorn sheep are a genetically distinct species that once formed at least 20 different bands, rambling through the steep canyons of the eastern Sierra Nevada, from Sonora Pass to Olancha.

Today, only 100 adults and 15 lambs remain. Several of the five herds left have only three females as a reproductive base, said John Wehausen, a biologist with the White Mountain Research Station, who has been tracking bighorn since 1974.

Their survival is linked to limiting kills by mountain lions, their traditional predator. Lions were hunted as trophy game until 1972, when the California Legislature made it illegal. During the next 18 years the bighorn population remained fragile but on the rise until 1990, when it began to drop precipitously. That was the year California voters approved a ballot initiative restricting state managers from killing mountain lions.

As the lion population increased, the predator increased its toll on the prey. Mountain lions killed some bighorn outright and forced others to move to higher ground where the forage for bighorn is poor, Wehausen said.

The April 20 emergency listing of Sierra bighorn sheep as endangered allows biologists to shoot mountain lions on federal land in spite of the ballot measure.

A separate bill introduced in the California Legislature would also give state biologists the power to kill lions to protect threatened or endangered species.

These are tools bighorn specialists need to have available but hope they don't have to use, said Torres.

"The last thing we want to do is kill mountain lions, but we can't afford to lose a single individual bighorn. The population is so small," Torres said, "it can't sustain any more losses."

The endangered listing also allows managers to restrict domestic sheep grazing on federal lands that affect bighorn sheep. Diseases carried by the domestic animals are second only to mountain lions in causing the decline of the bighorn, said Torres.

The ultimate goal is to reestablish the natural predator-prey relationship between lions and bighorn, and to allow it to operate without intervention, he said. It's a challenge in an ecological system like the Sierra, thrown out of balance by human disturbances and management of these and other native species.

"In an era when we continue to disturb natural systems, particularly for big mammals, it's our responsibility to keep them operating as naturally as we can," Torres said.

Using Paoha Island as a breeding ground would avoid an estimated $1 million in fencing, said Wehausen. If state and federal officials approve their plan, biologists hope to capture as many as five bighorn next winter and relocate them to the 2 square miles of rocky terrain in the middle of Mono Lake, where forage will be provided.

As lambs born in the protective safety mature, they will be reintroduced to the Sierra canyons and peaks, where they will face life's threats - including mountain lions - on their own.

Jane Braxton Little reports from Greenville, California.

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