ASPEN, Colo. — An environmental group wants to see wolves back in Colorado.

WildEarth Guardians of Santa Fe has filed papers with the U.S. Fish and Wildlife Service calling for wolf reintroduction in four Colorado areas. After being exterminated throughout the West decades ago, wolves have been reintroduced in Montana, Wyoming and Idaho.

WildEarth Guardians believes Colorado should come next. The group argues that wolves would help thin overpopulated elk herds, which would then lead to more young aspen trees.

"We believe that the Southern Rockies needs wolves, and wolves definitely need the Southern Rockies," said Rob Edward, carnivore recovery director for WildEarth Guardians, which also has offices in Denver and Phoenix.

Edward told the Aspen Daily News that while solitary wolves from the north and from New Mexico wander into Colorado, no sustainable wolf population is believed to have re-established itself within the state.

The group's proposal identifies four Colorado areas for wolf reintroduction — the Flat Tops north of Glenwood Springs, the Grand Mesa-Uncompahgre-Gunnison national forests near Pitkin County's western border, the San Juan Mountains and Weminuche Wilderness in southwestern Colorado and southern Colorado's Vermejo Park Ranch and Carson National Forest.

A Fish and Wildlife spokeswoman in Denver told the newspaper she had not heard about the petition, but the agency stands by its existing wolf recovery plan, which limits restoring the wolves to Wyoming, Idaho and Montana.

Edward argues that wolves could play a vital role in Colorado, especially by thinning a Flat Tops elk herd that is the nation's biggest. Biologists have said that elk herd is too large and is causing damage by munching on native vegetation such as young aspens.

Edward said opposition by ranchers and sheepherders shouldn't stop wolf reintroduction. "The bottom line is, regardless of the political opposition of a small, and growing ever smaller, part of our economy, we need wolves here, and we have an obligation to ensure that they are here, that they're restored," Edward said.

WildEarth Guardians has also sued the federal government over the Rocky Mountain National Park elk management plan for its decision to use hunters to cull elk herds rather than reintroduce wolves. The group's latest Colorado wolf petition was filed last week.

Managers at Rocky Mountain National Park have said the elk herd there is at the high end of a target range of 1,600 to 2,100 animals. The elk herd at Yellowstone National Park grew largely unchecked in part because of the loss of most predators. That changed when wolves were released there in 1995.

Edward said if the Fish and Wildlife Service does not take action on the Colorado wolf petition before the Bush administration leaves office in January, his group will ask the next administration to consider it.