DENVER – National Park Service employees and authorized agents will be permitted to kill up to 200 elk every year in Rocky Mountain National Park as part of a program announced Tuesday to control the growing herd of about 3,100 animals.

The National Park Service also detailed plans for fencing and other control measures in the $6 million, 20-year population management plan that has taken four years to complete.

Two conservation groups said they will sue the Park Service to block the plan, which they criticized as letting “politics and timidity triumph over science and common sense.”

The groups, Boulder-based Sinapu and Santa Fe, N.M.-based Forest Guardians, advocate restoring wolves to the area to manage the herds.

Although the elk herd is one of the main tourist attractions in the park about 70 miles west of Denver, it is responsible for nearly eliminating aspens, willows and other native plants. The goal is to reduce the herd to about 1,600 to 2,100 elk.

Park biologists have said the area’s elk densities – up to 285 per square mile in some prime winter range – are the highest recorded for a free-ranging herd in the Rockies.

“In the absence of the natural predators here, we have to replicate what they would do for us,” Superintendent Vaughn Baker said. “We are mandated to maintain natural process here in Rocky, and under current conditions we’re not doing that. And we need to step in, I guess, and help make that happen.”

Colorado state wildlife officials have urged the Park Service to allow hunters to help thin the herd, saying hunters would do it for free and use the meat.

A 1929 law bans hunting in the park.

Rep. Mark Udall, R-Colo., and Sen. Wayne Allard, R-Colo., sponsored bills authorizing the use of volunteer hunters. Udall said he is glad the Park Service is open to using hunters.

“However, I’m concerned the Park Service might give a higher priority to using people from other federal or state agencies,” Udall said. “I think if qualified sportsmen or sportswomen are willing to volunteer, they should be first in line.”

State and federal officials in North Dakota have asked that hunters be allowed to shoot elk in that state’s Theodore Roosevelt National Park, which faces similar problems with too many of the animals for the habitat.

Biologist Therese Johnson said that qualified volunteers, staffers from other agencies, Indian tribes or contractors authorized by the Park Service will be eligible to help with the culling. Johnson said the meat can’t be used by individuals but will be distributed to Indian tribes and nonprofit organizations after it has been tested for chronic wasting disease.

The disease, a brain-wasting ailment in deer and elk that’s in the same family as mad cow disease, has been detected in low levels in the park’s herd.

The earliest elk would be shot would be January 2009, Johnson said. Biologists will look at the size of the herd next fall and determine what reductions are necessary.

Up to 120 female elk will be tested for chronic wasting disease and given a birth-control agent during the first year.

The draft management plan recommended shooting elk at night with silencer-equipped rifles to keep the culling out of public view. Johnson said the final plan is more flexible, but the goal is to minimize the effect on the public.

Park officials have discussed releasing and intensively managing a few wolves to help keep the herd in check. One of the main benefits of using wolves is that the predators would keep the elk on the run so they couldn’t overgraze.

But Johnson said any move to restore wolves to Rocky Mountain National Park would have to be a regional effort because the wolves roam over large distances.

Environmentalists see the restoration of wolves to the area as the best answer and one that has worked in Yellowstone National Park.

Yellowstone’s elk herd grew largely unchecked in part because of the loss of most predators. That changed when wolves were released there in 1995.