DENVER (AP) - An environmental group is suing the federal government, saying releasing wolves was not seriously considered as an alternative to shooting elk to reduce the growing herd in Rocky Mountain National Park.

The lawsuit, filed Tuesday in federal court in Denver by WildEarth Guardians, a coalition of Western environmental groups, claims federal officials ignored scientific evidence showing that releasing wolves in Yellowstone National Park improved the ecosystem by returning the natural predator.

The lawsuit, filed with the help of student attorneys at the University of Denver law school, also contends the Park Service is obligated to conserve endangered species.

Wolves were native to Colorado but were eliminated from the state by the 1930s after ranchers, government agents and others shot, trapped and poisoned the predator.

"The Park Service should accept that their elk problem stems directly from a lack of wolves in the region," said Rob Edward of WildEarth Guardians. "It's time to restore the balance of nature in Rocky Mountain National Park.

The plan approved last year to cull the elk herd in Rocky Mountain National Park about 70 miles northwest of Denver calls for sharpshooters to kill up to 200 elk annually over 20 years. The number killed each year will depend on the herd's size, which fluctuates.

The herd, safe from hunters and most predators, has grown up to 3,000 elk. The goal is a herd of about 1,200 to 1,700 elk.

Park officials want to thin the herd because overgrazing by elk has nearly wiped out aspens and willows, prime habitat for beavers and birds. Elk also roam through the yards and gardens of homes outside the park, increasing chances for conflicts with people.

The preliminary plan for Rocky Mountain National Park said wolves would best meet environmental objectives and do the least damage, but did not recommend that option.