The Park Service and the wildlife advocacy group WildEarth Guardians made their cases at the University of Colorado Law School.
The wildlife advocacy group WildEarth Guardians sued the Park Service, saying it failed to give sufficient consideration to the use of wolves. The group also contends that allowing some specially trained volunteers to help Park Service employees in shooting and killing elk in the park to control the population numbers to hunting, which is prohibited in national parks.
The park sometimes has so many elk that they overgraze the trees, shrubs and grass, leaving other animals without enough food and habitat. Few natural predators are left there, and hunting is prohibited in all national parks, so little remains to keep the elk population in check.
The park uses National Park Service personnel and trained volunteers to periodically shoot and kill a specified number of elk. Officials rejected the idea of reintroducing wolves to prey on the elk and control their numbers, saying it was infeasible.
They cited a lack of support from other agencies, safety concerns of people who live nearby, expected conflicts between wolves and humans and the amount of attention that a wolf population would require of park officials.
WildEarth Guardians sued the Park Service in 2008, asking a Denver federal court judge to overturn the policy. The group said the Park Service didn't fully consider reintroducing wolves, and that using volunteers to kill elk amounts to hunting in the park, which would violate federal law.
The judge disagreed, upholding the policy in 2011, and WildEarth Guardians appealed to the 10th Circuit.
Rocky Mountain National Park launched a 20-year program in 2008 to thin the elk herd by shooting a number that varies from year to year, depending on the size of the park's overall herd. The program also includes fencing elk out of some areas to protect vegetation and redistributing some of the animals.
Park spokeswoman Kyle Patterson said 131 elk have been killed in the culling program during the winters of 2008, 2009 and 2010.
The current size of the elk herd in the park's lower elevations is 600 to 800, which is within the target range set by the program, so no elk were killed last year. The park website says weather, hunting outside the park and changes in elk movement patterns have helped keep the numbers low.
No decision has been made on whether or how many elk will be killed in the upcoming winter.
Wolves disappeared from many parts of the West after decades of hunting and government-backed extermination. They were re-introduced in Yellowstone National Park in Wyoming in 1995, and some advocates have argued for bringing them back elsewhere.