The Park Service and the wildlife advocacy group WildEarth Guardians made their cases Thursday before a federal appeals court hearing at the University of Colorado Law School in Denver.

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 amounts 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.

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