Two Montana-based environmental groups have filed a lawsuit seeking to force the U.S. Fish and Wildlife Service to protect a high-elevation pine tree whose nuts are an important food source for some threatened grizzly bears and other wildlife.

In 2011, the federal agency determined whitebark pine warranted listing under the Endangered Species Act, but it said other species were considered to be higher priorities. That finding was made after a 2010 lawsuit filed by the Natural Resources Defense Council.

The Helena-based Alliance for the Wild Rockies and the Missoula-based WildWest Institute filed a lawsuit in Missoula on Tuesday asking a federal judge to set aside the agency's decision or order it to set a date by which it would propose the trees for listing as a threatened or endangered species.