Billings, Mont. ? Environmental groups have filed notice they plan to sue the federal government to force officials to grant federal protections to plains bison as an endangered species.

Thursday’s notice said bison numbers have been reduced more than 99 percent over the last two centuries, from a population that once numbered in the tens of millions.

The notice was filed by the Center for Biological Diversity and the Western Watersheds Project. The Interior Department had rejected an earlier petition to list the animals as an endangered.

There are an estimated 400,000 bison in the United States but most are domestic animals used for meat production. About 20,000 wild bison inhabit at least 11 states.

North America’s largest terrestrial mammal, bison once ranged almost from coast to coast and from Canada to Mexico.