Two conservation groups have filed a lawsuit against the U.S. Forest Service and Regional Forester Tom Tidwell to stop prescribed burning and thinning on more than 900 acres in the Beaverhead-Deerlodge National Forest, saying the project would destroy habitat used by big game and sensitive wildlife species such as sage grouse.

The suit, filed Wednesday in federal court in Missoula, seeks to halt the Birch Creek fuels management project in the East Pioneer Mountains, about 20 miles northwest of Dillon. The project involves the slashing and burning of up to 930 acres of sage and juniper habitat, the Alliance for the Wild Rockies and the Native Ecosystems Council said in a news release.

The nonprofit groups say the project would destroy habitat that is critical to big game and other wildlife, including sage grouse, flammulated owls, pigmy rabbits, northern goshawks and Brewer's sparrows.

The groups also claim District Ranger Tom Osen applied an illegal ''categorical exclusion'' to the project, meaning it didn't have to undergo an environmental analysis. Such exclusions free the Forest Service from having to respond to public comment and are intended for projects that have no significant environmental impact, said Michael Garrity, executive director of the Helena-based Alliance for the Wild Rockies.

''The significant environmental impacts associated with this habitat destruction preclude use of a categorical exclusion,'' he said. ''The public needs to be allowed to participate in a full environmental analysis.''

Denise Germann, spokeswoman for the Forest Service's regional office in Missoula, said Wednesday she had not yet received a copy of the complaint and therefore couldn't comment on it. Jack de Golia, spokesman for the Beaverhead-Deerlodge Forest, declined to comment, saying it was against Forest Service policy to discuss pending litigation.

Dr. Sara Johnson, a former Gallatin National Forest wildlife biologist and director of the Three Forks-based Native Ecosystems Council, said sage grouse are ''highly dependent'' on dense sagebrush for nesting and on moderately dense sagebrush and broad-leafed herbs for brood-rearing habitat.

''Direct impacts to the sage grouse will be significant because of previous sagebrush burning projects in the Birch Creek project area,'' she said. ''Sage grouse is currently being considered by the U.S. Fish and Wildlife Service largely due to loss of habitat.''

She added that juniper habitat in the Birch Creek project area provides important big game winter range. Juniper trees also are ''drought-hardy'' and play a critical role in extending forest habitat into drier grassland regions, where they increase habitat diversity and are used extensively by loggerhead shrike and other ''species of concern.''

''Juniper trees establish only periodically during climatic wet cycles,'' she said. ''Therefore, the removal of highly beneficial wildlife habitat that juniper provides by slashing and burning in the Birch Creek project area may be relatively permanent. Re-establishment of juniper may not occur in the future due to the ongoing climatic drying associated with global warming.''

The groups also argue that thinning is unnecessary because the Birch Creek project is miles from any homes or structures.

''The Forest Service's own fire scientist, Dr. Jack Cohen, recommends that effective fuel modification for reducing potential wildland urban interface fire losses need only occur within a few tens of meters from a home, not hundreds of meters or more from a home,'' Garrity said.

On the Web:

Alliance for the Wild Rockies, www.wildrockiesalliance.org

Beaverhead-Deerlodge National Forest, http://www.fs.fed.us/r1/b-d/

U.S. Forest Service Northern Region, http://www.fs.fed.us/r1/