RENO, Nev. — A new study by an environmental group that wants the sage grouse listed as a threatened or endangered species shows less than 5 percent of what’s left of its dwindling habitat across the West is currently federally protected.

The new assessment found four-fifths of the chicken-sized game bird’s habitat is adversely affected by either livestock grazing, natural gas and oil development or invasive weeds.

‘‘Existing threats to sage grouse and their habitat are enormous,’’ said a copy of the report by WildEarth Guardians obtained by The Associated Press.

‘‘Livestock grazing, natural gas and oil development, agricultural conversion, roads, fences, power lines and pipelines, off-road vehicle use, urban sprawl, mining, unnatural fire and invasive weeds are destroying or degrading much of what remains,’’ the group said in the report being made public on Thursday.

It singled out livestock grazing — permitted on 91 percent of the bird’s range — as ‘‘the most ubiquitous use of sage grouse habitat on federal public land.’’

Critics of the report, including the head of the Nevada Department of Wildlife and others who oppose federal listing of the bird, said the study places too much emphasis on grazing and drilling while ignoring other threats to the species such as drought and West Nile virus.

‘‘Some of the things they are saying are true, but it is an anti-grazing bent. The situation is way more complicated than what they are talking about here,’’ NDOW Director Kenneth Mayer said. ‘‘West Nile and wildfires are the issue, not livestock grazing in my mind.’’

The sage grouse is found in 11 Western states on sagebrush plains and high desert from Colorado to California and north to the Canadian border. Its population has been declining for decades and it now occupies about half of its original, year-round habitat. The U.S. Fish and Wildlife Service estimated in 2005 there were 100,000 to 500,000 sage grouse.

Federal officials and private researchers have estimated that an ecosystem described as the sagebrush ‘‘steppe’’ covered anywhere from 150 million acres to 300 million acres of North America prior to white settlement.

Of the 80.1 million acres remaining that have been identified as most important to the sage grouse, only 4.35 percent is currently reserved for conservation or related purposes, according to the new report by the conservation group that formed earlier this year with the merger of the Arizona-based Forest Guardians, the Colorado-based Sinapu and the Sagebrush Sea Campaign.

‘‘This illustrates and quantifies, we believe for the first time, that the sagebrush steppe is one of the least — if not the least — protected landscapes in the United States,’’ said Mark Salvo, the group’s director based in Chandler, Ariz.

‘‘And yet it is rapidly becoming one of the most industrialized of all the ecosystems in the U.S. That is why sage grouse, and frankly dozens if not hundreds of other fish and wildlife and plants in that ecosystem, are showing declining numbers,’’ he said.

The U.S. Fish and Wildlife Service rejected a petition in 2005 to add the bird to the list of threatened or endangered species. But a judge in Idaho overturned the decision last December amid allegations Interior Department managers interfered with the science used in assessing the sage grouse and dozens of other endangered species.

State and federal wildlife officials have been assessing the status of the bird the past six months and USFWS is scheduled to issue a new listing decision in December.

Assistant Interior Secretary Stephen Allred said earlier this year that more than 16 million acres has burned in the Great Basin since 1990, much of it sage grouse habitat. Of the 22 million acres of sage grouse habitat that existed in Nevada in 1999, nearly 3 million acres has burned.

Cheatgrass, an invasive weed, overwhelms native sagebrush in the aftermath of wildfires.

The numbers of chicks per hen in Nevada hit a historic low of 0.58 last fall compared to a more typical figure of 1.8 to 2.0, but biologists are quick to remind that grouse populations operate in cycles.

‘‘Populations have improved overall the last five or six years. In the last couple years, the numbers are down because we are suffering through drought,’’ Mayer said. ‘‘We need to look at the long-term trend.’’

In Nevada, like most other states, the vast majority of the birds live on land managed by the U.S. Bureau of Land Management and U.S. Forest Service.

‘‘We continue to work with our local, state, and federal partners to ensure the sustainability of our sagebrush habitats,’’ said Christie Kalkowski, spokesperson for the Humboldt-Toiyabe National Forest spokesperson.

‘‘We do, however, recognize that there a variety of threats to these sagebrush ecosystems, and actively implement habitat improvement projects that will directly benefit these fragile ecosystems every year,’’ she added.

She said the Forest Service conducted two prescribed burns last spring on roughly 6,000 acres of sagebrush-and-pinyon landscapes, which she said will promote the new growth of sagebrush.