RENO -- Federal land managers working to keep a rare Nevada butterfly off the list of endangered species have closed scores of off-road vehicle trails at one of the biggest, most popular sand dunes in the West.

The closure affects six square miles of public land where an ancient lake existed.

The emergency order replaces a voluntary measure the U.S. Bureau of Land Management imposed two years ago on a portion of up to 200 miles of trails that run through shrubs and other vegetation that is home to the Sand Mountain blue butterfly, BLM officials said.

Agency officials could not say how many miles of trails were closed but said the main sand dunes at Sand Mountain where most motorized use occurs will remain open.

An estimated 50,000 outdoor enthusiasts in dune buggies, motorcycles and other off-road vehicles annually traverse the 600-foot tall, two-mile long dunes about 75 miles west of Reno.

The BLM and U.S. Fish and Wildlife Service have sought a compromise with off-road groups to restrict vehicles since conservationists started petitioning for federal protection of the butterfly in 2004.

The BLM published the closure order in the Federal Register on Friday and will begin posting signs where travel is prohibited, BLM spokesman Mark Struble said Tuesday.

"It's called an emergency restriction so when everybody hears the 'E' word they think we are shutting the whole mountain down. That is not the case," Struble said. "We're going from a wide-open, cross-country, go-over-any-of-the-bushes-if-they-get-in-your-way kind of situation, to trying to conserve as much of the vegetative resources that is out there as possible.

"If the old way continues, habitat will continue to degrade, and the species will be closer and closer to getting listed."

The Center for Biological Diversity and others filed a lawsuit in U.S. District Court in Sacramento, Calif., in January 2006 to try to force the agency to give the butterfly endangered status.

Fish and Wildlife officials might rule as soon as next month on the proposed listing.

"The closure is a good first step toward protecting the Sand Mountain blue butterfly, which exists nowhere else in the world," said Lisa Belenky, staff attorney with the Center for Biological Diversity in Tucson, Ariz.

"We hope that this closure is an indication that the bureau is finally stepping up to its responsibilities to preserve this area for future generations of Nevadans," said Charles Watson of the Nevada Outdoor Recreation Association.

Richard Hilton, president of Friends of Sand Mountain, said the closure was "kind of pushed on us" but probably the best deal his group of off-roaders could hope for.

"It was one of those things where we felt like any time an environmental group sues, off-road users usually lose," he said.

Karen Schambach of Public Employees for Environmental Responsibility, another plaintiff in the lawsuit, said she was encouraged by the closure order because BLM has been slow to respond and was "ignoring its own data that shows habitat for this species has been decimated by recreational excesses."

Some conservationists said there still will be too much intrusion and that more protection is warranted.

"The real test of the bureau's commitment to protecting butterfly habitat will be enforcement," Schambach said.

Ed Waldheim of the California Off Road Vehicle Association said off-roaders would be wise to follow the rules.

"Otherwise the next step is closing. This is something that we have learned the hard way in other areas," he said.

Violators will be subject to up to one year in prison, but Struble said the initial emphasis will be on distributing maps and educating visitors.

In all, the Sand Mountain Recreation Area consists of about 7.5 square miles of public land. The new order closes all but about a one-square-mile area.

"We are trying to work it to have the habitat, protect the species out there and have recreation out there, too," Struble said. "We need people's help and we weren't getting it under the volunteer system we had the first two years."