RENO, Nev. — 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 about six square miles of public land where an ancient lake once 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, the BLM 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 east of Reno.

The BLM and U.S. Fish and Wildlife Service have been seeking a compromise with off-road groups to restrict vehicles since conservationists first 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 told The Associated Press.

"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," he said.

"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 in January 2006 to try to force the agency to give the butterfly endangered status.

Fish and Wildlife officials are expected to 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.