NIMAS, N.M. — Ever since the great cattle drives of the Old West, ranching has been suspected of chewing up Western ecosystems. For decades, environmentalists have tried to limit grazing from public lands, where ranchers lease pastures from the government. But some scientists and conservationists are now saying that cattle ranches may be the last best hope for preserving habitat for many native species.

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The ranches could also be the best way to preserve grasslands and the periodic fires that keep brush and cactuses from taking over.

In recent studies published in peer-reviewed journals like BioScience, Conservation Biology, and Environmental Science and Policy, scientists have concluded that large, intact working cattle ranches are crucial puzzle pieces holding together an increasingly fragmented landscape.

When ranches are subdivided into "ranchettes" of 40 acres or less — a runaway trend — invasive species move in along with people and their pets, and fewer native species can live on the land. And it becomes much harder, if not impossible, to let fires burn across the land periodically, a process that is now thought to be essential in many ecosystems.

The studies emerge from a network of ecologists and ranchers, once at odds, but now increasingly working together in the West.

"There is this lore throughout the conservation community that ranching is bad, period," said Dr. James H. Brown, a professor of biology at the University of New Mexico and an expert on the ecology of the Southwest. "I think that is demonstrably wrong. And a number of people are gathering data to demonstrate that."

Dr. Brown noted, however, "It's clear some grazing practices have been enormously detrimental." Studies have found damage from grazing in and around streams in the desert West, for instance. But few studies have compared the alternatives to ranching on the lands that are home not only to ranchers but to many animal and plant species.

Dr. Richard L. Knight, a professor of wildlife biology at Colorado State University, recently did just that, comparing 93 sites on ranches, in wildlife refuges and in subdivisions with about one house per 40 acres.

He found that the ranches had at least as many species of birds, carnivores and plants as similar areas that are protected as wildlife refuges. Ranches also had fewer invasive weeds.

More important, the ranches provided a better habitat for wildlife than the ranchettes, which had fewer native species and more invasive species than ranches and refuges.

Like many ecologists, Dr. Knight had assumed that grazing hurt wildlife. "It finally dawned on me," he said. "We made a mistake."

Demographic trends in the West add a sense of urgency to the findings, Dr. Knight said. The population of the West is growing rapidly and much growth is in rural areas.

As ranches are carved up into subdivisions, the land consumption is growing at an even faster rate than population, said Dr. David M. Theobald, a geographer at Colorado State University. In the West, developed lands rose from almost 20 million acres in 1970 to 42 million in 2000.

Private ranch lands are often the most productive lands in the West, too. Ranches are usually located at lower elevations and have richer soils and more water than surrounding public lands.

Dr. Andrew J. Hansen, an associate professor of ecology at Montana State University, who studied ranch lands and ranchettes around Yellowstone National Park, found that some songbirds from higher elevation public lands used the private ranch lands as breeding grounds. But in the ranchettes, songbird death rates started to exceed birth rates, because houses draw magpies and other birds that prey on the songbirds.

Dr. Hansen speculated that the songbirds were getting squeezed between increasing development at lower elevations and protected but unproductive breeding grounds at higher elevations.

Grasslands, too, are getting pinched in the midelevations, said Dr. Charles G. Curtin, a zoologist and the director of the Arid Lands Project, a nonprofit research group. in Animas. And it is not just by subdivisions. Climate and weather trends along with firefighters have created good conditions for woody shrubs like dry thorny mesquite and have conspired against grasslands.

Rather than being too disturbed by cattle grazing, Dr. Curtin said, the grasslands in the boot heel of New Mexico, where he does his research, have not been disturbed enough, mainly because of the absence of periodic fires over the past century.

Dr. Curtin works with the Malpai Borderlands Group in southern New Mexico and Arizona. The group is made up of ranchers, scientists, conservationists and government land managers concerned about preserving species and returning periodic fires to a million acres of mountainous desert land, an area larger than Rhode Island and almost half the size of Yellowstone National Park.

Malpai is derived from the Spanish word for badlands; its craggy mountains, grassy plains and scrub-covered desert hills are home to more than 20 threatened species. Like most of the West, the area is a checkerboard of private, state and federal ownership. And it has subdivisions nibbling at its flanks. It is also dotted with 200 monitoring sites, where scientists are studying species of all kinds, including grasses and brush as well as rattlesnakes and jaguars.

On the Gray Ranch, a 321,700-acre spread run by the nonprofit Animas Foundation, Dr. Curtin has set up large test plots to study the effects of grazing and burning on the grassland and the species that live here. Dr. Curtin said that scientists, ranchers and conservationists here were trying to test "a vast untested hypothesis: that grazing is a viable landscape process and ranching is the most viable long-term method of protection."

Dr. Curtin said scientists had generally concluded that only some ecosystems could support long-term grazing. It seems to depend on rainfall and whether herbivores were present for thousands of years and thus were part of the system, as bison were here, he said.

In collaborations with other groups, Dr. Curtin hopes to conduct the same experiments on 20 ranches around the West, and in Africa as well. So far, Dr. Curtin said, his research indicates that grazing here does not have much of an effect on grasslands and shrubs.

Fire is more important in knocking down shrubs and encouraging grasses. But climate and weather are the major forces.

Dr. Brown, who has monitored the changing vegetation on experimental plots in nearby Portal, Ariz., for 24 years, agreed. An increase in winter precipitation driven by El Niño events has favored woody shrubs over grasses, he said. But with climate and weather being out of human control, "the only things you can really manage are fire and grazing," added Dr. Brown, a science adviser to the Malpai group.

The group is also experimenting with fire on a grand scale. Ranchers and federal land managers are working with scientists on a species habitat conservation plan that will set the stage for coordinated planning over the entire region, rather than for one endangered species at a time. One result is a "fire map" that shows where wildfires will be allowed to burn on private property with the landowner's consent.

In the Malpai area, wildfires can burn freely now on most of the land, up to the northern border, where real estate signs on newly divided land signal the end of any chance to keep natural forces at work.

"What you see is the result of 90 years of fire suppression," said Larry Allen, a retired Forest Service official who has worked with the Malpai group to plan prescribed burns. He pointed out an area that had not burned in many years and was thick with mesquite.

"If you do nothing, the mesquite will take over," Mr. Allen said. He then pointed to an area where a prescribed fire burned 12,000 acres in 1997 and grasses now grow thickly between widely spaced mesquite. "But you put a little fire in it," he said, "and it'll do miracles."

Bill McDonald, a rancher who is executive director of the Malpai group, said the two prescribed burns the group had managed to set since 1995 had helped restore grasslands.

"We just need another one," he said. "But the fire program has been set back by what happened in Los Alamos," he said, referring to the planned fire that got out control and burned homes two years ago. "They're so skittish."

Mr. McDonald said that scientists working in the area confirmed much of what local ranchers had long suspected about grazing and fire, except for one thing. "I'm surprised cattle grazing isn't a bigger impact for better or worse," Mr. McDonald said. "I guess it's not the biggest thing you see out there that is having the biggest impact."