JIDO EMILIANO ZAPATA, Mexico — Manuel López Gómez is watching the green world around him disappear, ravaged by people whose only path from starvation lies in slashing and burning the jungle to plant a patch of corn.

"We are out of balance here," said Mr. López, 60, a local farmer turned conservationist. "We are trying to stop the destruction. If nothing changes, all the land around here will be destroyed."

Five miles up a muddy trail from Emiliano Zapata, in southeastern Chiapas State, is Mexico's largest unpolluted lake, Laguna Miramar, and beyond that stands the last rain forest in Mexico. But today almost half a million poor people, speaking six different languages, live in that dying forest. For some here in Chiapas, the issue is turning from saving the trees to saving the people.

Luis Daniel Lopez Perez on the shores of Laguna Miramar, in the Montes Azules Biosphere Reserve.

Ejido Emiliano Zapata is near the last rain forest in Mexico.

A century of government reaching into this most remote corner of Mexico has left most citizens with next to nothing. President Vicente Fox's plans to build dams, railroads, highways and industries linking Chiapas to the outside world in a 21st-century free-trade network are grand but unrealized. And in Chiapas, development often means destruction.

Starting in the late 19th century, the government sold foreign companies the right to tear the great mahogany and cedar trees from Chiapas. In 1972, the government deeded what was left — a forest as big as Connecticut — to the tiny and untrammeled Lacandón tribe, a few hundred people, who farm by trimming the forest canopy, not erasing it.

Since then, more than two-thirds of the Lacandón forest has been sawed down, first by timber companies with heavy machinery, then by peasants — some from Chiapas, some from farther north — all seeking a little land by which to live.

In 1978, the government declared the remaining forest, 1,278 square miles of it, an international sanctuary: the Montes Azules Biosphere Reserve, presumably off limits to development. That pledge has been stretched to the breaking point by the pressures of poverty, population growth and political struggles.

Seen in satellite images, the green land of the bioreserve shrinks every year, like a lake slowly going dry. The trees are cut, the undergrowth is burned, the thin topsoil planted with corn until the crop fails, the land then grazed by cattle until the rains wash the earth away. Hundreds of settlements struggle in isolation, sharing little sense of community, rarely seeing eye to eye, often lacking a common language.

Under the ejido system, by which the Mexican revolution promised land to peasants, the earth in and around the preserve has been subdivided among farming families for five generations. It will not last much longer.

Now the struggle for land has started to pit the Zapatista rebel movement against ecologists who want to save the remains of the forest. The Zapatistas declared war on Mexico's government nearly nine years ago over the poverty of peasants in Chiapas. Today the movement criticizes efforts to conserve the bioreserve as a "war of extermination against our indigenous communities."

"They speak of the ecological danger which the indigenous people inside the bioreserve represent," the group said in a statement last spring.

But, the rebels argued, conservation serves "large multinational companies dedicated to exploiting biogenetic resources, covered with the masks of environmental foundations." (Biotech companies, aware that most of the species in the forest have never been cataloged, have contributed money to conservationists in Chiapas.)

The Zapatistas also denounced "shopkeepers trying to develop eco-tourism" in Chiapas as "fools trying to change our lives so that we will cease being what we are: indigenous peasants with our own ideas and culture."

The rebel group, whose headquarters lie close by, has faded from public view. It has little visible presence here in the town that shares its name. In Emiliano Zapata, the Mexican Army is more powerful than the Zapatistas. A new army base looms over the village. Throughout southern Chiapas, Humvees patrol mountain roads, policing the countryside, zooming past peasant women carrying backbreaking loads of firewood.

The question the Zapatistas have raised — control of the land — remains a burning issue. One answer may lie in ways that will bring a little money into villages like Emiliano Zapata, enough to end the dying tradition of scratching out a living raising corn and beans, without trampling a rural culture built on the ruins of the ancient Mayan civilization.

Villagers and outsiders have cut cornfields and cattle pastures into the reserve, but Mr. López, encouraged by conservationists, has persuaded many others to cultivate alternative crops, like organic coffee, under a program largely if silently financed by the United States Agency for International Development. A few tourists hike to Laguna Miramar, paying villagers for the privilege of experiencing a semblance of wilderness.

On the other side of the forest, that trickle of tourism is becoming a flood. At Frontera Corozal, an outpost established 23 years ago for Chol Indians who were ejected by the government from the bioreserve, 40,000 tourists a year, mostly Europeans, pass through town, spending the night — and thousands of dollars — to visit Mayan ruins.

This kind of economic development, financed by foreigners and moving peasants away from tradition and toward the modern world, may be the Zapatistas' worst nightmare. But right now it is about the only development around.

President Fox's plans so far amount to little beyond $10 billion worth of ideas on paper. Over the last three decades, the government has built roads, schools, health centers and power lines in the region. But its largest legacy in Chiapas is conflict.

Outsiders still make the rules here, said Ron Nigh, an anthropologist who has worked in the region for 17 years. "Some conservationists think there shouldn't be anybody living in the jungle," he said. "The local people basically have no say."

The only way to save the trees may be to change the political and economic conditions of the people living among them. But Chiapas remains a land of poverty. Illiteracy runs high. Education past grade school is rare. Hunger and anger are common.

And in Emiliano Zapata, Mr. López said, the conflict between development and preservation remains unresolved. "It's difficult to maintain a nature preserve in places where people want to live," he said.

lobal warming is forcing species around the world, from California starfish to Alpine herbs, to move into new ranges or alter habits in ways that could disrupt ecosystems, two groups of researchers say.

The two new studies, by teams at the University of Texas, Wesleyan, Stanford and elsewhere, are reported in today's issue of the journal Nature. Experts not associated with the studies say they provide the clearest portrait yet of a biological world driven into accelerating flux by warming caused at least in part by human activity.

Plants and animals have always had to adjust to shifting climates. But climate is changing faster now than in recent millenniums, and many scientists attribute the pace to rising concentrations of heat-trapping greenhouse gases.

In some cases, species' ranges have shifted 60 miles or more in recent decades, mainly toward the poles, according to the new analyses. In others, the timing of egg laying, migrations and the like has shifted weeks earlier in the year, creating the potential to separate species, in both time and place, from their needed sources of food.

L.&L. Langstroth

The population of the ochre sea star has declined in Monterey Bay, most likely a consequence, researchers say, of global warming.

One academic not associated with the studies, Dr. Richard P. Alley, an expert on past climate shifts who teaches at Pennsylvania State University, said that climate had changed more abruptly a few times since the last ice age and that nature had shifted in response. But, he noted, "the preindustrial migrations were made without having to worry about cornfields, parking lots and Interstates."

Citing the new work and studies of past climate shifts, Dr. Alley saw particular significance in the expectation that animals and plants that rely on one another were likely to migrate at different rates. Referring to affected species, he said, "You'll have to change what you eat, or rely on fewer things to eat, or travel farther to eat, all of which have costs."

The result in coming decades could be substantial ecological disruption, local losses of wildlife and extinction of some species, the two studies said.

The authors express their findings with a certainty far greater than in the last decade, when many of the same researchers contributed to reports on biological effects of warming that were published by the Intergovernmental Panel on Climate Change, the top international research group on the issue.

The authors of one of the new Nature papers, Dr. Camille Parmesan, a biologist at the University of Texas, and Dr. Gary Yohe, an economist at Wesleyan University, calculated that many ecological changes measured in recent decades had a 95 percent chance of being a result of climate warming and not some other factor.

"You're seeing the impact of climate on natural systems now," Dr. Yohe said. "It's really important to take that seriously."

Some butterflies have shifted northward in Europe by 30 to 60 miles or more, with the changes closely matching those in average warm-season temperatures, Dr. Parmesan said. The researchers were able to rule out other factors — habitat destruction, for example — as causes of the changes.

Some of these changes meshed tightly with variations in temperature over time. Dr. Parmesan cited bird studies in Britain. There, populations of the great tit adjusted their egg laying earlier or later as climate warmed early in the 20th century, then cooled in midcentury and warmed even more sharply after the 1970's.

Over all, Dr. Parmesan's study found that species' ranges were tending to shift toward the poles at some four miles a decade and that spring events, like egg laying or trees' flowering, were shifting 2.3 days earlier a decade.

Around Monterey Bay in California, warmer waters have caused many invertebrates to shift northward, driving some species out of the bay and allowing others to move in from the south.

Authors of both new papers said they were concerned that such significant ecological changes had already been detected even though global temperatures had risen only about one degree in the last century.

They noted that projections of global warming by 2100 ranged from 2.5 to 10 degrees above current levels, should concentrations of carbon dioxide and other heat-trapping gases, which flow mainly from smokestacks and tailpipes, continue to rise.

By comparison, the world took some 18,000 years to climb out of the depths of the last ice age and warm some five to nine degrees to current conditions.

"If we're already seeing such dramatic changes" among species, "it's really pretty frightening to think what we might see in the next 100 years," said Dr. Terry L. Root, an ecologist at Stanford University who was the lead author of one of the new studies.

The two teams of researchers used different statistical methods to analyze data on hundreds of species, focusing mainly on plants and animals that have been carefully studied for many decades, like trees, butterflies and birds. Both teams found, with very high certainty, a clear ecological effect of rising temperatures.

Several of the researchers said the effects of other, simultaneous human actions, like urban expansion and the introduction of invasive species, could greatly amplify the effects of climate change.

For example, the quino checkerspot butterfly, an endangered species with a small range in northern Mexico and Southern California, is being pushed out of Mexico by higher temperatures while also being pushed south by growing suburban sprawl around Los Angeles and San Diego, Dr. Parmesan said.

"The butterfly is caught between these two major human factors — urbanization in the north and warming in the south," said Dr. Parmesan, who has spent years studying shifting ranges of various checkerspot species.

Dr. Alley said the studies illustrated the importance of conducting much more research to anticipate impending harms and devise ways to maintain biological diversity, for instance with green "wildlife corridors" linking adjacent pockets of habitat.