CORUMBA, BRAZIL -- South American alligators called jacare bask by the dozens alongside ink-black ponds and lazy streams. Boa constrictors as fat as footballs and as long as Cadillacs stretch beneath bushes that line the riverbanks. Ostrich-like rhea sprint across grassy plains, while overhead fly squawking macaws, gaudy toucans and an array of long-legged storks, spoonbills and herons.

This is the Pantanal, meaning swamp. It is the world's largest wetland -- home to what ecologists call one of the richest and most varied collections of wildlife on the planet. Like almost all such delicately balanced environments, the Pantanal is under assault. Unlike many others, it may not yet be doomed.

Hunters are killing the jacare for their skins, taking as many as 1 million each year to be turned into shoes and handbags. Gold miners in the hills are pouring toxic mercury into the rivers. Farming activity in the surrounding highlands is bleeding pesticides into the water and eroding the soil, causing widespread silting.

And well-intentioned, camera-laden tourists are discovering the Pantanal, bringing with them tons of litter and a commotion of buses and boats that threaten to disrupt animal habitats.

Most experts agree that the Pantanal is still largely intact, but they also agree that an environmental and social balance that has held firm for two centuries is changing. Pressures of population and development are crowding in.

"Our whole way of life is under attack," said Manoel Martins, a cattle rancher who heads the Society for the Defense of the Pantanal, the area's leading environmental group.

Covering nearly 60,000 square miles, the Pantanal stretches into Bolivia and Paraguay but lies mostly in southwestern Brazil, in the states of Mato Grosso and Mato Grosso do Sul. It floods during the rainy southern summer, November to March, and partially drains during the dry winter.

With the exception of two government ranches and one small national park, the Brazilian Pantanal is privately owned. Europeans first moved in 200 years ago, finding the Pantanal ideal for cattle ranching. They assembled huge tracts of land -- a 50,000-acre spread is mid-sized -- over which their herds of humped zebu cattle range freely in what the ranchers have long claimed is coexistence with the native wildlife.

It is a landscape of swamps, bogs and streams, dotted with island-like elevated plains that stay dry year-round. The few roads through the area lead over creaking wooden bridges, some nearly a century old. The Paraguay River, into which the Pantanal's waters drain, is crisscrossed by ferryboats.

The rivers and ponds are lined with rushes and lily pads, the dry patches carpeted with grasses and punctuated by clumps of trees, like the one known here as the paratudo, easy to pick out this time of year because of its canopy of bright yellow blossoms. The name means "for everything," and locals say parts of the tree can be used to cure various ailments -- recommending, for example, an elixir made by boiling the bark as a balm for mosquito bites.

"Here we have the opportunity to protect a complete ecosystem," said Edith Wenger, a French scientist and World Wildlife Fund member who was in Corumba last month for the first international conference on the Pantanal, attended by representatives from the Brazilian, Paraguayan and Bolivian governments. "Here we can examine the largest flood plain in the world."

The Pantanal is home to about 650 species of birds, 230 of fish, 80 of mammals and 50 of reptiles. Included are many animals -- like the capybara, a rodent the size of a pig -- that are practically extinct in other parts of the continent.

Fishermen using illegal nets and taking much more than the prescribed limits threaten to deplete aquatic life. Thousands of exotic birds are captured for sale each year, such as the bright-blue hyacinth macaw, once common around Corumba but now rarely seen in the area. And hunters are killing animals for their skins -- boa constrictors, jaguars, capybara, but most of all the jacare.

From a converted old flour mill on the riverbank in Corumba, Col. Jose Leonel dos Santos commands the local detachment of the Forest Police, formed three years ago as a unit of the national Military Police. His men displayed scores of skins, mostly jacare, that they confiscated while arresting hunters. All such hunting is against the law.

The hunters work at night, shining a flashlight into the alligator's eyes to get it to freeze and then shooting it -- preferably with one bullet to the head, so as not to damage the skin. Dos Santos said the skins are generally smuggled out for sale in Paraguay, where relevant laws are rarely enforced.

Guilherme Mourao, who studies the jacare for a governmental agricultural research institute, said he is concerned about the hunting but thus far sees "no evidence that the population is declining substantially."

The pelt of the spotted jaguar is also valued. "We still have the jaguar here, but they will go, too, if the government does not take seriously this responsibility of protecting the environment, saving the wilderness," dos Santos said.

For dos Santos, to take the problem seriously means to provide more resources. There are 62 men in his headquarters unit, but most have administrative duties, and only about a dozen are available for field work. They have no airplanes or helicopters and must hunt the hunters in pickup trucks and flat-bottomed boats.

Dos Santos said he believes the fishing problem, at least, is now largely controlled. But one day recently, just miles from Corumba, a group of fishermen was camped by a riverbank in clear view of anyone using the main road through that part of the Pantanal, enthusiastically casting their illegal nets.

Another threat comes from gold mining in the highlands toward the north, especially around the cities of Cuiaba and Pocone. The miners use mercury, a highly poisonous metal, to separate gold from tailings. The mercury then finds its way into the Pantanal's waters.

For ranchers, one of the most pressing problems is the alteration of the terrain itself, due to silting. "There are thousands and thousands of acres that used to be dry during the winter and now are flooded year-round," said rancher Martins, 45, who has lived in the Pantanal all his life.

The silting comes from erosion caused by intensive farming activity in the surrounding areas. Joao Alberto Martins do Amaral, a scientist who studies the Pantanal for the Mato Grosso do Sul government, explained at the Corumba conference that silting along the Taquari River -- where the problem is most serious -- already threatens to "irrevocably harm the equilibrium of various ecosystems."

A final looming problem is tourism, spurred recently here by the success of a racy television soap opera called "Pantanal."

Ranch owners have discovered that opening their doors to tourists provides a new and steady source of income. A few weeks ago, local volunteers ventured into the Pantanal and trucked out eight tons of litter, most of it left by tourists. Some animals, particularly rare birds, have begun to stay away from the roads and the heavily traveled river areas.

"Tourism in itself is not a problem," said Urbano Gomes Pinto de Abreu, who heads a government research center for the Pantanal that helped organize the recent conference in Corumba. "The problem is uncontrolled tourism, tourism that does not have any regard for the environment. It is partly an educational problem."

President Fernando Collor de Mello focused new attention on the Pantanal earlier this year when he went there to announce that his government would adopt a new, more aggressive stance against environmental destruction.

In the Amazon, cattle ranchers are often seen as villains responsible for environmental destruction. Here, however, it was the ranchers who first recognized the dangers and formed the Society for the Defense of the Pantanal, by far the most active and respected private environmental group concerned with the marsh.

"It was a question of survival," Martins said, "given the assault we were facing, the attack against nature, our customs, our traditions, the concept of rural labor." The group has about 150 members, almost all of them ranchers with substantial holdings.

The society has been criticized for being exclusive -- a kind of club for wealthy ranchers -- and for seeking to monopolize the debate about the Pantanal. But the group's sincerity in seeking to conserve land and wildlife is not questioned by government agencies or such groups as the World Wildlife Fund.

Still, not all the ranchers are blameless. Some sell parcels of land to newcomers from the big cities and then grouse about "outsiders." Others, according to one source involved in the Corumba conference, finance the work of some of the gold miners, receiving a cut of what they take -- and then complain about the damage from mercury.

The threats amount to an invasion that is forcing open the doors of a self-contained world. Brazil is a nation of more than 140 million people, most of them now crowded into urban areas. The cities, though, have not fulfilled the promise that the poor once saw there.

The same combination of population pressure and gold-rush opportunity that leads some to turn Amazon rain forest into pastureland is leading others to hunt jacare and mine the Pantanal highlands.

The delegates to the conference in Corumba called on the three nations that share the Pantanal to develop an integrated plan for saving it -- a result that was considered modest but one that organizers hoped could lay the groundwork for more substantial action.