Given that history of destruction, it's not surprising that today less than 7 percent of the Atlantic forest remains, much of it in isolated patches, some less than six acres (2.4 hectares) in size. It's as if someone broke apart a strand of pearls, then stepped on each bead. Indeed, among biological hotspots—the environmentally threatened regions of the world with the highest amount of endemism, meaning species found nowhere else on the planet—Conservation International ranks the Atlantic forest as one of the top five.

Yet within those fragments many of the Mata Atlântica's unique species, includ­ing some of the world's rarest plants, birds, and other animals, have managed to survive. Among them is the maned sloth. Like other mammals stranded here on forested islands amid a sea of agriculture and development, the sloth seems doomed to genetic inbreeding—if not eventual extinction.

"We think the sloths' genetic variation has already decreased," Chiarello says. "In the past this population was connected to those in southern Bahia and northern Rio de Janeiro. But they've been separated for at least 50 years." To determine the amount of inbreeding in the three groups, one of Chiarello's students, Paula Lara Ruiz, has launched a study of their genetic makeup.

"We may need to relocate some sloths to maintain their viability," Chiarello says. "Before we can do that, we need to know what trees they prefer, how much deep forest they need to survive."

But nailing down the particularities of sloths is only a small part of what Chiarello has in mind. Like other biologists tracking species in the Mata Atlântica, he has a far grander vision. Never mind that some of the land around the fragmented forest has the look of an abandoned bombing range, or a Sahara-in-the-making. Never mind that farmers continue to expand their eucalyptus and coffee plantations. Chiarello and a growing coterie of conservationists are determined to bring back the Mata Atlântica by reconnecting as many fragments as they can.

By linking islands of natural landscapes with corridors of vegetation, these scientists believe the Atlantic forest can be partly restored and many of its species saved from extinction. The corridors, in essence, could provide a safe passage from one island to the next, enabling isolated populations of animals and birds to meet and mix. It's an idea that has been around since the 1960s. Although there's no absolute proof that corridors ensure a species' survival, they are currently being tested around the world, with projects under way in the Netherlands, Australia, the United States, and many other countries. "It just makes intuitive sense that corridors are beneficial," says Hugh Safford, an ecologist with the U.S. Forest Service who has worked extensively in the Mata Atlântica. "Any planting to restore a forest has to help."

In Brazil one goal is to develop a corridor that would link broken bits of forest along 500 miles (805 kilometers) of southeastern coastline, including both new forest and existing agriculture. Though native trees are preferable, just about any type of tree or shrub can be incorporated into a corridor. "Animals use the coffee and eucalyptus plantations to get from one fragment to another," says Chiarello's colleague and fellow mammalogist, Marcelo Passamani. Conservationists want farmers and ranchers to maintain plantings they already have and to join these with replanted stands of native trees. But is it really possible to bring back a rain forest?

"Theoretically, it can be done," says ecologist Rejan R. Guedes-Bruni, coordinator of the Atlantic Forest Program at the Botanical Garden of Rio de Janeiro. Every year since 1993, the Botanical Garden has overseen the planting of some 30,000 seedlings. "Of course, it's very difficult," she says. "But it is not too late to try this, and so we are doing it."

To get an idea of how much of the Atlantic forest has been destroyed, one has only to look at maps of the area. On many of them the forest is shown as dark splotches of green among the lighter greens and browns that depict agriculture, or the red swaths representing cities. In southeastern Brazil, along the coastline where the Mata Atlântica is most intact, maps typically show a broad stroke of deep green. But around the city of Rio de Janeiro and in the northern state of Bahia, the darker shade gives way to large patches of olive, sage, beige, and red. In many places on the maps, only specks of the richer green remain.

In one of those specks, about 60 miles (97 kilometers)from Rio, I join Marina Lapenta, a wildlife biologist with the Golden Lion Tamarin Association, as she and her assistant search for a group of adio-collared tamarins. The speck is named the Poço das Antas Biological Reserve and covers some 13,600 acres (5,504 hectares), about half forested. The rest will be, too, if the association has its way. Plans are afoot to connect the reserve to forested plots on nearby ranches via corridors that would nearly double the size of the tamarins' habitat here.

"All of this is secondary forest," Lapenta says, as we make our way through a tangle of vines, thorny palms, and spindly broad-leafed trees—the kind of trees and plants that sprout after an old-growth forest has been cut. Her assistant, Jadir Ramos, turns his radio antennas in a slow arc, homing in on the tamarins' signal. "They're coming this way," he says.

Right on cue, the air suddenly fills with the tamarins' high-pitched whistles, clucks, and warbles. They spy us and make a sharp alarm cry, then leap into the uppermost branches of a tree with such speed they look like flying cats. For a moment there is only a blur of red-orange, silky fur. Then curiosity gets the better of them, and they inch closer to peer down at us. "They'll get used to us," whispers Lapenta. "But they're nervous because another group is coming this way."