The Purus-Madeira moist forests are bounded not by mountains or coastlines but by water. The Purus River forms the western edge, the Madeira the eastern, and the Solimoes -- the upper Amazon -- closes the triangle to the north. Each of these rivers is large enough to function as a biogeographic barrier, and their seasonally flooded varzea forests create a buffer zone that many terrestrial species cannot cross. The result is an island of dry land in a continent of forest, isolated enough to produce its own evolutionary trajectory. Elevations range from just 20 meters above sea level, and the terrain is almost entirely flat -- a plain of soft sediments that emerged from the sea between two and five million years ago. Soils are acidic, sandy podzols or waterlogged clays, low in the nutrients that support intensive agriculture. The rivers that cross this landscape meander extravagantly, forming oxbow lakes by the dozens, and flood everything within reach for months at a time.

In the northern reaches of the ecoregion, the forest canopy rises to 30 meters, with emergent trees pushing up to 45. The understory is dense, tangled, and dim. Further south, the canopy opens and the understory thins until, at the extreme southern edge, moist forest gives way to patches of wooded savanna. The dominant tree families read like a roll call of tropical botany: Fabaceae, Sapotaceae, Lecythidaceae, Moraceae. Four species of palm are common, and the unusual Spathelia excelsa -- palm-like but unrelated -- fruits exactly once in its life and then dies. Couma utilis offers edible fruit to both animals and humans. Physocalymma scaberrimum towers above the canopy as a typical emergent, its red hardwood prized for furniture. Beneath the trees, the equatorial monsoonal climate maintains average temperatures around 27 degrees Celsius year-round, with 2,200 millimeters of annual rainfall peaking in March and dropping to a relative trickle of 33 millimeters in July.

The isolation that defines the Purus-Madeira ecoregion has produced extraordinary endemism. Among the 165 mammal species recorded here, three species of titi monkey are found nowhere else: Hershkovitz's titi, the ashy black titi, and the collared titi. The bare-eared squirrel monkey is endemic as well. Jaguars and cougars patrol territories that include some of the most intact habitat left in the Amazon. Giant anteaters lumber through clearings. Two species of sloth -- Hoffmann's two-toed and the brown-throated -- hang from cecropia branches. Giant otters, endangered and declining across their range, still hunt fish in the ecoregion's rivers. The birdlife is staggering: 572 species recorded, including toucans, Amazon parrots, macaws, quetzals, and 15 species of curassow. Among the endangered are the wattled curassow and the red-necked aracari. On the ground, fer-de-lance, bushmasters, and boa constrictors occupy the forest floor, while green iguanas bask on branches above.

The World Wildlife Fund's "Relatively Stable/Intact" assessment comes with a significant caveat. The southern edge of the ecoregion, where the Trans-Amazonian Highway crosses from Humaita to Labrea, has already lost forest to pasture and agriculture. Controlled and uncontrolled fires threaten the dry-season landscape. Mining along the upper Purus and Madeira rivers causes pollution and habitat destruction. Between 2004 and 2011, the ecoregion lost habitat at an annual rate of 0.32 percent -- a number that sounds small until you apply it to 17.4 million hectares. But the most sobering threat may be the one that no fence or patrol can address. As global temperatures rise, tropical species must migrate uphill to find tolerable conditions. In a mountain ecoregion, that means climbing a few hundred meters. In the Purus-Madeira moist forests, where the highest point barely exceeds 20 meters and the entire landscape is as flat as a table, there is nowhere to go.