Ecotones are transition zones - the narrow bands where two ecosystems overlap and exchange species. The South Amazon Ecotones Corridor tracks the enormous ecotone where the Amazon rainforest gives way to the cerrado savanna of the Brazilian interior. This is not a clean border. It is a mosaic of open rainforest, semi-deciduous seasonal forest, dry woodland, grassland, and palm swamps, interleaved in patterns that shift with every ridge and river. Biologists estimate 700 species of birds live here, along with 29 species of primates, among them several that exist nowhere else on Earth. The corridor encompasses parts of three distinct Amazon ecoregions, each with its own assemblage of species, each depending on the corridor to remain connected to neighboring populations. Cut the connections and genetic isolation begins its slow work; within decades, species vanish.

The corridor also runs along Brazil's most aggressive deforestation frontier. In the north of Mato Grosso and the south of Para, cattle ranches and soy fields have replaced forest at rates that once made global headlines and still do. This is the region where BR-163 cuts through the forest, where Santarem's port ships soybeans to China, where illegal loggers and land-grabbers have operated for decades with near impunity. The corridor was classified by its original planners as vulnerable but relatively stable - a polite bureaucratic way of saying it was not yet lost but was losing ground. The dominant threats identified were unsustainable logging and the continuing expansion of monocultures: soy, corn, cotton, all hungry for flat land cleared of trees.

The original 1997 proposal linked conservation units like beads on a string. The Jaru Biological Reserve in Rondonia. The Cristalino State Park in Mato Grosso, famous for its rainforest canopy tower. The Araguaia National Park, which protects part of Ilha do Bananal - believed to be the largest inland river island in the world. The Cantao State Park with its flooded igapo forests. The Ique Ecological Station. The Rio Roosevelt State Forest, named for the river that Theodore Roosevelt explored with Candido Rondon in 1914. Later revisions added the Rio Ronuro, Rio Roosevelt, and Rio Madeirinha ecological stations and the Apiacas Ecological Reserve. The expanded corridor spans Amazonas, Rondonia, Para, Mato Grosso, and Tocantins - five states, five political contexts, five different sets of local economic pressures pulling against the vision of a continuous protected landscape.

Brazil's Ministry of the Environment identified seven ecological corridors in 1997 as part of a national conservation framework. Only two received priority for initial development: the Central Amazon Corridor and the Central Atlantic Forest Corridor. These would serve as pilot programs, testing the concepts before the approach was extended to the remaining five corridors. That sequencing left the South Amazon Ecotones Corridor waiting. The delay has not been kind. Each year of waiting has meant more cleared land, more roads, more ranches occupying what the corridor would have protected. Four large blocks of remaining reserve and indigenous land now need reconnection through lands that are mostly private property. Reconnecting them would require buying back deforested parcels, restoring vegetation, and negotiating with farmers, ranchers, and the states themselves - a vast, slow effort. The corridor exists in documents. Whether it will ever exist on the land is one of the defining questions of Amazonian conservation.