Editor's Note: Why are ecologists concerned about the imminent destruction of the last of the tropical forests?

The answers are legion, so many as to seem diffuse, so powerful as to seem exaggerated and shrill, so fundamental as to seem obvious, and so demanding as to seem preemptive.

Forests are the great biotic flywheel that keeps the biosphere functioning more or less predictably. They are the major biotic component of the global carbon cycle, contain about three times as much carbon as the atmosphere, and their destruction contributes directly to the warming of the earth. Their presence determines the reflectivity of the earth over large areas, energy balance, water balance, nutrient fluxes, and air and water flows. They are, moreover, the major reservoir of biotic diversity on land: there is no habitat richer in species, none more promising as a source of succor for a swelling, scrambling, grasping human population uncertain as to where its great hopes lie. And yet, no habitat is being addressed more rapaciously than the tropical forests of Brazil.

Philip Fearnside is an ecologist with many years’ experience in research, writing, and teaching in Brazil. He writes here about one of the world's greatest tragedies and touches on the transitions in plant communities that accompany the process around the world. The shift from forest to grassland or lesser communities is common here as elsewhere under chronic disruption.