Ecosystems are naturally resilient, but human impact can reduce their ability to bounce back in many ways. Rain forests withstand some degree of cutting, for instance, but once forest fragments shrink beyond some unknown threshold, the entire system loses its ability to recover. PAGE refers to a recent study led by the University of Michigan's Lisa Curran, who contends that human activities such as logging may have doomed Indonesia's great dipterocarp trees, the anchor of its rain forests.

These trees reproduce by releasing huge masses of fruit in a synchronized fashion that is designed by nature to overwhelm the appetites of fruit and seedeaters and ensure that there are always some seeds left over to sprout. The strategy, called masting, worked for millions of years. Now, however, the forests in Borneo have been so reduced that humans and animals can consume all the dipterocarp fruit, with the result that no new dipterocarp trees are taking root in the areas studied by Curran and her colleagues. Since a host of creatures ranging from the orangutan to the boar are dependent on the dipterocarps, the trees' disappearance may ultimately doom Indonesia's rain-forest ecosystem. PAGE scientist Nigel Sizer of the World Resources Institute notes that similar problems associated with fragmentation loom over all but the largest remaining forests on Earth.

Halting the decline of the planet's life-support systems may be the most difficult challenge humanity has ever faced. The report specifies some common-sense steps in the right direction. For instance, governments can eliminate the estimated $700 billion in annual subsidies that spur the destruction of ecosystems. In Tunisia, water is priced at one-seventh of what it costs to pump, encouraging waste. In the mid-1980s, Indonesia spent $150 million annually to subsidize pesticide use. With access to cheap chemicals, Indonesian farmers poured pesticides onto their rice fields, killing pests, to be sure, but also causing human illness and wiping out birds and other creatures that ate the pests. When Indonesia ended the subsidies in 1986, pesticide use dropped dramatically with no ill effects on rice production. Corruption offers another target. PAGE notes that illegal logging accounts for half the timber harvest in Indonesia. Government officials have long looked the other way because of close financial ties to companies cutting the timber.

MISSION IMPROBABLE

An ecosystem's intricate, interdependent webs of life are hard to restore once they have become frayed. The U.S. is learning this lesson in its multibillion-dollar effort to halt the decline of the Everglades, the "river of grass" that once covered 4,500 sq. mi. (11,700 sq km) in Florida. Having spent much of this century channeling, damming and diverting Everglades water for urban and agricultural use, state and federal politicians have watched with growing alarm as these alterations threw the ecosystem into a tailspin. Wading-bird populations have plummeted; sport and commercial fish catches have fallen; 68 of the Everglades' resident species, including the manatee and the panther, have become endangered; and the capacity of the system to store water has shrunk even as human demand for it grows.

With Florida's water supply and a $14 billion annual tourist business in jeopardy, the Army Corps of Engineers put forward a $7.8 billion plan in 1998 to undo many of its earlier projects and restore the slow-moving sheet of water that made the Everglades a natural wonderland. Billions more will be spent removing phosphorus from agricultural runoff, restoring habitats and modifying development plans to reduce stress on the system, but there is no guarantee that even these efforts will bring back the Everglades. The unsettling prospect that the planet's richest nation may not have the wherewithal to restore a vital ecosystem underscores a theme that runs through the U.N. report and should guide development decisions in the coming years: it is far less expensive to halt destructive practices before an ecosystem collapses than it is to try to put things back together later.

In their joint editorial announcing the findings of PAGE, the heads of the World Bank, the U.N. Development Program, the U.N. Environment Program and the World Resources Institute confirm their "commitment to making the viability of the world's ecosystems a critical development priority for the 21st century." These are sweeping words, but the jury on this commitment will be composed of the world's ecosystems. The planet itself will let us know, in the harshest possible manner, if our words are not being backed by action.

Linden is the author of "The ParrotŐs Lament" and "The Future in Plain Sight"