Ecological lessons in survival

Natural History, April, 2005 by Jared Diamond

Societies normally endure minor rises and falls of fortune, even conquest bya neighbor, without undergoing a drastic change in total population or social complexity. But some societies have truly collapsed: their populations crashed and their complex social and economic organizations broke apart. Might such a fate befall our own society? Will tourists someday stare mystified at the rusting hulks of New York City's skyscrapers, much as we stare today at the overgrown ruins of Mayan cities?

Many collapses of the past appear to have been triggered, at least in part, by ecological problems: people inadvertently destroyed their environmental resources [see "Maya: A Classic Case," opposite page]. But societies are not doomed to collapse because of environmental damage. Some societies have coped with their problems, whereas others have not [see "Paths to Success," page 40]. But I know of no case in which a society's collapse can be attributed simply to environmental damage; there are always complicating factors. Among them are climate change, the role of neighbors (who can be friendly or hostile), and, most important, the ways people respond to their environmental problems.

In some respects we face greater risks than past societies did. Our technology (and its unintended destructive effects) is potent; our economy is global (so that now a collapse even in Somalia affects the United States and Europe); millions (and, soon, billions) of us depend on modern medicine for survival; and our population is much larger. No place is truly immune from environmental damage [see "Montana: Trouble in Paradise," page 42]. And for the first time in history, we face the risk of a global decline. Yet some of the same new conditions--technology, globalization, modern medicine--can help us find solutions to our problems.

Because we are the cause of most of our own environmental problems, we can choose to solve them. And we enjoy an unprecedented opportunity to learn quickly from developments everywhere in the world today, and from what has unfolded in times past.

Maya: A Classic Case

No ancient civilization is more commonly associated with the word "collapse" than that of the Maya of Central America. In the ninth century A.D., the Mayan population fell from at least 5 million people to a tenth that size or less. At about the same time, in some of the most vibrant centers of Mayan settlement, there's a sudden dearth of inscriptions featuring the names of kings or dates expressed in the "Long Count" calendar, signaling the disintegration of complex political and cultural institutions. Many cities were entirely abandoned and fell into ruin; then, overgrown with trees, they remained virtually unknown to the outside world for a thousand years.

Throughout the so-called Classic period of Mayan civilization, from about A.D. 250 to 900, Mayan society remained politically divided into small kingdoms. Typifying the Mayan collapse was a kingdom whose ruins now lie in western Honduras, at a site known as Copan. The best agricultural land in the kingdom, the fertile alluvial soil of a river valley, covered only ten square miles. Beginning in the fifth century A.D., the population of the valley rose rapidly, and by A.D. 650 people had begun to occupy and farm the surrounding hillsides. Archaeological evidence indicates that the hillsides were initially forested and less fertile than the river valley. But soon the forests were cut down, mostly for fuel, leaving the steep slopes open to soil erosion and probably also to the leaching of nutrients. Cultivation of the hillsides apparently proved worthwhile for only about a century. Erosion also carried the poorer soils from the slopes down into the valley, compromising the better agricultural zones. Furthermore, because forests play a major role in water recycling, the massive deforestation may have also contributed to drought.

At its height, in the ninth century A.D., Copan's population reached about 27,000; the last big buildings were erected around 800. The subsequent decline in population was not instantaneous--as late as A.D. 950 it was still about 15,000--but it was steadily dwindling. By about 1250 the valley was deserted.

Five strands, or major factors, contributing to the downfall of Copan can be tentatively identified. The strand was simply that population growth was outstripping the available resources. The second strand, already mentioned, compounding that mismatch, was the array of negative effects that were brought on by deforestation and hillside erosion.

The third strand was increased warfare, as neighboring kingdoms fought over their diminishing resources. Bringing matters to a head was a fourth strand: climate change. The worst drought to strike the region in 7,000 years began about A.D. 760 and peaked about 800. By then there were no unoccupied favorable lands to which people could move to save themselves. The ensuing declince in the Mayan population must have come about partly from starvation and warfare, as well as from a fall in the birthrate and in the survival rate of children.