PART FIVE

REVERSING OUR PROGRESS OVERNIGHT

The ruins of Pueblo Bunito, the largest structure in Chaco Canyon. This onetime Anasazi settlement is now Chaco Culture National Historical Park in New Mexico.

OUR SPECIES NOW COVERS THE EARTH AND commands a larger share of the planet’s productivity than ever before. That’s the good news. The bad news is that, as the next three chapters will show, we are now reversing our progress much more rapidly than we created it. Our power threatens our own existence. Will we suddenly blow ourselves up, or sink slowly into a stew of global warming, pollution, more mouths to feed with less food, and the loss of species we need to survive? And are these dangers really new ones that arose only after the Industrial Revolution of the late eighteenth and nineteenth centuries?

Most people believe that Nature exists in a state of balance: predators don’t exterminate their prey; grazing animals don’t overgraze their food sources. In this view, humans are the only misfits. If this were true, Nature would hold no lessons for us, because animals and their environments would never get out of balance.

It’s true that species don’t naturally become extinct as rapidly as we are now exterminating them, except under rare circumstances—such as the mass die-off sixty-five million years ago, possibly due to an asteroid crashing to earth, that finished the dinosaurs. Still, Nature offers many examples of species exterminating other species. This usually happens when a predator is introduced to a new environment, where it meets prey species that are not used to it. After exterminating some of these species, the predator survives by switching to others.

Rats, cats, goats, pigs, ants, and even snakes have become killers when they have been carried by humans to new environments. One example is a tree snake native to Australia. During World War II it was accidentally carried on ships or planes to the Pacific island of Guam, which had no snakes. By now the Australian tree snake has wiped out or brought to the brink of extinction most of Guam’s forest bird species, which had no chance to evolve behavior that would defend against snakes.

We humans are the prime example of a switching predator, one that can switch to new prey when one type of prey becomes scarce or extinct. We eat everything from snails and seaweed to whales, mushrooms, and strawberries. If we overharvest a species to the point of extinction, we just switch to another food source. For this reason, a wave of extinctions has followed us every time we have moved into an unoccupied part of the globe. Hawaiian bird species died out in great numbers after Polynesians reached Hawaii fifteen hundred years ago, for example.

What about animals? Do they ever destroy their own resource base? It doesn’t happen often, because animal populations tend to rise and fall along with their food supply. Still, some animal populations have eaten themselves out of food, and perished. In 1944, twenty-nine reindeer were introduced to St. Matthew Island in the Bering Sea. By 1963 they had multiplied to six thousand. But reindeer eat slow-growing lichens, and on the small island these plants had no chance to recover from reindeer grazing, because the animals had nowhere to migrate. When a harsh winter struck, all the reindeer died except forty-one females and one sterile male, leaving a doomed population on an island littered with skeletons.

Ecological suicide by animals happens when populations suddenly become free of the forces that usually control their numbers. Humans have recently escaped from the former controls on our numbers. We eliminated predation on us long ago. Modern medicine has greatly reduced the number of deaths from infectious disease. Behaviors that used to limit population size, such as killing our offspring and waging near-permanent war, have become socially unacceptable. Our population continues to grow, but the example of the St. Matthew reindeer teaches us that no population can grow indefinitely.

Our present condition can be compared to events in the animal world. Like many switching predators, we eliminate some prey species when we colonize a new environment or gain new destructive power. Like some animal populations that escape their growth limits, we risk destroying ourselves by destroying our resource base.

What about the view that humans lived in a state of ecological balance before the Industrial Revolution, and that only in modern times have we exterminated species and overused our environment? The remaining chapters examine this idea. We will look first at the belief in a golden age when humans supposedly lived in harmony with nature. Then we’ll look more closely at one of the biggest, most dramatic, and most controversial mass extinctions: the disappearance of many large mammals from the Americas, just as humans arrived. Finally, we’ll try to determine how many species we have already driven to extinction, and what that might mean for our own future.