Art of Transition: The Great Extinction

The Sixth Great Extinction

“Soon a millennium will end. With it will pass four billion years of evolutionary exuberance. Yes, some species will survive, particularly the smaller, tenacious ones living in places far too dry and cold for us to farm or graze. Yet we must face the fact that the Cenozoic, the Age of Mammals which has been in retreat since the catastrophic extinctions of the late Pleistocene is over, and that the Anthropozoic or Catastrophozoic has begun."

--Michael Soulè (1996)

The Crisis

The most important—and gloomy—scientific discovery of the twentieth century was the extinction crisis. During the 1970s, field biologists grew more and more worried by population drops in thousands of species and by the loss of ecosystems of all kinds around the world. Tropical rainforests were falling to saw and torch. Wetlands were being drained for agriculture. Coral reefs were dying from god knows what. Ocean fish stocks were crashing. Elephants, rhinos, gorillas, tigers, polar bears, and other “charismatic megafauna” were being slaughtered. Frogs were vanishing. Even Leviathan—the great whales—were being hunted down in their last redoubts of the Antarctic and Arctic seas, and their end was in sight. These staggering losses were in oceans and on the highest peaks; they were in deserts and in rivers, in tropical rainforests and Arctic tundra alike.

A few biologists—including geneticist Michael Soulè (who was later the founder of the Society for Conservation Biology) and Harvard's famed E. O. Wilson—put these worrisome anecdotes and bits of data together. They knew, through paleontological research by others, that in the 570 million years or so of the evolution of modern animal phyla there had been five great extinction events. The last happened 65 million years ago, at the end of the Cretaceous when dinosaurs became extinct. Wilson and company calculated that the current rate of extinction is one thousand to ten thousand times the background rate of extinction in the fossil record.

That discovery hit with all the subtlety of an asteroid striking Earth: RIGHT NOW, TODAY, LIFE FACES THE SIXTH GREAT EXTINCTION EVENT IN EARTH HISTORY. The cause is just as unsettling and unprecedented: eating, manufacturing, traveling, warring, consuming, and breeding by six billion human beings. For the first time in the history of life on Earth, one species is killing countless others. For the first time, one species—Homo sapiens; that's us—is waging a war against Nature.

The crisis we face is biological meltdown. Wilson (1992) warns that the proportion of species driven to extinction “might easily reach 20 percent by 2022 and rise as high as 50 percent or more thereafter.” Soulè (1980) has said that soon the only large mammals left will be those we consciously choose to protect; that, “[The twentieth] century will see the end of significant evolution of large plants and terrestrial vertebrates in the tropics.” He writes (1996), “The end of speciation for most large animals rivals the extinction crisis in significance for the future of living nature. As [Bruce Wilcox and I] said in 1980, ‘Death is one thing, an end to birth is something else.’”

Five Great Extinctions

The fossil record reveals five great extinction episodes in the last half-billion years. They are:

Ordovician—500 million years ago, 50 percent of animal families became extinct, including many trilobites (a dominant kind of marine organism that looked sort of like a horseshoe crab).

Devonian—345 million years ago, 30 percent of animal families became extinct, including some types of early fishes.

Permian—250 million years ago, 50 percent of animal families, 95 percent of marine species, many amphibians, and many trees became extinct.

Triassic—180 million years ago, 35 percent of animal families became extinct, including many reptiles and marine mollusks.

Cretaceous—65 million years ago, dinosaurs and many mollusks became extinct.

The Causes And Processes Of Extinction

Many things can push a species into the long, dark night of extinction. However, only a few things can cause mass extinction. For past mass extinctions, cataclysmic events—either terrestrial or extraterrestrial—so altered or harmed the biosphere that many species and whole groups of organisms died out. Scientists have found convincing evidence that the extinction of the dinosaurs 65 million years ago came suddenly (perhaps in a matter of days or weeks) when an asteroid struck Earth in a shallow sea where today's Yucatan Peninsula of Mexico lies.

But what causes “normal” extinctions, the kind that make up the background rate between the few big catastrophes? A species can become “extinct” by evolving into a new species or several new species (speciation driven by natural selection), or a species can become extinct by dying out and not continuing its evolutionary experiment. The latter is real extinction.

Extinction, or evolution into daughter species, is the fate of all species. Careful study of the fossil record of marine invertebrates shows that species usually last for one million to ten million years. What may cause species to become extinct? Michael Soulè lists the possible factors: rarity (low density); rarity (small, infrequent patches); limited dispersal ability; inbreeding; loss of heterozygosity (genetic diversity); founder effects; hybridization; successional loss of habitat; environmental variation; long-term environmental trends (such as climate change); catastrophe; extinction or reduction of mutualist populations; competition; predation; disease; hunting and collecting; habitat disturbance; and habitat destruction.

Soulè (1983) points out that some of these factors “do not become operative until one or more of the other factors have reduced the local populations to a very small size.” Note that he lumps the natural and human causes. Most of these factors are at play in today's mass extinction. Soulè warns, however, that “It is disappointing that we know so little about natural extinction.” Why does modern science know so little about this fascinating subject? It is because “no biologist has documented the extinction of a continental species of a plant or animal caused solely by nonhuman agencies.” The grim truth is that we humans are the cause of modern extinctions. How do we do it?

Extinction expert David Wilcove and his colleagues list five anthropogenic causes of extinction in the United States, in order of current importance: habitat destruction; non-native (alien) species; pollution; overexploitation; disease. (Worldwide, however, overexploitation is far more important than in the United States today.)

Here are a few examples of the ways humans cause extinction in each of these categories.

Habitat Destruction. We reduce, modify, degrade, or transform natural habitat upon which species depend by burning, agricultural clearing, logging, mining, grazing by domestic animals, preventing natural fire, damming rivers, dewatering rivers through irrigation diversion, drying up springs and streams through groundwater pumping, eliminating keystone species like beaver and prairie dogs whose activities create habitat for other species, and urban and suburban development. Furthermore, we fragment habitat—thereby disrupting necessary patterns of movement of many species—through the above activities and by building roads, clearing power-line rights-of-way, and driving vehicles.

Non-native (Alien) Species. As humans have spread into new lands, we have brought with us disruptive alien species that are generally well adapted to human disturbance and that outcompete native species, in part because their normal enemies, such as predators and diseases, are left behind. Such damaging invaders include plants and animals, both deliberately introduced species such as domestics or ornamentals, and accidentally introduced species such as weeds or pests. These non-native species include predators (cats, rats, pigs) and competitors (starlings, tamarisk, zebra mussels.

Pollution.

Pollution, whether localized or global (acid rain, greenhouse gases), can poison the waters and soils that are habitat for sensitive species, or leach away needed nutrients. Global warming and atmospheric ozone depletion—major threats to life forms worldwide—are caused largely by air pollution.

Overexploitation. Hunting, fishing, trapping, collecting, and government “pest” eradication programs have caused the extinction of many species and seriously endanger others today.

Disease.

As humans have spread around the world, we have brought exotic diseases with us. Global trade is spreading many new diseases. An exotic disease caused the loss of the American chestnut in the wild. The black-footed ferret was nearly wiped out by canine distemper, a disease not native to the Americas.

Ernst Mayr, perhaps the biological giant of the twentieth century, writes (2001):

Background extinction and mass extinction are drastically different in most respects. Biological causes and natural selection are dominant in background extinction, whereas physical factors and chance are dominant in mass extinction. Species are involved in background extinction, and entire higher taxa in mass extinction.

As the cause of today's mass extinction, we humans are no longer just a biological phenomenon, but are now a physical factor equivalent to an asteroid or continental drift in radically changing biological diversity. We are not exterminating only individual species, but “entire higher taxa.”

The Three Waves of Extinction

We can see the Sixth Great Extinction occurring in three waves, each caused by new groups of humans armed with new technologies spreading over new lands. The First Wave, the Spread of Modern Humans, ran from 40,000 to about 3,500 years ago as skilled big game hunters first entered lands where Homo sapiens had not previously existed. It continued from 3,000 years ago until 200 years ago, as Stone Age farmers found previously unpeopled islands in the Pacific and Indian oceans. The Second Wave, the Spread of Europeans, began in 1500 and ended around 1970 as European colonial and then industrial civilization spread over the world. The Third Wave, Overpopulation and Globalization, began about 1970 as human population exploded and new technologies and business practices tied the world into one exponentially expanding agro-techno-economy.

In the First Wave, extinctions were caused mostly by hunting, and perhaps by fire-setting and introductions of dogs and diseases into areas that had not previously experienced them. The victims were primarily large mammals, birds, and reptiles on continents and islands. In the second phase of the First Wave, Stone Age farmers settled Hawaii, New Zealand, Madagascar, and other islands, and extinctions were caused by agricultural clearing, fire-setting, hunting, and introductions of dogs, rats, pigs, goats, and diseases into areas that had not previously experienced them. The victims were primarily birds and reptiles.

The Second Wave was caused by hunting with guns; large-scale fishing; massive habitat destruction by agriculture, forestry, and domestic livestock grazing; river damming and diversion; introduction of exotic predators, browsers, grazers, parasites, and diseases; and later by industrial pollution. Islands lost birds, giant tortoises, and small mammals. On continents, some birds, fish, and large mammals have been driven into extinction, but many more species of birds, freshwater fish, and large mammals have had their numbers drastically reduced to possibly nonviable remnants. In the oceans, sea mammals, shellfish, and many fish have been wastefully exploited so that their populations are mere shadows of what they were 400 years ago.

The Third Wave has just begun. Its agents of extinction are those of the other waves, but now the human population explosion—from about 10 million 10,000 years ago to over six billion today—and a globalized agro-techno-economy spread over the whole Earth threaten everything from the last megafauna to plants to insects to coral reef ecosystems.

In 40,000 years, fully modern humans have spread across the Earth three times, with devastating consequences for the rest of life.