Extinction Events

Human activity has caused a few notable extinction events — and many more that go completely unremarked. Every species lost weakens biodiversity and the natural life support systems on which humans depend.

extinction events - ehrlichs

Noted ecologists Anne and Paul Ehrlich warn that human-driven extinction events are undermining the planet's life support systems.


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As we indicated in an earlier column, the extinction of non-human species is a matter of major concern to biologists ... and it should be of extreme concern to all human beings.

There are several reasons for this. First of all, the acceleration in extinction of populations or of entire species can be viewed as the removal of essential parts (sometimes analogized with rivets) of the life-support systems of Spaceship Earth more rapidly than natural "repair" processes can replace them. And, although there's considerable redundancy in such systems, the ultimate impact of continuing extinctions is clear: the loss of essential ecosystem services.

The services provided by ecosystems, as you may recall, include such essentials as the amelioration of the climate, the disposal of wastes and recycling of nutrients, the generation and maintenance of soils, and the control of the vast majority of crop pests and transmitters of human diseases. Humanity has no satisfactory way of replacing these free services should they be lost, and civilization cannot persist without them.

Furthermore, the accelerating rate of extinction is itself a measure of the increasing level of attack that humanity is launching on Earth's ecosystems. Each time a population is destroyed, one less component is available to help perform vital services for us. In short, biologists are worried in principle about the "popping of rivets" from our spaceship, and are frightened because they see the rivets disappearing at an ever more rapid pace.

There are other reasons for concern over the extinction of populations and species, as well ... including the fact that the disappearance of plants or animals often results in the loss of direct economic benefits (ecosystem services are considered indirect benefits) and the loss of aesthetic values. Moreover, in the name of plain old-fashioned compassion, we should take care not to eliminate any more of our fellow "passengers" than we already have in the short time man has trod this Earth.

The Passing of the Passenger Pigeon

The most dramatic cause of extinction is direct endangering, usually by overexploitation. And the classic example is the fate of the passenger pigeon. This fascinating, graceful, pretty bird—with a slate blue back and deep pink breast—didn't coo like a dove, but produced "shrieks and chatters and clucks." Its greatest claim to fame was the enormous size of its populations ... in fact, it may well have been the most abundant bird that ever existed. John James Audubon observed a flock that passed overhead for three days in a row, sometimes at the incredible rate of more than 300 million birds an hour. Alexander Wilson—co-founder, with Audubon, of American ornithology—estimated that another flock contained two billion birds! The passage of such huge numbers in flight created a roar of wings that could actually be heard half a dozen miles away!

These pigeons, which originally lived throughout eastern North America, fed on the fruits of forest trees, especially acorns and beechnuts. They nested in narrow colonies as much as 40 miles long and several miles across. Their droppings in roosting areas piled up inches thick, killing all the vegetation below the trees and—in time—the trees themselves.

This country's early settlers had no trouble, as you'd imagine, adding the passenger pigeon to their diets. The nesting grounds were so crowded that adult birds were always being injured or killed, and succulent squabs were constantly knocked out of their nests ... so a hungry person could simply wander through a colony and pick up dinner.

As the human population increased, however, two things began to happen: First, railroads pushed through the wilderness and enabled professional hunters to ship the birds to centers like New York. Second, the great oak and beech forests in which the birds nested began to be cleared.

Profit hunters devised ingenious ways of killing the pigeons in large numbers. They suffocated entire flocks by burning grass or sulfur under their roosts. The birds were also batted down with long sticks, blasted with shotguns, fed grain soaked in alcohol and picked up dead drunk, or netted (after which their heads were crushed with pincers). One trapping device depended upon a decoy bird—with its eyes sewed shut—tied to a perch called a "stool" ("stool pigeon" thereby became a part of the language).

The disappearance of the species was startlingly rapid. After the Civil War, so many millions of the birds were shipped from the Middle West to New York that live pigeons were even used as targets in shooting galleries. But the huge flocks were by then already gone from the coastal states and—by the 1880's—were dwindling everywhere. In 1878 one hunter shipped some three million birds from Michigan, the passenger pigeon's final stronghold. The last wild bird was seen in that state just 11 years later, and the final captive pigeon died in the Cincinnati Zoo in 1914. Her name was Martha.

A Delicate Balance

Economic extinction (reduction of the passenger pigeon population to a level where hunting was no longer profitable) preceded biological extinction. The last birds in the wild were not killed by Homo sapiens , but perished as a result of "natural causes." Habitat destruction was not a major factor, however, because even today large areas of forest suitable for the pigeons exist in the eastern United States. Apparently the ability to form huge flocks was essential to the birds' survival. Once their populations became too small to maintain sufficiently large breeding colonies, nesting failures, inbreeding, and mortality from predation must have escalated and pushed the species to extinction.

Other species, too, are known to have been wiped out by direct human intervention. The dodo, a flightless bird that once inhabited Mauritius, was eliminated a couple of centuries ago by early European settlers. Its story is not as well documented as is that of the passenger pigeon, but the fact is commemorated in the phrase "dead as a dodo." Furthermore, at least it's suspected that thousands of years ago—at the end of the last glacial epoch—Stone-Age hunters played a major role in the extermination of many species of large mammals, including mammoths, woolly rhinos, and ground sloths. Today, numerous other species are endangered as a result of human persecution. Elephants are slaughtered for their ivory; rhinos for their horns (thought by some cultures to have aphrodisiac qualities); whales for their meat; cheetahs, snow leopards, and other cats because they threaten livestock; and antelopes for "sport." Sea turtles are chased for their shells, and their nests are robbed of eggs. Collectors pay large sums for rare fishes, amphibians, reptiles, and especially birds. And, closer to home, the "cactus craze" has become so extreme that many species are faced with extinction. Even national parks are being stripped of these key desert plants.

Primates—monkeys, gibbons, chimps, and gorillas—are much sought for medical research. (The standard practice in collecting primates is to shoot a group of adults and then capture the young!) Unhappily, the research is much too often poorly designed and utterly useless. And saddest of all, only a small proportion of the specimens gathered for experiments or collections ever survive the journey.

Cruel Evictions

In spite of all this, the second major cause of extinction—habitat destruction—is much more important than the first. For every species and population endangered because of direct human action, thousands are threatened by the inadvertent loss of suitable places in which to live. Often these are relatively obscure organisms—insects and other invertebrates, small plants, and so on—whose jeopardy or eventual passing goes completely unmarked.

For example, the plight of the Bay checkerspot butterfly is typical of many temperate-zone organisms. The insects occupy patches of serpentine grassland in the San Francisco Bay area, where the food plants required by their caterpillars grow. Long-term research has shown that the separate populations of this insect are subject to periodic extinction by drought, and it's suspected that other colonies may occasionally be wiped out by the fires that maintain the local chaparral habitat.

But before the Bay Area was developed, the losses were not catastrophic. The vacant "islands" of serpentine grass would be repopulated, after a decade or so, by migrant individuals from a nearby island where the butterflies had survived. Thus, a shifting checkerboard of populations persisted.

Things began to change late in the last century, however, as the human population of San Francisco and its suburbs grew. Gradually, islands of serpentine became sites of human habitation. The entire process accelerated rapidly following World War II, as servicemen returning from the Pacific decided to settle in beautiful California. More and more checkerspot colonies disappeared under homes, shopping centers, fast food outlets, and freeways.

Then, during the mid-1970's, a drought pushed additional populations to extinction. As a result, the Bay checkerspot is now endangered. The few remaining populations are probably too far from the "empty" islands for migrant individuals to repopulate them before extinction overtakes the still existing colonies.

Habitat destruction, of course—in the form of the conversion of its river home into a lake behind the Tellico Dam—is the sole threat to the snail darter. The conversion of virgin land to cities, farms, and pasture is decimating the rich flora of South Africa's Cape region, threatening an orchid in Russia, and destroying a beautiful buttercup in New Zealand.

The Destruction Grows

The list is long and depressing. All the fish populations in some 300 Adirondack lakes have been exterminated by acid rains. Siberian tigers are disappearing as their forest home is cleared. Desert animals and plants in the American Southwest are being destroyed by offroad vehicles (a carefully driven dirt bike degrades an acre of land in a 20-mile journey ... a four-wheel-drive vehicle, more than three acres in the same distance). Rats, goats, burros, mongooses, and various other animal and plant "weeds" that have been moved around by humanity so degrade habitats that native species—especially on islands—go extinct wholesale.

All over the globe, at an ever-increasing pace, populations of all sorts of organisms are being paved over, plowed under, grazed away, flooded out, stripmined, poisoned, invaded, and—above all—destroyed as forests are cleared. This last cause of extinction is especially serious because of the mounting assault on tropical rain forests, which may contain two-fifths or more of all the Earth's species. If current trends continue, most of this enormous reservoir of life's diversity will be gone early in the next century ... and our spaceship may have lost enough rivets to make the survival of society impossible!

For more details on the way other organisms are forced to extinction and a discussion of ecosystem services and the consequences of their loss, see Extinction by P. Ehrlich and A. Ehrlich (Random House, 1981) upon which this column is based.  

In November 1981 Anne and Paul Ehrlich, and their friend and coauthor John Holdren, will join MOTHER EARTH NEWS for a unique South Pacific Seminar nn the idyllic islands of Bora Bora and Raïa-téa.   

Paul Ehrlich (Bing Professor of Population Studies and Professor of Biological Sciences, Stanford University) and Anne Ehrlich (Senior Research Associate, Department of Biological Sciences, Stanford) are familiar names to ecologists and environmentalists everywhere. As well they should be. Because it was Paul and Anne who—through their writing and research—gave special meaning to the words "population," "resources," and "environment" in the late 1960's. (They also coined the term coevolution, and did a lot to make ecology the household word it is today.) But while most folks are aware of the Ehrlichs' popular writing in the areas of ecology and overpopulation (most of us—for instance—have read Paul's book The Population Bomb), far too few people have any idea of how deeply the Ehrlichs are involved in ecological research (research of the type that tends to be published only in technical journals and college textbooks). That's why it pleases us to be able to present these semi-technical columns by authors/ecologists/educators Anne and Paul Ehrlich.