Mass Extinction and the Hazards of Earth's Vanishing Biodiversity
|7 in 10 biologists believe that mass extinction poses a colossal threat to human existence...In a staggering forecast, Wilson predicts that our present course will lead to the extinction of half of all plant and animal species by the year 2100.
MOTHER JONES MAGAZINE by Julia Whitty Apr 2007 4/27/2008
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From what we understand so far, five great extinction events have reshaped Earth in cataclysmic ways in the past 439 million years, each one wiping out between 50 and 95 percent of the life of the day, including the dominant lifeforms, the most recent event killing off the non-avian dinosaurs. Speciations followed, but an analysis published in Nature showed that it takes 10 million years before biological diversity even begins to approach what existed before a die-off.
Today we're living through the sixth great extinction, sometimes known as the Holocene extinction event. We carried its seeds with us 50,000 years ago as we migrated beyond Africa with Stone Age blades, darts, and harpoons, entering pristine Ice Age ecosystems and changing them forever by wiping out at least some of the unique megafauna of the times, including, perhaps, the saber-toothed cats and woolly mammoths. When the ice retreated, we terminated the long and biologically rich epoch sometimes called the Edenic period with assaults from our newest weapons: hoes, scythes, cattle, goats, pigs.
But as harmful as our forebears may have been, nothing compares to what's under way today. Throughout the 20th century the causes of extinction—habitat degradation, overexploitation, agricultural monocultures, human-borne invasive species, human-induced climate change—amplified exponentially, until now in the 21st century the rate is nothing short of explosive. The World Conservation Union's Red List—a database measuring the global status of Earth's 1.5 million scientifically named species—tells a haunting tale of unchecked, unaddressed, and accelerating biocide.
When we hear of extinction, most of us think of the plight of the rhino, tiger, panda, or blue whale. But these sad sagas are only small pieces of the extinction puzzle. The overall numbers are terrifying. Of the 40,168 species that the 10,000 scientists in the World Conservation Union have assessed, 1 in 4 mammals, 1 in 8 birds, 1 in 3 amphibians, 1 in 3 conifers and other gymnosperms are at risk of extinction. The peril faced by other classes of organisms is less thoroughly analyzed, but fully 40 percent of the examined species of planet Earth are in danger, including up to 51 percent of reptiles, 52 percent of insects, and 73 percent of flowering plants.
By the most conservative measure—based on the last century's recorded extinctions—the current rate of extinction is 100 times the background rate. But eminent Harvard biologist Edward O. Wilson and other scientists estimate that the true rate is more like 1,000 to 10,000 times the background rate. The actual annual sum is only an educated guess, because no scientist believes the tally of life ends at the 1.5 million species already discovered; estimates range as high as 100 million species on Earth, with 10 million as the median guess. Bracketed between best- and worst-case scenarios, then, somewhere between 2.7 and 270 species are erased from existence every day. Including today.
We now understand that the majority of life on Earth has never been—and will never be—known to us. In a staggering forecast, Wilson predicts that our present course will lead to the extinction of half of all plant and animal species by the year 2100.
You probably had no idea. Few do. A poll by the American Museum of Natural History finds that 7 in 10 biologists believe that mass extinction poses a colossal threat to human existence, a more serious environmental problem than even its contributor, global warming, and that the dangers of mass extinction are woefully underestimated by most everyone outside of science. In the 200 years since French naturalist Georges Cuvier first floated the concept of extinction, after examining fossil bones and concluding "the existence of a world previous to ours, destroyed by some sort of catastrophe," we have only slowly recognized and attempted to correct our own catastrophic behavior.
All these disappearing species are part of a fragile membrane of organisms wrapped around Earth so thin, writes E.O. Wilson, that it "cannot be seen edgewise from a space shuttle, yet so internally complex that most species composing it remain undiscovered." We owe everything to this membrane of life. Literally everything. The air we breathe. The food we eat. The materials of our homes, clothes, books, computers, medicines. Goods and services that we can't even imagine we'll someday need will come from species we have yet to identify. The proverbial cure for cancer. The genetic fountain of youth. Immortality. Mortality.
The living membrane we so recklessly destroy is existence itself.
p>Biodiversity is defined as the sum of an area's genes (the building blocks of inheritance), species (organisms that can interbreed), and ecosystems (amalgamations of species in their geological and chemical landscapes). The richer an area's biodiversity, the tougher its immune system, since biodiversity includes not only the number of species but also the number of individuals within that species, and all the inherent genetic variation—life's only army against the diseases of oblivion.
The loss of even one species irrevocably changes the desert (or the tundra, rainforest, prairie, coastal estuary, kelp forest, coral reef, and so on) as we know it, just as the loss of each human being changes his or her family forever.
Nowhere is this better proven than in a 12-year study conducted in the Chihuahuan Desert by James H. Brown and Edward Heske of the University of New Mexico. When a kangaroo rat guild composed of three closely related species was removed, shrublands quickly converted to grasslands, which supported fewer annual plants, which in turn supported fewer birds. Even humble players mediate stability.
So when you and I hear of this year's extinction of the Yangtze River dolphin, and think, how sad, we're not calculating the deepest cost: that extinctions lead to co-extinctions because most every living thing on Earth supports a few symbionts and hitchhikers, while keystone species influence and support a myriad of plants and animals. Army ants, for example, are known to support 100 known species, from beetles to birds. A European study finds steep declines in honeybee diversity in the last 25 years but also significant attendant declines in plants that depend on bees for pollination—a job estimated to be worth $92 billion worldwide. Meanwhile, beekeepers in 24 American states report that up to 70 percent of their colonies have recently died off, threatening $14 billion in U.S. agriculture. And bees are only a small part of the pollinator crisis.
One of the most alarming developments is the rapid decline not just of species but of higher taxa, such as the class Amphibia, the 300-million-year-old group of frogs, salamanders, newts, and toads hardy enough to have preceded and then outlived most dinosaurs. Biologists first noticed die-offs two decades ago, and since have watched as seemingly robust amphibian species vanished in as little as six months. The causes cover the spectrum of human environmental assaults, including rising ultraviolet radiation from a thinning ozone layer, increases in pollutants and pesticides, habitat loss from agriculture and urbanization, invasions of exotic species, the wildlife trade, light pollution, and fungal diseases. Sometimes stressors merge to form an unwholesome synergy; an African frog brought to the West in the 1950s for use in human pregnancy tests likely introduced a fungus deadly to native frogs. Meanwhile, a recent analysis in Nature estimates that in the last 20 years at least 70 species of South American frogs have gone extinct as a result of climate change.
In a 2004 analysis published in Science, author Lian Pin Koh and colleagues predict that an initially modest co-extinction rate will climb alarmingly as host extinctions rise in the near future. Graphed out, the forecast mirrors the rising curve of an infectious disease, with the human species acting all the parts: the pathogen, the vector, the Typhoid Mary who refuses culpability, and, ultimately, one of up to 100 million victims.
The protected lands we've made so far, 102,102 sites covering 7 million square miles of earth and water, total less than 4 percent of the planet's surface. Many if not most of these isolated fragments are surrounded by hostile neighbors: farms, used-car lots, urban sprawl, clearcuts.
Segregated wildlands experience the same challenges as the dwindling members of an endangered species. Spread too far apart or too genetically weakened, they're cut off from the vital contact that renews and refreshes them, and likewise suffer debilitating arrhythmias in their demographics. Initial species losses are followed by overcrowding, then by population crashes, and insularization, with its attendant biodiversity decline.
The picture is complicated by mysterious realities: that many species will not populate a small wilderness even though it's big enough for their needs. Others will not cross the openings that fragment wilderness, particularly roads, which prove impermeable barriers to many from beetles to bears, either because they refuse to cross or because they die trying. Fragmentation also produces a dreaded edge effect by breaching the protective skin of wilderness, disrupting microclimates, allowing pathogens, alien species, and human development inside, then sealing the edges through the scarification of weed growth.
Rewilding is bigger, broader, and bolder than humans have thought before. Many conservation biologists believe it's our best hope for arresting the sixth great extinction. E.O. Wilson calls it "mainstream conservation writ large for future generations." Because more of what we've done until now—protecting pretty landscapes, attempts at sustainable development, community-based conservation, and ecosystem management—will not preserve biodiversity through the critical next century. By then, half of all species will be lost, by Wilson's calculation. To save Earth's living membrane, we must put nature's shattered pieces back together. Only megapreserves modeled on a deep scientific understanding of continentwide ecosystem needs hold that promise. "What I have been preparing to say is this," wrote Thoreau more than 150 years ago, "in wildness is the preservation of the world." This, science finally understands.
The Wildlands Project calls for reconnecting wild North America in four broad megalinkages: along the Rocky Mountain spine of the continent from Alaska to Mexico; across the Arctic/boreal from Alaska to Labrador; along the Atlantic via the Appalachians; and along the Pacific via the Sierra Nevada into the Baja Peninsula. Within each megalinkage, core protected areas would be connected by mosaics of public and private lands providing safe passage for wildlife to travel freely. Broad, vegetated overpasses would link wilderness areas split by roads. Private landowners would be enticed to either donate land or adopt policies of good stewardship along critical pathways.
It's a radical vision, one the Wildlands Project expects will take 100 years or more to complete, and one that has won the project a special enmity from those who view environmentalists with suspicion. Yet the core brainchild of the Wildlands Project—that true conservation must happen on an ecosystemwide scale—is now widely accepted.
At its heart, rewilding is based on living with the monster under the bed, since the big scary animals that frightened us in childhood, and still do, are the fierce guardians of biodiversity. Without wolves, wolverines, grizzlies, black bears, mountain lions, and jaguars, wild populations shift toward the herbivores, who proceed to eat plants into extinction, taking birds, bees, reptiles, amphibians, and rodents with them. A tenet of ecology states that the world is green because carnivores eat herbivores. Yet the big carnivores continue to die out because we fear and hunt them and because they need more room than we preserve and connect. Male wolverines, for instance, can possess home ranges of 600 square miles. Translated, the entire state of Rhode Island would have room for only two.
But by far the most endangered wildlife linkage is the borderlands between the United States and Mexico. The Sky Islands straddle this boundary, and some of North America's most threatened wildlife—jaguars, bison, Sonoran pronghorn, Mexican wolves—cross, or need to cross, here in the course of their life travels. Unfortunately for wildlife, Mexican workers cross here too. Of late, Vacariu says, these immigrants have been traveling up the Chiricahuas. Men, women, and children, running at night, one-gallon water jugs in hand.
The problem for wildlife is not so much the intrusions of illegal Mexican workers but the 700-mile border fence proposed to keep them out. From an ecological perspective, it will sever the spine at the lumbar, paralyzing the lower continent.
Here, in a nutshell, is all that's wrong with our treatment of nature. Amid all the moral, practical, and legal issues with the border fence, the biological catastrophe has barely been noted. As if extinction is not contagious and we won't catch it.
Border fences have terrible consequences. One between India and Pakistan forces starving bears and leopards, which can no longer traverse their feeding territories, to attack villagers.
The truth is, wilderness is more dangerous to us caged than free—and has far more value to us wild than consumed. Wilson suggests the time has come to rename the "environmentalist" view the "real-world" view, and to replace the gross national product with the more comprehensive genuine progress indicator, estimating the true environmental costs of farming, fishing, grazing, mining, smelting, driving, flying, building, paving, computing, medicating, and so on. Until then, it's like keeping a ledger recording income but not expenses. Like us, Earth has a finite budget.