WE AWAKE IN OUR TENTS in the moonlight to what sounds like a dance troupe in wooden clogs practicing on rock under stunted juniper trees. It's a half-dozen Carmen mountain white-tailed deer, scraping at the ground with bootlike hooves, bending gracile necks to chew on wet soil and lick it dry. They're harvesting the minerals and moisture from our urine soaked into the parched earth of the high desert, the herd toiling through the night and into the morning in a pursuit tenacious enough to enlighten us to the wastefulness of our own bodies. Clearly, the three of us have squandered most of what we drank hiking to 7,400 feet on the south rim of Texas' Chisos Mountains. From the deer's point of view, our arrival here is the next best thing to rain.
Come morning, we pack camp and loiter on the precipice, staring across wracked ranges and sunburnt country to the Rio Grande thousands of feet below, and to the even higher country of Mexico's Sierra Madre. Here, in Big Bend National Park, one of America's truly wild places, there's barely a sign of human impact, and not a sound of it—not planes, cars, or human voices. The silence is so thick that our ears feel congested, and we jump when the quiet is pierced by the whistle of a peregrine falcon on its glide path through thin air.
We spend a couple of hours here with binoculars, map, and compass, scanning 100-mile visibility, scrutinizing the area below the rim and trying to find a trail we might travel another day. Although we don't know it, we're peering down into the place where a lost hiker is desperately trying to find the same trail and a freshwater spring midway along it. At this point he has been without water for three days. We don't see him stumbling through cholla and nopales cactus and writing farewell notes to loved ones—though he is likely staring up at the mirage of us silhouetted against the sky.
Ironically, this corner of the Chihuahuan Desert is lush at the moment, watered by rains two months ago that are still working their way through soils and roots and cells, so that many plants are blooming and an explosion of butterflies jams the breezes. The cacti are swollen with hoarded water. The Chisos oaks are dropping so many acorns that park rangers have closed trails where black bears are fattening on them. Countless millions of walking-stick insects are coupled in such dense mating congregations in the canopies of mesquites that entire trees appear to be walking through the sky. Everything is haloed in the golds, yellows, and greens of desert grasses, some taller than us, all bowed under heavy seed heads destined to feed and water kangaroo rats.
It is these same tall grasses that have woven closed the trail below and launched the lost hiker on his final wayward odyssey. Tomorrow, on his fourth day without water, he will be located by search-and-rescue personnel who find him alert and oriented, but too weak to stand. They administer oral and IV fluids, but he dies anyway. Like many before him, he succumbs to the peculiar capabilities of Homo sapiens that allowed him to enter Eden but not to survive its vicissitudes.
IN THE FINAL STAGES OF DEHYDRATION the body shrinks, robbing youth from the young as the skin puckers, eyes recede into orbits, the tongue swells and cracks. Brain cells shrivel and muscles seize. The kidneys shut down. Blood volume drops, triggering hypovolemic shock, with its attendant respiratory and cardiac failures. These combined assaults disrupt the chemical and electrical pathways of the body until all systems cascade toward death.
Such is also the path of a dying species. Beyond a critical point, the collective body of a unique kind of mammal or bird or amphibian or tree cannot be salvaged, no matter the first aid rendered. Too few individuals spread too far apart or too genetically weakened are susceptible to even small natural disasters. A passing thunderstorm. An unexpected freeze. Drought. At fewer than 50 members, populations experience increasingly random fluctuations until a kind of fatal arrhythmia takes hold. Eventually, an entire genetic legacy, born in the beginnings of life on Earth, is smote from the future.
Scientists recognize that species continually disappear at a background extinction rate estimated at about one species per million species per year, with new species replacing the lost in a sustainable fashion. Occasional mass extinctions convulse this orderly norm, followed by excruciatingly slow recoveries as new species emerge from the remaining gene pool until the world is once again repopulated by a different catalog of flora and fauna. 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.
Some nations move more slowly than others. In 1992, an international summit produced a treaty called the Convention on Biological Diversity that was subsequently ratified by 190 nations—all except the unlikely coalition of the United States, Iraq, the Vatican, Somalia, Andorra, and Brunei. The European Union later called on the world to arrest the decline of species and ecosystems by 2010. Last year, worried biodiversity experts called for establishing a scientific body akin to the Intergovernmental Panel on Climate Change to provide a united voice on the extinction crisis and urge governments to action.
Yet despite these efforts, the Red List, updated every two years, continues to show metastatic growth. There are a few heartening examples of so-called Lazarus species lost and then found: the Wollemi pine and the mahogany glider in Australia, the Jerdon's courser in India, the takahe in New Zealand, and, maybe, the ivory-billed woodpecker in the United States. But for virtually all others, the Red List is a dry country with little hope of rain, as species ratchet down the listings from secure to vulnerable to endangered to critically endangered to extinct.
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.