The Sixth Extinction (2014) chronicles the history of species extinction and shows how humans have had more than a hand in the rapidly decreasing numbers of animal species on earth. Through industrialization and deforestation, not to mention climate change, humans have damaged the environment and disrupted habitats, leading to a massive reduction in biodiversity.
Who is it for?
- Scientists, environmentalists or activists concerned with climate change
- People curious about how human activity affects animal survival
- Students examining theories of species extinction
Learn about the threat of extinction, and how we can still avoid it.
Our world has experienced five catastrophic species extinctions, a group that scientists call the big five. The disappearance of the dinosaurs, for example, was one of the five.
Yet today, even as you read this right now, a sixth extinction is happening. And it’s all our fault.
Humans are responsible, through industrialization, deforestation and the resulting climate changes, for speeding up the process of extinction for a serious number of animal species. Habitats have changed; oceans have acidified; biodiversity has dropped to alarming levels.
So what’s to be done? How can we turn the tide and reestablish some sort of balance in our world? These summaries will explain how we’ve had a hand in species extinction since Homo sapiens first threw a spear, and how if we don’t change our behavior, we just might go the way of the Neanderthal.
In these summaries, you’ll discover
- why a massive dust cloud might have made the dinosaurs extinct;
- how a lack of frozen real estate has put migratory polar bears in a bind; and
- how the ease of modern transportation has inspired a second Pangea.
How we live and how we travel the globe has directly resulted in animal species extinction.
Right now, many species of animal are endangered. Certain animals are threatened with extinction.
Yet have you ever considered how exactly a species disappears from the earth?
Historically, extinctions are rare and occur very slowly. Yet there have been periods of environmental change that have triggered mass extinctions, in which many species die in a shortened time period.
So while the “normal” rate of extinction – the background extinction rate – is generally slow, it does vary by animal group.
For instance, according to the background extinction rate for mammals, we should expect to see one species die out every 700 years. But during periods of mass extinction, this rate spikes. So far, we’re aware of five such episodes that the scientific community calls the “big five.” The extinction of dinosaurs roughly 64 million years ago, for example, was one of these five.
But mass extinctions aren’t just limited to prehistoric times. In fact, we might be experiencing one right now. We know this by looking at the actual rate of species extinction.
Take amphibians, one of the most endangered classes of animals. The actual rate of extinction today for amphibians is estimated to be 45,000 times higher than the background rate!
So the question is: What’s responsible for this disaster?
We are, actually. Humans are both directly and indirectly responsible for species extinction.
Consider modern transportation networks. Ships, planes and trains crisscross the globe, bridging continents and indirectly causing mass extinctions by introducing new organisms into environments where they can wreak havoc on existing species populations.
Panamanian golden frogs, for example, now struggle against a deadly fungus that likely came to Central America from Europe. But other species such as the great auk have been directly wiped out by hunters as well as by changes made to its habitat.
So we’re to blame for this mess. But could we have known what a profound effect our actions would have on the environment? To learn more, let’s dig into the history of evolution and extinction.
Extinction: slow or sudden? Theories have changed over the centuries as new info is unearthed.
The idea that a species is capable of dwindling and disappearing altogether is relatively new. In fact, we have for some time believed that the species here on earth would always remain the same.
So when did we finally understand the shifting nature of survival in the animal kingdom?
Back in the nineteenth century, a French naturalist named Georges Cuvier theorized that animal species could become extinct through cataclysmic environmental changes.
Cuvier’s theory was then challenged by British geologist Charles Lyell, who proposed that extinction occurs at the same pace as does environmental change. He said that if the environment changed slowly, so extinctions too would occur slowly – a concept favored over Cuvier’s theory of catastrophe.
But Cuvier’s theory gained traction much later, when in the 1980s geologist Walter Alvarez literally unearthed new information.
When digging through a layer of earth that corresponded with the end of the Cretaceous age, a period that ended approximately 66 million years ago, Alverez found that it contained an abnormal amount of iridium, a rare earth metal found most commonly in meteorites.
Based on this discovery, Alvarez proposed an idea to explain the circumstances leading to the extinction of the dinosaurs. He called his idea impact theory.
Impact theory postulates that so many millions of years ago, a ten-kilometer long meteor hit the earth; its impact kicked up so much dust that it blocked out the sun, leading to catastrophic climate change and the rapid demise of many species of dinosaurs.
According to current research, four of the “big five” mass extinctions were interestingly a result of climate change caused by shifts in the earth’s orbit, resulting from the gravitational pull of other planets in our solar system.
Yet we know that humans have had a hand in species extinction, too. But in what fashion?
Carbon dioxide has sped up the process of species extinction, through a warming climate.
To understand why we may be in a sixth period of species mass extinction, we need to understand what we’ve done to change the environment – with industrialization being a prime culprit.
For instance, industrial carbon dioxide emissions have dramatically acidified our oceans, which has led to a reduction in biodiversity.
But how exactly did this happen?
The oceans and the atmosphere are in constant exchange: gases from the atmosphere dissolve into water, while gases evaporating from the ocean mix with the air. The increasing level of carbon dioxide in the atmosphere is thus increasing the amount of carbon dioxide in the oceans.
Yet when carbon dioxide combines with water, it forms an acid. Studies have shown that our oceans are already 30 percent more acidic than when industrialization started in the late 1700s.
Acidification is a problem for a number of species. It changes the nutrient composition of the ocean, thereby reducing general biodiversity, as certain species can’t find enough to eat. Yet calcifiers, or organisms with a shell or external skeleton, are most at risk.
Greater levels of acidity in the oceans means reduced levels of calcium and carbonate ions, the building blocks of shells and external skeletons. If organisms can’t develop a protective shell, they’ll simply die out.
But that’s not the only problem. Carbon dioxide is also a greenhouse gas that causes global warming – a serious problem with repercussions which extend well beyond issues for cold-climate species.
Granted, if one habitat grows too warm, an animal could just migrate to a colder climate. But there’s a catch: even the coldest habitats on earth, such as where polar bears roam, are disappearing.
The unprecedented rate at which the earth is warming means that threatened species that might have previously been able to migrate to colder climes will now die out before they can find a place to live.
Increasing carbon dioxide levels are a serious problem for species extinction. But that’s not the only way humans are hastening extinction.
Trains, planes and automobiles have let us travel the globe, to the peril of vulnerable species.
In addition to carbon dioxide, deforestation and the negative effects of modern transportation systems are major factors contributing to species extinction.
As we destroy habitats through deforestation, we essentially force species into smaller population groups that are then more vulnerable, and more likely to become endangered or go extinct.
Consider if just one male and one female of a species exists, the death of one would doom the species entirely. It’s for this reason that islands typically boast less species diversity than do mainland habitats.
And it’s for this same reason that deforestation is a threat to biodiversity. Scientists have calculated that shrinking forests mean the extinction of about 5,000 species per year. This estimation assumes that the earth’s tropical forests, home to 2 million species, decrease annually in size by one percent.
Another problem is how easily we’re able to travel. In doing so, however, we help to redistribute species across the globe, which homogenizes existing species while reducing general biodiversity.
How exactly does this work?
For Charles Darwin, geographic barriers were essential to explaining why parts of the world that shared similar climates like Africa, Latin America and Oceania were home to entirely different species.
Yet in the late nineteenth century, paleontologists uncovered a curious correlation among fossils on different continents. The later theory of continental drift would explain this correlation: that the world’s continents used to be connected, forming a single landmass called Pangea.
Modern technology has in essence rebuilt these ancient land bridges. The process by which plants and animals used to spread depended on the slow effort of human migration; today, things move much faster and more frequently.
As a result, habitats have begun merging while species that used to exist securely in isolation are now threatened with competing organisms and may disappear entirely.
But while deforestation and modern transportation have affected the rate of extinction, we as humans have been encouraging extinction since our own early evolution.
Homo sapiens not only caused the extinction of the wooly mammoth, but perhaps our relatives, too.
It’s not just human activity since the industrial age that’s caused species mass extinctions. In fact, humans have had a hand in extinctions as far back as the origin of our own species, Homo sapiens.
Large mammals such as the rhinoceros reproduce slowly, but their size protects them from most natural predators – that is, except human predators. So as the human race spread over the last millennia and began to hunt, the population of the earth’s large animals declined.
Some researchers speculated that the population reduction was a result of climate change, but the connection was hard to establish. What they did find and easily prove, however, was that where humans lived, large animals died en masse.
Humans from our very beginnings have been hunters. Species like the wooly mammoth had no natural predators until humans entered the picture! Suddenly these large beasts were threatened, and their survival mechanisms were no longer effective.
Homo sapiens even had a hand in the extinction of the Neanderthals. When Homo sapiens traveled to areas where Neanderthals lived, the Neanderthals began to disappear.
But before the last Neanderthals passed away, Homo sapiens bred with them. As a result, some four percent of the human population today has some Neanderthal genes! This is especially true in Eurasia, the former habitat of the Neanderthal.
And while Neanderthals mostly kept put in Eurasia, Homo sapiens traveled far, reaching untouched parts of the world such as remote islands in the middle of the ocean.
Can you imagine how many people set out only to be lost at sea before Easter Island was finally discovered? Perhaps it’s this risk-taking behavior that explains the success of our species over our close Neanderthal relatives.
So interestingly, while humans are the cause of the sixth mass extinction, we may also be one of its victims. The serious environmental changes we’ve wrought could be the thing that sends us the way of the Neanderthals, after all.
The key message in this book:
For as long as humans have lived on the planet, we have inspired the extinction of other species. However, the environmental changes we’ve wrought have done more and more damage in recent centuries. If we don’t change our ways soon, it could mean the downfall of human civilization itself.
About the author
American journalist Elizabeth Kolbert is a frequent contributor to The New Yorker magazine and is also the author of Field Notes from a Catastrophe, a book on the effects of climate change, published in 2006. She was awarded the 2015 Pulitzer Prize for General Non-Fiction for The Sixth Extinction.
Elizabeth Kolbert is a staff writer at The New Yorker. She is the author of Field Notes from a Catastrophe: Man, Nature, and Climate Change. She lives in Williamstown, Massachusetts, with her husband and children.
Endangered Species, Natural History, Ecology, Biological Sciences, Science, History, Environment, Nature, Biology, Climate Change, Animals, Environmental Science, Life Sciences, Biological Diversity
Table of Contents
The Sixth Extinction
The Mastodon’s Molars
The Original Penguin
The Luck of the Ammonites
Welcome to the Anthropocene
The Sea Around Us
The Forest and the Trees
Islands on Dry Land
The New Pangaea
The Rhino Gets an Ultrasound
The Madness Gene
The Thing with Feathers
WINNER OF THE PULITZER PRIZE
ONE OF THE NEW YORK TIMES BOOK REVIEW’S 10 BEST BOOKS OF THE YEAR
A NEW YORK TIMES BESTSELLER
A NATIONAL BOOK CRITICS CIRCLE AWARD FINALIST
A major book about the future of the world, blending intellectual and natural history and field reporting into a powerful account of the mass extinction unfolding before our eyes
Over the last half a billion years, there have been five mass extinctions, when the diversity of life on earth suddenly and dramatically contracted. Scientists around the world are currently monitoring the sixth extinction, predicted to be the most devastating extinction event since the asteroid impact that wiped out the dinosaurs. This time around, the cataclysm is us.
In The Sixth Extinction, two-time winner of the National Magazine Award and New Yorker writer Elizabeth Kolbert draws on the work of scores of researchers in half a dozen disciplines, accompanying many of them into the field: geologists who study deep ocean cores, botanists who follow the tree line as it climbs up the Andes, marine biologists who dive off the Great Barrier Reef. She introduces us to a dozen species, some already gone, others facing extinction, including the Panamian golden frog, staghorn coral, the great auk, and the Sumatran rhino.
Through these stories, Kolbert provides a moving account of the disappearances occurring all around us and traces the evolution of extinction as concept, from its first articulation by Georges Cuvier in revolutionary Paris up through the present day. The sixth extinction is likely to be mankind’s most lasting legacy; as Kolbert observes, it compels us to rethink the fundamental question of what it means to be human.
Read an Excerpt/PDF Preview
Beginnings, it’s said, are apt to be shadowy. So it is with this story, which starts with the emergence of a new species maybe two hundred thousand years ago. The species does not yet have a name—nothing does—but it has the capacity to name things.
As with any young species, this one’s position is precarious. Its numbers are small, and its range restricted to a slice of eastern Africa. Slowly its population grows, but quite possibly then it contracts again—some would claim nearly fatally—to just a few thousand pairs.
The members of the species are not particularly swift or strong or fertile. They are, however, singularly resourceful. Gradually they push into regions with different climates, different predators, and different prey. None of the usual constraints of habitat or geography seem to check them. They cross rivers, plateaus, mountain ranges. In coastal regions, they gather shellfish; farther inland, they hunt mammals. Everywhere they settle, they adapt and innovate. On reaching Europe, they encounter creatures very much like themselves, but stockier and probably brawnier, who have been living on the continent far longer. They interbreed with these creatures and then, by one means or another, kill them off.
The end of this affair will turn out to be exemplary. As the species expands its range, it crosses paths with animals twice, ten, and even twenty times its size: huge cats, towering bears, turtles as big as elephants, sloths that stand fifteen feet tall. These species are more powerful and often fiercer. But they are slow to breed and are wiped out.
Although a land animal, our species—ever inventive—crosses the sea. It reaches islands inhabited by evolution’s outliers: birds that lay foot-long eggs, pig-sized hippos, giant skinks. Accustomed to isolation, these creatures are ill-equipped to deal with the newcomers or their fellow travelers (mostly rats). Many of them, too, succumb.
The process continues, in fits and starts, for thousands of years, until the species, no longer so new, has spread to practically every corner of the globe. At this point, several things happen more or less at once that allow Homo sapiens, as it has come to call itself, to reproduce at an unprecedented rate. In a single century the population doubles; then it doubles again, and then again. Vast forests are razed. Humans do this deliberately, in order to feed themselves. Less deliberately, they shift organisms from one continent to another, reassembling the biosphere.
Meanwhile, an even stranger and more radical transformation is under way. Having discovered subterranean reserves of energy, humans begin to change the composition of the atmosphere. This, in turn, alters the climate and the chemistry of the oceans. Some plants and animals adjust by moving. They climb mountains and migrate toward the poles. But a great many—at first hundreds, then thousands, and finally perhaps millions—find themselves marooned. Extinction rates soar, and the texture of life changes.
No creature has ever altered life on the planet in this way before, and yet other, comparable events have occurred. Very, very occasionally in the distant past, the planet has undergone change so wrenching that the diversity of life has plummeted. Five of these ancient events were catastrophic enough that they’re put in their own category: the so-called Big Five. In what seems like a fantastic coincidence, but is probably no coincidence at all, the history of these events is recovered just as people come to realize that they are causing another one. When it is still too early to say whether it will reach the proportions of the Big Five, it becomes known as the Sixth Extinction.
The story of the Sixth Extinction, at least as I’ve chosen to tell it, comes in thirteen chapters. Each tracks a species that’s in some way emblematic—the American mastodon, the great auk, an ammonite that disappeared at the end of the Cretaceous alongside the dinosaurs. The creatures in the early chapters are already gone, and this part of the book is mostly concerned with the great extinctions of the past and the twisting history of their discovery, starting with the work of the French naturalist Georges Cuvier. The second part of the book takes place very much in the present—in the increasingly fragmented Amazon rainforest, on a fast-warming slope in the Andes, on the outer reaches of the Great Barrier Reef. I chose to go to these particular places for the usual journalistic reasons—because there was a research station there or because someone invited me to tag along on an expedition. Such is the scope of the changes now taking place that I could have gone pretty much anywhere and, with the proper guidance, found signs of them. One chapter concerns a die-off happening more or less in my own backyard (and, quite possibly, in yours).
If extinction is a morbid topic, mass extinction is, well, massively so. It’s also a fascinating one. In the pages that follow, I try to convey both sides: the excitement of what’s being learned as well as the horror of it. My hope is that readers of this book will come away with an appreciation of the truly extraordinary moment in which we live.
The Sixth Extinction
The town of El Valle de Antón, in central Panama, sits in the middle of a volcanic crater formed about a million years ago. The crater is almost four miles wide, but when the weather is clear you can see the jagged hills that surround the town like the walls of a ruined tower. El Valle has one main street, a police station, and an open-air market. In addition to the usual assortment of Panama hats and vividly colored embroidery, the market offers what must be the world’s largest selection of golden-frog figurines. There are golden frogs resting on leaves and golden frogs sitting up on their haunches and—rather more difficult to understand—golden frogs clasping cell phones. There are golden frogs wearing frilly skirts and golden frogs striking dance poses and golden frogs smoking cigarettes through a holder, after the fashion of FDR. The golden frog, which is taxicab yellow with dark brown splotches, is endemic to the area around El Valle. It is considered a lucky symbol in Panama; its image is (or at least used to be) printed on lottery tickets.
As recently as a decade ago, golden frogs were easy to spot in the hills around El Valle. The frogs are toxic—it’s been calculated that the poison contained in the skin of just one animal could kill a thousand average-sized mice—hence the vivid color, which makes them stand out against the forest floor. One creek not far from El Valle was nicknamed Thousand Frog Stream. A person walking along it would see so many golden frogs sunning themselves on the banks that, as one herpetologist who made the trip many times put it to me, “it was insane—absolutely insane.”
Then the frogs around El Valle started to disappear. The problem—it was not yet perceived as a crisis—was first noticed to the west, near Panama’s border with Costa Rica. An American graduate student happened to be studying frogs in the rainforest there. She went back to the States for a while to write her dissertation, and when she returned, she couldn’t find any frogs or, for that matter, amphibians of any kind. She had no idea what was going on, but since she needed frogs for her research, she set up a new study site, farther east. At first the frogs at the new site seemed healthy; then the same thing happened: the amphibians vanished. The blight spread through the rainforest until, in 2002, the frogs in the hills and streams around the town of Santa Fe, about fifty miles west of El Valle, were effectively wiped out. In 2004, little corpses began showing up even closer to El Valle, around the town of El Copé. By this point, a group of biologists, some from Panama, others from the United States, had concluded that the golden frog was in grave danger. They decided to try to preserve a remnant population by removing a few dozen of each sex from the forest and raising them indoors. But whatever was killing the frogs was moving even faster than the biologists had feared. Before they could act on their plan, the wave hit.
I first read about the frogs of El Valle in a nature magazine for children that I picked up from my kids. The article, which was illustrated with full-color photos of the Panamanian golden frog and other brilliantly colored species, told the story of the spreading scourge and the biologists’ efforts to get out in front of it. The biologists had hoped to have a new lab facility constructed in El Valle, but it was not ready in time. They raced to save as many animals as possible, even though they had nowhere to keep them. So what did they end up doing? They put them “in a frog hotel, of course!” The “incredible frog hotel”—really a local bed and breakfast—agreed to let the frogs stay (in their tanks) in a block of rented rooms.
“With biologists at their beck and call, the frogs enjoyed first-class accommodations that included maid and room service,” the article noted. The frogs were also served delicious, fresh meals—“so fresh, in fact, the food could hop right off the plate.”
Just a few weeks after I read about the “incredible frog hotel,” I ran across another frog-related article written in a rather different key. This one, which appeared in the Proceedings of the National Academy of Sciences, was by a pair of herpetologists. It was titled “Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians.” The authors, David Wake, of the University of California-Berkeley, and Vance Vredenburg, of San Francisco State, noted that there “have been five great mass extinctions during the history of life on this planet.” These extinctions they described as events that led to “a profound loss of biodiversity.” The first took place during the late Ordovician period, some 450 million years ago, when living things were still mainly confined to the water. The most devastating took place at the end of the Permian period, some 250 million years ago, and it came perilously close to emptying the earth out altogether. (This event is sometimes referred to as “the mother of mass extinctions” or “the great dying.”) The most recent—and famous—mass extinction came at the close of the Cretaceous period; it wiped out, in addition to the dinosaurs, the plesiosaurs, the mosasaurs, the ammonites, and the pterosaurs. Wake and Vredenburg argued that, based on extinction rates among amphibians, an event of a similarly catastrophic nature was currently under way. Their article was illustrated with just one photograph, of about a dozen mountain yellow-legged frogs—all dead—lying bloated and belly-up on some rocks.
I understood why a kids’ magazine had opted to publish photos of live frogs rather than dead ones. I also understood the impulse to play up the Beatrix Potter–like charms of amphibians ordering room service. Still, it seemed to me, as a journalist, that the magazine had buried the lede. Any event that has occurred just five times since the first animal with a backbone appeared, some five hundred million years ago, must qualify as exceedingly rare. The notion that a sixth such event would be taking place right now, more or less in front of our eyes, struck me as, to use the technical term, mind-boggling. Surely this story, too—the bigger, darker, far more consequential one—deserved telling. If Wake and Vredenburg were correct, then those of us alive today not only are witnessing one of the rarest events in life’s history, we are also causing it. “One weedy species,” the pair observed, “has unwittingly achieved the ability to directly affect its own fate and that of most of the other species on this planet.” A few days after I read Wake and Vredenburg’s article, I booked a ticket to Panama.
The El Valle Amphibian Conservation Center, or EVACC (pronounced “ee-vac”), lies along a dirt road not far from the open-air market where the golden frog figurines are sold. It’s about the size of a suburban ranch house, and it occupies the back corner of a small, sleepy zoo, just beyond a cage of very sleepy sloths. The entire building is filled with tanks. There are tanks lined up against the walls and more tanks stacked at the center of the room, like books on the shelves of a library. The taller tanks are occupied by species like the lemur tree frog, which lives in the forest canopy; the shorter tanks serve for species like the big-headed robber frog, which lives on the forest floor. Tanks of horned marsupial frogs, which carry their eggs in a pouch, sit next to tanks of casque-headed frogs, which carry their eggs on their backs. A few dozen tanks are devoted to Panamanian golden frogs, Atelopus zeteki.
Golden frogs have a distinctive, ambling gait that makes them look a bit like drunks trying to walk a straight line. They have long, skinny limbs, pointy yellow snouts, and very dark eyes, through which they seem to be regarding the world warily. At the risk of sounding weak-minded, I will say that they look intelligent. In the wild, females lay their eggs in shallow running water; males, meanwhile, defend their territory from the tops of mossy rocks. In EVACC, each golden frog tank has its own running water, provided by its own little hose, so that the animals can breed near a simulacrum of the streams that were once their home. In one of the ersatz streams, I noticed a strings of little pearl-like eggs. On a white board nearby someone had noted excitedly that one of the frogs “depositó huevos!!”
EVACC sits more or less in the middle of the golden frog’s range, but it is, by design, entirely cut off from the outside world. Nothing comes into the building that has not been thoroughly disinfected, including the frogs, which, in order to gain entry, must first be treated with a solution of bleach. Human visitors are required to wear special shoes and to leave behind any bags or knapsacks or equipment that they’ve used out in the field. All of the water that enters the tanks has been filtered and specially treated. The sealed-off nature of the place gives it the feel of a submarine or, perhaps more aptly, an ark mid-deluge.
EVACC’s director is a Panamanian named Edgardo Griffith. Griffith is tall and broad-shouldered, with a round face and a wide smile. He wears a silver ring in each ear and has a large tattoo of a toad’s skeleton on his left shin. Now in his mid-thirties, Griffith has devoted pretty much his entire adult life to the amphibians of El Valle, and he has turned his wife, an American who came to Panama as a Peace Corps volunteer, into a frog person, too. Griffith was the first person to notice when little carcasses started showing up in the area, and he personally collected many of the several hundred amphibians that got booked into the hotel. (The animals were transferred to EVACC once the building had been completed.) If EVACC is a sort of ark, Griffith becomes its Noah, though one on extended duty, since already he’s been at things a good deal longer than forty days. Griffith told me that a key part of his job was getting to know the frogs as individuals. “Every one of them has the same value to me as an elephant,” he said.
The first time I visited EVACC, Griffith pointed out to me the representatives of species that are now extinct in the wild. These included, in addition to the Panamanian golden frog, the Rabb’s fringe-limbed tree frog, which was first identified only in 2005. At the time of my visit, EVACC was down to just one Rabb’s frog, so the possibility of saving even a single, Noachian pair had obviously passed. The frog, greenish brown with yellow speckles, was about four inches long, with oversized feet that gave it the look of a gawky teenager. Rabb’s fringe-limbed tree frogs lived in the forest above El Valle, and they laid their eggs in tree holes. In an unusual, perhaps even unique arrangement, the male frogs cared for the tadpoles by allowing their young, quite literally, to eat the skin off their backs. Griffith said that he thought there were probably many other amphibian species that had been missed in the initial collecting rush for EVACC and had since vanished; it was hard to say how many, since most of them were probably unknown to science. “Unfortunately,” he told me, “we are losing all these amphibians before we even know that they exist.”
“Even the regular people in El Valle, they notice it,” he said. “They tell me, ‘What happened to the frogs? We don’t hear them calling anymore.’ ”
When the first reports that frog populations were crashing began to circulate, a few decades ago, some of the most knowledgeable people in the field were the most skeptical. Amphibians are, after all, among the planet’s great survivors. The ancestors of today’s frogs crawled out of the water some 400 million years ago, and by 250 million years ago the earliest representatives of what would become the modern amphibian orders—one includes frogs and toads, the second newts and salamanders, and the third weird limbless creatures called caecilians—had evolved. This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs.
Most amphibians—the word comes from the Greek meaning “double life”—are still closely tied to the aquatic realm from which they emerged. (The ancient Egyptians thought that frogs were produced by the coupling of land and water during the annual flooding of the Nile.) Their eggs, which have no shells, must be kept moist in order to develop. There are many frogs that, like the Panamanian golden frog, lay their eggs in streams. There are also frogs that lay them in temporary pools, frogs that lay them underground, and frogs that lay them in nests that they construct out of foam. In addition to frogs that carry their eggs on their backs and in pouches, there are frogs that carry them wrapped like bandages around their legs. Until recently, when both of them went extinct, there were two species of frogs, known as gastric-brooding frogs, that carried their eggs in their stomachs and gave birth to little froglets through their mouths.
Amphibians emerged at a time when all the land on earth was part of a single expanse known as Pangaea. Since the breakup of Pangaea, they’ve adapted to conditions on every continent except Antarctica. Worldwide, just over seven thousand species have been identified, and while the greatest number are found in the tropical rainforests, there are occasional amphibians, like the sandhill frog of Australia, that can live in the desert, and also amphibians, like the wood frog, that can live above the Arctic Circle. Several common North American frogs, including spring peepers, are able to survive the winter frozen solid, like popsicles. Their extended evolutionary history means that even groups of amphibians that, from a human perspective, seem to be fairly similar may, genetically speaking, be as different from one another as, say, bats are from horses.
David Wake, one of the authors of the article that sent me to Panama, was among those who initially did not believe that amphibians were disappearing. This was back in the mid–nineteen-eighties. Wake’s students began returning from frog-collecting trips in the Sierra Nevada empty-handed. Wake remembered from his own student days, in the nineteen-sixties, that frogs in the Sierras had been difficult to avoid. “You’d be walking through meadows, and you’d inadvertently step on them,” he told me. “They were just everywhere.” Wake assumed that his students were going to the wrong spots, or that they just didn’t know how to look. Then a postdoc with several years of collecting experience told him that he couldn’t find any amphibians, either. “I said, ‘OK, I’ll go up with you, and we’ll go out to some proven places,’ ” Wake recalled. “And I took him out to this proven place, and we found like two toads.”
Part of what made the situation so mystifying was the geography; frogs seemed to be vanishing not only from populated and disturbed areas but also from relatively pristine places, like the Sierras and the mountains of Central America. In the late nineteen-eighties, an American herpetologist went to the Monteverde Cloud Forest Reserve in northern Costa Rica to study the reproductive habits of golden toads. She spent two field seasons looking; where once the toads had mated in writhing masses, a single male was sighted. (The golden toad, now classified as extinct, was actually a bright tangerine color. It was only very distantly related to the Panamanian golden frog, which, owing to a pair of glands located behind its eyes, is also technically a toad.) Around the same time, in central Costa Rica, biologists noticed that the populations of several endemic frog species had crashed. Rare and highly specialized species were vanishing and so, too, were much more familiar ones. In Ecuador, the Jambato toad, a frequent visitor to backyard gardens, disappeared in a matter of years. And in northeastern Australia the southern day frog, once one of the most common in the region, could no longer be found.
The first clue to the mysterious killer that was claiming frogs from Queensland to California came—perhaps ironically, perhaps not—from a zoo. The National Zoo, in Washington, D.C., had been successfully raising blue poison-dart frogs, which are native to Suriname, through many generations. Then, more or less from one day to the next, the zoo’s tank-bred frogs started dropping. A veterinary pathologist at the zoo took some samples from the dead frogs and ran them through an electron scanning microscope. He found a strange microorganism on the animals’ skin, which he eventually identified as a fungus belonging to a group known as chytrids.
Chytrid fungi are nearly ubiquitous; they can be found at the tops of trees and also deep underground. This particular species, though, had never been seen before; indeed, it was so unusual that an entire genus had to be created to accommodate it. It was named Batrachochytrium dendrobatidis—batrachos is Greek for “frog”—or Bd for short.
The veterinary pathologist sent samples from infected frogs at the National Zoo to a mycologist at the University of Maine. The mycologist grew cultures of the fungus and then sent some of them back to Washington. When healthy blue poison-dart frogs were exposed to the lab-raised Bd, they sickened. Within three weeks, they were dead. Subsequent research showed that Bd interferes with frogs’ ability to take up critical electrolytes through their skin. This causes them to suffer what is, in effect, a heart attack.
EVACC can perhaps best be described as a work-in-progress. The week I spent at the center, a team of American volunteers was also there, helping to construct an exhibit. The exhibit was going to be open to the public, so, for biosecurity purposes, the space had to be isolated and equipped with its own separate entrance. There were holes in the walls where, eventually, glass cases were to be mounted, and around the holes someone had painted a mountain landscape very much like what you would see if you stepped outside and looked up at the hills. The highlight of the exhibit was to be a large case full of Panamanian golden frogs, and the volunteers were trying to construct a three-foot-high concrete waterfall for them. But there were problems with the pumping system and difficulties getting replacement parts in a valley with no hardware store. The volunteers seemed to be spending a lot of time hanging around, waiting.
I spent a lot of time hanging around with them. Like Griffith, all of the volunteers were frog lovers. Several, I learned, were zookeepers who worked with amphibians back in the States. (One told me that frogs had ruined his marriage.) I was moved by the team’s dedication, which was the same sort of commitment that had gotten the frogs into the “frog hotel” and then had gotten EVACC up and running, if not entirely completed. But I couldn’t help also feeling that there was also something awfully sad about the painted green hills and the fake waterfall.
With almost no frogs left in the forests around El Valle, the case for bringing the animals into EVACC has by now clearly been proved. And yet the longer the frogs spend in the center, the tougher it is to explain what they’re doing there. The chytrid fungus, it turns out, does not need amphibians in order to survive. This means that even after it has killed off the animals in an area, it continues to live on, doing whatever it is that chytrid fungi do. Thus, were the golden frogs at EVACC allowed to amble back into the actual hills around El Valle, they would sicken and collapse. (Though the fungus can be destroyed by bleach, it’s obviously impossible to disinfect an entire rainforest.) Everyone I spoke to at EVACC told me that the center’s goal was to maintain the animals until they could be released to repopulate the forests, and everyone also acknowledged that they couldn’t imagine how this would actually be done.
“We’ve got to hope that somehow it’s all going to come together,” Paul Crump, a herpetologist from the Houston Zoo who was directing the stalled waterfall project, told me. “We’ve got to hope that something will happen, and we’ll be able to piece it all together, and it will all be as it once was, which now that I say it out loud sounds kind of stupid.”
“The point is to be able to take them back, which every day I see more like a fantasy,” Griffith said.
Once chytrid swept through El Valle, it didn’t stop; it continued to move east. It has also since arrived in Panama from the opposite direction, out of Colombia. Bd has spread through the highlands of South America and down the eastern coast of Australia, and it has crossed into New Zealand and Tasmania. It has raced through the Caribbean and has been detected in Italy, Spain, Switzerland, and France. In the U.S., it appears to have radiated from several points, not so much in a wavelike pattern as in a series of ripples. At this point, it appears to be, for all intents and purposes, unstoppable.
The same way acoustical engineers speak of “background noise” biologists talk about “background extinction.” In ordinary times—times here understood to mean whole geologic epochs—extinction takes place only very rarely, more rarely even than speciation, and it occurs at what’s known as the background extinction rate. This rate varies from one group of organisms to another; often it’s expressed in terms of extinctions per million species-years. Calculating the background extinction rate is a laborious task that entails combing through whole databases’ worth of fossils. For what’s probably the best-studied group, which is mammals, it’s been reckoned to be roughly .25 per million species-years. This means that, since there are about fifty-five hundred mammal species wandering around today, at the background extinction rate you’d expect—once again, very roughly—one species to disappear every seven hundred years.
Mass extinctions are different. Instead of a background hum there’s a crash, and disappearance rates spike. Anthony Hallam and Paul Wignall, British paleontologists who have written extensively on the subject, define mass extinctions as events that eliminate a “significant proportion of the world’s biota in a geologically insignificant amount of time.” Another expert, David Jablonski, characterizes mass extinctions as “substantial biodiversity losses” that occur rapidly and are “global in extent.” Michael Benton, a paleontologist who has studied the end-Permian extinction, uses the metaphor of the tree of life: “During a mass extinction, vast swathes of the tree are cut short, as if attacked by crazed, axe-wielding madmen.” A fifth paleontologist, David Raup, has tried looking at matters from the perspective of the victims: “Species are at a low risk of extinction most of the time.” But this “condition of relative safety is punctuated at rare intervals by a vastly higher risk.” The history of life thus consists of “long periods of boredom interrupted occasionally by panic.”
In times of panic, whole groups of once-dominant organisms can disappear or be relegated to secondary roles, almost as if the globe has undergone a cast change. Such wholesale losses have led paleontologists to surmise that during mass extinction events—in addition to the so-called Big Five, there have been many lesser such events—the usual rules of survival are suspended. Conditions change so drastically or so suddenly (or so drastically and so suddenly) that evolutionary history counts for little. Indeed, the very traits that have been most useful for dealing with ordinary threats may turn out, under such extraordinary circumstances, to be fatal.
A rigorous calculation of the background extinction rate for amphibians has not been performed, in part because amphibian fossils are so rare. Almost certainly, though, the rate is lower than it is for mammals. Probably, one amphibian species should go extinct every thousand years or so. That species could be from Africa or from Asia or from Australia. In other words, the odds of an individual’s witnessing such an event should be effectively zero. Already, Griffith has observed several amphibian extinctions. Pretty much every herpetologist working out in the field has watched several. (Even I, in the time I spent researching this book, encountered one species that has since gone extinct and three or four others, like the Panamanian golden frog, that are now extinct in the wild.) “I sought a career in herpetology because I enjoy working with animals,” Joseph Mendelson, a herpetologist at Zoo Atlanta, has written. “I did not anticipate that it would come to resemble paleontology.”
Today, amphibians enjoy the dubious distinction of being the world’s most endangered class of animals; it’s been calculated that the group’s extinction rate could be as much as forty-five thousand times higher than the background rate. But extinction rates among many other groups are approaching amphibian levels. It is estimated that one-third of all reef-building corals, a third of all freshwater mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion. The losses are occurring all over: in the South Pacific and in the North Atlantic, in the Arctic and the Sahel, in lakes and on islands, on mountaintops and in valleys. If you know how to look, you can probably find signs of the current extinction event in your own backyard.
There are all sorts of seemingly disparate reasons that species are disappearing. But trace the process far enough and inevitably you are led to the same culprit: “one weedy species.”
Bd is capable of moving on its own. The fungus generates microscopic spores with long, skinny tails; these propel themselves through water and can be carried far longer distances by streams, or in the runoff after a rainstorm. (It’s likely this sort of dispersal produced what showed up in Panama as an eastward-moving scourge.) But this kind of movement cannot explain the emergence of the fungus in so many distant parts of the world—Central America, South America, North America, Australia—more or less simultaneously. One theory has it that Bd was moved around the globe with shipments of African clawed frogs, which were used in the nineteen-fifties and sixties in pregnancy tests. (Female African clawed frogs, when injected with the urine of a pregnant woman, lay eggs within a few hours.) Suggestively, African clawed frogs do not seem to be adversely affected by Bd, though they are widely infected with it. A second theory holds that the fungus was spread by North American bullfrogs which have been introduced—sometimes accidentally, sometimes purposefully—into Europe, Asia, and South America, and which are often exported for human consumption. North American bullfrogs, too, are widely infected with Bd but do not seem to be harmed by it. The first has become known as the “Out of Africa” and the second might be called the “frog-leg soup” hypothesis.
Either way, the etiology is the same. Without being loaded by someone onto a boat or a plane, it would have been impossible for a frog carrying Bd to get from Africa to Australia or from North America to Europe. This sort of intercontinental reshuffling, which nowadays we find totally unremarkable, is probably unprecedented in the three-and-a-half-billion-year history of life.
Even though Bd has swept through most of Panama by now, Griffith still occasionally goes out collecting for EVACC, looking for survivors. I scheduled my visit to coincide with one of these collecting trips, and one evening I set out with him and two of the American volunteers who were working on the waterfall. We headed east, across the Panama Canal, and spent the night in a region known as Cerro Azul, in a guesthouse ringed by an eight-foot-tall iron fence. At dawn, we drove to the ranger station at the entrance to Chagres National Park. Griffith was hoping to find females of two species that EVACC is short of. He pulled out his government-issued collecting permit and presented it to the sleepy officials manning the station. Some underfed dogs came out to sniff around the truck.
Beyond the ranger station, the road turned into a series of craters connected by deep ruts. Griffith put Jimi Hendrix on the truck’s CD player, and we bounced along to the throbbing beat. Frog collecting requires a lot of supplies, so Griffith had hired two men to help with the carrying. At the very last cluster of houses, in the tiny village of Los Ángeles, the men materialized out of the mist. We bounced on until the truck couldn’t go any farther; then we all got out and started to walk.
The trail wound its way through the rainforest in a slather of red mud. Every few hundred yards, the main path was crossed by a narrower one; these paths had been made by leaf-cutter ants, making millions—perhaps billions—of trips to bring bits of greenery back to their colonies. (The colonies, which look like mounds of sawdust, can cover an area the size of a city park.) One of the Americans, Chris Bednarski, from the Houston Zoo, warned me to avoid the soldier ants, which will leave their jaws in your shin even after they’re dead. “Those’ll really mess you up,” he observed. The other American, John Chastain, from the Toledo Zoo, was carrying a long hook, for use against venomous snakes. “Fortunately, the ones that can really mess you up are pretty rare,” Bednarski assured me. Howler monkeys screamed in the distance. Griffith pointed out jaguar prints in the soft ground.
After about an hour, we came to a farm that someone had carved out of the trees. There was some scraggly corn growing, but no one was around, and it was hard to say whether the farmer had given up on the poor rainforest soil or was simply away for the day. A flock of emerald green parrots shot up into the air. After another several hours, we emerged into a small clearing. A blue morpho butterfly flitted by, its wings the color of the sky. There was a small cabin on the site, but it was so broken down that everyone elected to sleep outside. Griffith helped me string up my bed—a cross between a tent and a hammock that had to be hung between two trees. A slit in the bottom constituted the entryway, and the top was supposed to provide protection against the inevitable rain. When I climbed into the thing, I felt as if I were lying in a coffin.
That evening, Griffith prepared some rice on a portable gas burner. Then we strapped on headlamps and clambered down to a nearby stream. Many amphibians are nocturnal, and the only way to see them is to go looking in the dark, an exercise that’s as tricky as it sounds. I kept slipping, and violating Rule No. 1 of rainforest safety: never grab onto something if you don’t know what it is. After one of my falls, Bednarski pointed out to me a tarantula the size of my fist sitting on the next tree over.
Practiced hunters can find frogs at night by shining a light into the forest and looking for the reflected glow of their eyes. The first amphibian Griffith sighted this way was a San Jose Cochran frog, perched on top of a leaf. San Jose Cochran frogs are part of a larger family known as “glass frogs,” so named because their translucent skin reveals the outline of their internal organs. This particular glass frog was green, with tiny yellow dots. Griffith pulled a pair of surgical gloves out of his pack. He stood completely still and then, with a heronlike gesture, darted to scoop up the frog. With his free hand, he took what looked like the end of a Q-tip and swabbed the frog’s belly. He put the Q-tip in a little plastic vial—it would later be sent to a lab and analyzed for Bd—and since it wasn’t one of the species he was looking for, he placed the frog back on the leaf. Then he pulled out his camera. The frog stared back at the lens impassively.
We continued to grope through the blackness. Someone spotted a La Loma robber frog, which is orangey-red, like the forest floor; someone else spotted a Warzewitsch frog, which is bright green and shaped like a leaf. With every animal, Griffith went through the same routine: snatching it up, swabbing its belly, photographing it. Finally, we came upon a pair of Panamanian robber frogs locked in amplexus—the amphibian version of sex. Griffith left these two alone.
One of the amphibians that Griffith was hoping to catch, the horned marsupial frog, has a distinctive call that’s been likened to the sound of a champagne bottle being uncorked. As we sloshed along—by this point we were walking in the middle of the stream—we heard the call, which seemed to be emanating from several directions at once. At first, it sounded as if it were right nearby, but as we approached, it seemed to get farther away. Griffith began imitating the call, making a cork-popping sound with his lips. Eventually, he decided that the rest of us were scaring the frogs with our splashing. He waded ahead, and we stayed for a long time up to our knees in water, trying not to move. When Griffith finally gestured us over, we found him standing in front of a large yellow frog with long toes and an owlish face. It was sitting on a tree limb, just above eye level. Griffith was looking to find a female horned marsupial frog to add to EVACC’s collection. He shot out his arm, grabbed the frog, and flipped it over. Where a female horned marsupial would have a pouch, this one had none. Griffith swabbed it, photographed it, and placed it back in the tree.
“You are a beautiful boy,” he murmured to the frog.
Around midnight, we headed back to camp. The only animals that Griffith decided to bring with him were two tiny blue-bellied poison frogs and one whitish salamander, whose species neither he nor the two Americans could identify. The frogs and the salamander were placed in plastic bags with some leaves to keep them moist. It occurred to me that the frogs and their progeny, if they had any, and their progeny’s progeny, if they had any, would never again touch the floor of the rainforest but would live out their days in disinfected glass tanks. That night it poured, and in my coffinlike hammock I had vivid, troubled dreams, the only scene from which I could later recall was of a bright yellow frog smoking a cigarette through a holder.
Video and Podcast
*Starred Review* It didn’t take long for Homo sapiens to begin “reassembling the biosphere,” observes Kolbert, a Heinz Award–winning New Yorker staff writer and author of Field Notes from a Catastrophe: Man, Nature, and Climate Change (2006). By burning fossil fuels, we are rapidly changing the atmosphere, the oceans, and the climate, forcing potentially millions of species into extinction. Five watershed events in the deep past decimated life on earth, hence the designation “Sixth Extinction” for today’s human-propelled crisis. To lay the groundwork for understanding this massive die-off, Kolbert crisply tells the stories of such earlier losses as the American mastodon and the great auk and provides an orienting overview of evolutionary and ecological science. She then chronicles her adventures in the field with biologists, botanists, and geologists investigating the threats against amphibians, bats, coral, and rhinos. Intrepid and astute, Kolbert combines vivid, informed, and awestruck descriptions of natural wonders, from rain forests to the Great Barrier Reef, and wryly amusing tales about such dicey situations as nearly grabbing onto a tree branch harboring a fist-sized tarantula, swimming among poisonous jellyfish, and venturing into a bat cave; each dispatch is laced with running explanations of urgent scientific inquiries and disquieting findings. Rendered with rare, resolute, and resounding clarity, Kolbert’s compelling and enlightening report forthrightly addresses the most significant topic of our lives. – Donna Seaman
[The Sixth Extinction] is a wonderful book, and it makes very clear that big, abrupt changes can happen; they’re not outside the realm of possibility. They have happened before, they can happen again. – President Barack Obama
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