1-Page Summary

In The Sixth Extinction, journalist Elizbeth Kolbert argues that humans are rapidly changing the shape of the earth and the composition of the atmosphere, unleashing a mass extinction of most living things, quite possibly including ourselves. Scientists have identified five previous mass extinction events over 500 million years and many believe a Sixth Extinction, set in motion by humans, is underway.

Humans began impacting the world from the start. As modern humans spread from East Africa around the globe, they found archaic humans similar to themselves and gigantic animals (megafauna)—both of which they wiped out.

They—we—started demolishing forests to grow food and spreading animals, plants, and other life forms to new continents, thus changing the face of the earth. With the discovery of energy sources underground, we began our greatest and most deadly transformation—of the composition of the atmosphere and the oceans. Some plants and animals have survived by migrating. But many, perhaps millions, are stranded where they are unable, or lack time, to adapt. Extinction rates are skyrocketing. No other species has so drastically changed life on earth.

The Big Five Mass Extinctions

Until the eighteenth century, scientists and naturalists had no concept of extinction. They believed life was a long, unbroken “chain of being”—that the animals and other life forms existing at the time were the only ones that had ever existed or would exist. Then bones of huge creatures such as mastodons and mammoths began turning up and naturalists puzzled over why they had disappeared.

Some theorized there had been “lost worlds” of fantastic species that were obliterated by catastrophes. Others believed that extinction only happened slowly as part of the process of evolution—animals with non-beneficial traits eventually died out. Eventually, with the discovery in the 1980s of the site of an asteroid strike on the Yucatan Peninsula, the idea of sudden mass extinctions gained adherents.

Today, scientists believe in both gradual and sudden extinctions. They’ve identified five mass extinction events of the distant past—the “Big Five”—plus a number of smaller extinctions. Each of the Big Five suddenly decimated the earth’s diversity of life.

(Shortform note: The Big Five extinctions were:

1) The End Ordovician period, 444 million years ago, 86% of species lost. The cause was a sudden cooling of the climate (carbon dioxide levels and temperatures dropped and things froze—glaciation) plus a huge drop in sea levels plus an ocean chemistry change resulting from the drop in CO2.

2) Late Devonian, 375 million years ago, 75% of species lost.

3) End Permian, 251 million years ago, 96% of species lost. This extinction seems to have been triggered by a sudden warming of the climate. For unknown reasons, an enormous amount of carbon was released into the air. Temperatures shot up and the seas heated up and became acidified. Oxygen levels in the water plummeted, probably suffocating nearly all life.

4) End Triassic, 200 million years ago, 80% of species lost.

5) End Cretaceous, 66 million years ago, 76% of all species lost when an asteroid traveling at 45,000 miles an hour crashed into the Yucatan Peninsula. A scorching cloud spread across North America, vaporizing everything. Dust blocked much of the sunlight, creating an “impact winter” or prolonged cooling.)

The Anthropocene Epoch

Each of the Big Five had its own unique causes, but in every case, species faced drastic changes for which they had no time to adapt.

Scientists believe we’ve entered a new epoch: the Anthropocene or human-dominated geological epoch, characterized by man-made, planet-altering changes.

Among the first signs that our actions are leading to catastrophe was the disappearance of amphibians beginning in the 1980s. Researchers in Panama first noticed that an iconic local species, the Golden Frog, was dying. Then they realized frogs were disappearing all over the globe.

In 2008, citing the precipitous drop in amphibian populations, an article in the Proceedings of the National Academy of Sciences, asked, “Are We in the Midst of the Sixth Mass Extinction?” The authors concluded that, based on the extinction rates among amphibians, a sixth catastrophic event is underway.

Besides amphibians, animals are in trouble everywhere. Among those suffering steep declines are reef-building corals, sharks, rays, fresh-water mollusks, reptiles, mammals, and birds. While different animals are disappearing for seemingly different immediate reasons, in every case you can ultimately trace the cause to humans.

Global Warming

Most importantly, we’ve changed the composition of the atmosphere by adding vast amounts of carbon dioxide—over two hundred years, the level of carbon dioxide in the air has risen by 40%. As a result, the earth’s climate is likely to behave significantly differently for many millennia to come.

By burning fossil fuels, we’ve added 365 billion metric tons of carbon to the atmosphere. By cutting down forests, we’ve contributed another 180 billion tons and each year we add

9 billion tons more.

The concentration of carbon dioxide in the atmosphere—over four hundred parts per million—is higher than it’s been in more than a million years. At our current emissions rate, it will exceed five hundred parts per million by 2050, boosting temperatures, which will melt what remains of the glaciers and the Arctic ice cap and flood islands and coastal cities, such as New York and Washington, D.C.

Plant and animal species adjust to both short- (seasonal) and long-term temperature changes by migrating. During the multiple warming-cooling cycles of the ice ages, there were mass migrations—even insects moved thousands of miles. Scientists project that the temperature change in the next century will be comparable in magnitude to the temperature fluctuations of the ice ages.

Many species are already responding to climate change by adjusting their ranges. For instance, some tree species in Manu National Park in the Andes are “moving” to higher elevations as temperatures warm by dispersing their seeds up the mountain. The average genus (a group of closely related species) is moving eight feet higher per year. One species is even moving a hundred feet a year.

Habitat Destruction

Species need to migrate for survival. However, our transformation of the earth by fragmenting forests (dividing them by highways, cities, mining operations, cropland, and other human development) makes it difficult, if not impossible.

In addition, by cutting down forests entirely, we’ve reduced the amount of available habitat, which reduces species diversity by hindering their ability to reproduce and making the smaller populations more vulnerable to extinction.

Big animals like elephants, bears, and rhinos are threatened by both habitat loss and poaching. For example, humans have killed so many rhinos and destroyed so much of their habitat that all five species of rhinos are at risk.

Other large mammals that are also in trouble:

Ocean Acidification

Oceans absorb a lot of the carbon we’re pumping into the air—two-and-a-half-billion tons a year when this book was written in 2014—which is changing ocean chemistry.

In the past, there was a fairly even exchange of gases: the ocean absorbed gases from the atmosphere and also released dissolved gases back into the atmosphere. At this point, however, more CO2 is entering the oceans than they can release, resulting in acidification. (Carbon dioxide dissolves in water and forms carbonic acid.)

As a result, the pH of the oceans’ surface water has decreased, making them 30% more acidic than they were in 1800. The pH is on track to fall to 7.8 (from today’s average of 8.1) by the end of this century, making the oceans 150 percent more acidic than before the industrial revolution.

In terms of destructive effects, ocean acidification has been called global warming’s “evil twin.” There are numerous reasons, which add up to a steep loss of biodiversity, including:

Among the biggest victims are calcifiers—animals and plants that construct shells or external skeletons. They include starfish, sea urchins, mollusks (clams and oysters), barnacles, and many coral species (the ones that build reefs). Many kinds of seaweed, some algae, and some plants also are calcifiers.

To build shells and skeletons, they combine calcium ions and carbonate ions to create calcium carbonate. But to do so, they have to change the chemistry of the seawater. Acidification makes this more difficult, in part by decreasing the number of available carbonate ions. In addition, water with too much acid dissolves or eats holes in their shells.

Invasive Species

In the past, the range of many species was limited by geographic barriers such as oceans, rivers, and mountains. Today, however, species are being dispersed widely by humans, with disastrous consequences.

In the Anthropocene, there are no barriers to species’ travel when they hitch rides with humans. As a result, in some regions, non-native (invasive) plants have exceeded native species. At any given time, an estimated ten thousand species are traveling around the world in ships’ ballast water. Our constant reshuffling of species is unraveling millions of years of geographic separation.

The way we’re moving species around the world is a type of Russian roulette—sometimes nothing much happens; other times, catastrophes result. In the worst-case scenario, the new species thrives, reproduces, and becomes established, decimating local species through predation or by spreading new diseases.

In North America, for instance, bat populations have fallen victims to the dispersal of a European fungus, for which they have no defense. The foreign fungus causes a disease called white-nose syndrome, named after the white powder found on the faces of dead and dying bats. In some areas, as many as 90% of the bats have died—the dire consequence of a seemingly innocuous fungus that was accidentally imported to the U.S.

The Future

It’s possible that through our transformation of the earth, we’ll destroy ourselves. For a species, past longevity is no guarantee of future longevity. Marine creatures called ammonites lived for hundreds of millions of years before they suddenly disappeared. Regarding human prospects:

It’s also possible that human ingenuity will save us from human-created disaster. For instance, some scientists suggest we could restructure the atmosphere by dispersing sulfates to reflect sunlight into space. Or we could take up residence on other planets.

However, in the scheme of geologic time, saving ourselves isn’t the most important thing. It’s that our actions will set the direction of life long after we and everything we’ve created are gone and other life has inherited the earth.

Introduction

Around two hundred thousand years ago, in eastern Africa, a new species emerged. While not especially strong, fast, or prolific, the newcomer—Homo sapiens—proved to be uniquely inventive.

In The Sixth Extinction: An Unnatural History, journalist Elizbeth Kolbert argues that our species today is rapidly changing the shape of the earth and the composition of the atmosphere, in the process unleashing a mass extinction of most living things, quite possibly including ourselves. Scientists have identified five previous mass extinction events (plus smaller disasters) over 500 million years and many believe a sixth extinction, set in motion by modern humans, is underway.

Humans began impacting the world from the start. As their population grew, modern humans spread from East Africa into new regions, undeterred by climate or geographic barriers like rivers and mountains. Humans adapted to the conditions and food supply wherever they landed—for instance, along coasts, they ate shellfish, while inland, they hunted mammals.

When they arrived in Europe, they found Neanderthals, archaic humans similar to themselves, and interbred with them before wiping them out. As modern humans spread, they encountered gigantic animals—huge bears, elephant-sized turtles, fifteen-foot-tall sloths—which they also were able to wipe out by killing them faster than they could reproduce. Humans even found ways to cross the seas, reaching islands inhabited by fantastic creatures, such as giant skinks and birds that laid foot-long eggs, many of which they annihilated. (Shortform note: Learn more about how modern humans became the world’s dominant species in our summary of Sapiens.)

They—we—rapidly multiplied and started demolishing forests to grow food and spreading animals, plants, and organisms to new continents, thus changing the face of the earth. With the discovery of fossil fuels underground, we began our greatest transformation—of the makeup of the atmosphere and the oceans. Some plants and animals have survived by migrating. But many, possibly millions, are stranded where they are unable, or lack time, to adapt. Extinction rates are skyrocketing. No other species has so drastically changed life on earth.

The Big Five mass extinction events of the distant past each suddenly decimated the earth’s diversity of life, the most “recent” being an asteroid that wiped out dinosaurs and other life 66 million years ago.

(Shortform note: The “Big Five” mass extinctions were:

1) The End Ordovician period, 444 million years ago, 86% of species lost. The cause was a sudden cooling of the climate (carbon dioxide levels and temperatures dropped and things froze—glaciation) plus a huge drop in sea levels and an ocean chemistry change.

2) Late Devonian, 375 million years ago, 75% of species lost.

3) End Permian, 251 million years ago, 96% of species lost. This extinction seems to have been triggered by a sudden warming of the climate.

4) End Triassic, 200 million years ago, 80% of species lost.

5) End Cretaceous, 66 million years ago, 76% of all species lost when an asteroid traveling at 45,000 miles an hour crashed into the Yucatan Peninsula.)

By a twist of fate, just as we’re learning more about how these five mass extinctions occurred, we’re also beginning to grasp that our species is causing a Sixth Extinction.

This Pulitzer Prize-winning book traces our path to the Sixth Extinction by telling the stories of many unique species—some long gone and some going extinct as you read these words. Although the author reports from research outposts in the Amazon, the Great Barrier Reef, and the Andes, where scientists are trying to understand and slow the devastation, she notes that species die-offs are happening everywhere, probably including in your backyard. We live in an “extraordinary moment” that’s both riveting and terrifying.

Chapter 1: The Sixth Extinction

Amphibians such as frogs and toads have been around longer than mammals, birds, and even dinosaurs—yet today they’re on the leading edge of another mass extinction. Amphibians’ ancestors emerged from the water 400 million years ago and early forms of today’s amphibian orders appeared 250 million years ago.

“Amphibian” means having a “double life”—they start their lives in water and live on both water and land. Some frogs lay their eggs in streams while others lay them in vernal ponds; some make nests or carry their eggs on their bodies. Amphibian eggs have to stay wet in order to develop because they lack shells.

Amphibians live in a variety of habitats on every continent except Antarctica. Of the seven thousand species we’ve identified, the largest number live in tropical forests—however, one lives in the desert of Australia (the sandhill frog) and one can live above the Arctic Circle. Spring peepers and other North American frogs can revive in the spring after being frozen solid.

Today, amphibians are the most endangered of the six main classes of animals. Researchers started realizing frogs were in trouble in the late 1980s.

Golden Frogs Disappear

Panamanian golden frogs, native to the rainforests and higher-elevation cloud forests of western-central Panama, are less than two inches long and are bright yellow with dark brown spots. Their bright color warns of their extremely toxic skin—a single frog contains enough poison to kill twelve hundred mice.

sixth-extinction-frog.jpg

Golden frogs were once common in the village of El Valle de Anton in central Panama, where they were considered a symbol of luck. The golden frog’s image appeared on lottery tickets and stores sold figurines of the frogs in all kinds of poses. The frogs could easily be seen and heard in the hills around town. Then they started disappearing.

An American graduate student studying the golden frogs in western Panama went back to the U.S. to write her dissertation—and when she returned sometime later to her study area, she couldn’t find any frogs at all. She set up a new study area farther east, where she found some frogs; at first, they seemed healthy and then they, too, vanished.

By 2002, there weren’t any golden frogs left to the west of El Valle; in 2004, dead frogs were seen in the village of El Cope, close to El Valle. Something was causing the population to crash. Concerned biologists in the U.S. and Panama decided to try to save the species by capturing some and raising them indoors at a small facility named the El Valle Amphibian Conservation Center (EVACC).

In 2008, citing a precipitous drop in amphibian populations in general, an article in the Proceedings of the National Academy of Sciences asked, “Are We in the Midst of the Sixth Mass Extinction?”

The paper’s authors, David Wake and Vance Vredenburg, concluded that, based on the extinction rates among amphibians, a sixth catastrophic event is underway—which they attributed to “one weedy species”: humans.

Frogs disappeared not only from populated areas, but also from untouched areas like the Sierras and the mountains of Central America. In central Costa Rica, rare species, as well as common ones, were vanishing. In Ecuador, a backyard toad disappeared in just a few years. Northeastern Australia’s most common frog, the southern day frog, also disappeared.

Culprit Discovered

Similarly, at the National Zoo in Washington, D.C., blue poison-dart frogs, which the zoo was raising, began dying. A pathologist found a fungus on their skin belonging to a group known as chytrids. Chytrid fungi are common, but this was a species never seen before. The fungus, which was named Bd (Batrachochytrium dendrobatidis), was raised in a lab and tested on some blue frogs—they died. Researchers discovered that Bd keeps frogs from properly absorbing electrolytes through their skin that are needed for heart function.

The fungus can continue to live on after it has killed all the frogs in an area, which means that if frogs bred successfully in captivity—including the golden frogs at the EVACC in Panama—were released back into the wild, they’d die. Researchers at the center still hope to release them back into the forest someday, but no one knows how it could be done successfully.

With no sign of stopping, Bd has spread from El Valle into South America and to Australia, New Zealand, and Tasmania. It’s spread outward from several points in the U.S.

Two Types of Extinction

To grasp how rare this amphibian extinction is in the history of life on our planet, it’s important to understand how extinction works. Biologists talk about two types of extinction:

1) Background extinction is somewhat like the concept of background noise, occurring continuously in the background virtually unnoticed over geologic epochs. Background extinction rates are calculated in terms of extinctions per million species-years. For mammals, the rate is 0.25 per million species-years, meaning one species would disappear every 700 years. (Calculating the background extinction rate is complicated and requires fossils.)

2) Mass extinction is a sudden population crash. British scientists defined mass extinction as the elimination of “a significant portion of the world’s biota (life) in a geologically insignificant amount of time.” Another expert described these events as global in reach.

Scientists believe there have been smaller extinction events between the Big Five. When conditions—for example, ocean acidification—change rapidly by evolutionary standards, species don’t have time to adapt. In addition, traits that help a species handle typical threats can themselves be fatal in conditions never before encountered.

Most species have a low risk of extinction most of the time with rare moments of catastrophic risk. While a definitive background extinction rate hasn’t been determined for amphibians, possibly one species should become extinct every thousand years. You wouldn’t expect to see this happen in your lifetime. However EVACC director Edgardo Griffiths has seen several amphibians go extinct.

Amphibians aren’t the only animals rapidly losing ground. Animals are in trouble everywhere. Among those suffering steep declines are reef-building corals, sharks, rays, fresh-water mollusks, reptiles, mammals, and birds. While different animals are disappearing for seemingly different immediate reasons, in every case you can ultimately trace the cause to humans.

The fungus killing amphibians—Bd—is an example. It can move to a certain extent on its own. But it couldn’t have moved by itself to the corners of the world where it’s turned up. It may have have been spread in the fifties and sixties with shipments of African frogs used in pregnancy tests (injecting these frogs with the urine of a pregnant woman causes them to lay eggs within hours). Or it could have been spread by the export of North American bullfrogs for food. Both of these types of frogs are commonly infected with Bd but aren’t sickened by it.

Species’ capability, thanks to humans, of spreading rapidly from continent to continent is unprecedented. We’re not only witnessing a mass extinction event—we’re also causing it.

(Shortform note: Today, golden frogs are breeding successfully in U.S. zoos, such as San Diego, Atlanta, Maryland, and the National Zoo. However, the Panamanian golden frog is considered critically endangered and may be functionally extinct in the wild.)

Chapter 2: Tracing the Clues to Past Extinctions

Until the eighteenth century, scientists and naturalists had no concept of extinction. They believed life was a long, unbroken “chain of being”—that the animals and other life forms existing at the time were the only ones that had ever existed or would exist.

They believed this despite the existence of collections in London, Paris, and Berlin of the remains of creatures such as trilobites, belemnites, and ammonites. In addition, mammoth bones had been uncovered in Siberia, although they were thought to be from elephants.

Finally, in revolutionary France in the mid-1700s, a visionary naturalist, Georges Cuvier, began connecting the dots, starting with a giant molar found in New York state in 1705 and shipped to London, plus a cache of mastodon bones found in a sulfurous marsh along the Ohio River in 1739 by a French expedition. Today the site of the discovery is a state park in Kentucky called Big Bone Lick.

The mastodon bones, which included a three-and-a-half-foot thigh bone, a gigantic tusk, and several teeth, ended up at the Paris Museum of Natural History. A second shipment of bones from the site was sent to London. The bones seemed elephant-like, but naturalists were confused by the teeth, which were different from elephant teeth.

One scientist described it as a new animal, the American incognitum. Some naturalists thought the bones were parts of two or three different types of animals, such as an elephant and a hippo. In 1781, Thomas Jefferson also described it as a new animal, which he thought could still be living in unexplored territory—because naturalists believed nature had never allowed a species to become extinct.

A Lost World

Cuvier went to work at the Paris Museum and began studying the Kentucky bones in 1795. A year later, he presented his findings in a ground-breaking lecture. He contended that because of the different teeth, the Kentucky bones plus others found in Siberia belonged to two new species of animals, which he called “lost species” or extinct species, since no living animals had ever been found.

He continued searching for lost species and soon added others to the list:

Cuvier believed there had to be other extinct species. His radical assertion was that numerous species had died out over a widespread area, which he said proved that another world had previously existed and some kind of catastrophe had wiped it out.

Meanwhile, Cuvier sought specimens from other naturalists around Europe. By 1800, he identified 23 species he believed to be extinct, including a pygmy hippo, an elk with massive antlers, a giant bear, and a giant amphibian. All were similar to present-day animals, but something different was found in Bavaria, which lent momentum to the idea of a lost world—a strange flying reptile, which Cuvier called a ptero-dactyle.

The excitement of finding lost-world species extended across the Atlantic. Farmhands in Newburgh, N,Y, found a giant skeleton. Philadelphia naturalist Charles Wilson Peale reconstructed it and unveiled the eleven-foot tall creature (with the tusks incorrectly pointing down instead of up) on Christmas Eve 1801. It was an American mastodon, but they called it an incognitum and also a mammoth. Peale and his sons sent a second one from the same site to Europe for exhibition. Mammoths caught on in the popular imagination. Suddenly, people in the northeastern U.S. started describing large things as “mammoth”—for instance, merchants advertised “mammoth bread” and “mammoth cheese.”

However, Cuvier realized the Newburgh “mammoth” wasn’t really a mammoth—in an 1806 paper, he called it a mastodon and he identified four other mastodon species. Amateur “fossilists,” who hunted fossils for rich collectors, began uncovering other creatures. In 1812, Cuvier published a four-volume series on fossil animals, the number having grown to 49.

A Theory of Extinction

The big question of what had killed them remained.

The study of different layers of rock formed in different time periods—called stratigraphy—showed a progression of life. Fossils found near the surface of the earth—for instance, mastodons and cave bears—resembled animals still alive. In the next layer down, there were creatures unlike any modern animals and in the layer below that, there were immense reptiles.

Cuvier opposed the concept of evolution—that organisms had to adapt to changes in their environment over time to survive—which had begun gaining currency. He believed animals were designed specifically for their environment and if an animal was born with different features from its forebears, it wouldn’t survive. But, having rejected evolution, Cuvier couldn’t explain how the world ended up with different groups of animals at different times or how new types could appear (speciation).

In any case, he was most interested in extinction. First, he suggested that the mastodon, mammoth, and giant sloth had been wiped out by a one-time catastrophe. As his discoveries of extinct species grew, he theorized there were multiple catastrophes the animals couldn’t handle.

For instance, stratigraphy of rock formations around Paris showed that the region had been submerged at various times. Cuvier theorized there had been sudden “revolutions on the surface of the earth” or massive floods at the point just before human-recorded history.

However, scientists later learned that the Paris stratigraphy reflected slow changes in sea level and the effects of plate tectonics, not a sudden event like a flood. But Cuvier was right about past catastrophic events having disrupted the earth, causing mass extinctions. At these times, nature changed course and operated in an entirely different way.

He theorized that the American mastodon had been wiped out by the same catastrophe that killed the mammoth and sloth, which was almost correct. In fact, the American mastodon died off thirteen thousand years ago as part of what we now call the megafauna extinction—corresponding with and likely resulting from the spread of humans. We were the catastrophe.

(Shortform note: Supporting the belief that humans caused the extinction of mammoths is the discovery announced in November 2019 of traps in Mexico into which mammoths were driven and killed.)

Chapter 3: The Great Auk—an Observable Extinction

In the 19th century, those who believed in sudden mass extinctions were called catastrophists.

But geologist Charles Lyell, who greatly influenced Charles Darwin, believed in a slow process of extinction that accompanied evolution: the landscape changed gradually over millennia due to events and processes like volcanoes, erosion, and sedimentation. Some animals adapted to survive and others didn’t.

Those who believed only in gradual extinction were called uniformitarians. (Today, scientists believe the earth is subject to both gradual (background) extinction and sudden mass extinction.)

Darwin joined the five-year voyage of the Beagle in 1831, which headed for South America to survey the coast and improve existing maps. Through studies during and after the trip, he developed his theory of natural selection, which contends that life, like the physical environment, is always changing. Organisms better adapted to their environment produce more offspring, passing on the successful adaptations. This process of selecting and rejecting traits and variations is continuous and drives evolution.

Darwin believed that over vast amounts of time, new species—fish, birds, animals, humans—developed from old ones. This theory about how species began also accounted for how they vanished. Natural selection gradually rewarded the fit and eliminated the unfit. Both evolution and extinction were imperceptible processes—sudden catastrophes had nothing to do with species’ disappearances, in Darwin’s view.

Contradicting Darwin

However, during Darwin’s time, humans drove one of Europe’s most unusual species, the great auk, to extinction, contradicting his theory that extinction was always slow.

The great auk was a large, black-and-white, penguin-like bird (although it didn’t actually belong to the penguin family). It had a big beak and a white spot under each eye. The great auk inhabited the islands of the North Atlantic, where it bred and raised young in massive colonies. It was a great swimmer but couldn’t fly, which made the two-and-half-foot bird easy prey for passing sailors, who killed large numbers for food.

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Auks probably once numbered in the millions. When the first settlers arrived in Iceland from Scandinavia, they regularly killed auks for food. In addition, auks were used as fish bait, burned as fuel, and plucked for mattresses. On islands where they gathered, they were herded into stone pens or onto ships to be killed. By the late 1700s, as a result of mass slaughter, the great auk population was in steep decline. While environmental causes like a volcano on one of the breeding islands contributed to the decline, egg collectors delivered the final blow. The last known pair of auks, incubating an egg, was killed so the egg could be taken for collectors on the island of Eldey, off Iceland, in 1844.

Darwin had to have known of the great auk’s extinction—a colleague, Alfred Newton, was the one who determined the auk was gone for good. Darwin himself also received first-hand reports of the near extinction by humans of Charles Island tortoises on his Beagle voyage. In On the Origin of Species, he also referred to human-caused extermination of animals.

Despite the evidence of sudden, human-caused catastrophes in front of him, Darwin continued to insist that species went extinct only slowly, through competition and natural selection.

Chapter 4: Evidence of an Asteroid Strike

In the late nineteen-seventies in Italy, American geologist Walter Alvarez discovered traces of the asteroid that ended the Cretaceous period, causing the fifth mass extinction, which wiped out 75% of all species.

He was studying rock layers in the Gola Del Bottaccione gorge outside the town of Gubbio, Italy, which is north of Rome. The region once lay at the bottom of the sea. The remains of marine animals built up through millennia, eventually creating the Apennine Mountains and elevating limestone cliffs.

Between the diagonal bands of limestone reflecting different time periods, Alvarez saw a thin layer of clay that contained none of the marine lifeforms seen in the limestone layers below or above it. Something had wiped out the foraminifera—tiny creatures with calcite shells that fossilize—below the clay layer; when forams appeared later in the limestone layer above the clay, they were different species and much smaller. Alvarez determined that the larger forams seen below the clay layer had vanished at the time dinosaurs were known to have died off (the End Cretaceous period).

Further, Alvarez and his father Luis, a physicist at UC Berkeley, tested the samples from the clay layer and found it contained a huge amount of iridium, a chemical element rare on the surface of the earth but common in meteorites. Similarly, the Alvarezes found high levels of iridium in samples of late-Cretaceous clay in limestone cliffs in Denmark and from the South Island of New Zealand.

Finally, after ruling out numerous possible explanations for the astronomical iridium levels, they theorized that 65 million years ago, a six-mile-wide asteroid struck the earth. It exploded on impact, releasing energy equivalent to more than a million H-bombs. Dust including iridium spread around the earth, creating darkness and causing temperatures to plunge. There was mass extinction.

The Alvarezes published their asteroid strike theory in 1980 in an article in Science titled “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” which generated excitement in the science community and the popular press. However, many paleontologists rejected the theory, which came from outside their discipline (from geology/physics), still believing in the uniformitarian theory of gradual extinction.

More Evidence Backs Asteroid Theory

As far back as the mid-1800s, scientists had noticed a large gap in fossil records of tens of millions of years between plants and animals found in rocks from the late Cretaceous period and the start of the next period, the Tertiary.

For instance, late-Cretaceous sediment contained remains of numerous species of belemnites, squid-like creatures, but there weren’t any in more recent deposits. The same was true for sea creatures called ammonites, which created spiral shells.

But uniformitarians couldn’t imagine why these would disappear suddenly—so up to 1980, when the Alavarezes published their ground-breaking paper, most scientists continued to attribute the disappearances to an incomplete fossil record. Some even argued that a slow, continuous crisis led to mass extinction.

However, the Alvarezes found yet more evidence for the asteroid extinction hypothesis:

Based on the mounting evidence over the eleven years since the Alvarezes published their impact theory, uniformitarian paleontologists finally began changing their minds about sudden mass extinction. Also since then, dozens more sites, including one in suburban New Jersey, have been found showing an iridium layer; you can find ammonite fossils below it but not above it.

The Asteroid’s Effects

The Alvarezes theorized that the worst effect of the asteroid was the dust rather than the impact. The asteroid theory has since been tweaked and the date revised to 66 million years ago, but scientists generally agree that this is what happened:

The End of the Ammonites

Scientists don’t know what specific effect—for instance, whether heat, cold, or a change in ocean chemistry—eliminated the ammonites but not their cousins, the nautiluses.

Ammonites lived in the world’s oceans for more than 300 million years—their fossilized shells have been found worldwide. They built spiral shells with multiple chambers that ranged from palm-size to wagon-wheel size.

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No one is sure what the soft-bodied animals looked like because only their shells were fossilized, but they may have looked something like octopuses.

It’s theorized that ammonites may have died from ocean changes due to the asteroid strike, because their hatchlings floated near the surface of the water. In contrast, nautilus hatchlings may have survived because they hatch in deep water and stay there as they develop for at least a year.

Everything that’s alive today descended from an organism that survived the asteroid’s impact. But it was luck, including distance from impact, heat, tsunamis, and so on—not adaptation—that allowed some species to survive.

In a sudden mass extinction, there’s no time to adapt and pass on survival traits to offspring. Also, traits that have been beneficial for millions of years—like floating near the surface of the ocean as ammonites did—can suddenly become deadly.

Chapter 5: A New Science of Extinction

Today’s science of extinction developed from a series of paradigm shifts.

Sociologist Thomas Kuhn came up with the paradigm shift idea in 1962 to explain how a fundamental change in the basic concepts of scientific discipline leads to a totally new way of thinking. Kuhn showed through studies that when people receive “disruptive” information, which goes against their beliefs, they first try to fit it into their current thinking or framework. They disregard aspects that don’t fit for as long as possible.

When the inconsistencies between the new information and their old way of thinking become too great to ignore, they reach a crisis point. They have what psychologists call a “My God!” reaction and begin to assimilate the new reality.

Both individuals and entire fields of study experience this process. In science, new data that doesn’t fit accepted assumptions and principles is dismissed or rationalized. As the contradictory information grows, the explanations become crazier until people finally acknowledge the new reality. The old framework collapses and the paradigm shifts, opening the way for new insights.

The science of extinction evolved in this way. There wasn’t any concept of extinction until the latter part of the eighteenth century. When strange bones were found, naturalists tried to match them to something familiar—for instance, arguing that mammoths were a type of elephant. Then Cuvier changed the paradigm by showing through fossils that life had a progression and suggesting that species disappeared due to a catastrophic event.

However, so many extinct creatures were discovered in different time periods that the single-catastrophe framework weakened. Counter-arguments for slow extinction—the uniformitarian explanation—lasted for a century, until the discovery of the iridium layer, showing that a catastrophe did occur. And science faced a crisis undermining the uniformitarian view.

Today, the paradigm for extinction combines elements of both Cuvier’s and Darwin’s beliefs: Life on earth consists of long periods of almost imperceptible change punctuated by calamity. Or, as one researcher described it, “long periods of boredom interrupted occasionally by panic.”

But science hasn’t been able to come up with a “unified” theory for mass extinctions. The asteroid strike theory doesn’t explain the other big mass extinctions. It turns out that the Big Five extinctions had various causes, and climate change was a major player in at least two.

The First Extinction: End Ordovician

Scientists believe the first extinction occurred at the End Ordovician period 444 million years ago and eliminated 86% of marine species (there were no land animals). It was precipitated by some sort of spontaneous climate change, in this case, a cooling rather than an asteroid strike.

The Ordovician followed the Cambrian period, during which new life forms grew exponentially—for example, marine animal types tripled and the first plants started to appear on land. However, catastrophic change at the end of the Ordovician occurred as a result of: 1) sudden cooling of the climate (carbon dioxide levels and temperatures dropped and things froze—glaciation) and 2) a huge drop in sea levels plus an ocean chemistry change resulting from the drop in CO2.

Graptolites, a class of marine organisms, were nearly wiped out. However, a few survived; they repopulated the oceans in the next period—the Silurian—but they differed in shape from the graptolites that went extinct. The survivors can be seen in the form of fossils in the Southern Uplands of Scotland, at an outcropping of rock called Dob’s Linn.

The Third Extinction: End Permian

The End Permian extinction of 251 million years ago—the most devastating of the Big Five, eliminating 96% of species—seems to have been triggered by a sudden warming of the climate.

For unknown reasons, an enormous amount of carbon was released into the air. Temperatures shot up as much as 18 degrees; the seas heated up and became acidified. Oxygen levels in the water plummeted, probably suffocating nearly all life.

Possibly, bacteria that produce hydrogen sulfide suddenly multiplied in the oceans, killing marine life, and the oceans in turn released toxic gases into the air, which killed nearly everything else. Geologically, it was brief, lasting only one hundred thousand to two hundred thousand years, but it nearly eliminated multicellular life.

The Anthropocene Epoch

Every mass extinction event has its own unique causes. Suddenly, living organisms face drastically changed conditions, for which they’re not prepared. Eons later, signs left in rock reveal their demise and offer clues to the conditions that caused it.

One hundred million years from now, all our achievements—structures, monuments, works of art, cities—will be compressed to a paper-thin layer of sediment. The mass extinction event we are precipitating will leave its own evidence in rock—of nuclear fallout, river diversion, monoculture farming, ocean acidification, and our dispersal of species around the globe.

As a result of these man-made changes, scientists believe we’ve entered a new epoch—the Anthropocene or human-dominated geological epoch—named by Dutch chemist Paul Crutzen, who shared a Nobel Prize for research into ozone depletion.

Crutzen identified the following geologic-scale changes brought about so far by people:

Changing the Atmosphere

Most importantly, we’ve changed the composition of the atmosphere by adding vast amounts of carbon dioxide—over two hundred years, the level of carbon dioxide in the air has risen by 40%. As a result, Crutzen contends, the earth’s climate is likely to behave much differently for many millennia to come.

By burning fossil fuels, we’ve added 365 billion metric tons of carbon to the atmosphere. By cutting down forests, we’ve contributed another 180 billion tons; each year we add

9 billion tons more.

The concentration of carbon dioxide in the atmosphere—over four hundred parts per million—is higher than it’s been in more than a million years. At our current emissions rate, it will exceed five hundred parts per million by 2050, boosting the average global temperature by 3½ to 7 degrees Fahrenheit. This will melt what remains of the glaciers and the Arctic ice cap and flood islands and coastal cities, such as New York and Washington, D.C.

There’s also an equally destructive flipside to carbon emissions: ocean acidification.

Exercise: Your Carbon Footprint

Humans have changed the composition of the atmosphere by adding vast amounts of carbon dioxide—over two hundred years, the level of carbon dioxide in the air has risen by 40%. This is heating up the climate and acidifying the oceans, possibly setting in motion a sixth mass extinction.

Chapter 6: Impacts—Ocean Acidification

Oceans, covering 70% of the earth’s surface, absorb a lot of the carbon we’re pumping into the air—two-and-a-half-billion tons a year when this book was written in 2014—which is changing ocean chemistry.

In the past, there was a fairly even exchange of gases: the ocean absorbed gases from the atmosphere and also released dissolved gases back into the atmosphere. At this point, however, more CO2 is entering the oceans than they can release, resulting in acidification. (Carbon dioxide dissolves in water and forms carbonic acid.)

As a result, the pH of the oceans’ surface water has decreased, making them 30% more acidic than they were in 1800. The pH is on track to fall to 7.8 (from today’s average of 8.1) by the end of this century, making the oceans 150 percent more acidic than before the industrial revolution.

Ocean acidification may have played a major role in the most recent mass extinction event—the End Cretaceous. It’s believed to have been a factor in two more of the Big Five—the End Permian and End Triassic and possibly in two lesser extinction events.

Why Acidification is Dangerous

In terms of destructive effects, ocean acidification has been called global warming’s “evil twin.” There are numerous reasons, which add up to a steep loss of biodiversity, including:

Among the biggest victims will be calcifiers—animals and plants that construct shells or external skeletons. They include starfish, sea urchins, mollusks (clams and oysters), barnacles, and many coral species (the ones that build reefs). Many kinds of seaweed, some algae, and some plants also are calcifiers.

To build shells and skeletons, they combine calcium ions and carbonate ions to create calcium carbonate. But to do so, they have to change the chemistry of the seawater. Acidification makes this more difficult, in part by decreasing the number of available carbonate ions. In addition, water with too much acid dissolves or eats holes in their shells.

A Glimpse of the Future

An odd natural phenomenon occurring off the tiny Italian island of Castello Aragonese, west of Naples, provides a preview of ocean acidification’s devastating effects on marine life. For several hundred years, vents in the sea floor along the island have been spewing carbon dioxide. They’re a vestige of past underwater volcanic eruptions caused by two continental plates pressing together. Streams of gas erupt from the seafloor and dissolve in the water.

Scientists have been studying the effects along a natural pH gradient. On the Island’s eastern edge, the water is unaffected, but as you progress toward the vents, the acidity increases. The researchers divided the area into zones to count and track the species—for instance, mussels, barnacles, limpets, fish, sea urchins, and seaweeds—at different pH levels.

In the area farthest from the vents, they found 69 species of animals and 51 plants. In the area closest to the vents, where the pH is 7.8, there are a third fewer species. A pH of 7.8 is considered the tipping point for an ecological system crash, a point we are projected to reach in 2100. Calcifiers make up three-fourths of the species missing near the vents, including a normally ubiquitous barnacle, a robust mussel, sea snail, and seaweed. Limpets near the vents at Castello Aragonese have holes in their shells.

These and other experiments show that some species will thrive as acidification increases—for instance, picoplankton like acidified water but when they multiply, they use up nutrients, to the detriment of larger organisms. At-risk species include clownfish, Pacific oysters, and two organisms that are food sources for other species such as salmon and whales.

Reversing Geologic History

The oceans have absorbed about a third—150 billion metric tons—of the CO2 we’ve released into the atmosphere. But the pace is as critical as the volume.

It’s similar to drinking alcohol—how fast you drink it (over what period of time), in addition to how much your drink, affects your blood-alcohol content. Whether the oceans absorb large volumes of CO2 over a million years or over a hundred affects their chemistry differently.

CO2 releases of the past, for instance from volcanoes, can’t compare to the pace of man-made emissions from vehicles, power plants, and manufacturing. By burning fossil fuels, we’re releasing carbon into the air that’s been sequestered for hundreds of millions of years. We’re “running geologic history in reverse at warp speed.”

If we continue on this course, according to researchers writing in the journal Oceanography, the Anthropocene’s legacy will be “one of the most notable, if not cataclysmic events in the history of our planet.”

Chapter 7: Impacts—The End of Coral Reefs

Corals have endured for many geologic epochs, but researchers believe they won’t survive the Anthropocene. Instead, they’re on a course to be the first major ecological system to go extinct. The driving forces are acidification and climate change.

Some scientists project they’ll last out the century; others don’t give them even that long.

One paper in Nature predicted that visitors to the Great Barrier Reef in 2050 will find it rapidly disintegrating.

Coral reefs stretch around the middle of the globe. The largest is the Great Barrier Reef, which extends, with breaks, for more than fifteen hundred miles; in some places, it’s five hundred feet thick. The next largest is off the coast of Belize. There also are sizeable reefs in the Pacific, the Indian Ocean, and the Red Sea, plus smaller reefs in the Caribbean.

Reefs are enchanting in their beauty—Darwin described them as “amongst the wonderful objects of the world.” Biologically, they are even more distinctive. Yet they are under dire threat.

How Reef-Building Corals Work

Coral reefs are strong enough to destroy ships, yet they’re built by generations of tiny marine invertebrates called polyps that work together as communities, each secreting calcium carbonate to form a hard exoskeleton. Corals get nutrients from a microscopic plant (zooxanthellae) that lives in their tissues.

Among calcifiers, corals are master builders. Billions of individuals belonging to one hundred different species join forces to build a reef, which is a living, constantly growing structure.

Reefs are comparable to rainforests in the immense variety of life they support. Millions of marine species spend part of their lives on reefs, depending on them for protection or food. Additional species prey on those using reefs for protection or food. Researchers have cracked open small chunks of coral and found hundreds of species—one volleyball-sized piece contained fourteen hundred worms of one hundred and three species.

Coral are highly sensitive to ocean acidification. They need a certain “saturation state” or concentration of carbonate ions in seawater in order to create their exoskeletons. Acidification lowers carbonate ion concentrations.

Because reefs are always being eroded by waves and eaten by fish, they have to keep growing.

Research in Biosphere 2, a closed glass structure containing different habitats in Arizona, showed that coral grow fastest at a saturation state of five, and grow more slowly at four and three. They stop calcifying at level two. Currently, the saturation state virtually everywhere is four or less. Based on current trends, there won’t be any ocean areas above level 3.5 by 2026 and none above three by 2100.

Once a year, corals engage in mass spawning, in which the polyps release eggs and sperm together in bundles that break open after release. Ideally, eggs and sperm connect and produce larvae. However, acidification and lower saturation states reduce fertilization. They also hinder larval development and establishment to begin new colonies.

Other research has confirmed that diversity plummets as acidification increases and causes the saturation state to drop. According to one study, coral cover in the Great Barrier Reef has decreased by 50% over the past thirty years. Researchers project that at current emission rates, all reefs will stop growing and begin dissolving in the next fifty years.

Climate Change and Other Threats

Acidification’s “evil twin,” climate change, also is a significant threat to reef-building corals because it increases water temperatures. When the water temperature rises too high, zooxanthellae, the microscopic plant that lives in coral and provides it with carbs through photosynthesis, starts producing harmful concentrations of oxygen radicals. The coral then eject the zooxanthellae. Because these organisms also give reefs their color, the reefs turn white, which is referred to as coral bleaching; they stop growing and die. Significant bleaching occurred in 1998, 2005, and 2010 and is expected to increase as temperatures continue to rise.

Research reported in Science in 2008 found that, of eight hundred reef-building coral species studied, a third are at risk of extinction, primarily due to increasing water temperatures. The researchers wrote that “the proportion of coral species listed as threatened exceeds that of most terrestrial animal groups apart from amphibians.” Thanks to global warming, reef-building corals are one of the planet’s most endangered groups.

Corals also face other threats, some of which are more immediate, including:

Stresses make coral vulnerable to diseases, such as white-band disease, a bacterial infection that has devastated two species of Caribbean coral, both of which are listed as critically endangered.

(Shortform note: The author doesn’t explicitly discuss the larger implications of the loss of coral reefs, but we see two themes through the book. First, coral reefs can be like the canary in the coal mine, warning us of eventual dangers to ourselves. Secondly, they’re a key part of the ocean ecosystem where they exist, and their loss could have consequences throughout the food chain.)

Chapter 8: Impacts—Rainforests and Biodiversity

Most people think of global warming primarily as a threat to cold-climate species such as polar bears, penguins, and seals. Their worlds are changing dramatically as polar sea ice declines and gets thinner. And as the ice declines, the larger areas of open water absorb more heat, which melts more ice.

Half of the Arctic’s perennial sea ice has disappeared in the last thirty years and the rest may be gone in thirty more. Perennial sea ice, also known as multiyear ice, is thicker ice that survives the summer season. As the earth warms, the outlook for species that rely on the ice is grim.

But according to researchers, global warming will have an even greater impact in the tropics because that’s where the most species live. In Canada’s boreal forest of nearly a billion acres, there are only about twenty species of trees. In the U.S., eastern deciduous forests contain fifty to two hundred species. In contrast, Belize, in Central America, has some 700 native tree species. Manu National Park, in the Andes of Peru, is a forest reserve where researchers have counted more than a thousand species of trees.

The same pattern—fewer species in cold climates and many more in warmer climates—applies to birds, butterflies, frogs, and fungi. Biodiversity is lowest at the poles and increases as you go from the poles to the equator. There are three theories for why more species live in tropical rainforests:

Species in Manu National Park in the Andes are already responding to climate change. The reserve is a biodiversity “hot spot”—for instance, the cloud forest (a higher-elevation rainforest) is home to one in every nine bird species. Researchers have found thirty new tree species and another three hundred species they believe are new but haven’t yet classified.

They’ve divided part of the forest into 17 study areas at different elevations with different temperatures. Their research has shown that some tree species are “moving” to higher elevations as temperatures warm, by dispersing their seeds up the mountain. Researchers calculated that global warming is pushing the average genus (a group of closely related species) eight feet higher per year. However, they found that one species is moving a hundred feet a year.

Extreme Temperature Swings

Plants and animals everywhere adjust to brief or cyclical temperature changes—for instance, day and night, wet and dry periods, and seasonal changes. They have physiological responses such as panting and growing or shedding fur. They may also migrate or hibernate.

However, over vast amounts of time—a million or more years—species have confronted drastic changes in climate (long-term temperature changes). The earth has been cooling for forty million years, starting in the late Eocene when there wasn’t any ice. Thirty-five million years ago, glaciers began forming in Antarctica. Three million years ago, ice also formed in the Arctic.

A half-million years later, at the beginning of the Pleistocene Epoch, the world experienced a series of ice ages or glaciations, which scientists believe resulted from changes in Earth’s orbit. The amount of sunlight varied depending on the latitude and time of the year. Snow and ice accumulated in the north, carbon dioxide levels dropped, and the ice grew. When Earth’s orbit changed again, the ice began melting, CO2 increased, and further melting occurred.

This cycle recurred twenty times. Plants and animals responded with mass migrations—even insects moved thousands of miles. Scientists project that the temperature change in the next century will be comparable in magnitude to the temperature fluctuations of the ice ages. At the current emissions rate, temperatures in the Andes will soar as much as nine degrees.

Faster Warming Today

While the magnitude of current change is similar to that of the ice ages, the rate of temperature change is much faster. Today, the planet is warming at ten times the rate at the end of the last ice age. Species will have to migrate ten times as fast. Only time will tell how many succeed. However, many won’t.

In 2004, scientists used a concept known as the species-area relationship, which is expressed by a formula, to estimate the extinction risk at different levels of global warming. The idea is that the larger your sample area, the more species you’ll find. Researchers compiled data on the ranges of over one thousand species of plants and animals and correlated the ranges with the current climate. Then they considered two climate warming scenarios: one in which species stayed put (and the area habitable for them declined), and one in which they were mobile. Here’s what they determined for each scenario:

Scenario 1—Species stay put:

Scenario 2—Species are highly mobile:

When researchers averaged the two scenarios and factored in a mid-range warming projection, they calculated that 24% of all species would be headed for extinction. These findings, featured on the cover of Nature, received wide attention. The BBC boiled the research down with this headline: “Climate Change Could Drive a Million of the World’s Species to Extinction.”

More recent studies have contended the article’s projections are either understated or overstated. Either way, the researchers say, the potential impacts support the notion that we have entered the Anthropocene, in which humans are significantly altering the earth’s geology, atmosphere, and ecosystems.

A Domino Effect on Species

A cloud forest, like a coral reef, is the backbone of an ecosystem of interdependent species. Specific insects depend on specific tree species. Certain birds eat certain insects and they shelter in trees. Birds and animals also serve the trees by pollinating them and spreading seeds.

Global warming continues to change the ecosystem dynamic. Among plants, some species will benefit from warmer temperatures and higher carbon dioxide levels, but others will languish and disappear.

Forest reserves like Manu National Park can protect biodiversity against mining, logging, and other threats by establishing protective borders. But global warming knows no borders. And when species need to move to survive, reserves won’t be big enough to save them.

In the long, long run, a warmer world may be as diverse or more diverse than today’s world. But for present-day species, the outlook is grim. Aside from the issue of habitat, their ability to deal with a much warmer climate is doubtful. Today’s species descended from ancestors who survived the ice ages—they’re cold-adapted. Since temperatures haven’t gotten that much warmer, they haven’t developed adaptations to additional heat.

Yet by the end of the century, carbon dioxide could reach levels unprecedented in 50 million years.

Chapter 9: Impacts—Fragmentation of Habitat

As humans have reshaped the earth’s landmass, we’ve constrained the ability of other species to survive the life-altering effects of global warming.

Currently, the earth contains about fifty million square miles of land not covered by ice. We’ve transformed more than half—27 million square miles—into cities, highways and shopping centers, cropland and pasture, logging and mining operations, and manufacturing plants. That leaves 23 million square miles, which are mostly (three-fifths) forest; the other two-fifths are high mountains, tundra, and desert.

Another way of looking at how we use the planet is by dividing the landmass into “anthromes,” or human-altered zones, such as urban, irrigated cropland, and populated forest. Researchers have identified eighteen anthromes covering 39 million square miles. That leaves 11 million square miles of wildland, including parts of the Amazon, Siberia, northern Canada, and the deserts.

However, there’s virtually no area left that’s truly untouched—roads, logging, and mining have sliced up and cut off every wild area to some extent. There are two results: 1) species lose the ability to move or flee, and 2) their ability to reproduce declines and extinction becomes more likely.

Brazil’s Reserve 1202, a twenty-five-acre block of untouched Amazon rainforest surrounded by cut-over forest and brush, illustrates the survival challenges we’ve created for other species.

It’s part of a string of forest “islands” known as the Biological Dynamics of Forest Fragments Project or BDFFP, an experiment enabled by cooperation between ranchers and conservationists. In the 1970s, the Brazilian government gave ranchers a stipend as an incentive to create new ranchland by clearing rainforest. At the same time, they had to leave at least half the forest on their property intact. Scientists now known as “fragmentologists” have been managing and studying these untouched fragments for thirty years. What they’re learning is how species respond to isolated, shrinking habitats.

Birds are an example. Reserve 1202 has at least thirteen hundred bird species. At first, birds from the surrounding deforested areas took shelter in the fragment, increasing the number of species. But gradually, the number of bird species declined. Some species, for instance, wouldn’t cross roads or cleared areas to get to the reserve. Ultimately, in all of the fragments, birds and other species experienced steady declines. Overall biodiversity dropped.

Similarly, ocean islands typically have less biodiversity. To start with, some species probably don’t have enough room—for instance, a large cat that needs an average of forty square miles can’t thrive when confined to a twenty-square-mile area. But the problem isn’t just the amount of habitat. Species don’t level off—they keep declining.

Smaller populations are more prone to local extinction because they have a harder time recovering from chance disasters. For instance, the last breeding pair of a certain bird species might see its nest destroyed by a storm one year and raided by predators the next. The following year, the chicks might die of disease or turn out to be all males. So, the species eventually dies out. Depending on how accessible the habitat fragment is, new members of the species may or may not be able to recolonize it. Local extinctions can eventually become regional and then global.

Forest fragment researchers in the Amazon islands have found:

As noted in regard to forest reserves in the Andes, species that need to move to adjust to continually warming temperatures will run out of room to move in habitat fragments. Man-made barriers—cities, highways, deforested areas—will impede movement as well, further challenging many species’ ability to survive even as man-made changes compel them to move.

Chapter 10: Impacts—Dispersal of Species

In the past, the range of many species was limited by geographic barriers such as oceans, rivers, and mountains. Today, however, species are being dispersed widely by humans, with disastrous consequences.

Darwin believed each species originated in one place. It spread by dispersing seed via the wind or it moved under its own power. With a lot of time, any organism could eventually spread widely. However, geographic features like oceans, mountains, and deserts set limits, which explained why flora and fauna on one continent could be different from those on another—they’d evolved separately.

However, Darwin struggled to answer the question of how the original colonizers got started. Also, his theory didn’t explain why fossils of the same types of reptiles and plants were found on different continents. In later years, scientists wondered whether land bridges had once spanned oceans, allowing travel, or whether the continents had once been larger and then separated and shifted. The latter theory suggested there was originally one giant continent, Pangaea.

In the Anthropocene, humans are, in a sense, reuniting the continents into a New Pangaea by dispersing species all around the globe via various means of transportation—and with unprecedented speed. There are no barriers to species’ travel when they hitch rides with humans. As a result, in some regions, non-native (invasive) plants have exceeded native species. At any given time, an estimated ten thousand species are traveling around the world in ships’ ballast water. Our constant reshuffling of species is unraveling millions of years of geographic separation.

Invasive Species Spread

The way we’re moving species around the world is a type of Russian roulette—sometimes nothing much happens; other times, catastrophes result.

In the no-harm-done scenario, the new species doesn’t survive because the climate is inhospitable, it can’t find food, or it gets eaten by predators. This probably is what happens most of the time. But in the worst-case scenario, the new species thrives, reproduces, and becomes established. Some might stick around the place where they landed; others might spread wildly.

Japanese beetles are an example of the worst-case scenario. In 1916, they were found in a nursery in New Jersey and, by the following year, had spread over three square miles. They covered seven square miles the next year and forty-eight the year after that. Today they’ve spread south to Alabama and west to Montana.

By one estimate, between five and fifteen invasive species out of every one hundred will become established. Of those, one will be the bullet in the game of Russian roulette, causing havoc.

Invasives that spread wildly often do so because they lack competitors and predators in the new environment. An example is purple loosestrife, a flowering plant that arrived in the northeastern U.S. from Europe in the early nineteenth century.

In its native habitat, many species of beetles and weevils helped to contain it. However, without enemies in the U.S., it has spread coast to coast. In the early 1990s, some of the beetle predators were introduced in the U.S. to control its spread and so far they’ve been effective.

However, introducing further invasives to stop invasives can be another instance of playing Russian roulette—it works in some instances, while in others it’s disastrous. An example of a disastrous predator introduction is the rosy wolf snail. It was introduced in Hawaii in the late 1950s to control the African snail, which had become an agricultural threat. Instead of harassing the African snail, however, the wolf snail attacked native snails—90% of Hawaii’s native snail species are now extinct or in sharp decline

Along with the benefit of escaping their old predators, invasive species often find local species to be easy pickings. An example is the brown tree snake of Papua New Guinea, which was introduced to Guam in the 1940s, probably in military cargo. After wiping out most of Guam’s native bird species, as well as some mammals, it ran out of native victims and now feeds on more recent newcomers.

Introduced pathogens behave the same way—when they find new hosts that lack defenses, they’re deadly. The American chestnut, which dominated deciduous eastern forests in the 1800s, fell victim to a fungus, chestnut blight, probably imported from Japan in the early 1900s.

It killed nearly 100% of the chestnuts—some four billion trees. By virtue of its newness, the chytrid fungus is having a similar effect on amphibians, which have no defense.

Science writer David Quammen has observed that the brown tree snake and other invasives do exactly what humans have done around the world—thrive at the expense of other species.

Bats Fall Victim to a Deadly Import

In 2007, biologists in New York who studied bats began finding huge numbers of dead bats in the caves where they hibernated. The dead and dying bats had a white powder on their faces.

The next winter, the so-called white-nose syndrome was found on bats in thirty-three caves in four states. In some cases, as many as 90% of the bats died. The die-off continued to spread as this book was being written. (Shortform note: Twelve species of bats in the U.S. have been affected by white-nose syndrome, including two endangered species and one threatened species.)

The fungus causing white-nose syndrome is a cold-climate fungus. In 2010, it was traced to Europe, where it also is widespread. One European species, the greater mouse-eared bat, carries white-nose but doesn’t get sick. Somehow, the fungus was accidentally imported to the U.S., where researchers named it Geomyces destructans.

Bats’ habit of hibernating close together in clusters helped spread the disease. Their slow reproduction rate—a pair has one pup per year—will make it hard for numerous bat species to recover even if some become resistant to the fungus or if scientists can stop the spread.

Scientists aren’t sure how the fungus kills bats. It may irritate their skin to the point that it wakes them up and they fly around expending energy that uses up fat stores. Starving, they then fly outside to hunt for insects in winter and they die. Another theory is that the fungus dehydrates them, which wakes them up to search for water, again using up energy stores.

(Shortform note: Scientists have since reported limited success with spraying bats with a bacteria that kills the fungus. Also, in 2015, researchers reported successfully treating small numbers of bats with gases expelled by certain bacteria. And in 2019, scientists reported some success with an oral vaccine. Read more here.)

Some of This Was Intentional

The human-driven dispersal of species may have started as far back as one hundred twenty thousand years ago, when humans left Africa. When they crossed the Bering land bridge and reached North America thirteen thousand years ago, they brought dogs, which they had domesticated. Fifteen hundred years ago, Polynesians carried rats, lice, and pigs to Hawaii.

However, those small-scale species introductions pale in comparison to the huge exchange of species—called the Columbian Exchange—that followed Europeans’ discovery of the New World. Groups known as acclimatization societies intentionally spread species to areas where they hadn’t existed before.

For example, in 1890, a New York group imported European starlings, releasing a hundred in Central Park. We now have over two hundred million. (Shortform note: European starlings are considered our worst nuisance bird species, ravaging crops, transmitting diseases to livestock, causing sanitation problems in cities, and forcing out native birds.)

We are still purposefully introducing new non-native species—for instance, for gardens and aquariums. Researchers who track invasive species say that more non-native species of mammals, birds, amphibians, turtles, and so on are brought into the U.S. as pets than there are native species of these groups.

Meanwhile, accidental imports have grown along with international trade. California is getting a new invasive species every sixty days, while Hawaii gets one every month. In contrast, a new species established itself in Hawaii roughly once every ten thousand years before humans settled there. In North American coastal waters, invasive species have increased exponentially in recent years due to a greater volume of goods being transported at a faster rate.

The introduction of new species has increased local diversity. For example, before humans arrived, Hawaii lacked rodents, amphibians, terrestrial reptiles, and hoofed animals. There were also no ants, aphids, or mosquitoes. However, Hawaii had thousands of its own unique species, many of which are now gone or declining.

In addition, as local diversity has increased, global diversity has declined as plants and animals come into contact with new species and compete with them or become their prey. In a “thought experiment,” scientists calculated that a single megacontinent with all landmasses physically connected would have 66% fewer mammal species and 50% fewer bird species.

The New Pangaea—Invasives Everywhere

Introduced species are now so prevalent that you can see some just by looking out your window—for instance:

One study found that nearly a third of all plant species in Massachusetts are “naturalized newcomers.”

Earthworms are also immigrants from Europe. New England had none before settlers arrived—they’d been wiped out by the last ice age. Earthworms change soil composition by eating leaf litter. Although gardeners like them, they may have contributed to a decline in native salamanders in the Northeast.

Other invasives currently spreading rapidly include:

The problem of invasives is global. A database in Europe tracks more than twelve thousand species. Even in Antarctica, researchers found that visitors (including other researchers) brought with them over seventy thousand seeds from other continents. A grass from Europe has established itself there. Welcome to the New Pangaea.

Exercise: Invasive Species

Humans have spread invasive species around the world through various means of travel, shipping, and trade. Often, the invasives wipe out local species by predation, damaging crops, or spreading new diseases.

Chapter 11: Last of the Megafauna

A great variety of supersized animals—megafauna—once stalked the earth. Near the end of the Cretaceous period, there were many groups of huge dinosaurs besides Tyrannosaurus. Members of the Saltasaurus group weighed around seven tons. A member of the Therizinosaurus group was thirty feet long,

Near the end of the last ice age, there were enormous animals all over the world. In Europe, the roster included woolly rhinos, cave bears, giant elk, and hyenas. North America had mastodons, mammoths, giant camels, grizzly-size beavers, saber-toothed cats, and a giant ground sloth. South America had glyptodonts, which resembled armadillos the size of small cars. Australia’s even weirder animals included diprotodons, a group of huge marsupials called rhinoceros wombats; a marsupial lion, and a ten-foot-tall kangaroo. New Zealand had giant birds—the South Island giant moa was twelve feet tall.

Being very big was an evolutionary advantage—large animals had no predators. So the question of why the megafauna died out has puzzled scientists dating back to Cuvier’s day, when the fossils of huge unknown creatures began turning up.

Scientists have debated whether megafauna disappeared due to climate change (possibly multiple events) or were killed by humans. But most researchers today lean toward blaming humans. The big animals’ disappearance in Australia and the Americas coincided with the appearance of humans. There’s also evidence that Maori killed off the giant birds in New Zealand: the remains of outdoor ovens and “middens” containing bones of large birds.

The advantage of being too big to have predators disappeared when humans came on the scene. At that point, the flipside of being extra-large—being slow to reproduce—became a disadvantage. If humans killed off large numbers of a species—or even killed small numbers continually for millennia—the rest wouldn’t have been able to reproduce fast enough to avoid extinction.

Big Animals in Trouble Today

Human pressure plus slow reproduction are also why big animals like elephants, bears, and rhinos are in trouble today. Take rhinos, for example. Humans have killed so many rhinos and destroyed so much of their habitat that, ironically, only extraordinary human efforts can save them.

The double-horned Sumatran rhino is the smallest and oldest of the five species of rhino that still exist. It once ranged from the foothills of the Himalayas through Myanmar, Thailand, Cambodia, and beyond. But by the early 1980s, its population had shrunk to a few hundred and was heading toward extinction due to habitat loss as southeast Asia’s forests were cut down.

In 1984, conservationists started a captive breeding program to try and save the species. They caught forty and sent seven to zoos in the U.S. Only three in the U.S. lived and the two females and a male were consolidated at the Cincinnati Zoo. There, with the help of the zoo’s Center for Conservation and Research of Endangered Wildlife, three offspring eventually were born. Along with one other rhino, they’ve been the only four captive Sumatran rhinos born in the last thirty years. Meanwhile, the number in the wild has declined sharply since the 1980s.

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Other rhino species are also at risk:

In fact, almost all large mammals are in trouble:

Chapter 12: Pruning the Family Tree

From the day that modern humans migrated from Africa to the Middle East a hundred twenty thousand years ago, something in humans’ genes set them apart from all other species, driving them to cross oceans, explore, and take over new areas, by killing off the locals and by altering the environment as they chose.

It might be called a restlessness or insanity gene, in that we’ve been pressing the envelope ever since, exploring the outer reaches of space but also altering our own world in ways that are driving other species—and maybe eventually ourselves—extinct.

Pre-Modern Humans Discovered

We share DNA with an archaic human, the Neanderthal, who didn’t survive contact with us. The first bones of these distant relatives were found in 1856 in the Neander Valley in Germany is north of Cologne. The valley was lined with limestone cliffs, which were being quarried when workers discovered the bones in a cave.

They tossed the bones aside and they might have been lost, if the quarry’s owner hadn’t heard about them. Believing they were from a cave bear, the owner passed them on to a fossilist, who recognized them as resembling human bones and called them “a primitive member of our race.”

This was around the time Darwin’s On the Origin of Species had been published. Opponents of evolution dismissed the claims that the bones were a species of human. But over the next few decades, more bones of the same type turned up.

In 1908, a nearly complete skeleton was discovered in France. The Neanderthals (named after the valley where they were first discovered) were deemed to be more like apes than humans, although there’s no evidence they were hairy, as they were depicted in drawings at the time.

However, starting in the 1950s, scientists began piecing together a different, more human-like story. They concluded that Neanderthals walked upright with a human-like gait. A set of bones found in the 1960s in Iraq had a head injury, which had healed, suggesting the victim been cared for by others. Another skeleton had been buried with flowers, a researcher contended, although the seeds found with the bones could have carried there by rodents.

Neanderthals Meet Humans

Since the original discovery, Neanderthal bones have been found all over Europe, where it’s believed they lived for at least a hundred thousand years. To survive in the ice age, they probably created shelters in caves, used fire, and made clothing. Tools were found with their bones, including axes and scrapers probably used to cut meat.

Then, about thirty thousand years ago, Neanderthals disappeared. Scientists first suggested climate change or disease as theories for their disappearance. In recent decades, however, other researchers have concluded that humans killed them off, as they did the megafauna.

Around forty thousand years ago, humans showed up in Europe. Archaeological evidence suggests that whenever they arrived in a region inhabited by Neanderthals, the latter disappeared.

Researchers in Germany sequenced the entire Neanderthal genome (complete set of DNA) so they could compare the Neanderthal and human genomes’ similarities and differences. It turned out that Europeans and Asians shared DNA with Neanderthals, while Africans didn’t.

In a paper published in Science in 2010, the genome researchers theorized that modern humans who migrated from Africa interbred with Neanderthals before driving them to extinction. This produced children, who escaped to Europe, Asia, and the New World. Thirty thousand years later, all non-Africans have between 1% and 4% Neanderthal DNA. Africans whose ancestors didn’t migrate and didn’t interbreed don’t have Neanderthal DNA.

More Archaic Humans

Skeletons of two additional archaic humans were found in the early 2000s. A small hominid species called Homo floresiensis, popularly referred to as a hobbit, was found in 2004 on the island of Flores in Indonesia. Researchers dated a tooth, which indicated the species was about seventeen thousand years old, but they weren’t able to extract any DNA to determine any relationship to modern humans.

In 2005, forty thousand-year-old bones of yet another pre-modern group were found in a cave in Siberia. Scientists called the species Denisovans after the name of the cave where the bones were found. Scientists were able to extract DNA, which indicated modern humans must have bred with them—New Guineans have up to 6% Denisovan DNA.

There’s no evidence that humans killed off the hobbits and Denisovans, but the timing of their disappearance in the late Pleistocene along with other extinctions suggests humans were the cause. Like the megafauna, they had a low reproductive rate, so pressure on the population from humans could have resulted in extinction.

Last of the Great Apes?

We’re putting our next-closest relatives, the great apes, under similar pressure today. Over fifty years, the wild population of chimpanzees has declined by half. Mountain gorillas are experiencing a similar decline.

Lowland gorillas have declined by 60% over the last twenty years under pressure from poaching, disease, loss of habitat, and wars. Sumatran orangutans are listed as “critically endangered,” or at high risk of extinction in the wild.

In effect, we’re pruning our own family tree. We started with Neanderthals and Denisovans tens of thousands of years ago; today, we’re continuing the job with our cousins, the great apes. One day, the only remaining representative of the great apes may be us.

For more than a hundred thousand years, Neanderthals lived in Europe without any significant impact on the planet. Had humans not shown up, Neanderthals might still exist, along with the woolly rhinos and wild horses they drew on the walls of their caves.

We don’t know what small mutation from the Neanderthals gave humans the insatiable desire to explore, along with the capacity to change and destroy the environment. But it’s literally made a world of difference.

Chapter 13: Saving Species, Saving Ourselves

At the San Diego Zoo’s Institute for Conservation Research, researchers are maintaining cell cultures of critically endangered and extinct species preserved in vials in tanks of nitrogen. One of them is the black-faced honeycreeper from Maui, believed to have gone extinct in the early 2000s.

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In its Frozen Zoo, the institute has saved cell lines of about a thousand species, most of which still exist. The Cincinnati Zoo is doing something similar, as is England’s University of Nottingham. (Shortform note: According to Wikipedia, genetic material can be stored in nitrogen indefinitely for possible use for artificial insemination, in vitro fertilization, embryo transfer, and cloning.)

While humans as a species have been destructive and shortsighted, researchers and conservationists also have joined together in heroic efforts, like the Frozen Zoo and the El Valle Amphibian Conservation Center in Panama, to save threatened species. Examples include:

But no matter how much people care about saving species from extinction, the fact remains that our presence changes the world—we’ve done so since we migrated out of Africa—and we’ve set in motion a mass extinction event. We’re all contributors, whether a poacher in Africa, a logger in the Amazon, a commuter in Washington, D.C., or the reader of a book made of paper.

In the Sixth Extinction, what does the future hold for us?

One possibility, of course, is that through our transformation of the earth, we’ll destroy ourselves. We’re still dependent on the earth’s atmospheric and geochemical composition and its biological processes. By altering these systems—pumping CO2 into the atmosphere, acidifying the oceans, cutting down forests—we’re threatening our own survival.

For a species, past longevity is no guarantee of future longevity. Ammonites lived for hundreds of millions of years before they disappeared. Regarding human prospects:

It’s also possible that human ingenuity will save us from our own foolishness. For instance, some scientists suggest we could restructure the atmosphere by dispersing sulfates to reflect sunlight into space. Or we could take up residence on other planets. In that regard, one author advises that “as long as we keep exploring, humanity is going to survive.”

However, in the scheme of geologic time, saving ourselves isn’t the most important thing. It’s that our actions will set the direction of life long after we and everything we’ve created are gone and other life has inherited the earth.

Exercise: Your Representative’s Record

The U.S. Congress has voted on 257 environment-related bills this session.