PDF Summary:The Sixth Extinction, by Elizabeth Kolbert

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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...

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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...

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

• A skeleton found in Brazil, which he concluded was a sort of giant sloth (he named it Megatherium).
• The so-called Maastricht animal, whose remains with shark-like teeth had been found in a Dutch quarry. This was much later determined to be a marine reptile—a mosasaur.
• Remains of several species of hoofed animals plus a marsupial found in gypsum quarries north of Paris.

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...

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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,...

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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...

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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...

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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...

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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....

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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...

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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...

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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...

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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...

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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...

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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:

• Anthropologist Richard Leakey suggested, “Homo sapiens might not only be the agent of the sixth extinction, but also risks being one of its victims.”
• Stanford University ecologist Paul Erlich described the future in even starker terms: “In pushing other species to extinction, humanity is busy sawing off the limb on which it perches.”

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...