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The Sixth Extinction

An Unnatural History

Elizabeth Kolbert
Paperback edition 2015

Life on earth is 3.8 billion years old. Our planet has experienced five mass extinction events during that time, the last of which occurred some 66 million years ago when a six-mile-wide asteroid collided with earth, wiping out the dinosaurs and dramatically changing the biodiversity of the planet: marine ecosystems essentially collapsed, and about 75 percent of all plant and animal species disappeared. We are now facing a sixth mass extinction — the current spate of plant and animal loss threatens to eliminate 20 to 50 percent of all living species on earth within this century. Humans are the agents of this extinction, and we can make it stop.

At times long stretches of the sea off the west coast of Africa turn a milky white, then an iridescent shade of jade green. Any marine creature that can do so crawls onto land to escape the water which has become laced with hydrosulfuric acid. Hydrogen sulfide gas – heavier than air, foul, and poisonous – bubbles out of the sea. The air for miles around fills with a noxious odor. Around 225 million years ago, during the largest extinction event in Earth’s history, this was the condition of all of the world’s oceans.

Atlantic Ocean off the coast of Africa from space

A recent aerial photograph of the iridescent ocean off the west coast of Africa.

This deadly chemical change, caused by the depletion of most of the oxygen in the sea water, lead to the “end-Permian” mass extinction event, sometimes referred to as “The Great Dying.” Oceans absorbed huge quantities of carbon dioxide from the atmosphere triggering a world-wide bloom of anaerobic, sulfur-producing organisms. It was more cataclysmic even than the later extinction of the dinosaurs.

The Great Dying is one of five mass extinctions since the “Cambrian Explosion” of about 500 million years ago. Extinctions great and small happened before the Cambrian period, but conditions on Earth then were so different that they’re not relevant here. By contrast, the five mass extinctions since then – when at least 75% of all species on land and in the seas disappeared – are the subject of active research.

A far-reaching extinction event is in progress right now. The Sixth Extinction, by Elizabeth Kolbert, places the present extinction in line with the previous five. It’s an account of the author’s personal witness to this loss, of the studies undertaken to understand its causes, and of some of the attempts being made to save some species.

While saving a species in captivity prevents its annihilation, it takes an extraordinary effort to return it to the wild. California condors have been brought back from the brink, but before being released they need to be inoculated against bird flu, trained not to fly into power lines, and are then often treated for lead poisoning from swallowed gun pellets. Similar efforts will not be possible for the more than 1,200 other bird species in danger of extinction. For some animals, “extinct in the wild” means extinct, period.

Darwin believed that extinction is a gradual process. In contrast, Kolbert recounts how the early French paleontologist Georges Cuvier understood that extinction could be sudden and catastrophic. Cuvier’s insight into the nature of the enormous fossils that were turning up led him to theorize that the world was once dominated by giant reptiles, and that these creatures were destroyed by a flood of biblical proportions. There was a catastrophe: 66 million years ago, when the dinosaurs died within a few years of the impact of an asteroid the size of Mt. Everest, in Chicxulub, near the Yucatan peninsula.

But the asteroid didn’t wipe out the dinosaurs – they lived on the other side of the world to the Yucatan. The lethal climate following its impact killed them all, along with almost anything else that lived. Asteroid impacts were not the cause of the other mass extinctions but climate change appears always to have been involved. Although not instantaneous like a collision with an asteroid, the Earth’s climate changed in all the other mass extinctions many times faster than any species’ ability to adapt.

Mass extinctions do create opportunities for new species to develop, but the recovery of biodiversity takes millions of years. As a gruesome analogy, think of a terrorist bomb planted at the finish line of a marathon. Depending on the timing of the explosion, the runners bringing up the rear might be unharmed, and the best runners wiped out.

The present extinction has these same random and rapid properties, but it’s unique in that it’s caused entirely by the actions of a single species – humans. This “Holocene Extinction” is named for the present geological epoch that began after the last Ice Age ended around 12,000 years ago. The impact that Homo sapiens’ proliferation has had on the natural world has been rapid and profound. Of the planet’s 50 million square miles of ice-free land, ½ has been converted to agriculture, industry, and housing. We have altered the composition of the atmosphere, and consequently altered the Earth’s climate in ways that are yet to be determined.

The Most Deadly Predator

The hallmark of the Holocene extinction is the vanishing of flourishing species: the passenger pigeon, the American bison, and some species of whale, to name but a few. Kolbert describes the extinction of the Great Auk, which was once so numerous that they were even burned for firewood. At one time this bird was hunted for its feathers used to adorn clothing, and, it turns out many were not killed outright, but plucked and left to die of exposure! Some species today are killed for similarly obtuse reasons: the rhinoceros has been thriving for 15 million years, but will certainly be poached to extinction in the wild because its horn is erroneously believed to be an aphrodisiac.

Archaic humans grew in numbers and expanded their territory in the same manner as the Earth’s other large species. Neanderthals lived in Europe for more than 100,000 years and had no more impact on the landscape than any big mammal. But modern humans and megafauna have never been in balance together; Neanderthals may well have been the first species eliminated by Homo sapiens. Our newly evolved cognitive abilities tipped the scales of competition fatally in our favor, both for prey species and for other humans.

Megafauna, such as the wooly mammoth and saber tooth tiger, disappeared in every part of the world soon after Homo sapiens arrived.  There is some evidence of wholesale slaughter, but wanton overkills were not necessarily what finished the mammoths. Studies have found that a small band of hunters taking just one adult megabeast per year will clean a continent of that species in a couple of thousand years. Early humans had no way of knowing this: the megafauna were too large to be prey; their low birth rates were a liability once human hunters arrived.

You Don’t Know What You’ve Got ‘Til it’s Gone

Current Global Forest Distribution map

Current Global Forest Distribution in 2009 (courtesy of World Resources Institute).

The Brazilian government in 1979 set aside tracts of land in rainforests that were slated to be cleared for agriculture. This ongoing program has provided naturalists with a wealth of information, including how species respond to shrinking habitats. The pattern of habitat loss is important to this understanding – roads, power lines and the like create an enclosure effect even in preserves. Some species –  plant as well as animal – move easily across gaps in the landscape, while others have evolved to remain bounded in their immediate habitat.

All Earth’s species descend from those that survived numerous ice ages and the rapid warming periods that followed. They survived by migrating to new areas similar to the climate and ecology in which they evolved. Today some plants and animals are already relocating in response to climate change, but for many species the globe may be warming too fast – it’s happening faster than any previously known warming – and the habitat may already be too fragmented to accommodate similar migrations. As Kolbert puts it, “One of the defining features of the Anthropocene is that the world is changing in ways that compel species to move, (but) … prevents them from doing so.”

The Body Snatchers

While loss of habitat is the greatest single cause of the reduction in biodiversity, loss due to competition by invasive species is not far behind. Invaders arrive without their predators, giving them an enormous advantage over native species in the struggle for resources.  Every year more non-native mammals, birds, amphibians, and so forth arrive in the USA than there are native species. In the last five years, white nose syndrome, a fungal infection from Europe has killed 95% of the bats in the eastern US. Since bats devour massive numbers of agricultural insect pests, the infestation is expected to drive up food prices.

Ten years ago the golden frog was the signature species of the Panamanian rainforest.  Today it probably exists only in captivity, a victim of another fungus known as Bd that attacks the frog’s skin, basically suffocating it. There are 7,000 species of amphibians world-wide, mostly frogs, and it’s estimated that 30% of all amphibians are infected with the Bd fungus. They were already stressed by habitat loss and pollution, and with the added stress from Bd, amphibians have become the most endangered class of animal, dying at perhaps 45,000 times their average rate.

Since the Bd fungus lives in the soil it infects animals that are released to the wild, so how can rescued amphibians ever be returned to their habitat? The fungus is spread by people transporting types of frogs that had developed immunity: the African Clawed Frog, for example, is a host for Bd but is itself immune. It was widely used for decades in a test for human pregnancy. Still, how the fungus is in so many remote and formerly pristine locations, like the Panama rainforest, is a mystery.

Humans have connected the biology of even the remotest places to everyplace else. A count taken during a single summer in Antarctica revealed that tourists and researchers inadvertently brought with them more than 70,000 seeds!

In Hot Water

We have increased the amount of CO2 in the atmosphere to levels higher than at any time since humans first evolved, greater than any time in the last 15 million years. This will likely increase average global temperatures by 2°C to 4°C, enough to melt the Arctic ice cap and inundate coastal areas in the ensuing rise in sea level. Climate change will intensify all of the existing threats to wildlife and encourage predation.

Additionally, the change in climate presents its own unique dangers. Coral reefs are under heavy and increasing pressure from a variety of sources. Reefs are the marine equivalent of tropical rainforests, supporting 25% of marine species. Today it’s difficult to find an undamaged reef in Indonesia or the Philippines, for example, where it’s common for the locals to fish using dynamite. “Coral Bleaching,” spurred by higher water temperature, has contributed to a 50% decline in the Great Barrier Reef in the last 30 years, and by 80% for reefs in the Caribbean.

Acidification is another peril for reefs and to clams, oysters, starfish, types of seaweed and plankton.  Oceans absorb 30% of the CO2 produced by human activities. This absorption is slowing global warming but it is making seawater 30% more acidic than it was 200 years ago. One can already observe the consequences of acidification not only, as mentioned before, in the ocean off the west coast of Africa but at a natural source near an island in the Mediterranean which emits carbon dioxide into the surrounding sea. An underwater vent turns the water gradually more and more acidic the closer one approaches. When the pH drops to 7.8, a tipping point is reached at which the population of native crustaceans drops by 1/3, and the shells of those that survive are thin and almost transparent. Kolbert states that if the current rate of CO2 absorption continues, all of the world’s reefs will be dead or dying in less than 50 years. Remember, ocean acidification was likely the coup-de-grâce of the end-Permian or “The Great Dying” event, which was the largest mass extinction in Earth’s history; but oceans today are acidifying even faster than they did then.

“Hope is the Thing with Feathers” – Emily Dickinson

On one hand, a young man shot the first breeding whooping crane released back into the wild. On the other, volunteers in ultralight aircraft (even Vladimir Putin) help cranes bred in captivity find their way to their breeding sites. What kind of species are we, anyway?

The Holocene extinction, which may expand to include us, is not a natural event. There is no massive outburst of gases from gigantic volcanoes poisoning the air, nor has the Earth’s orbit shifted so that its surface is covered in ice, both of which have happened. Humans are the agents of this event, and we can make it stop. We have the ability to at least mitigate the effects of what we have done. Many people are working to do so, and more people need to be convinced that they must cooperate. We have to change the way we deal with nature, or nature will surely change the way it deals with us.

Appendix – The Magnitude of the Holocene Extinction

The most widely accepted counts of endangered species are produced by the International Union for the Conservation of Nature. The annual IUCN Red List provides seven categories of threat, from recently Extinct down to Least Concern. Species are sorted into eight higher taxonomic groups (classifications) for an overview of the current state of the natural world.

Endangered species chart

Data from the IUCN Red List of 2007. Bars indicate the percentage of the total number of species in each group in a category of risk. A conservative approach is taken to listing a species; still, the number of species in each risk category has increased significantly in the years since 2007.

The threat to mammals most captures our attention. There are about 5,500 species of mammals, including ourselves. Recent estimates list 165 as critically endangered and another 275 as endangered. In official terms, “critically endangered” equates to “possibly already extinct in the wild,” and “endangered” means “expected to soon become extinct in the wild.” These two categories contain such familiar animals as two humped-camels, mountain gorillas, giant pandas and chimpanzees, the latter likely to be gone from the wild in 10 years. Add in another 900 or so species listed as vulnerable (“likely to become endangered”), and fully ¼ of the mammals on Earth are threatened.

But what if the Holocene extinction is more far reaching? At what point for our species does this change from tragedy to peril? It might not take many species lost, compared to the extinctions of the past. 90% of wheat varieties have no resistance to a new type of rust that is now spreading from African tropics into Asia. Wheat supplies 30% of humanity’s calories, a kind of dependence completely unknown to our evolutionary ancestors. Human civilization is more fragile than the human species. Along with crows, cockroaches, and coyotes, humans will likely survive any mass extinction, but, unless many more of us start doing something now, they may spend their days picking through rubble.

About the Author: Elizabeth Kolbert has been a staff writer for the New Yorker since 1999. Prior to that she was a reporter for the New York Times. She received the American Association for the Advancement of Science’s magazine writing award for the New Yorker series on which this book is based. The Sixth Extinction: An Unnatural History draws on the work of scores of researchers in half a dozen disciplines–geologists who study deep ocean cores, botanists who follow the tree line as it climbs up the Andes, and marine biologists who dive off the Great Barrier Reef. The author accompanies many of these researchers into the field, and introduces the reader to a dozen species–some already gone, others facing extinction.

It's OK to Be Smart Video on the 6th extinction

The Sixth Extinction

It’s Okay to Be Smart

This time, we’re the asteroid. A history of mass extinctions.

Video lecture from Elizabeth Kolbert

Watch: The Sixth Extinction: Elizabeth Kolbert

Elizabeth Kolbert, Heinz Award-winning staff writer for the New Yorker and author of Field Notes from a Catastrophe: Man, Nature, and Climate Change (2006), discusses her book The Sixth Extinction at OSU.

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