The Sixth Extinction: humanity busy sawing off the limb on which it perches

Though it might be nice to imagine there once was a time when man lived in harmony with nature, it’s not clear that he ever really did.

Elizabeth Kolbert is a journalist who writes on science matters for the New Yorker. She has written two books, the first being Field Notes from a Catastrophe: A Frontline Report on Climate Change. I’ve just read her most recent book, The Sixth Extinction, which is only partly concerned with climate change: travelling across the world to report on the latest of the mass extinctions that have occurred on Earth in the last half billion years, she reveals how this sixth and most devastating extinction is all down to human impact – but climate change is only a part of it.

Atelopus zeteki, the Panamanian Golden Frog

Kolbert begins her journey across the planet in search of the evidence for the latest mass extinction in central Panama, home to Atelopus zeteki, the Panamanian Golden Frog, once so numerous hereabouts that, she writes:

One creek not far from El Valle was nicknamed Thousand Frog Stream. A person walking along it would see so many golden frogs sunning themselves on the banks that, as one herpetologist who made the trip many times put it to me, “it was insane—absolutely insane.”

Then the frogs started to disappear. And the Golden Frogs were not the only ones: in the last two decades, across the world, amphibians of all kinds have begun to disappear.  Not only rare and highly specialized species are vanishing; so, too, are much more familiar ones. What makes the situation so mystifying and alarming is the geography: frogs are vanishing everywhere – from Australia to North America – and not only from populated and disturbed areas but also from relatively pristine places, like the mountains of Central America.

At first, scientists were sceptical of this amphibian apocalypse. As Kolbert writes:

Amphibians are, after all, among the planet’s great survivors. The ancestors of today’s frogs crawled out of the water some 400 million years ago, and by 250 million years ago the earliest representatives of what would become the modern amphibian orders – one includes frogs and toads, the second newts and salamanders, and the third weird limbless creatures called caecilians – had evolved. This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs.

So why are amphibians rapidly disappearing from the planet?  In one sense the answer is: fungus. Wherever frogs are dying, a strange micro-organism has been found on the animals’ skin, a fungus that interferes with their ability to take up critical electrolytes through their skin, causing them to suffer what is, in effect, a heart attack.  But in another, crucial sense, there is a man-made reason why frogs are dying.

The infection has spread rapidly across the globe, through the highlands of South America and down the eastern coast of Australia, and across into New Zealand and Tasmania. It has raced through the Caribbean and has been detected in Italy, Spain, Switzerland, and France. In North America, it has radiated out across the continent in a series of ripples. At this point, the fungus appears to be unstoppable, leaping across oceans and mountain ranges, devastating quite different species of amphibians.  The causal link is human:

Without being loaded by someone onto a boat or a plane, it would have been impossible for a frog carrying the fungus to get from Africa to Australia or from North America to Europe. This sort of intercontinental reshuffling, which nowadays we find totally unremarkable, is probably unprecedented in the three-and-a-half-billion-year history of life.

The sudden collapse of an order of animals that has survived for 400 million years is just one part of what is now accepted by scientists as the steady unfolding of the sixth extinction. Over the last half-billion years, there have been five mass extinctions, when the diversity of life on earth suddenly and dramatically contracted. The sixth extinction is predicted to be the most devastating extinction event since the asteroid impact that wiped out the dinosaurs. This time, however, the cataclysm is us.

Having caused her readers’ hairs to stand on end with the story of the frog catastrophe, Kolbert backtracks to fill in some of the history of extinction as a scientific  idea. While it’s now accepted that there have been five great mass extinctions during the history of life on this planet, that’s not always been the case.  Until the 19th century, the general belief was of a natural order on Earth unchanging  since the time of God’s creation.  Today, however, as Kolbert observes, ‘extinction may be the first scientific idea that kids have to grapple with’ – given toy dinosaurs to play with, toddlers will soon understand that were once lots of kinds of dinosaurs  and that they all died off long ago. Kolbert traces how the idea of extinction – of ‘a world previous to ours’ – slowly gained acceptance among scientists in the 19th century, steadily reinforced by evidence from rocks and fossils, and the theories of men such as  Georges Cuvier, Jean-Baptiste Lamarck, and Charles Darwin.

Today, it is accepted that there have been five mass extinctions on earth – events that led to ‘a profound loss of biodiversity’. The first took place during the late Ordovician period, some 450 million years ago, when living things were still mainly confined to the water. The most devastating – an event sometimes referred to as ‘the mother of mass extinctions’ or ‘the great dying’ – took place at the end of the Permian period, some 250 million years ago, and it came perilously close to emptying the earth out altogether.

The most recent – and best-known – mass extinction came at the close of the Cretaceous period, when an asteroid about the size of Manhattan ploughed into the Earth at 45,000 mph in the Yucután Peninsula, Mexico . The asteroid sent a vast cloud of scalding vapour and pulverised rock high into the atmosphere, killing almost everything unprotected by soil, rock or deep water. About three-quarters of animals, including the dinosaurs were wiped out as a result. It took millions of years for life to recover.

Today, Kolbert writes, we are witnessing a similar mass extinction event happening in a time frame that amounts to a geologic blink of an eye. She refers to the assessment of the biologist E O Wilson that the present extinction rate in the tropics is ‘on the order of 10,000 times greater than the naturally occurring background extinction rate (the average extinction rates in the evolutionary time scale of planet Earth)’ and will reduce biological diversity to its lowest level since the last great extinction.  However, some groups are becoming extinct much faster, notably the amphibians.

Humans are affecting the Earth’s systems in many ways, and have been doing so continually for decades, indeed centuries. The sixth extinction may seem like a long drawn-out affair to us, but compared with many natural processes, it is virtually instantaneous. Kolbert tells us how scientists have now agreed that the millennia during which humans have inhabited the earth deserves to be classed as a new geological era -the Anthropocene, one during which we have used our skills to wreak havoc, despite the fact that our achievements will be reduced to a future sedimentary layer the thickness of a cigarette paper.

In her prologue, Kolbert presents a concise summary of the changes wrought by humans in our own Anthropocene era:

The members of the species are not particularly swift or strong or fertile. They are, however, singularly resourceful. Gradually they push into regions with different climates, different predators, and different prey. None of the usual constraints of habitat or geography seem to check them. They cross rivers, plateaus, mountain ranges. In coastal regions, they gather shellfish; farther inland, they hunt mammals. Everywhere they settle, they adapt and innovate. On reaching Europe, they encounter creatures very much like themselves, but stockier and probably brawnier, who have been living on the continent far longer. They interbreed with these creatures and then, by one means or another, kill them off.

The end of this affair will turn out to be exemplary. As the species expands its range, it crosses paths with animals twice, ten, and even twenty times its size: huge cats, towering bears, turtles as big as elephants, sloths that stand fifteen feet tall. These species are more powerful and often fiercer. But they are slow to breed and are wiped out.

Although a land animal, our species—ever inventive—crosses the sea. It reaches islands inhabited by evolution’s outliers: birds that lay foot-long eggs, pig-sized hippos, giant skinks. Accustomed to isolation, these creatures are ill-equipped to deal with the newcomers or their fellow travelers (mostly rats). Many of them, too, succumb.

The process continues, in fits and starts, for thousands of years, until the species, no longer so new, has spread to practically every corner of the globe. At this point, several things happen more or less at once that allow Homo sapiens, as it has come to call itself, to reproduce at an unprecedented rate. In a single century the population doubles; then it doubles again, and then again. Vast forests are razed. Humans do this deliberately, in order to feed themselves. Less deliberately, they shift organisms from one continent to another, reassembling the biosphere.

Meanwhile, an even stranger and more radical transformation is under way. Having discovered subterranean reserves of energy, humans begin to change the composition of the atmosphere. This, in turn, alters the climate and the chemistry of the oceans. Some plants and animals adjust by moving. They climb mountains and migrate toward the poles. But a great many—at first hundreds, then thousands, and finally perhaps millions—find themselves marooned. Extinction rates soar, and the texture of life changes.

No creature has ever altered life on the planet in this way before, and yet other, comparable events have occurred. Very, very occasionally in the distant past, the planet has undergone change so wrenching that the diversity of life has plummeted. Five of these ancient events were catastrophic enough that they’re put in their own category: the so-called Big Five. In what seems like a fantastic coincidence, but is probably no coincidence at all, the history of these events is recovered just as people come to realize that they are causing another one. When it is still too early to say whether it will reach the proportions of the Big Five, it becomes known as the Sixth Extinction.

Why does this matter? Because, we are altering environmental conditions on our planet so swiftly and dramatically that a large proportion of other species cannot adapt. And because, too, we are risking our own future, by fundamentally altering the integrity of the climate balance that has persisted in more or less the same configuration since the end of the last ice age, and which has fostered the flourishing of human civilization.

The extinct great auk

One of the case studies of human depredation advanced by Kolbert is that of the great auk. During its brief acquaintance with humans – from the 16th century to the mid 19th – the auk, the original ‘penguin’, was so numerous that large flocks thronged entire outcrops of Iceland and Newfoundland. Flightless birds, like the dodos, they were ready for the taking, providing meat and mattress stuffing. ‘You do not give yourself the trouble of killing them,’ reported an English sailor, ‘but lay hold of one and pluck the best… You then turn the poor penguin adrift, with his skin half naked and torn off, to perish at his leisure.’  The last hapless auk perished one June evening in 1844, strangled by Icelandic hunters. It joined a long line of animals driven to extinction by man, a line that is getting longer every day. By the latest estimation, says Kolbert, one third of reef corals, one third of freshwater molluscs, one third of sharks and rays, a fifth of all reptiles, a quarter of all mammals and a sixth of all birds will go the way of the auk this century.

Kolbert places her account of the demise of the great auk in the context of the debates between Catastrophists and Uniformitarians. Catastrophists, such as Georges Cuvier, the first naturalist to argue that animals had gone extinct, believed that extinction resulted from sudden geological changes in the past. Uniformitarians, by contrast, typified by the geologist Charles Lyell, argued for continuous, gradual change persisting into the present. Strongly influenced by Lyell, Charles Darwin thought that extinctions were a prolonged, almost undetectable process. Kolbert notes that Darwin was reluctant to accept the role of human agency in recent extinctions, holding to his opinion even when the extinction of the great auk was announced nine years after he had returned from the Beagle voyage.

Kolbert tells how it was not until the 1980s that the idea of species turnover as an extended, continuous process was overturned as the dominant paradigm. Then came the evidence from physicist Luis Alvarez and his geologist son, Walter, that 65 million years ago a 6-mile-wide asteroid struck the Yucatán Peninsula. The impact, they claimed, led to a catastrophic extinction that wiped out the dinosaurs and a large proportion of other species. At first paleontologists derided the Alvarezes’ findings, but they gradually changed their minds as the geological evidence accumulated.

But now there is a new paradigm. The sixth mass extinction that Kolbert chronicles in her book is explained by a more recent paradigm, one in which human action – not evolution over vast stretches of time or catastrophic natural events – has pushed species over the brink. In a key chapter, she explains the species-area relationship, a mathematical key to understanding what humans are doing to the world.  As species become confined to shrinking areas – say by the expansion of farmland or suburbs – a proportion of the species that once inhabited the terrain will be lost – roughly ten percent over a period of time.

But add into the equation global warming and, where species are trapped, surrounded by corridors of human activity they cannot cross, and calculations suggest that if warming reaches the point considered a  likely maximum, between 38% and 52%  of species could be fated to disappear by the middle of this century. Kolbert expresses the horror of this figure in the words of Anthony Barnosky, a paleontologist at the University of California-Berkeley:

Look around you.  Kill half of what you see. Or, if you’re feeling generous, just kill about a quarter of what you see.  That’s what we could talking about.

This is what we did to our closest relations, the Neanderthals. Whether by predation or simply by driving them into more difficult terrain, successful human expansion  imposed a sustained downward pressure on the number of breeding adults in Neanderthal populations.  And, writes Kolbert, the process continues:

And the same holds true for our next-closest kin, which is why, with the exception of humans, all the great apes today are facing oblivion. The number of chimpanzees in the wild has dropped to perhaps half of what it was fifty years ago, and the number of mountain gorillas has followed a similar trajectory. Lowland gorillas have declined even faster; it’s estimated the population has shrunk by sixty percent just in the last two decades. Causes of the crash include poaching, disease, and habitat loss; the last of these has been exacerbated by several wars, which have pushed waves of refugees into the gorillas’ limited range. Sumatran orangutans are classified as “critically endangered,” meaning they’re at ”extremely high risk of extinction in the wild.” In this case, the threat is more peace than violence; most of the remaining orangutans live in the province of Aceh, where a recent end to decades of political unrest has led to a surge in logging, both legal and not. One of the many unintended consequences of the Anthropocene has been the pruning of our own family tree. Having cut down our sister species – the Neanderthals – many generations ago, we’re now working on our first and second cousins. By the time we’re done, it’s quite possible that there will be among the great apes not a single representative left, except,that is, for us.

Kolbert writes of her visit to the Grotte des Combarelles in the Dordogne, a cave first entered by humans twelve or thitreen thousand years ago, where the walls are covered by hundreds of engravings.  All the images are of animals, most of them now extinct: mammoths, aurochs, woolly rhinos:

It is often speculated that the humans who sketched on the walls of the Grotte des Combarelles thought their images had magical powers, and in a way they were right. The Neanderthals lived in Europe for more than a hundred thousand years and during that period they had no more impact on their surroundings than any other large vertebrate. There is every reason to believe that if hum.ans had not arrived on the scene, the Neanderthals would be there still, along with the wild horses and the woolly rhinos. With the capacity to represent the world in signs and symbols comes the capacity to change it, which, as it happens, is also
the capacity to destroy it. A tiny set of genetic variations divides us from the Neanderthals, but that has made all the difference.

Towards the end, Kolbert writes:

Right now we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will forever be closed. No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy.

Reviewing The Sixth Extinction for the London Review of Books, Luke Mitchell wrote:

Palaeontologists have found Neanderthal bones everywhere from Israel to Wales, and agree that the species died out suddenly, about thirty thousand years ago, which is suspiciously close to the time that Homo sapiens began its expansion from Africa. One theory is that clever man simply murdered his stronger cousin. But there are other theories. Maybe we simply out-hunted our cousins, or carried a disease that was novel to them. Or maybe our contribution to their demise was even more indirect; animals with a long reproductive cycle are vulnerable to even the slightest of disruptions. John Alroy, an American palaeobiologist, has run computer simulations that suggest it would take just a tiny bit of interference with the Neanderthal birth rate, over the course of a few thousand years, to drive it to extinction. Alroy called this a ‘geologically instantaneous ecological catastrophe too gradual to be perceived by the people who unleashed it’. Such imperception is no longer possible.

In another example of the species-area relationship at work, since the onset of the Industrial Revolution, argues Kolbert, human beings have changed the composition of the atmosphere by burning subterranean sources of energy and releasing carbon dioxide. Some plants and animals adjust by moving to more hospitable terrain, but many, she says, find themselves marooned and extinction rates soar.

Her most urgent warning is about the condition of the oceans. Since the beginning of the Industrial Revolution, carbon dioxide absorbed by the sea has increased by 30 per cent. The result of this is to prevent calcifiers  – animals from corals to bivalves, and even some plants – from forming their structures, with disastrous effects for the marine food chain. The acidification of the oceans means that all coral reefs – which support up to nine million other species – will have dissolved within 50 years.

Elizabeth Kolbert in the Hall of Biodiversity at the American Museum of Natural History

Towards the end of her book, Elizabeth Kolbert describes how the Hall of Biodiversity at the American Museum of Natural History is arranged around a central plaque that notes there have been five major extinction events since complex animals evolved over five hundred million years ago. According to the plaque, ‘Global climate change and other causes, probably including collisions between earth and extraterrestrial objects, were responsible for these events. Right now we are in the midst of the Sixth Extinction, this time caused solely by humanity’s transformation of the ecological landscape.’

Kolbert continues:

Radiating out from the plaque are sheets of heavy-duty Plexiglas, and beneath the sheets the fossilized remains of a handful of exemplary casualties. The Plexiglas has been scuffed by the shoes of the tens of thousands of museum visitors who have walked across it, probably for the most part oblivious of what’s beneath their feet. But crouch down and look closely and you can see that each of the fossils is labelled with the name of the species as well as the extinction event that brought its lineage to an end. The fossils are arranged in chronological order, so that the oldest- graptolites from the Ordovician – are close to the centre, while the youngest – Tyrannosaurus rex teeth from the late Cretaceous – are farther away. If you stand at the edge of the exhibit, which is really the only place from which to view it, you are positioned right where the victims of the Sixth Extinction should go.

In an extinction event of our own making, what happens to us? One possibility-the possibility implied by the Hall of Biodiversity-is that we, too, will eventually be undone by our ‘transformation of the ecological landscape’. The logic behind this way of thinking runs as follows: having freed ourselves from the constraints of evolution, humans nevertheless remain dependent on the earth’s biological and geochemical systems. By disrupting these systems – cutting down tropical rainforests, altering the composition of the. atmosphere, acidifying the oceans – we’re putting our own survival in danger. Among the many lessons that emerge from the geologic record, perhaps the most sobering is that in life, as in mutuals, past performance is no guarantee of future results. When a mass extinction occurs it takes out the weak, but also lays low the strong.

Kolbert concludes with the words of ecologist Paul Ehrlich:

In pushing other species to extinction, humanity is busy sawing off the limb on which it perches.

See also

• What killed the Neanderthals? Luke Mitchell reviews The Sixth Extinction (LRB)
• The Sixth Extinction review (Guardian)
• The Sixth Extinction: A Conversation With Elizabeth Kolbert (National Georgraphic)
• The Sixth Extinction: article by Elizabeth Kolbert (New Yorker, 2009)
• More articles by Elizabeth Kolbert for New Yorker

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