The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

Ocean acidification is sometimes referred to as global warming’s “equally evil twin.”

No single mechanism explains all the mass extinctions in the record, and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous). There’s strong evidence for ocean acidification during an extinction event known as the Toarcian Turnover, which occurred 183 million years ago, in the early Jurassic, and similar evidence at the end of the Paleocene, 55 million years ago, when several forms of marine life suffered a major crisis.

Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to begin with. To extend the construction metaphor, imagine trying to build a house while someone keeps stealing your bricks. The more acidified the water, the greater the energy that’s required to complete the necessary steps. At a certain point, the water becomes positively corrosive and solid calcium carbonate begins to dissolve. This is why the limpets that wander too close to the vents at Castello Aragonese end up with holes in their shells.

Continuing along this path for much longer, the pair continued, “is likely to leave a legacy of the Anthropocene as one of the most notable, if not cataclysmic events in the history of our planet.”

Researchers now believe it won’t last out the Anthropocene. “It is likely that reefs will be the first major ecosystem in the modern era to become ecologically extinct” is how a trio of British scientists recently put

Reefs are often compared to rainforests, and in terms of the sheer variety of life, the comparison is apt.

Tropical waters tend to be low in nutrients, like nitrogen and phosphorus, which are crucial to most forms of life. (This has to do with what’s called the thermal structure of the water column, and it’s why tropical waters are often so beautifully clear.) As a consequence, the seas in the tropics should be barren—the aqueous equivalent of deserts. Reefs are thus not just underwater rainforests; they are rainforests in a marine Sahara.

Reefs—or, really, reef creatures—have developed a fantastically efficient system by which nutrients are passed from one class of organisms to another, as at a giant bazaar. Corals are the main players in this complex system of exchange, and, at the same time, they provide the platform that makes the trading possible. Without them, there’s just more watery desert.

Ocean acidification is, of course, not the only threat reefs are under. Indeed, in some parts of the world, reefs probably will not last long enough for ocean acidification to finish them off.

All of these stresses make corals susceptible to pathogens. White-band disease is a bacterial infection that, as the name suggests, produces a band of white necrotic tissue. It afflicts two species of Caribbean coral, Acropora palmata (commonly known as elkhorn coral) and Acropora cervicornis (staghorn coral), which until recently were the dominant reef builders in the region. The disease has so ravaged the two species that both are now listed as “critically endangered” by the International Union for Conservation of Nature. Meanwhile coral cover in the Caribbean has in recent decades declined by close to eighty percent.

Tropical reefs need warmth, but when water temperatures rise too high, trouble ensues.

But global warming is going to have just as great an impact—indeed, according to Silman, an even greater impact—in the tropics. The reasons for this are somewhat more complicated, but they start with the fact that the tropics are where most species actually live.

One thousand and thirty-five tree species have been counted there, roughly fifty times as many as in all of Canada’s boreal forest.

As a general rule, the variety of life is most impoverished at the poles and richest at low latitudes.

“This is a new species,” he said. Along the trail, Silman and his students have found thirty species of trees new to science. (Just this grove of discoveries represents half again as many species as in Canada’s boreal forest.) And there are another three hundred species that they suspect may be new, but that have yet to be formally classified. What’s more, they’ve discovered an entirely new genus.

ANY species (or group of species) that can’t cope with some variation in temperatures is not a species (or group) whose fate we need be concerned about right now, because it no longer exists. Everywhere on the surface of the earth temperatures fluctuate. They fluctuate from day to night and from season to season.

Even after the idea of ice ages was generally accepted—it was first proposed in the eighteen-thirties by Louis Agassiz, a protégé of Cuvier—no one could explain how such an astonishing process could take place.

Climate change alone “is unlikely to generate a mass extinction as large as one of the Big Five,” he wrote. However, there’s a “high likelihood that climate change on its own could generate a level of extinction on par with, or exceeding, the slightly ‘lesser’ extinction events” of the past.

By the official count, there are something like thirteen hundred species of birds in the Amazon, but Cohn-Haft thinks there are actually a good many more, because people have relied too much on features like size and plumage and not paid enough attention to sound. Birds that might look more or less identical but produce different calls often turn out, he told me, to be genetically distinct.

But if a lack of suitable habitat were the only issue, land-bridge islands should pretty quickly stabilize at a new, lower level of diversity. Yet they don’t. They keep on bleeding species—

Small populations, of course, aren’t confined to islands. A pond may have a small population of frogs, a meadow a small population of voles. And in the ordinary course of events, local extinctions occur all the time. But when such an extinction follows from a run of bad luck, the site is likely to be recolonized by members of other, more fortunate populations wandering in from somewhere else. What distinguishes islands—and explains the phenomenon of relaxation—is that recolonization is so difficult, in many cases, effectively impossible. (While a land-bridge island may support a small remnant population of, say, tigers, if that population winks out, new tigers presumably aren’t going to paddle over.)

Yet another possible explanation for why observations don’t match predictions is that humans aren’t very observant. Since the majority of species in the tropics are insects and other invertebrates, so, too, are the majority of anticipated extinctions. But as we don’t know, even to the nearest million, how many tropical insect species there are, we’re not likely to notice if one or two or even ten thousand of them have vanished.

One of the defining features of the Anthropocene is that the world is changing in ways that compel species to move, and another is that it’s changing in ways that create barriers—roads, clear-cuts, cities—that prevent them from doing so.

The bat die-off continued the following winter, spreading to five more states. It continued the winter after that, in three additional states, and, although in many places there are hardly any bats left to kill off, it continues to this day. The white powder is now known to be a cold-loving fungus—what’s known as a psychrophile—that was accidentally imported to the U.S., probably from Europe. When it was first isolated, the fungus, from the genus Geomyces, had no name. For its effect on the bats it was dubbed Geomyces destructans

WITHOUT human help, long-distance travel is for most species difficult, bordering on impossible.

By the time I learned about what was going on, white-nose syndrome, as it had become known, had spread as far as West Virginia and had killed something like a million bats.

Roy van Driesche, an expert on invasive species at the University of Massachusetts, has estimated that out of every hundred potential introductions, somewhere between five and fifteen will succeed in establishing themselves. Of these five to fifteen, one will turn out to be the “bullet in the chamber.”

This sort of it-takes-an-invasive-to-catch-an-invasive strategy has a decidedly mixed record. In some cases it’s proven highly successful; in other it’s turned out to be another ecological disaster.

So thorough has been the devastation wrought by the brown tree snake that it has practically run out of native animals to consume; nowadays it feeds mostly on other interlopers, like the curious skink, a lizard also introduced to Guam from Papua New Guinea. The author David Quammen cautions that while it is easy to demonize the brown tree snake, the animal is not evil; it’s just amoral and in the wrong place. What Boiga irregularis has done in Guam, he observes, “is precisely what Homo sapiens has done all over the planet: succeeded extravagantly at the expense of other species.”

Even before the cause of white-nose syndrome was identified, Al Hicks and his colleagues suspected an introduced species. Whatever was killing the bats was presumably something they’d never encountered before, since the mortality rate was so high.

(Plantains—Plantago major—seem to have arrived with the very first white settlers and were such a reliable sign of their presence that the Native Americans referred to them as “white men’s footsteps.”

According to a study of specimens in Massachusetts herbaria, nearly a third of all plant species documented in the state are “naturalized newcomers.”

From the standpoint of the world’s biota, global travel represents a radically new phenomenon and, at the same time, a replay of the very old. The drifting apart of the continents that Wegener deduced from the fossil record is now being reversed—another way in which humans are running geologic history backward and at high speed. Think of it as a souped-up version of plate tectonics, minus the plates. By transporting Asian species to North America, and North American species to Australia, and Australian species to Africa, and European species to Antarctica, we are, in effect, reassembling the world into one enormous supercontinent—what biologists sometimes refer to as the New Pangaea.

the science writer Alan Burdick has called Homo sapiens “arguably the most successful invader in biological history”

Still today, Americans often deliberately import “foreign varieties” they think “might prove useful or interesting.”

It attributed the accelerating pace to the increased quantities of goods being transported and also to the increased speed with which they travel. The Center for Invasive Species Research, which is based at the University of California-Riverside, estimates that California is now acquiring a new invasive species every sixty days. This is slow compared to Hawaii, where a new invader is added each month. (For comparison’s sake, it’s worth noting that before humans settled Hawaii, new species seem to have succeeded in establishing themselves on the archipelago roughly once every ten thousand years.)

But Hawaii was, in its prehuman days, home to thousands of species that existed nowhere else on the planet, and many of these endemics are now gone or disappearing.

Bats’ sociability has turned out to be a great boon to Geomyces destructans. In winter, when they cluster, infected bats transfer the fungus to uninfected ones. Those that make it until spring then disperse, carrying the fungus with them. In this way, Geomyces destructans passes from bat to bat and cave to cave.

The total came to 112. This was about a thirtieth of the bats that used to be counted in the hall in a typical year.

She noted that lucis reproduce very slowly—females produce only one pup per year—so even if some bats ultimately prove resistant to white-nose, it was hard to see how populations could rebound.

Not long ago, I called Scott Darling to get an update. He told me that the little brown bat, once pretty much ubiquitous in Vermont, is now officially listed as an endangered species in the state. So, too, are northern long-eared and tricolored bats. “I frequently use the word ‘desperate,’” he said. “We are in a desperate situation.”

Genetic analysis has shown that the Sumatran is the closest living relative of the woolly rhino, which, during the last ice age, ranged from Scotland to South Korea. E. O. Wilson, who once spent an evening at the Cincinnati Zoo with Suci’s mother and keeps a tuft of her hair on his desk, has described the Sumatran rhino as a “living fossil.”

Since the mid–nineteen-eighties, the number of Sumatran rhinos in the wild has declined precipitously, to the point that there are now believed to be fewer than a hundred left in the world. In an ironic twist, humans have brought the species so low that it seems only heroic human efforts can save it.

(In fact, rhinos are most closely related to horses.)

Very big animals are, of course, very big for a reason. Already at birth, Suci weighed seventy pounds. Had she been born on Sumatra, at that point she could have fallen victim to a tiger (though nowadays Sumatran tigers, too, are critically endangered). But probably she would have been protected by her mother, and adult rhinos have no natural predators. The same goes for other so-called megaherbivores; full-grown elephants and hippos are so large that no animal dares attack them. Bears and big cats are similarly beyond predation.

If climate change drove the megafauna extinct, then this presents yet another reason to worry about what we are doing to global temperatures. If, on the other hand, people were to blame—and it seems increasingly likely that they were—then the import is almost more disturbing. It would mean that the current extinction event began all the way back in the middle of the last ice age. It would mean that man was a killer—to use the term of art an “overkiller”—pretty much right from the start.

The megafauna extinction, it’s now clear, did not take place all at once, as Lyell and Wallace believed it had. Rather, it occurred in pulses.

It’s hard to see how such a sequence could be squared with a single climate change event. The sequence of the pulses and the sequence of human settlement, meanwhile, line up almost exactly.

Even clearer is the evidence from New Zealand. When the Maori reached New Zealand, around the time of Dante, they found nine species of moa living on the North and South Islands. By the time European settlers arrived, in the early eighteen hundreds, not a single moa was to be seen. What remained were huge middens of moa bones, as well as the ruins of large outdoor ovens—leftovers of great, big bird barbecues. A recent study concluded that the moas were probably eliminated in a matter of decades.

Like the V-shaped graptolites or the ammonites or the dinosaurs, the megafauna weren’t doing anything wrong; it’s just that when humans appeared, “the rules of the survival game” changed.