The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

define mass extinctions as events that eliminate a “significant proportion of the world’s biota in a geologically insignificant amount of time.”

Probably, one amphibian species should go extinct every thousand years or so. That species could be from Africa or from Asia or from Australia. In other words, the odds of an individual’s witnessing such an event should be effectively zero.

Today, amphibians enjoy the dubious distinction of being the world’s most endangered class of animals; it’s been calculated that the group’s extinction rate could be as much as forty-five thousand times higher than the background rate. But extinction rates among many other groups are approaching amphibian levels. It is estimated that one-third of all reef-building corals, a third of all freshwater mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion.

etiology

The cause

amplexus

Frog sex

Extinction finally emerged as a concept, probably not coincidentally, in revolutionary France. It did so largely thanks to one animal, the creature now called the American mastodon, or Mammut americanum, and one man, the naturalist Jean-Léopold-Nicolas-Frédéric Cuvier, known after a dead brother simply as Georges.

Darwin’s theory about how species originated doubled as a theory of how they vanished. Extinction and evolution were to each other the warp and weft of life’s fabric, or, if you prefer, two sides of the same coin. “The appearance of new forms and the disappearance of old forms” were, Darwin wrote, “bound together.” Driving both was the “struggle for existence,” which rewarded the fit and eliminated the less so.

lacuna

Gap

Whatever the explanation, the contrasting fate of the two groups raises a key point. Everything (and everyone) alive today is descended from an organism that somehow survived the impact. But it does not follow from this that they (or we) are any better adapted. In times of extreme stress, the whole concept of fitness, at least in a Darwinian sense, loses its meaning: how could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history?

giant sea scorpions, which, as best as can be determined, looked like something out of a nightmare).

The current theory is that the end-Ordovician extinction was caused by glaciation. For most of the period, a so-called greenhouse climate prevailed—carbon dioxide levels in the air were high and so, too, were sea levels and temperatures. But right around the time of the first pulse of extinction—the one that wreaked havoc among the graptolites—CO2 levels dropped. Temperatures fell and Gondwana froze.

The oceans’ chemistry changed, too; among other things, colder water holds more oxygen.

Human activity has transformed between a third and a half of the land surface of the planet. • Most of the world’s major rivers have been dammed or diverted. • Fertilizer plants produce more nitrogen than is fixed naturally by all terrestrial ecosystems. • Fisheries remove more than a third of the primary production of the oceans’ coastal waters. • Humans use more than half of the world’s readily accessible fresh water runoff. Most

As head of the Anthropocene Working Group, Zalasiewicz is hoping to bring a proposal to a vote by the full body in 2016. If he’s successful and the Anthropocene is adopted as a new epoch, every geology textbook in the world immediately will become obsolete.

Like many small places, Castello Aragonese is a product of very large forces, in this case the northward drift of Africa, which every year brings Tripoli an inch or so closer to Rome. Along a complicated set of folds, the African plate is pressing into Eurasia, the way a sheet of metal might be forced into a furnace. Occasionally, this process results in violent volcanic eruptions. (One such eruption, in 1302, led the entire population of Ischia to take refuge on Castello Aragonese.) On a more regular basis, it sends streams of gas bubbling out of vents in the sea floor. This gas, as it happens, is almost a hundred percent carbon dioxide.

Every day, every American in effect pumps seven pounds of carbon into the sea.

What is this calculated frlom? Just averagde?

Thanks to all this extra CO2, the pH of the oceans’ surface waters has already dropped, from an average of around 8.2 to an average of around 8.1. Like the Richter scale, the pH scale is logarithmic, so even such a small numerical difference represents a very large real-world change. A decline of .1 means that the oceans are now thirty percent more acidic than they were in 1800.

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. “Oh, ocean acidification,” Zalasiewicz had told me at Dob’s Linn. “That’s the big nasty one that’s coming down.”

ROUGHLY one-third of the CO2 that humans have so far pumped into the air has been absorbed by the oceans. This comes to a stunning 150 billion metric tons. As with most aspects of the Anthropocene, though, it’s not only the scale of the transfer but also the speed that’s significant.

Prior to the industrial revolution, all of the world’s major reefs could be found in water with an aragonite saturation state between four and five. Today, there’s almost no place left on the planet where the saturation state is above four, and if current emissions trends continue, by 2060 there will be no regions left above 3.5. By 2100, none will remain above three.

Tropical reefs need warmth, but when water temperatures rise too high, trouble ensues. The reasons for this have to do with the fact that reef-building corals lead double lives. Each individual polyp is an animal and, at the same time, a host for microscopic plants known as zooxanthellae. The zooxanthellae produce carbohydrates, via photosynthesis, and the polyps harvest these carbohydrates, much as farmers harvest corn. Once water temperatures rise past a certain point—that temperature varies by location and also by species—the symbiotic relation between the corals and their tenants breaks down. The zooxanthellae begin to produce dangerous concentrations of oxygen radicals, and the polyps respond, desperately and often self-defeatingly, by expelling them. Without the zooxanthellae, which are the source of their fantastic colors, the corals appear to turn white—this is the phenomenon that’s become known as “coral bleaching.” Bleached colonies stop growing and, if the damage is severe enough, die.

LIKE the Jews, the corals of the Great Barrier Reef observe a lunar calendar.

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.

Canada’s boreal forest is huge; it stretches across almost a billion acres and represents roughly a quarter of all the intact forest that remains on earth. But diversity in the boreal forest is low. Across Canada’s billion acres of it, you will find only about twenty species of tree, including black spruce, white birch, and balsam fir.

At around thirteen degrees south latitude, it passes to the west of Silman’s tree plots. In his plots, which collectively have an area roughly the size of Manhattan’s Fort Tryon Park, the diversity is staggering. One thousand and thirty-five tree species have been counted there, roughly fifty times as many as in all of Canada’s boreal forest. And what holds for the trees also holds for birds and butterflies and frogs and fungi and just about any other group you can think of (though not, interestingly enough, for aphids). As a general rule, the variety of life is most impoverished at the poles and richest at low latitudes. This pattern is referred to in the scientific literature as the “latitudinal diversity gradient,”

insuperable

Impossible to overcome

Very roughly speaking, he found that global warming was driving the average genus up the mountain at a rate of eight feet per year.

(One member of the group, Schefflera arboricola, from Taiwan, commonly known as the dwarf umbrella tree, is often grown as a houseplant.) Trees in Schefflera, Feeley found, were practically hyperactive; they were racing up the ridge at the astonishing rate of nearly a hundred feet a year. On the opposite extreme were trees in the genus Ilex. These have alternate leaves that are usually glossy, with spiky or serrated edges. (The genus includes Ilex aquifolium, which is native to Europe and known to Americans as Christmas holly.) The trees in Ilex were like kids who spend recess sprawled out on a bench. While Schefflera was sprinting upslope, Ilex was just sitting there, more or less inert.

estivate

Dormant for a season

Wood storks cool off by defecating on their own legs. (In very hot weather, wood storks may excrete on their legs as often as once a minute.)

It is now generally believed that ice ages are initiated by small changes in the earth’s orbit, caused by, among other things, the gravitational tug of Jupiter and Saturn. These changes alter the distribution of sunlight across different latitudes at different times of year. When the amount of light hitting the far northern latitudes in summer approaches a minimum, snow begins to build up there. This initiates a feedback cycle that causes atmospheric carbon dioxide levels to drop. Temperatures fall, which leads more ice to build up, and so on. After a while, the orbital cycle enters a new phase, and the feedback loop begins to run in reverse. The ice starts to melt, global CO2 levels rise, and the ice melts back farther.

The correlation between the number of species and the size of the area is not linear. Rather, it’s a curve that slopes in a predictable way. Usually, the relationship is expressed by the formula S = cAz, where S is the number of species, A is the size of the area, and c and z are constants that vary according to the region and taxonomic group under consideration (and hence are not really constants in the usual sense of the term). The relationship counts as a rule because the ratio holds no matter what the terrain.

recent estimates suggest there are at least two million tropical insect species and perhaps as many as seven million. By comparison, there are only about ten thousand species of birds in the entire world and only fifty-five hundred species of mammals.

ordure

Dung

“geologically instantaneous ecological catastrophe too gradual to be perceived by the people who unleashed it.”

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.

Even now, at least thirty thousand years after the fact, the signal is discernible: all non-Africans, from the New Guineans to the French to the Han Chinese, carry somewhere between one and four percent Neanderthal DNA.

They vaccinated every single condor—today there are about four hundred—against West Nile virus,

a disease, it’s worth noting, for which a human vaccine has yet to be developed.

chelation

Chemical metal compound containing a ligand

It doesn’t much matter whether people care or don’t care. What matters is that people change the world.

IN PUSHING OTHER SPECIES TO EXTINCTION, HUMANITY IS BUSY SAWING OFF THE LIMB ON WHICH IT PERCHES.