Fact or fiction?: The sixth massextinction can be stopped

A giant tortoise named "Lonesome George" is seen in the Galapagos islands, an archipelago off Ecuador's Pacific coast.Lonesome George, the late reptile prince of the Galapagos Islands, may be dead, but scientists now say he may not bethe last giant tortoise of his species after all. AP Photo/Galapagos National Park, File

The most famous mass extinction came from space, but the biggest might have beenbecause of carbon dioxide. Cataclysms, whether the asteroid that ended the dinosaurs’reign or the volcanism that may have caused the Great Dying, drove the first five massextinctions in Earth’s history, in which 75 percent or more of the planet’s life died out. Thesixth mass extinction may now be beginning — and the apocalypse this time is us.

During the last several centuries we have burned through eons worth of fossilizedsunshine, changing the climate for our fellow species. We use more than half of theplanet’s unfrozen land for cities, logging or food, eliminating the habitats of our fellowanimals and plants. Before we even achieved civilization, we had already helped hunt thebiggest, fiercest animals — woolly mammoths, giant kangaroos and giant sloths — toextinction.

Biologists and paleoecologists estimate that humans have driven roughly 1,000 speciesextinct in our 200,000 years on the planet. Since 1500, we have killed off at least 322 typesof animals, including the passenger pigeon, the Tasmanian tiger and, most recently, the

By David Biello, Scientific American on 09.21.14 Word Count 899

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baiji, a freshwater dolphin in China. Another 20,000 or more species are now threatenedwith extinction, according to the International Union for the Conservation of Nature, whichkeeps a list of all the known endangered plants and animals on the planet. The populationof any given animal among the five million or so species on the planet is, on average, 28percent smaller, thanks to humans. And as many as one third of all animals are eitherthreatened or endangered, a new study in Science finds.

In the jargon it's an "Anthropocene defaunation," or sixth mass extinction, and one causedby humans. Scientists can't be sure of the current die-off rate, perhaps because much of itis happening to beetles and other insects that are notoriously overlooked. But according tothat new study in Science, the total number of such invertebrates fell by half over the past35 years while the human population doubled. Other recent studies suggest that thecurrent extinction rate is roughly 1,000 times faster than the average pace in Earth'shistory. That makes this the fastest extinction event on record, even if it is not yet a massdie-off.

The biggest, fiercest animals still left on the planet — elephants, tigers, whales, amongothers — are most at risk. And we humans have shown no inclination to stop the activities— overexploitation for food, habitat destruction and others — that drive extinction.

And yet it's not too late. In the past few decades humans brought the black-footed ferretback from just seven individuals; vaccinated and hand-reared condors to relativeabundance; and battled to preserve and restore populations of hellbender salamanders, toname just a few in just North America alone. According to another new analysis in Science,people have physically moved 424 species of plants and animals to protect them fromextinction.

For such assisted migration efforts to succeed, careful attention must be paid both togenetics and habitat. There is no point in bringing back the baiji, for example, if theYangtze River remains polluted and overfished. But conservation efforts can work. Fishescan rebound when fishing pressure is removed, just as Maine haddock and Washingtonstate coho salmon both have. The reforesting of the U.S. eastern seaboard shows thatwhen farms go away, woodlands return, and coyotes, deer, turkey and other wildlife moveback in. The animals and plants of the Amazon rain forest have benefited from Brazil'sefforts to curb deforestation. And in what might prove an enduring lesson in conservation,paleoecologists have shown that 20 out of 21 large mammals in India — from leopards tomuntjac deer — have survived there for the past 100,000 years alongside one of thelargest human populations on the planet.

To avoid the sixth mass extinction we will probably have to employ more aggressiveconservation, such as moving species to help them cope with a changing climate. Thinkre-wilding: reintroducing species like wolves or beavers that were once present in a givenecosystem but have since disappeared. Aggressive conservation might also mean killingoff newcomer species to preserve or make room for local flora and fauna; in New Zealand,rat extirpations have helped kakapos survive.

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In the most extreme case aggressive conservation could involve bringing in new animalsto fill the role of animals that have gone extinct. For example, European sailors ate theirway through the Indian Ocean islands of Mauritius, killing off the dodo and the localtortoise species. But closely related tortoises from the neighboring Seychelles archipelagohave been imported recently, and they have helped restore the island ecosystem,including bringing back the endangered local ebony trees. As a result of that success,similar projects are being considered from Caribbean islands to Madagascar. There iseven some hope of bringing back entirely extinct species in the future using the new toolsof synthetic biology. (De-extinction or even ecological replacement could cause some ofthe same problems as invasive species, so careful management is required.)

But we are not doomed to cause a sixth mass extinction, at least not yet, despiteconsuming our way through the world's remaining big wild animals. Based on an estimatepublished in Nature in 2011, we have a century or two at present rates before ourdepredations assure a mass extinction. Unlike an asteroid, we could choose to changecourse.

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