Predictions of a Man-Made Sixth Mass Extinction May Be Exaggerated
Urbanization, forest, and agricultural trends point in a more hopeful direction
"The Sixth Great Extinction Is Underway—and We're to Blame," Time's headline declared. "Earth in the midst of sixth mass extinction," echoed USA Today. The headline-writers at The Mirror adopted an even more apocalyptic tone: "End of the world predicted by scientists who warn Earth's animals are heading for mass extinction."
They were all reacting to a study in Science reporting researchers' concerns about "mass defaunation." (Defaunation is a fancy way to say that human beings are killing off lots of animals.) The authors are comparing their estimates of future species loss to the five prior mass extinctions of the past 540 million years, in each of which around 75 percent of all then-living species died off. The most famous extinction episode—likely triggered by an asteroid crashing into the earth—killed off the dinosaurs 65 million years ago. This time, instead of a killer asteroid, there's us.
The research team, led by the Stanford biologist Rodolfo Dirzo, reviewed the scientific literature on species abundance and extinction. They report that 322 species of vertebrates (birds, mammals, fish, reptiles, and other animals with backbones) have gone extinct since 1500. In addition, the populations of the remaining vertebrates have dropped an average of 25 percent over the past 500 years. For invertebrates—animals without backbones, such as insects and mollusks—the trends for the species we monitor are even worse, with population declines averaging 45 percent. "Such animal declines will cascade onto ecosystem functioning and human well-being," Dirzo and his colleages warn. The ongoing defaunation, they conclude, is "both a pervasive component of the planet's sixth mass extinction and also a major driver of global ecological change."
Why are so many wildlife species in danger of going extinct? Overexploitation, habitat destruction, invasive species, and now man-made climate disruption, comes the answer. Based on a conservative estimate of 5 to 9 million animal species on the planet, the scientists cite studies that suggest the world is "likely losing 11,000 to 58,000 species annually." That would mean we're losing at least one animal species per hour. At the higher rate, something like 40 percent of all animal species will be gone by 2050.
To slow and hopefully halt the wave of extinctions, they continue, requires the "mitigation of animal overexploitation and land-use change." Put more simply, they want us to stop eating so many wild animals and stop cutting down forests. They also offer a bit of somewhat gratuitous socioeconomic advice when they assert that "human population growth and increasingly uneven per capita consumption…ultimately drive all these threats," concluding that we therefore need "reduced and more evenly distributed global resources consumption."
This is not the first time biologists have sounded the alarm over allegedly accelerated extinctions. In 1970, S. Dillon Ripley, secretary of the Smithsonian Institution, predicted that in 25 years, somewhere between 75 and 80 percent of all the species of living animals would be extinct. That is, between 75 and 80 percent of all species of animals that were alive in 1970 would be extinct by 1995. In 1975, the biologists Paul and Anne Ehrlich wrote that "since more than nine-tenths of the original tropical rainforests will be removed in most areas within the next 30 years or so, it is expected that half of the organisms in these areas will vanish with it." Nearly 40 years later, nowhere near 90 percent of the rainforests have been cut and no one thinks that half of the species inhabiting tropical forests have vanished.
In his 1979 book The Sinking Ark, the Oxford biologist Norman Myers stated that 40,000 species per year were going extinct and that 1 million species would be gone by the year 2000. At a symposium at Brigham Young University that year, Thomas Lovejoy—who would go on to draw up the first projections of worldwide extinction rates for the Global 2000 Report to the President—predicted between 15 and 20 percent of all species alive in 1980 would be extinct by the end of the century. That was his "conservative" estimate. No one believes that extinctions of this magnitude have occurred over the last three decades.
Make no mistake about it: Human beings have been pretty good at killing off other species. As the last major ice age was ending, our hunter-gatherer ancestors spread across the world killing off megafaunal populations already stressed by climate change. By one reckoning, 178 mammal species that weigh more than 100 pounds disappeared, drastically reducing the total mammalian biomass of the planet. After humans arrived in North America, more than 30 different groups of large mammals disappeared, including horses, camels, mammoths, and mastodons. In South America, we lost 100 percent of mammals weighing more than a ton, including ground sloths, armadillo-like glyptodonts and rhinoceros-like toxodons. Eighty percent of those weighing more than 100 pounds went extinct. A 2013 study estimates that Polynesian wayfarers killed off 1,300 species of bird as they colonized the isolated islands of the Pacific Ocean. The arrival of Europeans eliminated an additional 40 Pacific bird species.
But are we really heading for the mass die-off that Dirzo's team predicts?
Oddly, when Dirzo and company calculated their projects, they didn't use the more recent estimate of species extinction rates, which Science published in May. That group of researchers, led by the Duke biologist Stuart Pimm, concluded that current extinction rates "are about 1000 times the likely background rate of extinction." The background rate is what the rate of extinction would be without human influence; Pimm's team reckons that it's about 0.1 species per million species-years. This means that if one followed the fates of one million species, one would expect to observe about one species going extinct every 10 years. If the rate really is a thousand times higher, that would mean that 100 species are going extinct per million species years. Let's assume that the world contains about 5,000,000 species, which would suggest that 500 are going extinct every year. Still bad, but far less scary than the 11,000 to 58,000 in Dirzo's forecast.
Meanwhile, some countervailing trends suggest the future of animal species may not be so grim. These trends include urbanization, slowing deforestation, the global increase in protected areas, and the advent of peak farmland.
Transforming hundreds of millions of hardscrabble subsistence farmers scattered across the landscape into urban dwellers would greatly reduce the pressure on wildlife and their habitats. The good news is that that process is happening. According to the United Nations Population Division's World Urbanization Prospects 2014 report, 54 percent of people live in cities—up from 30 percent in 1950. The U.N. demographers forecast that proportion will rise to 66 percent by 2050. If world population follows the medium fertility trend, rising to 9.5 billion by 2050, that means that 3.2 billion people will still live in rural areas, down slightly from 3.4 billion today. In that scenario, population pressure will not be pushing the expansion of the human frontier into more wildlands. And if population growth traverses along the U.N.'s low fertility trend, the year 2050 will see human numbers peaking at 8.3 billion people. If 66 percent live in cities, only 2.8 billion people will be living on the landscape.
Looking further into the future, the U.N. expects that 80 percent of people will be urbanites by 2100. Medium and low fertility population projections suggest that would mean that only 2.2 to 1.3 billion rural dwellers.
Since most species live in forests, chiefly tropical forests, we should take a look at global forest cover trends. Happily, the deforestation rate is slowing. The Food and Agriculture Organization's State of the World's Forests 2012 report notes that the global rate of deforestation slowed from 0.2 percent per year between 1990 and 2000 to 0.14 percent between 2005 and 2010. Between 2000 and 2010, a total of 130 million hectares were cut, but 78 million hectares returned to forests. So globally, forests declined on average by 5.2 million hectares per year—at which rate, the report notes, "It will take 775 years to lose all of the world's forests." It adds, "This would seem to provide enough time for actions to slow or stop global deforestation." And indeed, researchers in 2006 found that more and more countries are passing through a "forest transition" in which their forest area starts expanding. Roger Sedjo, a forest ecologist at Resources for the Future, predicts that by 2050 most of the world's industrial wood will be grown on forest plantations covering only 5 to 10 percent of the extent of today's global forests.
One dark blot on forest restoration trends is biofuel mandates in rich countries, which have spurred tropical countries to chop down forests to grow palm oil to produce biodiesel. By one estimate, 87 percent of the deforestation in Malaysia and 118 percent in Indonesia occurred as result of expanding palm oil plantations.
Another beneficial trend is that protected areas such as parks and marine preserves are expanding at a remarkably fast pace. The World Bank notes that protected areas have nearly doubled from 8.5 percent of the world's total land area in 1990 to 14.3 percent in 2012. That's an area twice the size of the entire United States. Marine protected areas have increased from 4.7 percent of territorial waters in 1990 to 10 percent in 2012. Under the Convention on Biological Diversity, governments of the world have committed to protecting 17 percent of terrestrial and inland water areas and 10 per cent of coastal and marine areas by 2020.
Considering that agriculture is the most expansive and intensive way in which people transform natural landscapes, the really good news is that the amount of land globally devoted to food production may be falling as population growth slows and agricultural productivity increases. "We believe that projecting conservative values for population, affluence, consumers, and technology shows humanity peaking in the use of farmland," concludes Jesse Ausubel, the director of the Human Environment Program at the Rockefeller University. In a 2013 article titled "Peak Farmland and the Prospect for Land Sparing," Ausubel and his colleagues write: "Global arable land and permanent crops spanned 1,371 million hectares in 1961 and 1,533 million hectares in 2009, and we project a return to 1,385 million hectares in 2060."
As a result of these trends, humanity will likely restore at least 146 million hectares of land, an area two and a half times that of France, or ten Iowas—and possibly much more. Relaxing those biofuel mandates would spare an additional 256 million hectares from the plow, the researchers estimate. That mean nearly 400 million hectares—an area nearly double the size of the United States east of the Mississippi River—could be restored to nature by 2060.
The late 20th century's predictions of imminent mass extinction happily proved wrong. The positive trends cited above provide good grounds to believe that the new ones will also turn out to be exaggerated.