In short, the problem of determining the number of modern species that have become extinct seems at first to be simple and to lead to modest estimates—for example, only five or six extinct bird species in all of North America plus Europe. On reflection, though, we appreciate two reasons why published lists of species known to be extinct must be gross underestimates of the actual numbers involved. Firstly, by definition the published lists consider only named species, whereas the great majority of species (except in well-studied groups like birds) have not even been named. Secondly, outside North America and Europe and except for birds, the published lists consist only of those few named species which some biologist happened to get interested in for one or another reason and found to be extinct. Among all those remaining species of unknown status, many are likely to be extinct or nearly so—for example, about half in the case of Malaysian freshwater fishes.
Now let's move on to the second step in evaluating the mass extinction debate. Our estimates up to this point have concerned only those species exterminated since 1600 AD, when scientific classification of species was beginning. These exterminations have taken place because the world's human population has grown in numbers, reached previously uninhabited areas, and invented increasingly destructive technologies. Did these factors spring up suddenly in 1600, after several million years of human history? Were there no exterminations before 1600? Of course not. Until fifty thousand years ago, humans were confined to Africa plus the warmer areas of Europe and Asia. Between then and 1600 AD our species underwent a massive geographic expansion that took us to Australia and New Guinea around 50,000 years ago, Siberia around 20,000 years ago, most of North and South America around 11,000 years ago, and most of the world's remote oceanic islands only since 2000 BC. We also underwent a massive expansion in numbers, from perhaps a few million people 50,000 years ago to about half a billion in 1600. Our destructiveness also increased, with the development of improved hunting skills in the last 50,000 years (Chapter Two), polished stone tools and agriculture in the last 10,000 years (Chapters Ten and Fourteen), and metal tools in the last 6,000 years.
In every area of the world that paleontologists have studied and that humans first reached within the last 50,000 years, human arrival approximately coincided with massive instances of prehistoric species' extinction waves. For Madagascar, New Zealand, Polynesia, and the Americas I have described those instances in the preceding two chapters. After people reached Australia, that continent lost its giant kangaroos, its 'marsupial lion', and other giant marsupials. Around the time that Indians reached North America 11,000 years ago, it lost lions, cheetahs, native wild horses, mammoths, mastodonts, giant ground sloths, and several dozen other large mammals. Mediterranean islands like Crete and Cyprus lost dwarf elephants and pygmy hippos, while Madagascar lost giant lemurs and flightless elephant birds. New Zealand lost its giant flightless moas, and Hawaii its flightless geese and dozens of smaller birds, when the Polynesians arrived around 1000 and 500 AD, respectively.
Ever since scientists became aware of these prehistoric extinction waves associated with human arrival, they have argued over whether people were the cause or just happened to arrive while animals were succumbing to climate changes. In the case of the extinction waves on Polynesian islands, there is now no reasonable doubt that Polynesian arrival in one way or another caused them. Bird extinction waves and Polynesian arrival coincided within a few centuries at a time when no big climate change was happening, and bones of thousands of roasted moas have been found in Polynesian ovens. The coincidence of timing is equally convincing for Madagascar. But the causes of the earlier extinction waves, especially those in Australia and the Americas, are still being debated. As I explained on America's extinction waves in Chapter Eighteen, the evidence seems to me overwhelming that humans also played a role in those prehistoric cases of extinction outside Polynesia and Madagascar. In each part of the world an extinction wave occurred after the first arrival of humans, but did not occur simultaneously in other areas undergoing similar climate swings, and did not occur in the same area whenever si rnilar climate swings had occurred previously.
Hence I doubt that climate did it. Instead, all of you who have visited Antarctica or the Galapagos Islands know how tame are the animals there, being unaccustomed to humans until recently. Photographers can still walk up to those naive animals as easily as hunters used to. I assume that the first arriving hunters similarly walked up to naive mammoths and moas elsewhere in the world, while rats that came with the first hunters walked up to naive little birds of Hawaii and other islands.
It is not just in those areas of the world previously unoccupied by humans that prehistoric humans probably exterminated species. Within the last 20,000 years species also became extinct in the areas long occupied by humans—in Eurasia, woolly rhinos, mammoths, and giant deer ('Irish elk') died out, and Africa lost its giant buffalo, giant hartebeest, and giant horse. These big beasts may also have been among the victims of prehistoric humans who had already been hunting them for a long time, but who now were able to hunt them with better weapons than ever before. Eurasia's and Africa's big mammals were not unused to humans, but they disappeared for the same two simple reasons that California's
Can we at least estimate how many species were involved in these prehistoric extinction waves? No one has ever tried to guess the number of plants, invertebrates, and lizards exterminated by prehistoric habitat destruction, but virtually all oceanic islands explored by paleontologists have yielded remains of recently extinct bird species. Extrapolation to those islands not yet paleontologically explored suggests that about 2,000 bird species—one fifth of all the birds that existed a few thousand years ago—were island species already exterminated prehistorically. That does not include birds that may have been exterminated prehistorically on the continents. Among genera of large mammals, about seventy-three, eighty, and eighty-six per cent respectively became extinct in North America, South America, and Australia at the time of or after human arrival. The remaining step in evaluating the mass extinction debate is to predict the futufe. Is the peak of the extinction wave that we have caused already past, or is most still to come? There are a couple of ways to assess this question.
A simple way is to reason that tomorrow's extinct species will be drawn from today's endangered species. How many species that still exist have populations already reduced to dangerously low levels? The ICBP estimates that at least 1,666 bird species are either endangered or at imminent risk of extinction—almost twenty per cent of the world's surviving birds. I said 'at least 1,666 , because this number is an underestimate for the same reason I mentioned that the ICBP's estimate of extinct species was an underestimate. Both numbers are based just on those species whose status caught a scientist's attention, rather than on a reappraisal of the status of all bird species.
The alternative way of predicting what is to come is to understand the mechanisms by which we exterminate species. Extinction of species caused by humans may continue accelerating until human population and technology reach a plateau, but neither shows any signs of plateauing. Our population, which grew ten fold from half a billion in 1600 to over five billion now, is still growing at close to two per cent per year. Every day brings new technological advances for changing the earth and its denizens. There are four main mechanisms by which our growing population exterminates species: by overhunting, species introductions, habitat destruction, and ripple effects. Let's see if these four mechanisms have plateaued.
Overhunting—killing animals faster than they can breed—is the main mechanism by which we have exterminated big animals, from mammoths to California grizzly bears. (The latter appears on the flag of California, the state in which I live, but many of my fellow Californians do not recall that we exterminated our state's symbol long ago.) Have we already killed off all big animals that we might kill off? Obviously not. While the low numbers of whales led to an international ban on whaling for commercial reasons, Japan thereupon announced its decision to triple the rate at which it kills whales 'for scientific reasons'. We have all seen photos of the accelerating slaughter of Africa's elephants and rhinos, for their ivory and horns respectively. At current rates of change, not just elephants and rhinos but most populations of most other large mammals of Africa and Southeast Asia will be extinct outside game parks and zoos in a decade or two. The second mechanism by which we exterminate is through intentionally or accidentally introducing certain species to parts of the world where they did not previously occur. Familiar examples of introduced species now firmly established in the US are Norway rats, European starlings, boll weevils, and the fungi causing Dutch elm disease and chestnut blight. Europe too has acquired introduced species, of which the misnamed Norway rat is an example (it originated in
Asia, not Norway). When species are introduced from one region to another, they often proceed to exterminate some of the new species they encounter, by eating them or causing diseases. The victims evolved in the absence of the introduced Pests and never developed defences against them. American chestnut trees have already been virtually exterminated in this way by chestnut b'ght, an Asian fungus to which Asian chestnut trees are resistant. milarly, goats and rats have exterminated many plants and birds on oceanic islands.
