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Our species is now at the pinnacle of its numbers, its geographic extent, its power, and the fraction of the Earth's productivity that it commands. That is the good news. The bad news is that we are also in the process of reversing all that progress much more rapidly than we created it. Our power threatens our own existence. We do not know whether we shall suddenly blow ourselves up before we would otherwise expire in a slow stew caused by global warming, pollution, habitat destruction, more mouths to feed, less food to feed those mouths, and extermination of other species that form our resource base. Are these dangers really new ones that arose since the Industrial Revolution, as widely assumed? It is a common belief that species in a state of nature live in balance with each other and with their environment. Predators do not exterminate their prey, nor do herbivores overgraze their plants. According to this view, humans are the unique misfit. If this were true, Nature would hold no lessons for us.

There is something to this view, insofar as species do not go extinct under natural conditions as rapidly as we are exterminating them now, except under rare circumstances. Such a rare event was the mass fatality sixty-five million years ago, possibly due to an asteroidal impact, that finished the dinosaurs. Since evolutionary multiplications of species are very slow, natural extinctions obviously must also be slow, otherwise we would have been left with no species long ago. Expressed alternatively, the vulnerable species get eliminated quickly, and what we see persisting in Nature are the robust combinations of species.

That broad conclusion still leaves us with many instructive examples of species exterminating other species. Almost all known cases prove to combine two elements. Firstly, the cases involve species reaching environments where they did not occur before, and where they encounter prey populations that are naive about the threat of those invading predators. By the time that the ecological dust settles and a new equilibrium is reached, some of the new-found prey may have been exterminated. Secondly, the perpetrators of such exterminations prove to be so-called switching predators, which are not specialized to eat only a single prey species but can feed on many different ones. Although the predator exterminates some prey, it survives by switching to others.

Such exterminations often occur when humans intentionally or accidentally transfer a species from one part of the globe to another. Rats, cats, goats, pigs, ants, and even snakes are among these transferred killers. For instance, during the Second World War a tree snake native to the Solomon Islands was accidentally transported on ships or planes to the previously snake-free Pacific island of Guam. This predator has already exterminated or brought to the brink of extinction most of Guam's native forest bird species, which had had no opportunity to evolve behavioural defences against snakes. Yet the snake is in no danger itself despite having virtually eliminated its bird prey, because it can switch to bats, rats, lizards, and other victims. As another example, cats and foxes introduced into Australia by humans have been eating their way through Australia's small native marsupials and rats without endangering themselves, because there remain abundant rabbits and other prey species on which to feed.

We humans furnish the prime example of a switching predator. We eat everything from snails and seaweed to whales, mushrooms, and strawberries. We can overharvest some species to the point of extinction, and then just switch to other food. A wave of extinctions has ensued every time that humans have reached a previously unoccupied part of the globe. The dodo, whose name has become synonymous with extinction, formerly lived on the island of Mauritius, half of whose land and freshwater bird species became extinct following the island's discovery in 1507. Dodos in particular were big, edible, flightless, and easily caught by hungry sailors. Hawaiian bird species similarly died out en masse following Hawaii's discovery by Polynesians 1,500 years ago, as did America's large mammal species after ancestral Indians arrived 11,000 years ago. Extinction waves have also accompanied major improvements of hunting technology in lands long occupied by humans. For example, wild populations of the Arabian oryx, a beautiful antelope of the Near East, survived one million years of human hunting, only to succumb to high-powered rifles in 1972.

Thus, there are numerous animal precedents for our propensity to exterminate individual prey species but to sustain ourselves by switching to others. Is there any precedent for an animal population destroying its entire resource base and eating its way into extinction? This outcome is uncommon, because animal numbers are regulated by many factors that tend automatically to lower birth rates or increase death rates when the animal is numerous, and vice versa when it is rare. For example, mortality due to external factors like predators, diseases, parasites, and starvation tends to increase at high population densities. Responses of the animal itself to high densities also contribute, such as infanticide, postponed breeding, and increased aggression. These responses and external factors generally reduce the animal's population and relieve its pressure on its resources before they can be exhausted.

Nevertheless, some animal populations actually have eaten themselves into extinction. One example involves the progeny of twenty-nine reindeer that were introduced in 1944 to St Matthew Island in the Bering Sea. By 1957 they had multiplied nearly fifty-fold to 1,350, by the year 1963 another four-fold to 6,000. But reindeer depend for food on slow-growing lichens, which on St Matthew had no chance to recover from reindeer grazing, since the animals had nowhere to migrate. When a harsh winter struck in 1963-64, all the animals except forty-one females and one sterile male starved to death, leaving a doomed population on an island littered with thousands of skeletons. A similar example was the introduction of rabbits to Lisianski Island west of Hawaii in the first decade of this century. Within a decade the rabbits had eaten themselves into oblivion by consuming every plant on the island except two morning glories and a tobacco patch. These and other similar examples of ecological suicide all involve populations that suddenly became free of the usual factors regulating their numbers. Rabbits and reindeer are normally subject to predators, and reindeer on continents use migration as a safety valve to leave an area and allow its vegetation to recover. But Lisianski and St Matthew Islands lacked predators, and emigration was impossible, so that the animals bred and ate unchecked. On reflection, it is clear that the entire human species has been equally successful in recently escaping from the former controls on our numbers. We eliminated predation on ourselves long ago; twentieth-century medicine has greatly reduced our mortality from infectious disease; and some of our leading behavioural techniques of population control, such as infanticide, chronic war, and sexual abstinence, have become socially unacceptable. Our population is now doubling about every thirty-five years. Granted, that is not as fast as the St Matthew reindeer, and Island Earth is bigger than St Matthew Island, and some of our resources are more elastic than lichens (though other resources, like oil, are less elastic). Yet the qualitative conclusion remains the same: no population can grow indefinitely.

Thus, our present ecological predicament has familiar animal precursors. Like many switching predators, we exterminate some prey species when we colonize a new environment or acquire new destructive power. Like some animal populations that suddenly escaped their former limits on growth, we risk destroying ourselves by destroying our resource base. What about the view that we were in a state of relative ecological equilibrium until the Industrial Revolution, and that only since then have we begun seriously to exterminate species and overexploit our environment? That Rousseau-esque fantasy will be taken up in the three chapters of Part Five. Chapter Seventeen examines the widespread belief in a former Golden Age, when we supposedly lived as noble savages practising a conservation ethic and in harmony with Nature. In reality, mass extinctions have coincided with each major extension of human lebensraum during the last ten thousand years and possibly much longer. Our direct responsibility for the extinctions is clearest in the case of the most recent expansions, where the evidence is still freshEuropeans' expansion over the globe since 1492, and the earlier colonization of oceanic islands by Polynesians and the Malagasy. Older expansions such as the first human occupation of the Americas and Australia were also accompanied by mass extinctions, though the trail of evidence has had much more time to fade and so conclusions about cause and effect are necessarily weaker. It is not just the case that the Golden Age was blackened by mass extinctions. While no large human population has eaten itself out of existence, some populations on small islands have done so, and many large populations have damaged their resources to the point of economic collapse. The clearest examples come from isolated cultures, such as the collapses of Easter Island and Anasazi civilizations. But environmental factors also drove the major shifts in Western civilization, including the successive collapses of the Middle Eastern, then Greek, then Roman hegemonies. Self-destructive abuse of our environment, far from being a modern invention, has long been a prime mover of human history. Chapter Eighteen looks more closely at the biggest, most dramatic, and most controversial of these

'Golden Age mass extinctions'. Around 11,000 years ago most of the large mammals of two entire continents, North America and South America, became extinct. Around the same time appears the first unequivocal evidence for human occupation of the Americas, by the ancestors of American Indians. It was the biggest expansion of human territory since Homo erectus spread out of Africa to colonize Europe and Asia a million years ago. The temporal coincidence between the first Americans and the last big American mammals, the lack of mass extinctions elsewhere in the world at that same time, and proofs that some of the now-extinct beasts were hunted have suggested what is termed the New World blitzkrieg hypothesis. According to this interpretation, as the first wave of human hunters multiplied and spread from Canada to Patagonia, they encountered big animals that had never seen humans before, and they exterminated as they marched. However this theory's critics are at least as numerous as its backers; Chapter Eighteen 'will try to make sense of the debate.

The concluding chapter will seek to put approximate numbers on the count of species that we have already driven into extinction. We shall start with the firmest numbers: the species whose extinctions occurred in modern times and were well documented, and for which the search for survivors has been so thorough as to leave no doubt that there are no survivors. Next come estimates of three less certain numbers: the modern species that have not been seen alive for some time and that became extinct before anyone was aware of it; the modern species that have not even been 'discovered' and received a name; and the species that humans exterminated before the rise of modern science. That background will let us appraise the main mechanisms by which we exterminate, and the number of species that we are likely to exterminate within my sons' lifetimeif we proceed at our current rate.