The risks of a nuclear holocaust and of an environmental holocaust constitute the two really pressing questions facing the human race today. Compared to these two clouds, our usual obsessions with cancer, AIDS, and diet pale into insignificance, because those problems do not threaten the survival of the human species. If the nuclear and environmental risks should not materialize, we shall have plenty of leisure time to solve bagatelles like cancer. If we fail to avert those two risks, solving cancer will not have helped us anyway.
How many species have humans really driven into extinction already? How many more are likely to become extinct within our children’s lifetimes? If more do become extinct, so what? How much do wrens contribute to our gross national product? Are not all species destined to become extinct sooner or later? Is the claimed mass extinction crisis an hysterical fantasy, a real risk for the future, or a proven event that is already well underway?
We need to go through three steps if we are to arrive at realistic estimates of the numbers involved in the mass extinction debate. Firstly, let’s see how many species have become extinct in modern times (that is, since 1600). Secondly, let’s estimate how many other species had become extinct before 1600. As the third step, let’s try to predict how many further species are likely to become extinct within the lifespans of ourselves, our children, and our grandchildren. Finally, let’s ask what difference it all makes to us anyway.
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The first step, that of calculating the number of species that have become extinct in modern times, seems easy when one initially thinks about it. Just take some group of plants or animals, count up in a catalogue the total number of species, mark off the ones known to have become extinct since 1600, and add them up. As a group on which to try this exercise, birds have the advantage that they are easy to see and identify, and hoards of bird-watchers watch them. As a result, more is known about them than about any other group of animals.
Approximately 9,000 species of birds exist today. Only one or two previously unknown species are still being discovered each year, so virtually all living birds have already been named. The leading agency concerned with the status of the world’s birds – the International Council for Bird Preservation (ICBP) – lists 108 species of birds, plus many additional subspecies, as having become extinct since 1600. Virtually all these cases of extinction were caused in one way or another by humans – more of that later. One hundred and eight is about one per cent of that total number of bird species: 9,000. That is where the one per cent figure I mentioned earlier comes from.
Before we take that as the final word on the number of modern birds that have become extinct, let’s understand how the number of 108 was arrived at. The ICBP decides to list a species as extinct only after that bird has been specifically looked for in areas where it was previously known to occur or might have turned up, and after it has not been found for many years. In many cases, birders have watched a population dwindle down to a few individuals and have followed the fates of those last individuals. For example, the most recent subspecies of bird to have become extinct in the US was the dusky seaside sparrow that lived in marshes near Titusville, Florida. As its population shrank due to destruction of the marshes where it lived, wildlife agencies put identification bands on the few remaining sparrows so that they could be individually recognized. When only six remained, they were brought into captivity in order to protect and breed them. Unfortunately, one after another died. The last individual, and with it the subspecies itself, died on 16 June 1987.
Thus, there is no doubt that the dusky seaside sparrow is extinct. Equally little doubt attaches to the many other subspecies and the 108 full species of birds listed as extinct. The full species listed as having vanished in North America since European settlement, and the years in which the last individual of each died, are the great auk (1844), spectacled cormorant (1852), Labrador duck (1875), Carolina parakeet (1914), and passenger pigeon (1914). The great auk also formerly occurred in Europe, but no other European bird species is listed as having become extinct since 1600, though some species have disappeared within Europe while surviving on other continents.
What about all those remaining bird species that did not fulfil the ICBP’s rigorous criteria for extinction? Can we be certain that they still exist? For most North American and European birds the answer is ‘yes’. Hundreds of thousands of fanatical bird-watchers monitor all bird species on these continents every year. The rarer the species, the more fanatical is the annual search for it. No North American or European bird species could possibly drift into extinction unnoticed. There is only one North American bird species whose current existence is uncertain, the Bachman’s warbler, last definitely recorded in 1977, but the ICBP hasn’t given up hope for it because of more recent unconfirmed records. (The ivory-billed woodpecker may also be extinct, but the North American population is ‘only’ a subspecies; a few individuals of the other subspecies of this woodpecker survive in Cuba.) Thus, the number of North American bird species that have suffered extinction since 1600 is surely not less than five nor more than six. Every species but Bachman’s warbler can be assigned to one of two categories – those that are ‘definitely extinct’, or ‘definitely in existence’. Similarly, the number of European bird species extinct since 1600 is surely one – not two, not zero, but one.
Consequently we have an exact, unequivocal answer to the question of how many North American and European bird species have become extinct since 1600. If we could be equally definite for other groups of species, our first step in assessing the mass extinction debate would be complete. Unfortunately, this cut-and-dried situation does not apply to other groups of plants and animals, nor does it apply elsewhere in the world – least of all in the tropics, where the overwhelming majority of species lives. Most tropical countries have few or no bird-watchers, and no annual monitoring of birds. Many tropical areas have never again been monitored since they were first explored biologically many years ago. The status of many tropical species is unknown, because no one has seen them again or specifically looked for them since they were discovered. For instance, among the New Guinea birds that I study, Brass’s friarbird is known only from eighteen specimens shot at one lagoon on the Idenburg River between 22 March and 29 April 1939. No scientist has revisited that lagoon, so we know nothing about the current status of Brass’s friarbird.
At least, we know where to look for that friarbird. Many other species were described from specimens collected by nineteenth-century expeditions that provided only vague indications of the collecting site, such as, ‘South America’. Try resolving the status of some rare species when you have only that broad hint where to look! The songs, behaviour, and habitat preferences of such species are unknown. Hence we do not know where to seek them, nor how to identify them if we glimpsed or heard them.
The status of many tropical species cannot be classified either as ‘definitely extinct’ or ‘definitely in existence’, but just as ‘unknown’. Instead, it becomes a matter of chance which species happens to attract the attention of some ornithologist, becomes the object of a specific search, and hence may be recognized as possibly extinct.
Here is an example. The Solomon Islands are another of my favourite bird-watching areas in the tropical Pacific Ocean, and will be recalled by older Americans and Japanese as the site of some of the fiercest fighting in the Second World War. (Remember Guadalcanal, Henderson Field, President Kennedy’s PT boat, the Tokyo Express?) The ICBP lists one Solomon bird species, Meek’s crowned pigeon, as extinct. Yet when I tabulated all recent observations of all 164 known Solomon bird species, I noticed that twelve of those 164 species had not been encountered since 1953. Some of those twelve species are surely extinct, because they were formerly abundant and conspicuous. Several Solomon islanders told me that those birds had been exterminated by cats.
Twelve species possibly extinct out of 164 still may not sound like much to worry about. However, the Solomons are in much better shape environmentally tha
n most of the remaining tropical world, because they have relatively few people, few bird species, little economic development, and much natural forest. More typical of the tropics is Malaysia, which is rich in species and has had most of its lowland forest cut down. Biological explorers had identified 266 fish species dependent on fresh water in Malaysia’s forest rivers. A recent search that lasted four years was able to find only 122 of those 266 species – less than half. The other 144 Malaysian freshwater fish species must either be extinct, rare, or very local. They reached that status before anyone noticed it.
Malaysia is typical of the tropics in the pressure it faces from humans. Fish are typical of all species other than birds, in that they attract only patchy scientific attention. The estimate that Malaysia has already lost (or nearly lost) half of its freshwater fish is therefore a reasonable ballpark figure for the status of plants, invertebrates, and vertebrates other than birds in much of the rest of the tropics.
That is one complication in trying to pinpoint the number of extinctions since 1600: the status of many or most named species is unknown. But there is a further complication. So far, we have been trying to assess the extinction only of those species that had already been discovered and described (named). Could any species have become extinct before they were even described?
Of course they could, since sampling procedures suggest that the actual number of the world’s species is near thirty million, but less than two million species have been described. Two examples illustrate the certainty of other species becoming extinct before description. Botanist Alwyn Gentry surveyed the plants of an isolated ridge in Ecuador called Centinela, where he found thirty-eight new species confined to that ridge. Shortly afterwards, the ridge was logged and those plants were exterminated. On Grand Cayman Island in the Caribbean, zoologist Fred Thompson discovered two new species of land snails confined to forest on a limestone ridge that was completely cleared a few years later for a housing development.
The fact that Gentry and Thompson accidentally visited those ridges before rather than after they were cleared means that we have names for those extinct species. But most tropical areas that are being developed are not first surveyed by biologists. There must have been land snails on Centinela, and plants and snails on innumerable other tropical ridges, that we exterminated before we discovered them.
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.
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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 milar 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 grizzly bear, and Britain’s bears, wolves, and beavers, succumbed only in recent times, after thousands of years of human persecution. Those reasons were more people, and better weaponry.
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 b
y 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.
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The remaining step in evaluating the mass extinction debate is to predict the future. 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.