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 dr 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 sp*ecies 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.
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 m 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 BP'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 birdwatchers 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, jve 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 than 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 edge. 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.
