To judge the scale of the problem facing conservation managers we need to know the total number of species that occur in the world, the rate at which these are going extinct and how this rate compares with that of prehuman times. Unfortunately, there are considerable uncertainties in our estimates of all these things.
About 1.8 million species have so far been named (Alonso et al., 2001), but the real number is very much larger. Estimates have been derived in a variety of ways (see May, 1990). One approach is based on a general observation that for every temperate or boreal mammal or bird (taxa where most species have now apparently been described) there are approximately two tropical counterparts. If this is assumed also to hold for insects (where there are many undescribed species), the grand total would be around 3-5 million. Another approach uses information on the rate of discovery of new species to project forward, group by group, to a total estimate of up to 6-7 million species in the world. A third approach is based on a species size-species richness relationship, taking as its starting point that as one goes down from terrestrial animals whose characteristic linear dimensions are a few meters to those of around 1 cm, there is an approximate empirical rule that for each 10-fold reduction in length there are 100 times the number of species. If the observed pattern is arbitrarily extrapolated down to animals of a characteristic length of 0.2 mm, we arrive at a global total of around 10 million species of terrestrial animals. A fourth approach is based on estimates of beetle species richness (more that 1000 species recorded in one tree) in the canopies of tropical trees (about 50,000 species), and assumptions about the proportion of nonbeetle arthropods that will also be present in the canopy plus other species that do not occupy the canopy; this yields an estimate of about 30 million tropical arthropods. The uncertainties in estimating global species richness are profound and our best guesses range from 3 to 30 million or more.
An analysis of recorded extinctions during the modern period of human history shows that the majority have occurred on islands and that birds and mammals have been particularly badly affected (Figure 7.16). The percentage of extant species involved appears at first glance to be quite small, and moreover, the extinction rate appears to have dropped in the latter half of the 20th century, but how good are these data?
Once again, these estimates are bedevilled by uncertainty. First, the data are much better for some taxa and in some places than others, so the patterns in Figure 7.16 must be viewed with a good deal of scepticism. For example, there may be serious underestimates even for the comparatively well-studied birds and mammals because many tropical species have not received the careful attention needed for fully certified extinction. Second, a very large number of species have gone unrecorded and we will never know how many of these have become extinct. Finally, the drop in recorded extinctions in the latter half of the 20th century may signal some success for the conservation movement. But it might equally well be due to the convention that a species is only denoted extinct if it has not been recorded for 50 years. Or it may indicate that many of the most vulnerable species are already extinct. Balmford et al. (2003) suggest that all our attention should not be focused on extinction rates, but that a more meaningful view of the scale of the problem of species at risk will come from the long-term assessment of changes (often significant reductions) in the relative abundance of species (which have not yet gone extinct) or of their habitats.
An important lesson from the fossil record is that the vast majority, probably all, of extant species will become extinct eventually - more than 99% of species that ever existed are now extinct (Simpson, 1952). However, given that individual species are believed, on average, to have lasted for 1-10 million years (Raup, 1978), and if we take the total number of species on earth to be 10 million, we would predict that only an average of between 100 and 1000 species (0.001-0.01%) would go extinct each century. The current observed rate of extinction of birds and mammals of about 1% per century is 100-1000 times this 'natural' background rate. Furthermore, the scale of the most powerful human influence, that of habitat destruction, continues to increase and the list of endangered species in many taxa is alarmingly long (Table 7.4). We cannot be complacent. The evidence, whilst inconclusive because of the unavoidable difficulty of making accurate estimates, suggests that our children and grandchildren may live through a period of species extinction comparable to the five 'natural' mass extinctions evident in the geological record (see Chapter 21).
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