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The Dimensions of Life on Earth

Robert M. May
Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK

Introduction.

This paper aims to give estimates of the numbers of living and distinct species of eukaryotes that have been named and recorded. As will be seen, the factual numbers are accurate only to within 10% or more, mainly because we lack a well-documented and synoptic catalog of all named species. Next, I survey estimates of the total numbers of eukaryotic species on Earth today. Here, our ignorance is such that defensible estimates have a range of a factor of over 100—from a few million to 100 million or more. I conclude by asking what fraction of species that have ever lived on Earth are with us today and outlining an approach to an answer that avoids the huge uncertainties in absolute species numbers.

On the one hand, this paper builds on Wilson's (this volume) scene-setting account of the evolutionary and ecological causes and consequences of biological diversity, seeking to quantify the resulting abundance of life forms. On the other hand, the concluding part of the paper prepares the ground for Pimm and Brooks's (this volume) assessment of likely future rates and patterns of extinction.

Throughout the paper, the focus is on species, and eukaryotic species at that.

Why species? As discussed elsewhere (Collar 1997; Groombridge 1992; Heywood 1995; May 1994a; Wilson 1992), biological diversity exists on many levels, from the genetic diversity in local populations of a species or between geographically distinct populations of a species, all the way up to communities or ecosystems. Any level can be predominant, depending on the questions being asked. At the most basic level, genetic diversity in a species is the raw stuff on which

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evolutionary processes work their wonders. At the opposite extreme, “we do not have to embrace the wilder poetic flights of Gaians to acknowledge that ecosystems can usefully be regarded as supraorganisms for many discussions of the way biological and physical processes entwine to maintain the biosphere as a place where life can flourish” (May 1994a). A different kind of stratification is oriented toward taxonomy, from races and subspecies through genera and families to phyla and kingdoms.

Given the variety of ways of measuring the dimensions of life on Earth, I nevertheless believe that species are usually the best place to begin. For one thing, there is the practical reason that effective conservation action needs public support, and the public identifies more easily with tangible biological species than with abstractions such as gene pools or ecosystems. For another thing, although it is undoubtedly more important to preserve habitats and ecosystems than individual species, the choices that we will increasingly be forced to make are likely ultimately to be species-based (Claridge and others 1997; Wilcove 1994).

Why eukaryotic species? A molecular biologist could justifiably argue that plants, animals and fungi represent only a recently diversified tip of an evolutionary tree whose main flowering is among bacteria and archaea. But what is meant by species among bacteria and the like is vastly different from what is meant among plants and animals (see, for example, Bisby and Coddington 1995; Vane-Wright 1992). For instance, different strains of what is currently classified as a single bacterial species, Legionella pneumophila, have nucleotide-sequence homologies (as revealed by DNA hybridization) of less than 50%; this is as large as the characteristic genetic distance between mammals and fishes (Selander 1985). Relatively easy exchange of genetic material among different “species” of such microorganisms means, I think, that basic notions about what constitutes a species are necessarily different between animals and bacteria. That holds even more strongly for viral species, many of which are best regarded as “quasispecies swarms” (Eigen and Schuster 1977; Nowak 1992). Of course, even within well-studied groups of plants and animals, some workers recognize many more species than others, especially when the organisms in question can reproduce asexually; thus, some taxonomists recognize around 200 species of the parthenogenetic British blackberry, others see only around 20, and a “lumping” invertebrate taxonomist might concede only two or three.

Be this as it may, in what follows I restrict attention to numbers of distinct species of living eukaryotic organisms. In academic fashion, I begin by dwelling on a range of problems before turning to group-by-group assessments of known and suspected total numbers.

Numbers of Named Species

Patterns of Effort

From Linnaeus's time to our own, it has often been noted that some groups have received much more taxonomic attention than others (see, for example, Hawksworth 1997). One indication of this is the rates at which species are being

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recorded. Over the span 1978–1987, an average of five new species of birds was described each year, representing an annual average growth rate in the bird species list of 0.05%. For insects, nematodes, and fungi, the corresponding annual averages for newly added species were 7,222, 364, and 1,700, respectively, representing species-list growth rates of 0.76%, 2.4%, and 2.4% (Hammond 1992, table 4.6). From an academic dean's view, the typical bird or mammal species gets about 1.0 scientific paper per year, other vertebrate species get about 0.5 paper per year, and the average invertebrate species is lucky to average 0.1 paper per year and more likely to get 0.01 (May 1988, table 3).

That pattern of attention among groups reflects the distribution of the taxonomic workforce, as summarized in table 1 (condensed from Gaston and May 1992). Taking a very conservative estimate of 3 million invertebrate species as the global total, table 1 shows that the ratio of taxonomists to species is an order of magnitude greater for vertebrates than for plants and two orders of magnitude greater for vertebrates than for invertebrates. This is no way to run a business. It reflects intellectual fashions and bears no relation to the relative importance of taxa either in the sweep of the evolutionary story or in the delivery of ecosystem services.

Reorganizing our priorities rapidly, to learn more about the little things that arguably run a lot of the natural world, will not be easy. Fascination with the furries and featheries goes deep: in the UK, the Royal Society for the Protection of Birds (RSPB) has almost 1 million members; the analogous society for plants (the Botanical Society of the British Isles) has around 10,000; and there is no corresponding society to express affection for nematodes.

Problems with Synonyms

Despite the gross incompleteness of, and biases in, the taxonomic record, a colleague in the physical sciences might reasonably expect that we could at least say how many living species have been named and recorded. It is a simple fact, ascertainable in principle. But the lack of synoptic databases for most groups means that such factual totals are not generally available. Hence the embarrassing situation that “the figures for described species given, even in high profile re-

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ports and ostensibly authoritative works, vary considerably (for examples see Gaston 1991a,b; Hammond 1992, 1995b), and are almost always, and notably with respect to some of the larger invertebrate animal groups, out-of-date, due to delays in cataloguing” (Hammond 1995a).

This is one reason why I cannot provide a crisp and definitive table of recorded species numbers, group by group. There is, however, a more fundamental and nastier problem. The count of recorded species is inflated by synonyms: single species have been independently and differently named and recorded on two or more occasions. Given, for example, that some 40% of all named beetle species are known from only one geographic site, and that no intercollated database exists, the synonymy problem should not surprise us.

For the better-studied groups—such as birds, mammals and many plant families—synonyms have usually been fairly thoroughly resolved. In contrast, among the more poorly known groups, which tend to contain many more species than the better-known ones, synonymy rates can run high. Hammond (1995a) notes that in 1979 some 2,116 beetle species were newly described and 426 named beetle species were recognized as synonymous with others; thus the net gain in known beetle species in 1979 was roughly 80% of the number newly described. Gaston (1991a) surveyed known synonymy rates for the four major insect orders—Coleoptera, Lepidoptera, Hymenoptera, and Diptera—for the period 1986–1989; he found that the rates varied, but averaged about one-third of the number of species newly described over the same period. Another study of particular groups, mainly insects, found typical synonymy rates of around 20% with some groups exceeding 50% (Gaston and Mound 1993). Bland Findlay (Natural Environmental Research Council, Lake Windermere, UK, pers. comm.) and collaborators have focused on six recent taxonomic revisions of six species-rich genera of ciliates and have found that 584 previously recognized species were reduced to 293 when synonyms were removed; this represents a synonymy rate of 50%. The recently published checklist of Nearctic insects (Poole and Gentili 1996) recognizes 95,694 distinct species but acknowledges 152,079 species names, for an overall rate of resolved synonymy of 37%; the rates in individual orders range from 49% for lepidoptera to around 20% for mecoptera, megaloptera, and trichoptera.

Moreover, any such assessment of synonymy rates must be a lower limit; other synonyms are yet to be uncovered or to accumulate in new work. Solow and others (1995) have made a start on estimating the true rate of synonymy. They used the records of thrip (Thysanoptera) species as published each year since 1901. Some 197 workers have named thrip species (and 28 of these have had all their names relegated to synonymy). Of the total of 6,112 thrip names, 1,326 are currently recognized as synonyms, for an observed synonymy rate of 22%. We also know what proportion of the names published each year are known to be synonyms. Not surprisingly, there is a much higher rate among the names assigned in earlier years; it takes time to uncover aliases. Using this information, Solow and others fitted a probability distribution to the time taken to uncover a synonym and then estimated how many more have yet to be revealed. They conclude that the true proportion of synonyms is around 39%, roughly double the observed rate. They also estimate that on the average it takes around 43 years to

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identify a synonym. Although there can be some technical quibbles about the details of the calculation (May and Nee 1995), it is clearly indicative.

A more serious question concerns the extent to which the thrip data are representative of other groups. Altaba (1996) has noted the great variations in synonymy rates among mollusk taxa in Mediterranean regions: for melanopsids (relatively large freshwater snails), recent work suggests an observed synonymy rate of roughly 40%; for unionoids (freshwater mussels), he estimates a rate of around 93%; but for hydrobiids (minute snails, often living in springs and subterranean waters), he estimates a rate of 5% or less.

Even for mammals, things are not really as simple as suggested above. Using the database for Neotropical mammals that he is compiling, Patterson (1996) notes that three-fourths of the names for all species recognized since 1980 had earlier been regarded as synonyms. Over that period, the number of species resurrected from earlier relegation to synonymy (173) was three times the number newly banished as synonyms (62) or the number newly described (60). These reappraisals derive more from changing emphases in taxonomic research (in particular, the relatively recent shift toward phylogenetic concepts to replace earlier biological species concepts) than from the independent rediscoveries that account for many insect synonyms. But the complexities that they introduce into the listing of numbers of distinct species are nonetheless real.

In summary, even if we could pull together all the catalogs scattered among museums and other institutions around the world, an accurate assessment of the total number of distinct species currently named and recorded would elude us. The synonymy problem varies from group to group, and it tends to be worst for the most species-rich groups. In light of the work of Solow and others, it could be argued that an overall discount factor of something like 20% might be applied to existing species lists (Hammond 1992, 1995a). But other people are entitled to other guesses.

Numbers of Named and Distinct Eukaryotic Species

The list of numbers of described and extant species in table 2 is derived largely from the thorough work of Hammond (1992, 1995a), itself based on wide consultation. Hammond's estimates were around 1.7 million in 1992 and around 1.74 million in 1995; the largest components of the latter assessment are listed in table 2. Hammond (1995a) also estimated that a total of “13,000 or so” new species are described each year, and that this number had been strikingly constant over the preceding decades. Allowing for synonyms, I would place the true rate of addition of new and distinct species at around 10,000 per year (which roughly reconciles Hammond's 1992 and 1995 estimates).

The right-hand column in table 2 gives my own current assessment, modified in the light of discussion at, and immediately arising from, the meeting on which this volume is based. For some of the groups synonymy might not pose a problem, but it undoubtedly does for the species-rich groups that dominate the overall number (particularly insects, but also crustaceans, nematodes, arachnids, and fungi). My estimated total count of distinct living species is 1.5 million, and this number probably contains an uncertainty of about 10% or so.

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The estimate of 1.5 million is essentially identical with Wilson's (1988) widely cited figure of 1.4 million (based mainly on expert opinions for various groups), if we update to allow for adding around 10,000 new and distinct species each year over the last decade.

Before commenting on some individual entries in the right-hand column of table 2, it is helpful to draw back and consider the more coarsely grained picture presented in table 3 of metazoan species in different phyla, subdivided by broad habitat (marine, freshwater, symbiotic, and terrestrial). Here we see order-of-magnitude assessments of species numbers, which highlight how any overall estimate of recorded species diversity is dominated by a few groups. Terrestrial arthropod species are roughly ten times more numerous than any other group, and benthic arthropods and annelids, with mollusks and platyhelminths, account for most of the remaining animal species. The table also underlines how diversity measured by species numbers is very different from diversity in terms of basic body plans (reflected at the phylum level). Although more than 85% of all recorded species are terrestrial (Barnes 1989; Briggs 1994), phyla are predominantly aquatic: 32 of 33 are found in the sea (21 are exclusively marine), whereas only 12 are found on land (only one exclusively).

Before presenting some telegraphic comments on table 2, I emphasize that (with a few exceptions for arithmetic clarity) I have given all numbers to only two significant figures. In some cases, the second digit is reasonably secure (for example, the number of distinct plant species currently described is probably 270,000 rather than 280,000 or 260,000), but in other cases—especially the overwhelmingly important insects—even the first digit is unsure. Systematists and conservation biologists have an unfortunate tendency to present estimates that convey a mislead-

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ing sense of precision; for example, Wilson's actual estimate in 1988 was 1,392,485 named species rather than 1.4 million. This should be avoided.

Table 2 shows that my assessment of 1.5 million species differs from Hammond's (1995a) 1.74 million by virtue of my estimating 0.23 million fewer insect species, and 0.01 million fewer nematode species. Hammond's 950,000 insect species

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comprise 400,000 beetle species, 150,000 lepidopteran species, 130,000 hymenopteran species, 120,000 dipteran species, and 150,000 other species. Although Hammond gives a good discussion of the problems of synonymy (referred to above), I believe that he does not adequately discount the totals. My suggested 720,000 insect species in table 2 are 300,000 beetles; 300,000 lepidopteran, hymenopteran, and dipteran species combined; and 120,000 other species. This accords roughly with Nielsen's (Australian National Insect Collection, CSIRO, Canberra, Australia, pers. comm.) estimate of around 750,000 insect species and brings the present estimate into accord with Wilson's (1988) earlier one. I have reduced the nematode species total from 25,000 to 15,000 on the basis of discussions and other published estimates.

My other numbers in table 2 agree with Hammond's (1995a) estimates. Most seem reasonably agreed on among the relevant experts. The roughly 80,000 species of Protoctista (protozoans and algae) are mainly in Bacillariophyta (12,000), Foraminifera (10,000), Gamophyta (10,000), Rhodophyta (5,000), Actinopoda (6,000), Ciliophora (8,000), and Sporozoa (5,000). The estimated 270,000 plant species (embryophytes) are mainly in Spermatophyta (240,000), Pteridophytes (10,000), and mosses and liverworts (16,000). The estimate of 70,000 distinct species of mollusks strikes me as having an uncertainty of about 10%. The same is true for the estimate of 75,000 species of arachnids; an estimate of 36,000 distinct spider species is fairly sure, but the very rough estimate of 40,000 distinct mite species might have an uncertainty of 10% or more.

Numbers of Species Extant Today

The true total of extant species, as distinct from those we have named and recorded, is hugely uncertain. Table 4 shows Hammond's (1995a) excellent summary of the range of estimates of the possible totals in the major groups of eukaryotes and his own “working figures”.

My current estimate is presented in the right-hand column of table 4. The most important discrepancies between my best guesses and Hammond's are in my lower numbers for fungi (1 million fewer species) and for insects (4 million fewer). There are other minor differences, but those two account for essentially all the difference between Hammond's estimate of roughly 12 million and mine of roughly 7 million species. Hammond's (1995a, table 3.1.2) estimated total was actually 13.6 million, but this included 1.4 million bacteria and viruses.

Before briefly discussing table 4, I emphasize the great uncertainty in many of its numbers. The overall range of estimates runs from 3 million to more than 100 million species, with a conservative estimate of the likely range being 5–15 million eukaryotic species. Hammond's 12.2 million best guess is remarkably close to Briggs's (1994) independent estimate of 12.3 million, although they differ considerably in detail (Briggs has 10 million insects, 1 million nematodes, but essentially no fungi).

As discussed much more fully elsewhere (May 1988, 1990, 1994a; Hammond 1992, 1995a), there are many ways to estimate species totals. They include subjective expert opinion, extrapolation of trends, assessments of ratios of unknown

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to known species in previously unstudied places, and other methods that combine evidence with various degrees of theoretical argument. The remainder of this section outlines some of the salient points of the various approaches, particularly in relation to my choice of lower estimates in table 4.

Insects

As reviewed by May (1994a) and Hammond (1995a), extrapolation of past trends and surveys of expert opinion tend to put insect species totals in the rough range of 5–10 million. Estimates based on detailed keying-out of the fraction of species new to science in previously unexplored regions tend to give lower numbers—around 3 million (for example, Hodgkinson and Casson 1993). Conversely, estimates reached by using a chain of theoretical arguments to scale from numbers of beetle species in the canopies of individual tropical tree species to tropical insect species totals about 30 million (Irwin 1984); reappraisal of such theoretical arguments has, however, suggested totals more like 3 million (May 1988, 1990; Stork 1988).

I have chosen a best guess of 4 million (rather than Hammond's 8 million, or the lower 2 million guess by Nielsen and Mound, this volume) largely on the basis of the new approach developed by Gaston and Hudson (1994). This original method first asks what fractions of the species in particular taxa are found in each

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of nine biogeographic realms (these nine realms represent a slight extension of the conventional Wallace scheme); the reference taxa range from general categories (such as higher plants, amphibians, birds, and mammals) to very particular ones (such as dragonflies, tiger beetles, and swallowtails). Gaston and Hudson then take a range of estimated total numbers of insect species in the Nearctic and in Australia and scale them up to global totals on these biogeographic bases. For example, given that Nearctic higher plants represent 6.5% of the global total, an estimated total of 200,000 Nearctic insect species would imply around 3 million insect species in total. For their fairly wide range of estimators, Gaston and Hudson arrive at global insect totals in the range of 1–10 million. I favor an assessment of around 150,000–250,000 Nearctic insects (with Australian insect totals less sure), and use of the higher plants as the biogeographic template, which gives 2–4 million insects in total. This estimate tends to accord with those from empirical studies, such as those of Hodgkinson and Casson (1993); hence my choice of 4 million insect species in table 4. It also accords with Erwin's (Smithsonian Institution, Washington, DC, pers. comm.) recent estimate that preliminary keying-out of some of his tropical-canopy beetle collection suggests that around 80% of the species are new; this implies multiplying the insect total in table 2 by 5, which again gives around 4 million.

Fungi

Observing that there are about six to seven fungal species for each indigenous plant species in the United Kingdom, Hawksworth (1991) suggested that the global total of around 270,000 plant species should be scaled up to yield around 1.5 million species of fungi. Given that only some 72,000 fungal species have yet been named, that would imply that 95% remain to be discovered. Put another way, we might expect that in collections from previously unstudied places, only 5% of fungal species would be known, which is very discordant with the facts (May 1991). Seemingly in support of the 1.5 million estimate (Hawksworth and Rossman 1997), Mibey and Hawksworth (1997) cite 43 species new of 61 species of Meliolaceae and 10 new of 14 Asterinaceae studied in Kenya: but if the 71% figure were representative, it would scale from the known 72,000 fungal species to only around 250,000.

I think the inconsistencies here are associated with problems in simply scaling from UK fungus-plant ratios to global totals. As discussed more carefully, and with other examples elsewhere, such scaling up assumes, among many other things, that fungal species and flowering-plant species characteristically have similar geographic ranges and latitudinal distributions (May 1990). I think it more likely that typical fungal species have wider geographic distributions than typical plant species. Witness the study by Rossman and Farr (1997) of four representative groups of fungi, of which the North American species represented 40–50%, 16%, 54%, and 68% of the world total. The corresponding figure for North American flowering-plant species is 6.5%: maybe the North American fungi are vastly better known than those of other parts of the world, but surely not to this extent. Also, the flowering-plant diversity of the United Kingdom is depauperate, still recovering from the last ice age.

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Such considerations undercut many other scaling-up exercises. A count of Heliconius butterfly species to Passiflora species in typical Neotropical sites, scaled against the roughly 360 species of Passiflora in South America, would suggest around 500 species of Heliconius. There are in fact only 66. The same butterflies use different Passiflora species in different places. There are many other such cautionary tales (May 1990).

Other Taxa

Some other “high” entries in table 4 also come from scaling-up of one kind or another. Grassle and Maciolek (1992) have suggested 10 million or more marine macrofaunal species (mostly mollusks, crustaceans, and polychaete worms) on the basis of a different kind of extrapolation. As pointed out on ecological (May 1992) and statistical (Solow 1995) grounds, such projections must be treated with considerable caution.

Apart from insects and fungi, my estimates in table 4 differ little from those discussed fully by Hammond (1995a). I have revised protozoa, algae, and mollusks down a bit and nematodes up a bit as a result of input from this forum. Influenced by Platnick (1997), I have revised arachnids down to around two-thirds of Hammond's estimate. These changes, however, have little effect on my best guess of about 7 million species, some 5 million lower than Hammond's (1995a).

Species Alive Today as a Fraction of the Historical Total

Given the great uncertainties in how many species are alive today, any estimate of the total numbers ever to have lived, or of likely future numbers of extinctions over the coming century, is even more imprecise.

There is, however, an alternative approach that asks about the fraction of species alive today, or about comparative rates of extinction (in terms of probabilities that species in particular groups became extinct recently, or under various assumptions about the future relative to average extinction probabilities over the sweep of the geological record). Such assessments involve dimensionless ratios and thereby factor out the gross uncertainties associated with absolute numbers of species, permitting quite accurate statements to be made.

For an assessment of f, the fraction of all species to have lived since the Cambrian dawn of hard-bodied fossils (some 600 million years ago) that are alive today, we first ask what is the average life span of a species in the fossil record, from origination to extinction. Such life spans vary greatly, both within and among groups. Raup (1978) brought together several studies and then analyzed some 8,500 cohorts of fossil genera to conclude that the average life span of invertebrate species is around 11 million years. A later, and particularly thoughtful, review by Sepkoski (1992) suggests that 5 million might be a better estimate. The top part of table 5 summarizes the studies surveyed by Sepkoski (1992) and some others, giving an overall impression that the average species has a life span of around 5–10 million years, but with much variability (May and others 1995).

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Cocks (Natural History Museum, London, UK, pers. comm.) has recently compiled a somewhat wider range of estimated species life spans, arguing broadly for a shorter average figure than those above. Graptolites in the Lower Palaeozoic seem to evolve particularly quickly: a collection of more than 30 species from the Silurian of Kazakhstan has examples of three successive species within a single graptolite zone, the duration of which is probably 500,000 years; thus, individual species life spans could be as short as 150,000 years. Likewise, Cambrian trilobites in the Acado-Baltic realm show 25 species with an average life span of 500,000 years. Brachiopods also can be short lived, with particular examples (such as Eocoelia intermedia) having life spans less than 500,000 years. Turning to vertebrates, small mammals have evolved at such speeds that most rodent species have life spans of less than 1 million years, with even shorter durations (300,000–400,000 years) in times of rapid dispersal. Perrissodactyls also typically have life spans of less than 500,000 years. Insectivore species live longer, averaging maybe

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3 million years. A sample of 175 species of tertiary to recent Corals has species life spans ranging from 200,000 years to 7 million years, with an average of about 4 million years. Moving on up to longer life spans, we find an analysis of 131 species of benthic Foraminifera with average life spans of 14–16 million years, although some have shorter spans, around 7 million years. Coccoliths have comparable longevity. Perhaps the longest-lived species that is well documented is a bryozoan that ranges from the early Cretaceous to the present, a span of around 85 million years (PBT Taylor, Natural History Museum, London, UK, pers. comm.). These estimates, and supporting references, are set out in the lower part of table 5.

In short, there is very great variability—over a range of a factor of 100—among species life spans in the documented fossil record. If one is to speak of an average, it might be better to offer a range like 1–10 million years. Forced to produce a more definite guess, Cocks and his colleagues in the Natural History Museum in London produce a figure of 4–5 million years.

If the sweep of the fossil record is around 600 million years and the average life span from origin to extinction of individual species averages around 4–5 million years, then we might conclude that the species living today—or at any other specific instant—represent just under 1% of the total ever to have lived; that is, f is about 0.01.

Such an estimate, however, assumes total species numbers to have been roughly constant over the 600 million years. That, of course, is not so. As has been argued by Sepkoski (1992), and more recently by Rosenzweig (1997), on the grounds of apparent trends, and by others from more recondite analyses (some involving power laws and fractal measurements; for example, Solé and others (1997), in a very broad outline the history of the fossil record is one of roughly linear increase in species numbers. That implies that the number of species living today is roughly twice the average over the fossil record, which suggests that they make up more like 2% of those ever to have lived, or an f of around 0.02. Benton (1995, 1997) has gone further, marshaling evidence in support of an exponential increase in terrestrial species diversity since the end of the Precambrian; I read this work as arguing for an f of 0.03 or higher.

The latter estimate is subject both to the uncertainties in species life spans and to other complications. For instance, given that most living species are terrestrial insects, whose origins were more like 400 million years ago (and whose average life spans might be somewhat longer than the overall average—see May and others 1995), f could be somewhat larger than 0.02.

Whatever the details, today's evolutionary heritage of living species is not a negligible fraction of those ever to have graced the planet. By the same token, only relatively few past species have exited in dramatic mass extinctions (by the above estimate, the “big five” mass extinctions, even if they had each wiped out virtually all extant species, account for only 5%, or at most 10%, of all endings). The sixth wave, on whose breaking tip we stand, is an uncommon evolutionary event, when judged against the geological record.

Pimm and Brooks (this volume) extend earlier work by May and others (1995) and themselves (Pimm and others 1995), applying similar arguments based on

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comparative species life spans to estimate recent and likely future changes in extinction rates, as seen against the background average of the fossil record.

Coda.

Emphasizing the uncertainties, I have estimated that the number of distinct eukaryotic species alive on Earth today lies in the 5–15 million range, with a best guess of around 7 million. Of these, roughly 1.5 million have been recognized. Allowing for the resolution of synonyms, new species are being recorded at around 10,000 each year. At that rate, it will take over 500 years to complete the catalog.

Such a 500-year estimate is, of course, misleading on several grounds. For one thing, recent and likely future extinction rates point toward qualitative reductions in the catalog. Even more important, I believe that advances in automating molecular sequencing, along with more systematic and computerized handling of phylogenetic information, will revolutionize the basic task of taxonomy in ways that we can yet barely imagine. I guess that within 50 years, and possibly much sooner, we will put a small DNA sample from a newly collected specimen into a machine and be told its exact location in a synoptic tree of living species.

The task of inventorying is sometimes mistaken for “stamp collecting” by thoughtless colleagues in the physical sciences. But such information is a prerequisite to the proper formulation of evolutionary and ecological questions, and essential for rational assignment of priorities in conservation biology (Nee and May 1997; Vane-Wright and others 1990). Lacking basic knowledge about the underlying taxonomic facts, we are impeded in our efforts to understand the structure and dynamics of food webs, patterns in the relative abundance of species, or, ultimately, the causes and consequences of biological diversity.

It is interesting to speculate whether the denizens of other inhabited planets—if there are any—share the vagaries of our intellectual history: a fascination with the fate of the universe and the structure of the atom, lagging well behind interest in the living things with which we share our world. A different, but related, question lies in human institutions' difficulties in taking action to address longterm problems at the expense of short-term interests (witness climate change). Such questions do not come readily under Medawar's rubric of science as “the art of the soluble”, but they go to the heart of humanity's future, which unwittingly entrains the rest of life on Earth.

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