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REVIEW| VOLUME 27, ISSUE 9, P501-510, SEPTEMBER 01, 2012
What we know and don’t know about Earth's missing biodiversity
Published:July 11, 2012​DOI:​https://doi.org/10.1016/j.tree.2012.05.008
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Estimates of non-microbial diversity on Earth range from 2 million to over 50 million species, with great uncertainties in numbers of insects, fungi, nematodes, and deep-sea organisms. We summarize estimates for major taxa, the methods used to obtain them, and prospects for further discoveries. Major challenges include frequent synonymy, the difficulty of discriminating certain species by morphology alone, and the fact that many undiscovered species are small, difficult to find, or have small geographic ranges. Cryptic species could be numerous in some taxa. Novel techniques, such as DNA barcoding, new databases, and crowd-sourcing, could greatly accelerate the rate of species discovery. Such advances are timely. Most missing species probably live in biodiversity hotspots, where habitat destruction is rife, and so current estimates of extinction rates from known species are too low.
How many species are there?
This deceptively simple question has a rich pedigree. In 1833, Westwood [1] speculated ‘On the probable number of species of insects in the Creation’. Over recent decades, many have grappled with the question, reaching widely varying conclusions [2, 3, 4, 5, 6]. Clearly, far more species exist than taxonomists have named; most are missing from the taxonomic catalog. Alas, taxonomists have complicated matters by inadvertently giving multiple names to many known species. Chapman's recent, thorough compilation of estimates [7], plus new studies embracing novel methods of estimation, motivate our synthesis of recent progress.
Here, we highlight previous work and ask: How many missing species are left to discover? Where do these species live? What ecological traits might they possess? And, how can unresolved challenges in documenting diversity be best approached? We do not, however, conjecture about the total number of species on Earth. For some taxa, the numbers and their uncertainties are well known. For others, including insects and fungi, the estimates vary so widely as to overwhelm any simple attempt to estimate a grand total for all species.
Human activities currently drive species to extinction at 100–1000 times their natural rate [8]. It is likely that biologists will not discover many missing species before they vanish and so will underestimate the magnitude of the contemporary biodiversity crisis [3, 8, 9]. The need to discover and describe species has never been more urgent [8, 10]. Optimizing where to focus conservation interventions requires, in part, counting species accurately and knowing where they live [3]. Unfortunately, current conservation efforts work from an incomplete biodiversity catalog [11].
Today, describing the unknown animal species might cost US$263 billion [12] and require centuries to complete. Given such obvious impracticalities, there is little choice but to rely on current estimates of total species numbers and their probable geographic distribution, using the best available information [2, 3, 13, 14, 15].
How many species are known?
 Known species counts
Table 1 simplifies Chapman's [7] compilation of species numbers. We add additional data to illustrate key debates. As have others, we restrict our analyses to metazoans (fungi, plants, and animals) because for viruses, bacteria, and other microorganisms, the definition of ‘species’ is unclear. The column ‘Currently Catalogued’ counts known species within various taxonomic groupings, and represents the work of many thousands of taxonomists across hundreds of years. Despite this massive undertaking, simply adding up the numbers of ‘known’ species, even for well-studied groups such as birds, is itself not straightforward. (See ‘Described Species Range’, which shows the range of variability for different groups). Synonymy is the problem.
Table 1Chapman's (2009) estimates of species numbers 7, with other noteworthy estimates discussed in the main text
KingdomPhylum/DivisionWithin PhylumMajor divisionData from Chapman 7 and sources thereinEstimatedOther Refs
Currently cataloguedDescribed species range
Fungi98 99845 173–300 0001.5 M
611 0006
9.9 M38
3.5–5.1 M39
1.62 M35
Plants310 129∼390 800
215 644298 0006
Vascular plantsMagnoliophyta(∼268 600)223 300–315 903(∼352 000)
Monocots and selected non- monocots352 000 + 15%9, 13
Gymnosperms(∼1021)846–1021(∼1050)
Ferns and allies(∼12 000)10 000–15 000(∼15 000)
Bryophyta16 23613 370–23 000∼22 750
Algae12 27212 205–12 272NA
AnimalsAll terrestrial1 233 5008 740 0006
All marine193 7562 210 0006
Porifera∼60005500–10 000∼18 000
Cnidaria97959000–11 000N/A
Mollusca∼85 00050 000–120 000∼200 000
Annelida16 763120,00–16,763∼30 000
AnthropodaTropical arthropods3.6–11.4 M33
Arachnida102 24860 000–102 248∼600 000
Myriapoda16 0728160–17 923∼90 000
Insecta∼1 M720 000–>1 M5 M
Coleoptera360 000–400 0001.1 M
Diptera152 956240 000
Hemiptera80 000–88 000
Hymenoptera115 000>300 000
Lepidoptera174 250300 000–500 000
Crustacea47 00025 000–68 171150 000
20 00020 000–25 000(∼80 000)
Platyhelminthes<25 00012 000–80 000∼500 000
Nematoda>1 M22
70036100–7003∼14 000
Echinodermata12 673N/A∼20 000
Other invertebrates64 788∼80 500
ChordataMammals54874300–5487∼5500
Birds99909000–9990>10 000
10 052
a
Reptiles87346300–8734∼10 000
Amphibians65154950–6515∼15 000
Fishes31 26925 000–31 269∼40 000
a http://www.birdlife.org/datazone/info/taxonomy​.
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 The problem of synonymy
A range of estimates arises because taxonomists have described some species many times. This is not surprising. The descriptions of species come from different taxonomists on different continents in different generations. Fixing this problem requires considerable effort. For flowering plants, for example, the highest estimate of known species is twice that of the lowest; synonymy is suspected to be upwards of 60–78% for many plant groups [16]. Because estimates of missing species use the number of known species as their basis, these uncertainties are fundamental.
Taxonomists recognize the seriousness of synonyms. Major botanic gardens now collaborate to produce the unique and continuously updated World Checklist of Selected Plant Families [17], which has largely resolved the problem of synonymy for approximately 110 000 species (all monocots, plus selected non-monocot families). Other rigorous attempts to confront synonymy include the 2011 symposium to eliminate separate names for asexual and sexual stages of certain fungi [18]. An estimated 66% of fungal names are synonymous [19].
How many species are unknown?
The completeness of global inventories varies greatly (see ‘Estimated’ in Table 1). Completeness ranges from approximately 97% for mammals, 80–90% for flowering plants, 79% for fish, 67% for amphibians, roughly 30% for arthropods and <4% for nematodes [11, 13, 20, 21, 22] (Table 1). Across these groups, levels of completeness decline with the currently known numbers of species. Taxonomic effort is distributed approximately evenly among vertebrates, plants, and invertebrates, yet plants have approximately ten times, and invertebrates 100 times, more known species than do vertebrates [23, 24].
Global inventories (Table 1 ‘Estimated’) come from various methods, including the expert opinions of taxonomists specialized on the various taxa. Differing methods result in widely varying estimates; for instance, estimates for fungi vary nearly 20-fold. Methods fall into three basic categories: extrapolations from fractions, extrapolations from taxonomic-scaling patterns and mechanistic model estimates.
 Extrapolations from fractions of unknown species
Early, often-controversial estimates of missing species used fractions of missing species in samples. Hodkinson and Casson [25] used such fractions to predict the number of insect species globally. Finding that 62.5% of hemipteran species in a location were unknown, the authors projected the existence of 1.84–2.57 million insect species globally. Using an alternative extrapolation, Adamowicz and Purvis [26] calculated a correction factor for three different sources of diversity underestimation (differential taxonomic effort among biogeographical regions; multi-continental distributions of species; and morphology and genetics) and concluded that the number of known branchiopod crustaceans should more than double.
In such sample-based estimates, the key assumption that described species form a random, unbiased subset of all species will rarely hold. For example, for well-known terrestrial and marine taxa, a few species have large geographical ranges, and many have small ones, with the former being more common locally than are the latter [27]. Inevitably, common and widespread and, thus, taxonomically known, species will predominate in small samples, leading to a spurious confidence in taxonomic completeness that declines as samples become more complete. As we show later, even well-known vertebrate taxa are now yielding surprising numbers of new species, overlooked because of their small ranges or because of cryptic species complexes (Box 1).
Box 1
 The hyper-estimates of species numbers
The greatest uncertainties involve ‘hyper-estimates’, by which we mean individual totals of 5 million species or more. For example, Grassle and Maciolek [28] used the relationship between the numbers of seafloor invertebrates in samples of increasing area to extrapolate the total number of species in the deep sea. They estimated that the deep seafloor worldwide could have up to 10 million species, a total several orders of magnitude larger than that found in their geographically restricted samples.
Large numbers capture the public imagination and invite scientific controversy. In the case of marine invertebrates, such extrapolations from local to global seafloor diversity were unwarranted because of the obvious doubts about scaling up from small to a much larger geographical scales [29]. Several authors have highlighted the limitations of scaling up from estimates collected at a single spatial scale [3, 15, 30, 31].
For tropical insects, the best-known hyper-estimate was Erwin's [4] astounding conjecture of 30 million species. His approach started with the number of beetle species associated uniquely with a single species of tropical rainforest tree in Panama. This generated criticism, primarily from those concerned about the assumptions underlying such a ‘small to large’ extrapolation, but spawned considerable interest and research. Fundamental was the degree of host specificity of herbivorous insects on their food plants, which Erwin assumed to be high. Novotny et al. [32], ØDegaard [15] and others found considerably lower host specificity, perhaps by a factor of four or five. The resulting global estimate of insect species richness has accordingly dropped sharply.
Recently, Hamilton et al. [21] highlighted the sensitivity of Erwin's model to its input parameters. As with earlier studies, their estimate requires values for the average effective specialization of herbivorous beetle species across all tree species, a correction factor for beetle species that are not herbivorous, the proportion of canopy arthropod species that are beetles, the proportion of all arthropod species found in the canopy, and the number of tropical tree species. Their approach uniformly and randomly sampled plausible ranges for each of these numbers. Approximately 90% of these parameter combinations resulted in estimates of between 3.6 and 11.4 million species [33]. Although their parameter distributions are untested assumptions, Hamilton et al.’s approach suggests that Erwin's estimate is exceedingly improbable.
Fungi are poorly known [34] and their diversity is hotly debated. Hawksworth [35] started with the 6:1 ratio of fungi to flowering plant species found in Britain, where both groups are well known, and extrapolated this ratio to the global total for flowering plants, yielding an estimate of 1.62 million species of fungi globally. May [36] was sharply critical because it again involved a small- to large-scale extrapolation. His key concern was that the species-rich tropics would not have the ratio of fungi to plants found in Britain. If so, then in tropical collections over 95% of the species encountered would be new, given that only approximately 70 000 fungi had been catalogued globally. The actual percentages of new species from tropical samples were much smaller. An alternative approach by Mora et al. [6] estimates 611 000 fungal species globally and seemingly supports May's more conservative estimate of approximately 500 000 fungal species.
Such low estimates of fungi have spawned strident criticism. First, small, quickly obtained samples will not be random ones, but dominated by well-known, widespread species. Second, Hawksworth [35] emphasized not only fungal and plant associations, but also the strong associations of fungi with insects. Each beetle species might have its own unique fungus. Third, Bass and Richards [37] point out that, over the past decade, new methods in molecular biology and environmental probing have substantially increased the rate of descriptions of species.
Cannon [38] estimates approximately 9.9 million species of fungi, whereas O’Brien et al. [39] estimate 3.5–5.1 million species. Very high genetic diversity in soil samples (491 distinct genomes in pine-forest soil samples and 616 in soils from mixed-hardwood forests) underlay these hyper-estimates. They emerge from extrapolating from local to global scales, so previous concerns about scaling also apply here. At present, there are no comparable genomic surveys in tropical moist forests showing exceptional fungal richness, as would be expected if the above hyper-estimates are correct. Moreover, no one has yet shown how communities of fungal genomes change over large geographical areas. In an important potential advance to this debate, Blackwell [40] lists locations and hosts known to contain rich, and poorly known, fungal communities. These are places where one might test key hypotheses.
 Scaling the tree of life
Another way to estimate the numbers of species links them to the numbers of higher taxonomic levels, such as families and orders. Arguments arise over the correct level of higher taxonomic unit to use, as well as the previous criticisms of using ratios to extrapolate from one place to another [41].
Ricotta et al. [42] described scaling patterns across different taxonomic levels of seed plants, asserting that they could use such relationships ‘to predict species richness in a given area with considerable accuracy’. Mora et al. [6] slightly modified this approach to estimate the total number of species ‘on earth and in the ocean’. They used the rates of description to fit asymptotic regression models to taxon-accumulation curves over time for different taxonomic levels. Using the asymptotic estimates for animals, the ratios of classes per phyla, orders per class, families per order, and genera per family were strikingly similar. On this basis, they posited that the ratio of species per genus would be the same globally and so predicted 8 750 000 terrestrial and 2 210 000 marine species. There is no particular theoretical reason to make this final supposition, but to the extent that they could compare the best available estimates of numbers of species within phyla, there was broad agreement with their predictions.
Bass and Richards [37] and Blackwell [40] criticized Mora et al.’s [6] estimates for fungi, because the numbers of higher fungal taxa, including even phyla, are still increasing. The number of genera is not asymptoting, making it hard to use Mora et al.’s approach. Moreover, synonymy is also a potential problem in fungal genera and families.
 Mechanistic models of taxonomists as predators, missing species as prey
None of the work described thus far incorporates a mechanistic understanding of species discovery. Early efforts estimated the asymptotic number of species over time assuming that the curve's first derivative, the rate of description, will decline [6, 11, 43, 44, 45]. By analogy, one views this as a model with ‘predators’ (taxonomists) exploiting a continually declining ‘prey’ population (the numbers of missing species). For birds globally, and for some taxa regionally, the rates of description are indeed slowing and asymptotic approaches provide reasonable estimates. Mora et al. [6] also used this approach to estimate the numbers of higher taxa.
For most taxa, not only are the rates of species description increasing, but they are also doing so exponentially, so ruling out estimates of asymptotes. The numbers of taxonomists are also increasing exponentially [46]. To account for this, a more truly mechanistic model accounts for taxonomic effort and taxonomic efficiency required to document previously unknown species [13, 20, 47, 48]. Joppa et al. [9, 13] initially proposed this strategy, noting it shares an intellectual lineage with traditional ‘catch per unit effort’ approaches used in fishery models. It defines ‘taxonomic effort’ as the number of taxonomists involved in describing species and ‘taxonomic efficiency’ as an increase in the number of species described per taxonomist, adjusted for the continually diminishing pool of as-yet-unknown species. Importantly, the model uses maximum likelihood techniques, allowing confidence intervals about any estimate.
How well does this model perform? Validating it by expert opinion revealed broad agreement with its predictions [13], but the method encounters two problems. First, in some cases, the numbers of species described per taxonomist remain approximately constant even as the pool of missing species inevitably declines. Individual taxonomists probably describe only so many species in a year, regardless of how many missing species there are, and perhaps work through the backlog methodically, genus by genus. Second, although taxonomic efficiency increases in a broadly linear way for most taxa, for others, more complex patterns suggest alternative functions [9, 13].
Where are the missing species?
Knowing where species live is vital for setting international priorities for conservation. Incomplete information might leave one unable to prioritize effectively where to allocate conservation efforts. For example, the ‘biodiversity hotspots’ [49] combine a measure of habitat destruction (<30% habitat remaining) with the numbers of known endemic flowering plant species (>1500). These areas have become international priorities for conservation, with large resources allocated for their preservation [50]. The incomplete catalog of flowering plants begs our asking: Will knowing where the missing species are located alter conservation priorities? Are missing species concentrated in imperiled habitats where they are at risk of extinction? If so, can they be found before they go extinct?
Several studies identify areas of high missing biodiversity for prioritizing future conservation efforts [9, 20, 47, 48, 51, 52, 53, 54]. Recently, Joppa et al. [9] suggest that missing plant species will concentrate in the biodiversity hotspots (Figure 1), places such as Central America, the northern Andes and South Africa, where, by definition, the threat of habitat loss is greatest. These predictions have limitations, obviously, because factors such as remoteness or political instability reduce the rate of species description in some regions. Expanding on Joppa et al. [9], Laurance and Edwards [55] highlighted their probable underestimation of the importance of the Asia-Pacific region, such as the Philippines and New Guinea, as centers of missing plant species (Figure 1). The Asia-Pacific region might also have many unknown amphibian and mammal species [20]. Despite such limitations, biodiversity hotspots will surely sustain large numbers of missing species. As we discuss below, missing species tend to have small geographical ranges.
Figure 1The percentages of missing plant species predicted to occur in various regions suggest that existing biodiversity hotspots, such as southern Africa, Central America, and especially the northern Andes, hold the greatest numbers. However, unexpectedly low predicted numbers in places such as New Guinea might reflect their inaccessibility to scientists, causing missing-species numbers to be underestimated. Stephanie Pimm Lyon photographed these three orchids in the genus Corybas; similar to most of the members of this genus that she collected in New Guinea in 2012, she considers these probably new to science. Adapted, with permission, from [9].
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These findings bring both good and bad news. The good news is that most missing species occur in places that are already global conservation priorities. The bad news is that most of these species are in areas already under dire threat of habitat loss. By instilling an appropriate sense of urgency, focusing species-discovery efforts on hotspots would result in ‘taxonomy that matters’ [56]. Discovering unknown species in hotspots would help to underscore their exceptional biological diversity and uniqueness. Invaluable insights would also be gained into the traits these species display and the services they could potentially provide (Box 2).
Box 2
Are missing species different?
To extend our analogy of ‘taxonomists as predators’, taxonomists are surely searching for the most obvious ‘prey’, inadvertently selecting species with traits that are most conducive for discovery. As the pool of missing species diminishes, one would expect those remaining to have traits that make them harder to find (Figure 2). For example, the unique biota of deep-sea hydrothermal vents was discovered only during the late 1970s, whereas a nocturnal stream-dwelling lizard from high in the Peruvian Andes was described only this year [57]. This begs the question: are missing species functionally different from those already described?
Figure 2Many undiscovered species are difficult to find because they are cryptic, small in size or have small geographic ranges. Shown are (a) a recently discovered burrowing caecilian species from India, (b) a newly discovered chameleon (Brookesia micra) from Madagascar that is the smallest lizard in the world, and (c) a locally endemic waterfall frog (Barbourula kalimantanesnis) from Borneo. Photographs reproduced, with permission, from Biju Das (a), Frank Glaw (b), and David Bickford (c).
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Certainly, the first European expeditions across the African savannahs had little trouble in finding and describing large-bodied wildebeests, giraffes, and elephants. The remaining unknown mammal species are smaller. Similarly, taxonomists have described larger-bodied species sooner in a variety of animals, including British beetles [58], South American songbirds [59], and Neotropical mammals [60]. However, this trend evidently varies among taxa. Body size in most animal groups is highly right-skewed [61, 62] and, thus, the tendency for newly described species to be small bodied might simply reflect a random sample of the overall size distribution, rather than small-bodied animals being harder to find or describe [63]. Body size and year of description strongly correlate in insects, but this phenomenon varies considerably among different insect taxa [2, 64].
However, scientists still find larger-bodied species in remote or poorly studied parts of the world (Figure 3). Many islands in the Philippines, for instance, remain unexplored. Recent discoveries there include a 2-m long monitor lizard (Varanus olivaceus) [65] and a large-bodied fruit-bat (Styloctenium mindorensis) [66]. Local communities hunt both. Along with small body size, geographical remoteness affects the rate at which taxonomists discover species.
Figure 3Relatively large, conspicuous species are still being discovered in remote or poorly studied areas. Shown are (a) an undescribed jay species (Cyanocorax sp.) from the Amazon basin, (b) a recently discovered fruit bat (Styloctenium mindoroensis) and (c) monitor lizard (Varanus bitatawa) from the Philippines. Photographs reproduced, with permission, from Mario Cohn-Haft (a), H.J.D. Garcia/Haribon Foundation (b), and Joseph Brown (c).
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Unknown species might also be less colorful or obvious than their described brethren. We hypothesize, for instance, that taxonomists will describe brightly colored bird species earlier than drab, earth-toned bird species. Species with cryptic behaviors also tend to be discovered later (Figure 2). For instance, as-yet-undescribed shore fish are likely to be those that hide in deeper waters [51], whereas researchers recently discovered a fossorial caecilian, representing an entirely new family (Chikilidae), only after 1100 h of digging holes in the ground [67]. Animals with elusive life histories can be discovered even in the best-studied parts of the world. A recently described fossorial salamander in the southeastern USA not only represents a new genus (Urspelerpes), but is also among the smallest salamander species ever found [68].
Finally, taxonomists describe small-ranged species later than more widely distributed ones [63, 64, 69]. Such trends are evident in holozooplankton [69], fleas [70], leaf beetles [71], Palaearctic dung beetles [72], South American oscine songbirds [59], and Neotropical mammals [60].
Missing species will typically be more vulnerable than are described species. Most often, two key factors combine to determine the threat level for a species under the IUCN Red List criteria: its geographical range size and the amount of its habitat loss. We have already emphasized that missing species are generally concentrated in the places where habitat loss is greatest. In showing that missing species also tend to have small ranges, we can be certain that many will eventually be listed as ‘threatened’; that is, if they do not become extinct first.
The high vulnerability of missing species is evident in Brazil, which has the largest number of amphibian species globally (Figure 4). Although local amphibian diversity is especially high in the western Brazilian Amazon, the greatest concentration of species with small geographic ranges is in the coastal hotspot of the Atlantic forest [27]. Taxonomists described most of these small-ranged species only within the past two decades, a pattern similar to that for mammals in Brazil [47]. Missing species, such as those only recently discovered, will probably also be in such vulnerable areas. Only approximately 7% of the original Brazilian Atlantic forest remains [27].
Figure 4Although amphibian species reach their peak local diversity in the Brazilian Amazon, the greatest concentration of small-ranged species is in the Brazilian Atlantic forests (a). (b) The number of small-ranged species in Brazil is increasing exponentially, with most discovered only recently. Reproduced, with permission, from [27] (a) and [47] (b); photograph reproduced with permission from Luis A. Mazariegos.
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All this signals that researchers are underestimating the magnitude of the current extinction crisis, because many undiscovered species will both have small ranges and occur in threatened hotspots [20]. Including estimates of missing species increases the percentage of threatened plants to 27–33% of all plant species [13]. If many species are cryptic (Box 1), the figure could be even higher.
Prospects
Relative to the task at hand, taxonomists are describing species slowly. Although the catalog of flowering plants should be compete in a few decades [13], recent estimates suggest another 480 years is needed to describe all the species on Earth [23], or possibly 1000 years just to describe all fungi [40]. Yet, the outlook is considerably brighter than one might suppose, for several reasons.
Herbaria and museums might harbor many of the missing species. For example, Bebber et al. [73] found that existing herbarium material typically took decades to describe. They estimated that perhaps half of all missing plant species were already in herbaria.
Recent advances in DNA barcoding make it easier to discriminate similar species [74, 75], thereby accelerating species descriptions and generally aiding better taxonomy. Barcoding is also inherently a quantitative technique, allowing statistical sampling methods to estimate what fraction of samples are missing species and how species turn over geographically. Potentially, barcoding can address many of the methodological concerns we have highlighted here. Nonetheless, the use of ‘floating barcodes’ (ones without associated morphological descriptions of organisms) generates considerable debate.
The genetic methods used to detect fungi discussed above are rapidly expanding knowledge of what could be an extremely diverse group, but one poorly sampled by traditional morphological approaches.
Many communities of taxonomists are now addressing the tedious but vital issue of synonymy and placing their lists and taxonomic decisions into the public domain. These include websites for flowering plants (​http://www.kew.org/wcsp/​), spiders (​http://research.amnh.org/oonopidae/catalog/​) amphibians (​http://research.amnh.org/herpetology/amphibia/index.php​), birds (​http://www.birdlife.org/datazone/info/taxonomy​), and mammals (​http://www.bucknell.edu/msw3/​). Global efforts to catalogue all species, such as All-Species (​http://www.allspecies.org​), GBIF (http://www.gbif.org), Species 2000 (www.sp2000.org), and Tree of Life (​http://www.tolweb.org/tree/phylogeny.html​), are also now readily available online.
Efforts to map where species occur are progressing. The most obvious advance is using smartphones and software-website applications such as iNaturalist (​http://www.inaturalist.org​) that link data directly into the IUCN Red Lists, the Global Biodiversity Information Facility, and other pre-existing databases. Crowd-sourcing of species mapping could greatly expand these databases, which are major contributions to knowledge of where species live. Such databases are already promoting the discovery of missing species, revealing those that do not fit known descriptions.
Finally, even if it might not be practical or even desirable to describe every species, cataloguing carefully selected taxa, locations, or regions might generate important insights [56]. Better quantification of the number and locations of known species afford a fighting chance to set effective conservation priorities, even if the taxonomic catalog is incomplete.
Acknowledgments
We thank Peter Raven, David Hawksworth, and two anonymous referees for helpful comments and for supporting our decision not to provide yet another conjectural estimate of the total number of species on Earth. A Singapore International Graduate Award and Wildlife Reserves Singapore Conservation Fund grant supported B.R.S. and an Australian Laureate Fellowship supported W.F.L.
References
Westwood J.
On the probable number of species of insects in the creation; together with descriptions of several minute Hymenoptera.
Magaz. Nat. Hist. J. Zool. Bot. Mineral. Geol. Meterol. 1833; 6: 116-123
View in Article
Google Scholar
Gaston K.J.
The magnitude of global insect species richness.
Conserv. Biol. 1991; 5: 283-296
View in Article
Scopus (313)
Crossref
Google Scholar
Stork N.E.
How many species are there?.
Biodivers. Conserv. 1993; 2: 215-232
View in Article
Scopus (161)
Crossref
Google Scholar
Erwin T.L.
Tropical forests: their richness in coleoptera and other arthropod species.
Coleopt. Bull. 1982; 36: 74-75
View in Article
Google Scholar
May R.M.
Beverton R.J.H.

How many species?.
Philos. Trans. R. Soc. B. 1990; 330: 293-304
View in Article
Crossref
Google Scholar
Mora C.
et al.

How many species are there on earth and in the ocean?.
PLoS Biol. 2011; 9: e1001127
View in Article
Scopus (1213)
PubMed
Crossref
Google Scholar
Chapman A.D.
Numbers of Living Species in Australia and the World.
2nd edn. Australian Biodiversity Information Services, 2009
View in Article
Google Scholar
Pimm S.L.
et al.

The future of biodiversity.
Science. 1995; 269: 347-350
View in Article
Scopus (1431)
PubMed
Crossref
Google Scholar
Joppa L.N.
et al.

Biodiversity hotspots house most undiscovered plant species.
Proc. Natl. Acad. Sci. U.S.A. 2011; 108: 13171-13176
View in Article
Scopus (137)
PubMed
Crossref
Google Scholar
Dirzo R.
Raven P.H.

Global state of biodiversity and loss.
Annu. Rev. Environ. Resour. 2003; 28: 137-167
View in Article
Scopus (729)
Crossref
Google Scholar
Mora C.
et al.

The completeness of taxonomic inventories for describing the global diversity and distribution of marine fishes.
Proc. R. Soc. B. 2008; 275: 149-155
View in Article
Scopus (136)
PubMed
Crossref
Google Scholar
Carbayo F.
Marques A.C.

The costs of describing the entire animal kingdom.
Trends Ecol. Evol. 2011; 26: 154-155
View in Article
Scopus (41)
PubMed
Abstract
Full Text
Full Text PDF
Google Scholar
Joppa L.N.
et al.

How many species of flowering plants are there?.
Proc. R. Soc. B. 2010; 278: 554
View in Article
Scopus (152)
PubMed
Crossref
Google Scholar
Bebber D.P.
et al.

Predicting unknown species numbers using discovery curves.
Proc. R. Soc. B. 2007; 274: 1651-1658
View in Article
Scopus (101)
PubMed
Crossref
Google Scholar
ØDegaard F.
How many species of arthropods? Erwin's estimate revised.
Biol. J. Linn. Soc. 2000; 71: 583-597
View in Article
Crossref
Google Scholar
Scotland R.W.
Wortley A.H.

How many species of seed plants are there?.
Taxon. 2003; 52: 101-104
View in Article
Scopus (121)
Crossref
Google Scholar
WCSP
World Checklist of Selected Plant Families.
Royal Botanic Gardens, 2008
View in Article
Google Scholar
Hawksworth D.L.
et al.

The Amsterdam declaration on fungal nomenclature.
IMA Fungus. 2011; 2: 105-112
View in Article
Scopus (250)
PubMed
Crossref
Google Scholar
Hawksworth D.L.
The need for a more effective biological nomenclature for the 21st century.
Bot. J. Linn. Soc. 1992; 109: 543-567
View in Article
Scopus (29)
Crossref
Google Scholar
Giam X.
et al.

Reservoirs of richness: least disturbed tropical forests are centres of undescribed species diversity.
Proc. R. Soc. B. 2011;
https://doi.org/10.1098/rspb.2011.0433
View in Article
Scopus (81)
PubMed
Crossref
Google Scholar
Hamilton A.J.
et al.

Quantifying uncertainty in estimation of tropical arthropod species richness.
Am. Nat. 2010; 176: 90-95
View in Article
Scopus (162)
PubMed
Crossref
Google Scholar
Creer S.
et al.

Ultrasequencing of the meiofaunal biosphere: practice, pitfalls, and promise.
Mol. Ecol. 2010; 19: 4-20
View in Article
Scopus (241)
PubMed
Crossref
Google Scholar
May R.M.
Why worry about how many species and their loss?.
PLoS Biol. 2011; 9: e1001130
View in Article
Scopus (47)
PubMed
Crossref
Google Scholar
Gaston K.J.
May R.M.

The taxonomy of taxonomists.
Nature. 1992; 356: 281-282
View in Article
Scopus (200)
Crossref
Google Scholar
Hodkinson I.D.
Casson D.

A lesser predilection for bugs: Hemiptera (Insecta) diversity in tropical rain forests.
Biol. J. Linn. Soc. 1991; 43: 101-109
View in Article
Scopus (100)
Crossref
Google Scholar
Adamowicz S.J.
Purvis A.

How many branchiopod crustacean species are there? Quantifying the components of underestimation.
Global Ecol. Biogeogr. 2005; 14: 455-468
View in Article
Scopus (64)
Crossref
Google Scholar
Pimm S.L.
Jenkins C.N.

Extinctions and the practice of preventing them.
in: Sodhi N.S. Ehrlich P.R. Conservation Biology for All. Oxford University Press, 2010: 181-196
View in Article
Scopus (31)
Crossref
Google Scholar
Grassle J.F.
Maciolek N.J.

Deep-sea species richness: regional and local diversity estimates from quantitative bottom samples.
Am. Nat. 1992; 139: 313-341
View in Article
Scopus (557)
Crossref
Google Scholar
Lambshead P.J.D.
Boucher G.

Marine nematode deep-sea biodiversity – hyperdiverse or hype?.
J. Biogeogr. 2003; 30: 475-485
View in Article
Scopus (147)
Crossref
Google Scholar
Gering J.C.
et al.

Scale dependence of effective specialization: its analysis and implications for estimates of global insect species richness.
Divers. Distrib. 2007; 13: 115-125
View in Article
Scopus (12)
Crossref
Google Scholar
May R.M.
The dimensions of life on Earth.
in: Raven P.H. Williams T. Nature and Human Society: The Quest for a Sustainale World. National Academy of Sciences Press, 2000: 30-45
View in Article
Google Scholar
Novotny V.
et al.

Low beta diversity of herbivorous insects in tropical forests.
Nature. 2007; 448: 692-695
View in Article
Scopus (199)
PubMed
Crossref
Google Scholar
Hamilton A.J.
et al.

Correction.
Am. Nat. 2011; 177: 544-545
View in Article
Scopus (21)
Crossref
Google Scholar
Anonymous
New insights into global fungal species numbers?.
IMA Fungus. 2011; 2: 59-60
View in Article
Google Scholar
Hawksworth D.L.
The fungal dimension of biodiversity: magnitude, significance, and conservation.
Mycol. Res. 1991; 95: 641-655
View in Article
Scopus (927)
Crossref
Google Scholar
May R.M.
A fondness for fungi.
Nature. 1991; 352: 475-476
View in Article
Scopus (79)
Crossref
Google Scholar
Bass D.
Richards T.A.

Three reasons to re-evaluate fungal diversity ‘on Earth and in the ocean’.
Fungal Biol. Rev. 2011; 25: 159-164
View in Article
Scopus (66)
Crossref
Google Scholar
Cannon P.F.
Diversity of the Phyllachoraceae with special reference to the tropics.
in: Hyde K.D. Biodiversity of Tropical Microfungi. Hong Kong University Press, 1997: 255-278
View in Article
Google Scholar
O’Brien H.E.
et al.

Fungal community analysis by large-scale sequencing of environmental samples.
Appl. Environ. Microbiol. 2005; 71: 5544-5550
View in Article
Scopus (625)
PubMed
Crossref
Google Scholar
Blackwell M.
The Fungi: 1, 2, 3…5.1 million species?.
Am. J. Bot. 2011; 98: 426-438
View in Article
Scopus (691)
PubMed
Crossref
Google Scholar
Williams P.H.
Gaston K.J.

Measuring more of biodiversity: can higher-taxon richness predict wholesale species richness?.
Biol. Conserv. 1994; 67: 211-217
View in Article
Scopus (306)
Crossref
Google Scholar
Ricotta C.
et al.

Using the scaling behaviour of higher taxa for the assessment of species richness.
Biol. Conserv. 2002; 107: 131-133
View in Article
Scopus (13)
Crossref
Google Scholar
Solow A.R.
Smith W.K.

On estimating the number of species from the discovery record.
Proc. R. Soc. B. 2005; 272: 285-287
View in Article
Scopus (29)
PubMed
Crossref
Google Scholar
Costello M.J.
Wilson S.P.

Predicting the number of known and unknown species in European seas using rates of description.
Global Ecol. Biogeogr. 2011; 20: 319-330
View in Article
Scopus (54)
Crossref
Google Scholar
Wilson S.P.
Costello M.J.

Predicting future discoveries of European marine species by using a non-homogeneous renewal process.
Appl. Stat. 2005; 54: 897-918
View in Article
Google Scholar
Joppa L.N.
et al.

The population ecology and social behaviour of taxonomists.
Trends Ecol. Evol. 2011; 26: 551-553
View in Article
Scopus (77)
PubMed
Abstract
Full Text
Full Text PDF
Google Scholar
Pimm S.L.
et al.

How many endangered species remain to be discovered in Brazil?.
Nat. Conserv. 2010; 8: 71-77
View in Article
Google Scholar
Scheffers B.R.
et al.

The world's rediscovered species: back from the brink?.
PLoS ONE. 2011; 6: e22531
View in Article
PubMed
Google Scholar
Myers N.
et al.

Biodiversity hotspots for conservation priorities.
Nature. 2000; 403: 853-858
View in Article
Scopus (16968)
PubMed
Crossref
Google Scholar
Dalton R.
Biodiversity cash aimed at hotspots.
Nature. 2000; 406: 818
View in Article
Google Scholar
Zapata F.A.
Robertson R.D.

How many species of shore fishes are there in the tropical eastern Pacific?.
J. Biogeogr. 2007; 34: 38-51
View in Article
Google Scholar
Ceballos G.
Ehrlich P.R.

Discoveries of new mammal species and their implications for conservation and ecosystem services.
Proc. Natl. Acad. Sci. U.S.A. 2009; 106: 3841-3846
View in Article
PubMed
Google Scholar
Medellín R.A.
Soberón J.

Predictions of mammal diversity on four land masses.
Conserv. Biol. 1999; 13: 143-149
View in Article
Google Scholar
Giam X.
et al.

The extent of undiscovered species in Southeast Asia.
Biodivers. Conserv. 2010; 19: 943-954
View in Article
Google Scholar
Laurance W.F.
Edwards D.P.

The search for unknown biodiversity.
Proc. Natl. Acad. Sci. U.S.A. 2011; 108: 12971-12972
View in Article
PubMed
Google Scholar
Joppa L.N.
et al.

Taxonomy that matters: response to Bacher.
Trends Ecol. Evol. 2011; 27: 66
View in Article
Google Scholar
Chávez G.
Vásquez D.

A new species of Andean semiaquatic lizard of the genus Potamites (Sauria, Gymnophtalmidae) from southern Peru.
ZooKeys. 2012; 168: 31-43
View in Article
PubMed
Google Scholar
Gaston K.J.
Body size and probability of description: the beetle fauna of Britain.
Ecol. Entomol. 1991; 16: 505-508
View in Article
Scopus (75)
Crossref
Google Scholar
Blackburn T.M.
Gaston K.J.

What determines the probability of discovering a species? A study of South American oscine passerine birds.
J. Biogeogr. 1995; 22: 7-14
View in Article
Google Scholar
Patterson B.D.
Accumulating knowledge on the dimensions of biodiversity: systematic perspectives on Neotropical mammals.
Biodivers. Lett. 1994; 2: 79-86
View in Article
Google Scholar
Blackburn T.
Gaston K.

The distribution of mammal body masses.
Divers. Distrib. 1998; 4: 121-133
View in Article
Google Scholar
Blackburn T.
Gaston K.J.

The distribution of body sizes of the world's bird species.
Oikos. 1994; 70: 127-130
View in Article
Scopus (98)
Crossref
Google Scholar
Gaston K.J.
Blackburn T.

Are newly described bird species small-bodied?.
Biodivers. Lett. 1994; 2: 16-20
View in Article
Google Scholar
Gaston K.J.
et al.

Which species are described first? The case of North American butterflies.
Biodivers. Conserv. 1995; 4: 119-127
View in Article
Google Scholar
Welton L.J.
et al.

A spectacular new Philippine monitor lizard reveals a hidden biogeographic boundary and a novel flagship species for conservation.
Biol. Lett. 2010; 6: 654-658
View in Article
PubMed
Google Scholar
Esselstyn J.A.
A new species of stripe-faced fruit bat (Chiroptera: Pteropodidae: Styloctenium) from the Philippines.
J. Mammal. 2007; 88: 951-958
View in Article
Google Scholar
Kamei R.G.
et al.

Discovery of a new family of amphibians from northeast India with ancient links to Africa.
Proc. R. Soc. B. 2012; 279: 2396-2401
View in Article
PubMed
Google Scholar
Camp C.D.
et al.

A new genus and species of lungless salamander (family Plethodontidae) from the Appalachian highlands of the south-eastern United States.
J. Zool. 2009; 279: 86-94
View in Article
Google Scholar
Gibbons M.J.
et al.

What determines the likelihood of species discovery in marine holozooplankton: is size, range or depth important?.
Oikos. 2005; 109: 567-576
View in Article
Google Scholar
Krasnov B.
et al.

What are the factors determining the probability of discovering a flea species (Siphonaptera)?.
Parasitol. Res. 2005; 97: 228-237
View in Article
Scopus (12)
PubMed
Crossref
Google Scholar
Baselga A.
et al.

Which leaf beetles have not yet been described? Determinants of the description of western Palaearctic Aphthona species (Coleoptera: Chrysomelidae).
Biodivers. Conserv. 2007; 16: 1409-1421
View in Article
Google Scholar
Cabrero-Sañudo F.J.
Lobo J.M.

Estimating the number of species not yet described and their characteristics: the case of Western Palaearctic dung beetle species (Coleoptera, Scarabaeoidea).
Biodivers. Conserv. 2003; 12: 147-166
View in Article
Google Scholar
Bebber D.P.
et al.

Herbaria are a major frontier for species discovery.
Proc. Natl. Acad. Sci. U.S.A. 2010; 107: 22169-22171
View in Article
Scopus (196)
PubMed
Crossref
Google Scholar
Godfray H.C.J.
Knapp S.

Introduction.
Philos. Trans. R. Soc. B. 2004; 359: 559-569
View in Article
PubMed
Google Scholar
Smith M.A.
et al.

DNA barcodes reveal cryptic host-specificity within the presumed polyphagous members of a genus of parasitoid flies (Diptera: Tachinidae).
Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 3657-3662
View in Article
Scopus (457)
PubMed
Crossref
Google Scholar
Hebert P.D.N.
et al.

Ten species in one: DNA barcoding reveals cryptic species in the Neotropical skipper butterfly Astraptes fulgerator.
Proc. Natl. Acad. Sci. U.S.A. 2004; 101: 14812-14817
View in Article
Scopus (2371)
PubMed
Crossref
Google Scholar
Hibbett D.S.
et al.

Progress in molecular and morphological taxon discovery in Fungi and options for formal classification of environmental sequences.
Fungal Biol. Rev. 2011; 25: 38-47
View in Article
Scopus (208)
Crossref
Google Scholar
Bickford D.
et al.

Cryptic species as a window on diversity and conservation.
Trends Ecol. Evol. 2007; 22: 148-155
View in Article
Scopus (2115)
PubMed
Abstract
Full Text
Full Text PDF
Google Scholar
Pfenninger M.
Schwenk K.

Cryptic animal species are homogeneously distributed among taxa and biogeographical regions.
BMC Evol. Biol. 2007; 7: 121
View in Article
PubMed
Google Scholar
Dejean T.
et al.

Persistence of environmental DNA in freshwater ecosystems.
PLoS ONE. 2011; 6: e23398
View in Article
PubMed
Google Scholar
Ficetola G.F.
et al.

Species detection using environmental DNA from water samples.
Biol. Lett. 2008; 4: 423-425
View in Article
Scopus (726)
PubMed
Crossref
Google Scholar
Beattie A.J.
Discovering new biological resources: chance or reason?.
Bioscience. 1992; 42: 290-292
View in Article
Google Scholar
Chivian E.
et al.

The threat to cone snails.
Science. 2003; 302: 391
View in Article
PubMed
Google Scholar
Bogin O.
Venom peptides and their mimetics as potential drugs.
Modulator. 2005; 19: 14-20
View in Article
Google Scholar
Livett B.G.
et al.

Therapeutic applications of conotoxins that target the neuronal nicotinic acetylcholine receptor.
Toxicon. 2006; 48: 810-829
View in Article
Scopus (118)
PubMed
Crossref
Google Scholar
Article Info
Publication History
Published online: July 11, 2012
Identification
DOI: https://doi.org/10.1016/j.tree.2012.05.008
Copyright
© 2012 Elsevier Ltd. Published by Elsevier Inc. All rights reserved.
ScienceDirect
Access this article on ScienceDirect
Figures
Figure ITen species of cryptic caterpillars in the
Figure 1The percentages of missing plant species predicted to occur in various regions suggest that existing biodiversity hotspots, such as southern Africa, Central America, and especially the northern Andes, hold the greatest numbers. However, unexpectedly low predicted numbers in places such as New Guinea might reflect their inaccessibility to scientists, causing missing-species numbers to be underestimated. Stephanie Pimm Lyon photographed these three orchids in the genus
Figure IExamples of cone shells. Chivian et al. [
Figure 2Many undiscovered species are difficult to find because they are cryptic, small in size or have small geographic ranges. Shown are
Figure 3Relatively large, conspicuous species are still being discovered in remote or poorly studied areas. Shown are
Figure 4Although amphibian species reach their peak local diversity in the Brazilian Amazon, the greatest concentration of small-ranged species is in the Brazilian Atlantic forests
Tables
Table 1Chapman's (2009) estimates of species numbers 7, with other noteworthy estimates discussed in the main text
Linked Article
Erratum to: “What we know and don’t know about Earth's missing biodiversity”: [ 27 (2012) 501–510]
Scheffers et al.
Trends in Ecology & EvolutionOctober 8, 2012
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