What Percentage Of Animal Life Lives On Land

• Journal Listing
• PLoS Biol
• v.9(8); 2011 Aug
• PMC3160336

PLoS Biol. 2011 Aug; ix(8): e1001127.

How Many Species Are At that place on Earth and in the Ocean?

Camilo Mora

¹Section of Biological science, Dalhousie Academy, Halifax, Nova Scotia, Canada

²Department of Geography, University of Hawaii, Honolulu, Hawaii, Us

Derek P. Tittensor

¹Department of Biological science, Dalhousie University, Halifax, Nova Scotia, Canada

³United nations Surroundings Programme World Conservation Monitoring Heart, Cambridge, United Kingdom

^fourMicrosoft Enquiry, Cambridge, United Kingdom

Sina Adl

¹Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

Alastair G. B. Simpson

^oneDepartment of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

Boris Worm

^aneDepartment of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

Georgina G. Mace, Academic Editor

Received 2010 Nov 12; Accepted 2011 Jul thirteen.

Supplementary Materials

Effigy S1: Abyss of the higher taxonomy of kingdoms of life on Globe.

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Figure S2: Sensitivity analysis due to changes in higher taxonomy.

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Effigy S3: Assessing the effects of information incompleteness.

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Effigy S4: Comparing of the fits of the hyperexponential, exponential, and ability functions to the relationship between the number of higher taxa and their numerical rank.

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Abstruse

The diversity of life is one of the most striking aspects of our planet; hence knowing how many species inhabit World is among the most central questions in science. Notwithstanding the reply to this question remains enigmatic, as efforts to sample the globe's biodiversity to date have been limited and thus take precluded direct quantification of global species richness, and because indirect estimates rely on assumptions that have proven highly controversial. Hither we prove that the higher taxonomic classification of species (i.e., the assignment of species to phylum, grade, order, family, and genus) follows a consequent and predictable pattern from which the total number of species in a taxonomic grouping can be estimated. This approach was validated against well-known taxa, and when applied to all domains of life, information technology predicts ∼eight.7 million (±1.three million SE) eukaryotic species globally, of which ∼2.2 million (±0.xviii million SE) are marine. In spite of 250 years of taxonomic classification and over 1.2 million species already catalogued in a central database, our results propose that some 86% of existing species on World and 91% of species in the ocean all the same look clarification. Renewed interest in further exploration and taxonomy is required if this significant gap in our knowledge of life on Earth is to be closed.

Writer Summary

Knowing the number of species on World is one of the most basic yet elusive questions in science. Unfortunately, obtaining an accurate number is constrained by the fact that almost species remain to be described and because indirect attempts to answer this question accept been highly controversial. Here, we document that the taxonomic classification of species into higher taxonomic groups (from genera to phyla) follows a consistent blueprint from which the total number of species in any taxonomic grouping can be predicted. Assessment of this pattern for all kingdoms of life on World predicts ∼8.7 million (±1.three one thousand thousand SE) species globally, of which ∼two.2 million (±0.xviii million SE) are marine. Our results propose that some 86% of the species on World, and 91% in the bounding main, still wait description. Closing this knowledge gap volition require a renewed interest in exploration and taxonomy, and a continuing endeavor to catalogue existing biodiversity data in publicly available databases.

Introduction

Robert May [1] recently noted that if aliens visited our planet, one of their first questions would be, "How many singled-out life forms—species—does your planet have?" He also pointed out that nosotros would exist "embarrassed" past the uncertainty in our reply. This narrative illustrates the central nature of knowing how many species there are on Earth, and our limited progress with this inquiry topic thus far [1]–[iv]. Unfortunately, limited sampling of the earth's biodiversity to date has prevented a directly quantification of the number of species on Earth, while indirect estimates remain uncertain due to the use of controversial approaches (see detailed review of available methods, estimates, and limitations in Table one). Globally, our best approximation to the total number of species is based on the stance of taxonomic experts, whose estimates range betwixt 3 and 100 million species [1]; although these estimations probable represent the outer premises of the full number of species, adept-opinion approaches have been questioned due to their limited empirical basis [5] and subjectivity [5]–[6] (Tabular array 1). Other studies have used macroecological patterns and biodiversity ratios in novel ways to ameliorate estimates of the total number of species (Table 1), but several of the underlying assumptions in these approaches have been the topic of sometimes heated controversy ([3]–[17], Table 1); moreover their overall predictions concern simply specific groups, such as insects [ix],[18]–[19], deep sea invertebrates [13], large organisms [6]–[vii],[10], animals [7], fungi [xx], or plants [21]. With the exception of a few extensively studied taxa (east.thousand., birds [22], fishes [23]), we are still remarkably uncertain every bit to how many species exist, highlighting a significant gap in our basic knowledge of life on Earth. Here we present a quantitative method to guess the global number of species in all domains of life. We report that the number of higher taxa, which is much more completely known than the total number of species [24], is strongly correlated to taxonomic rank [25] and that such a pattern allows the extrapolation of the global number of species for whatever kingdom of life (Figures 1 and ​ two).

Predicting the global number of species in Animalia from their higher taxonomy.

(A–F) The temporal accumulation of taxa (black lines) and the frequency of the multimodel fits to all starting years selected (graded colors). The horizontal dashed lines indicate the consensus asymptotic number of taxa, and the horizontal greyness area its consensus standard mistake. (G) Relationship between the consensus asymptotic number of higher taxa and the numerical hierarchy of each taxonomic rank. Blackness circles represent the consensus asymptotes, green circles the catalogued number of taxa, and the box at the species level indicates the 95% confidence interval around the predicted number of species (see Materials and Methods).

Validating the higher taxon approach.

Nosotros compared the number of species estimated from the higher taxon approach implemented hither to the known number of species in relatively well-studied taxonomic groups as derived from published sources [37]. Nosotros also used estimations from multimodel averaging from species accumulation curves for taxa with near-complete inventories. Vertical lines indicate the range of variation in the number of species from different sources. The dotted line indicates the 1∶ane ratio. Note that published species numbers (y-axis values) are mostly derived from skillful approximations for well-known groups; hence there is a possibility that those estimates are subject field to biases arising from synonyms.

Table ane

Bachelor methods for estimating the global number of species and their limitations.

Case Study	Limitations
Macroecological patterns
Body size frequency distributions. By extrapolation from the frequency of big to small species, May [seven] estimated 10 to fifty million species of animals.	May [7] suggested that in that location was no reason to look a simple scaling constabulary from large to small species. Further studies confirmed unlike modes of evolution amongst small species [4] and inconsistent body size frequency distributions amongst taxa [4].
Latitudinal gradients in species. By extrapolation from the better sampled temperate regions to the tropics, Raven [x] estimated 3 to 5 1000000 species of big organisms.	May [2] questioned the assumption that temperate regions were better sampled than tropical ones; the arroyo likewise causeless consistent diversity gradients across taxa which is not factual [4].
Species-surface area relationships. By extrapolation from the number of species in deep-sea samples, Grassle & Maciolek [13] estimated that the earth'south deep seafloor could contain upward to 10 one thousand thousand species.	Lambshead & Bouchet [12] questioned this interpretation by showing that high local multifariousness in the deep ocean does not necessarily reverberate loftier global biodiversity given low species turnover.
Diversity ratios
Ratios between taxa. Past bold a global 6∶i ratio of fungi to vascular plants and that at that place are ∼270,000 species of vascular plants, Hawksworth [20] estimated one.6 1000000 fungi species.	Ratio-like approaches have been heavily critiqued because, given known patterns of species turnover, locally estimated ratios between taxa may or may not exist consistent at the global scale [3],[12] and because at least one group of organisms should be well known at the global scale, which may non always be truthful [15]. Bouchet [6] elegantly demonstrated the shortcomings of ratio-based approaches by showing how even for a well-inventoried marine region, the ratio of fishes to total multicellular organisms would yield ∼0.five one thousand thousand global marine species whereas the ratio of Brachyura to total multicellular organisms in the same sampled region would yield ∼one.v one thousand thousand species.
Host-specificity and spatial ratios. Given 50,000 known species of tropical trees and assuming a five∶i ratio of host beetles to trees, that beetles stand for 40% of the awning arthropods, and that the awning has twice the species of the footing, Erwin [ix] estimated xxx million species of arthropods in the tropics.
Known to unknown ratios . Hodkinson & Casson [18] estimated that 62.v% of the bug (Hemiptera) species in a sampled location were unknown; by assuming that vii.5%–10% of the global multifariousness of insects is bugs, they estimated betwixt 1.84 and 2.57 million species of insects globally.
Taxonomic patterns
Time-species accumulation curves. By extrapolation from the discovery record it was estimated that there are ∼xix,800 species of marine fishes [23] and ∼11,997 birds [22].	This approach is non widely applicable considering it requires species accumulation curves to arroyo asymptotic levels, which is just truthful for a small number of well-described taxa [22]–[23].
Authors-species accumulation curves. Modeling the number of authors describing species over time allowed researchers to estimate that the proportion of flowering plants however to be discovered is xiii% to 18% [21].	This is a very recent method and the event of a number of assumptions remains to be evaluated. One is the extent to which the description of new species is shifting from using taxonomic expertise alone to relying on molecular methods (particularly among small organisms [26]) and the other that not all authors listed on a manuscript are taxonomic experts, particularly in recent times when the number of coauthors per taxa described is increasing [21],[38], which could exist due to more collaborative research [38] and the acquittance of technicians, field assistants, specimen collectors, and and so on as coauthors (Philippe Bouchet, personal communication).
Analysis of skilful estimations. Estimates of ∼5 meg species of insects [15] and ∼200,000 marine species [xiv] were arrived at past compiling opinion-based estimates from taxonomic experts. Robustness in the estimations is causeless from the consistency of responses among different experts.	Erwin [v] labeled this approach equally "non-scientific" due to a lack of verification. Estimates can vary widely, even those of a single expert [5],[6]. Bouchet [6] argues that adept estimations are often passed on from one adept to some other and therefore a robust estimation could be the "same guess copied over again and again".

Higher taxonomy information have been previously used to quantify species richness within specific areas by relating the number of species to the number of genera or families at well-sampled locations, and then using the resulting regression model to judge the number of species at other locations for which the number of families or genera are better known than species richness (reviewed by Gaston & Williams [24]). This method, however, relies on extrapolation of patterns from relatively small areas to approximate the number of species in other locations (i.due east., blastoff diversity). Matching the spatial scale of this method to quantify the Earth's total number of species would require knowing the richness of replicated planets; not an option equally far as we know, although May's aliens may disagree. Hither we clarify higher taxonomic data using a different approach past assessing patterns beyond all taxonomic levels of major taxonomic groups. The existence of predictable patterns in the higher taxonomic classification of species allows prediction of the full number of species within taxonomic groups and may help to better constrain our estimates of global species richness.

Results

Nosotros compiled the total taxonomic classifications of ∼ane.2 million currently valid species from several publicly attainable sources (meet Materials and Methods). Amongst eukaryote "kingdoms," assessment of the temporal accumulation curves of higher taxa (i.e., the cumulative number of species, genera, orders, classes, and phyla described over fourth dimension) indicated that higher taxonomic ranks are much more completely described than lower levels, equally shown by strongly asymptoting trajectories over time ([24], Figure 1A–1F, Effigy S1). However, this is not the case for prokaryotes, where there is piddling indication of reaching an asymptote at any taxonomic level (Figure S1). For near eukaryotes, in contrast, the charge per unit of discovery of new taxa has slowed along the taxonomic hierarchy, with clear signs of asymptotes for phyla (or "divisions" in botanical nomenclature) on one hand and a steady increase in the number of species on the other (Effigy 1A–1F, Figure S1). This prevents direct extrapolation of the number of species from species-accumulation curves [22],[23] and highlights our current dubiety regarding estimates of total species richness (Figure 1F). Even so, the increasing completeness of college taxonomic ranks could facilitate the estimation of the total number of species, if the old predicts the latter. We evaluated this hypothesis for all kingdoms of life on Earth.

First, we accounted for undiscovered higher taxa by fitting, for each taxonomic level from phylum to genus, asymptotic regression models to the temporal accumulation curves of higher taxa (Figure 1A–1E) and using a formal multimodel averaging framework based on Akaike'due south Data Criterion [23] to predict the asymptotic number of taxa of each taxonomic level (dotted horizontal line in Figure 1A–11E; come across Materials and Methods for details). Secondly, the predicted number of taxa at each taxonomic rank down to genus was regressed against the numerical rank, and the fitted models used to predict the number of species (Effigy 1G, Materials and Methods). Nosotros applied this approach to 18 taxonomic groups for which the total numbers of species are thought to exist relatively well known. Nosotros constitute that this approach yields predictions of species numbers that are consistent with inventory totals for these groups (Figure two). When applied to all eukaryote kingdoms, our approach predicted ∼7.77 million species of animals, ∼298,000 species of plants, ∼611,000 species of fungi, ∼36,400 species of protozoa, and ∼27,500 species of chromists; in total the arroyo predicted that ∼viii.74 one thousand thousand species of eukaryotes exist on Globe (Table two). Restricting this arroyo to marine taxa resulted in a prediction of ii.21 million eukaryote species in the globe's oceans (Table 2). We also applied the arroyo to prokaryotes; unfortunately, the steady stride of description of taxa at all taxonomic ranks precluded the adding of asymptotes for higher taxa (Figure S1). Thus, we used raw numbers of higher taxa (rather than asymptotic estimates) for prokaryotes, and as such our estimates correspond only lower bounds on the diversity in this group. Our approach predicted a lower bound of ∼ten,100 species of prokaryotes, of which ∼1,320 are marine. It is important to note that for prokaryotes, the species concept tolerates a much college caste of genetic dissimilarity than in nearly eukaryotes [26],[27]; additionally, due to horizontal cistron transfers amongst phylogenetic clades, species take longer to isolate in prokaryotes than in eukaryotes, and thus the sometime species are much older than the latter [26],[27]; equally a result the number of described species of prokaryotes is small (only ∼10,000 species are currently accepted).

Table 2

Currently catalogued and predicted total number of species on Earth and in the ocean.

Species	Earth			Ocean
	Catalogued	Predicted	±SE	Catalogued	Predicted	±SE
Eukaryotes
Animalia	953,434	7,770,000	958,000	171,082	ii,150,000	145,000
Chromista	13,033	27,500	30,500	four,859	7,400	9,640
Fungi	43,271	611,000	297,000	one,097	5,320	eleven,100
Plantae	215,644	298,000	eight,200	8,600	16,600	9,130
Protozoa	eight,118	36,400	six,690	eight,118	36,400	6,690
Full	1,233,500	8,740,000	1,300,000	193,756	2,210,000	182,000
Prokaryotes
Archaea	502	455	160	1	1	0
Bacteria	10,358	9,680	iii,470	652	one,320	436
Full	x,860	10,100	3,630	653	ane,320	436
Grand Total	1,244,360	8,750,000	1,300,000	194,409	2,210,000	182,000

Assessment of Possible Limitations

We recognize a number of factors that tin can influence the interpretation and robustness of the estimates derived from the method described here. These are analyzed beneath.

Species definitions

An important caveat to the estimation of our results concerns the definition of species. Different taxonomic communities (e.g., zoologists, botanists, and bacteriologists) use different levels of differentiation to define a species. This implies that the numbers of species for taxa classified according to dissimilar conventions are non directly comparable. For example, that prokaryotes add only 0.1% to the total number of known species is not so much a statement well-nigh the diversity of prokaryotes every bit it is a statement about what a species means in this group. Thus, although estimates of the number of species are internally consistent for kingdoms classified nether the same conventions, our aggregated predictions for eukaryotes and prokaryotes should be interpreted with that caution in mind.

Changes in college taxonomy

Increases or decreases in the number of higher taxa will touch on the raw data used in our method and thus its estimates of the total number of species. The number of higher taxa can alter for several reasons including new discoveries, the lumping or splitting of taxa due to improved phylogenies and switching from phenetic to phylogenetic classifications, and the detection of synonyms. A survey of 2,938 taxonomists with expertise across all major domains of life (response rate xix%, see Materials and Methods) revealed that synonyms are a major trouble at the species level, merely much less and then at higher taxonomic levels. The percentage of taxa names currently believed to be synonyms ranged from 17.ix (±28.seven SD) for species, to 7.38 (±fifteen.8 SD) for genera, to v.v (±34.0 SD) for families, to three.72 (±45.two SD) for orders, to 1.fifteen (±8.37 SD) for classes, to 0.99 (±7.74 SD) for phyla. These results propose that by not using the species-level data, our higher-taxon approach is less sensitive to the problem of synonyms. Still, to appraise the extent to which whatever changes in higher taxonomy will influence our electric current estimates, we carried out a sensitivity analysis in which the number of species was calculated in response to variations in the number of higher taxa (Figure 3A–3E, Figure S2). This analysis indicates that our electric current estimates are remarkably robust to changes in higher taxonomy.

Assessment of factors affecting the higher taxon arroyo.

(A–Due east) To test the furnishings of changes in higher taxonomy, nosotros performed a sensitivity analysis in which the number of species was calculated later on altering the number of college taxa. We used Animalia as a exam case. For each taxonomic level, nosotros added or removed a random proportion of taxa from 10% to 100% of the current number of taxa and recalculated the number of species using our method. The exam was repeated 1,000 times and the average and 95% confidence limits of the simulations are shown as points and night areas, respectively. Light grey lines and boxes indicate the currently estimated number of species and its 95% prediction interval, respectively. Our current estimation of the number of species announced robust to changes in higher taxonomy as in most cases changes in college taxonomy led to estimations that remained inside the electric current estimated number of species. The results for changes in all possible combinations of taxonomic levels are shown in Effigy S2. (F–J) The yearly ratio of new college taxa in Animalia (blackness points and reddish line) and the yearly number of new species (grey line); this reflects the fraction of newly described species that besides represent new higher taxa. The contrasting patterns in the description of new species and new higher taxa suggest that taxonomic effort is probably non driving observed flattening of accumulation curves in higher taxonomic levels as there is at least sufficient endeavor to maintain a abiding description of new species. (K–O) Sensitivity assay on the completeness of taxonomic inventories. To appraise the extent to which incomplete inventories affect the predicted consensus asymptotic values obtained from the temporal accumulation of taxa, we performed a sensitivity assay in which the consensus asymptotic number of taxa was calculated from curves at different levels of abyss. Nosotros used the aggregating curves at the genus level for major groups of vertebrates, given the relative completeness of these data (i.e., reaching an asymptote). Vertical lines indicate the consensus standard error. (P–T) Frequency distribution of the number of subordinate taxa at different taxonomic levels. For display purposes nosotros present simply the information for Animalia; lines and exam statistics are from a regression model fitted with a power function.

Changes in taxonomic effort

Taxonomic effort can be a strong determinant of species discovery rates [21]. Hence the estimated asymptotes from the temporal aggregating curves of higher taxa (dotted horizontal line in Effigy 1A–1E) might be driven by a decline in taxonomic effort. We presume, however, that this is not a major factor: while the discovery charge per unit of higher taxa is failing (black dots and red lines in Figure 3F–3J), the rate of description of new species remains relatively abiding (grey lines in Effigy 3F–3J). This suggests that the asymptotic trends among higher taxonomic levels practise not issue from a lack of taxonomic endeavour as there has been at least sufficient try to describe new species at a constant rate. Secondly, although a bulk (79.4%) of experts that we polled in our taxonomic survey felt that the number of taxonomic experts is decreasing, information technology was pointed out that other factors are counteracting this trend. These included, amid others, more amateur taxonomists and phylogeneticists, new sampling methods and molecular identification tools, increased international collaboration, improve access to information, and access to new areas of exploration. Taken together these factors have resulted in a constant rate of description of new species, as evident in our Figure 1, Figure 3F–3J, and Figure S1 and suggest that the observed flattening of the discovery curves of college taxa is unlikely to be driven past a lack of taxonomic effort.

Completeness of taxonomic inventories

To account for nevertheless-to-be-discovered college taxa, our approach fitted asymptotic regression models to the temporal aggregating curve of higher taxa. A disquisitional question is how the completeness of such curves will affect the asymptotic prediction. To address this, we performed a sensitivity analysis in which the asymptotic number of taxa was calculated for aggregating curves with different levels of abyss. The results of this test indicated that the asymptotic regression models used here would underestimate the number of predicted taxa when very incomplete inventories are used (Figure 3K–3O). This underestimation in the number of higher taxa would lower our prediction of the number of species through our college taxon approach, which suggests that our species estimates are conservative, particularly for poorly sampled taxa. We reason that underestimation due to this consequence is severe for prokaryotes due to the ongoing discovery of college taxa (Figure S1) but is likely to exist minor in most eukaryote groups because the rate of discovery of higher taxa is apace failing (Figure 1A– ​ 3E, Figure S1, Figure 3F–3J).

Since higher taxonomic levels are described more completely (Effigy 1A–1E), the resulting fault from incomplete inventories should decrease while rise in the taxonomic bureaucracy. Recalculating the number of species while omitting all data from genera yielded new estimates that were more often than not within the intervals of our original estimates (Effigy S3). Notwithstanding, Chromista (on Earth and in the ocean) and Fungi (in the ocean) were exceptions, having inflated predictions without the genera data (Effigy S3). This inflation in the predicted number of species without genera data highlights the loftier incompleteness of at least the genera data in those 3 cases. In fact, Adl et al.'southward [28] survey of skilful opinions reported that the number of described species of chromists could be in the order of 140,000, which is almost 10 times the number of species currently catalogued in the databases used here (Table 1). These results suggest that our estimates for Chromista and Fungi (in the sea) need to be considered with circumspection due to the incomplete nature of their data.

Subjectivity in the Linnaean system of classification

Dissimilar ideas almost the correct nomenclature of species into a taxonomic hierarchy may distort the shape of the relationships we describe here. Even so, an assessment of the taxonomic hierarchy shows a consistent pattern; we found that at whatever taxonomic rank, the diversity of subordinate taxa is full-bodied within a few groups with a long tail of low-diversity groups (Figure 3P–3T). Although nosotros cannot refute the possibility of capricious decisions in the classification of some taxa, the consistent patterns in Effigy 3P–3T imply that these decisions do not obscure the robust underlying human relationship between taxonomic levels. The mechanism for the exponential relationships between nested taxonomic levels is uncertain, but in the case of taxa classified phylogenetically, it may reflect patterns of diversification likely characterized by radiations within a few clades and little cladogenesis in most others [29]. We would like to caution that the database nosotros used here for protistan eukaryotes (mostly in Protozoa and Chromista in this work) combines elements of various classification schemes from different ages—in fact the very division of these organisms into "Protozoa" and "Chromista" kingdoms is not-phylogenetic and not widely followed amongst protistologists [28]. It would exist valuable to revisit the species estimates for protistan eukaryotes once their global catalogue can be organized into a valid and stable higher taxonomy (and their catalogue of described species is more complete—see above).

Discussion

Knowing the total number of species has been a question of great interest motivated in role by our commonage curiosity about the diversity of life on Earth and in function by the need to provide a reference bespeak for electric current and time to come losses of biodiversity. Unfortunately, incomplete sampling of the world's biodiversity combined with a lack of robust extrapolation approaches has yielded highly uncertain and controversial estimates of how many species there are on World. In this newspaper, we describe a new approach whose validation against existing inventories and explicit statistical nature adds greater robustness to the estimation of the number of species of given taxa. In general, the arroyo was reasonably robust to various caveats, and we hope that future improvements in data quality will further diminish problems with synonyms and incompleteness of data, and atomic number 82 to fifty-fifty better (and likely higher) estimates of global species richness.

Our current estimate of ∼8.7 million species narrows the range of iii to 100 million species suggested by taxonomic experts [ane] and information technology suggests that after 250 years of taxonomic classification only a pocket-size fraction of species on Earth (∼14%) and in the bounding main (∼9%) accept been indexed in a key database (Table 2). Endmost this knowledge gap may still take a lot longer. Considering current rates of description of eukaryote species in the final 20 years (i.e., six,200 species per year; ±811 SD; Figure 3F–3J), the average number of new species described per taxonomist'southward career (i.e., 24.8 species, [30]) and the estimated average cost to describe brute species (i.eastward., US$48,500 per species [thirty]) and assuming that these values remain abiding and are general among taxonomic groups, describing Earth'southward remaining species may have as long as i,200 years and would crave 303,000 taxonomists at an approximated toll of Usa$364 billion. With extinction rates now exceeding natural groundwork rates past a cistron of 100 to ane,000 [31], our results also suggest that this slow accelerate in the clarification of species volition lead to species becoming extinct before nosotros know they even existed. High rates of biodiversity loss provide an urgent incentive to increment our knowledge of Earth's remaining species.

Previous studies have indicated that electric current catalogues of species are biased towards conspicuous species with large geographical ranges, trunk sizes, and abundances [4],[32]. This suggests that the bulk of species that remain to be discovered are likely to be small-ranged and perhaps concentrated in hotspots and less explored areas such as the deep bounding main and soil; although their minor body-size and cryptic nature advise that many could be institute literally in our own "backyards" (subsequently Hawksworth and Rossman [33]). Though remarkable efforts and progress have been made, a further closing of this knowledge gap volition require a renewed interest in exploration and taxonomy by both researchers and funding agencies, and a continuing effort to catalogue existing biodiversity information in publicly available databases.

Materials and Methods

Databases

Calculations of the number of species on Earth were based on the classification of currently valid species from the Catalogue of Life (www.sp2000.org, [34]) and the estimations for species in the sea were based on The World'due south Annals of Marine Species (www.marinespecies.org, [35]). The latter database is largely contained within the erstwhile. These databases were screened for inconsistencies in the higher taxonomy including homonyms and the classification of taxa into multiple clades (east.thousand., ensuring that all diatom taxa were assigned to "Chromista" and non to "plants"). The Globe's prokaryotes were analyzed independently using the most recent classification bachelor in the List of Prokaryotic Names with Standing in Nomenclature database (http://world wide web.bacterio.cict.fr). Additional information on the year of description of taxa was obtained from the Global Names Index database (http://world wide web.globalnames.org). We merely used data to 2006 to foreclose bogus flattening of accumulation curves due to recent discoveries and descriptions not withal being entered into databases.

Statistical Analysis

To business relationship for higher taxa yet to exist discovered, we used the following approach. First, for each taxonomic rank from phylum to genus, nosotros fitted vi asymptotic parametric regression models (i.e., negative exponential, asymptotic, Michaelis-Menten, rational, Chapman-Richards, and modified Weibull [23]) to the temporal accumulation bend of college taxa (Figure 1A–1E) and used multimodel averaging based on the small-sample size corrected version of Akaike'south Information Criteria (AIC_c) to predict the asymptotic number of taxa (dotted horizontal line in Figure 1A–1E) [23]. Ideally data should exist modeled using only the decelerating part of the aggregating curve [22]–[23], however, frequently at that place was no obvious breakpoint at which accumulation curves switched from an increasing to a decelerating rate of discovery (Effigy 1A–1E). Therefore, we fitted models to data starting at all possible years from 1758 onwards (data before 1758 were added every bit an intercept to prevent a spike due to Linnaeus) and selected the model predictions if at least 10 years of data were bachelor and if 5 of the vi asymptotic models converged to the subset data. And so, the estimated multimodel asymptotes and standard errors for each selected year were used to estimate a consensus asymptote and its standard mistake. In this approach, the multimodel asymptotes for all cut-off years selected and their standard errors are weighted proportionally to their standard mistake, thus ensuring that the doubtfulness both within and amid predictions were incorporated [36].

To guess the number of species in a taxonomic group from its higher taxonomy, we used To the lowest degree Squares Regression models to relate the consensus asymptotic number of college taxa confronting their numerical rank, so used the resulting regression model to extrapolate to the species level (Figure 1G). Since data are not strictly independent across hierarchically organized taxa, nosotros also used models based on Generalized Least Squares bold autocorrelated regression errors. Both types of models were run with and without the inverse of the consensus approximate variances as weights to account for differences in certainty in the asymptotic number of higher taxa. We evaluated the fit of exponential, power, and hyperexponential functions to the data and obtained a prediction of the number of species by multimodel averaging based on AIC_c of the best type of function. The hyperexponential function was called for kingdoms whereas the exponential function for the smaller groups was used in the validation assay (encounter comparison of fits in Figure S4).

Survey of Taxonomists

We contacted four,771 taxonomy experts with email addresses equally listed in the World Taxonomist Database (world wide web.eti.uva.nl/tools/wtd.php); 1,833 were faulty e-mails, hence almost 2,938 experts received our asking, of which 548 responded to our survey (response rate of 18.7%). Respondents were asked to identify their taxon of expertise, and to comment on what percentage of currently valid names could be synonyms at taxonomic levels from species to kingdom. Nosotros also polled taxonomists about whether the taxonomic try (measured as numbers of professional taxonomists) in their area of expertise in recent times was increasing, decreasing, or stable.

Supporting Information

Figure S1

Completeness of the higher taxonomy of kingdoms of life on Earth.

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Figure S2

Sensitivity analysis due to changes in higher taxonomy.

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Effigy S3

Assessing the effects of information incompleteness.

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Effigy S4

Comparing of the fits of the hyperexponential, exponential, and power functions to the human relationship between the number of higher taxa and their numerical rank.

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Acknowledgments

Nosotros thank David Stang, Ward Appeltans, the Catalogue of Life, the World Register of Marine Species, the Listing of Prokaryotic Names with Standing Classification, the Global Names Index databases, the World Taxonomist Database, and all their constituent databases and uncountable contributors for making their information freely bachelor. We besides thank the numerous respondents to our taxonomic survey for sharing their insights. Finally, nosotros are indebted to Stuart Pimm, Andrew Solow, and Catherine Muir for helpful and effective comments on the manuscript and to Philippe Bouchet, Frederick Grassle, and Terry Erwin for valuable discussion.

Footnotes

The authors have declared that no competing interests exist.

Funding was provided by the Sloan Foundation through the Demography of Marine Life Program, Futurity of Marine Animate being Populations project. The funders had no part in study blueprint, data drove and analysis, determination to publish, or training of the manuscript.

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3160336/

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