How Many Species Are There on Earth and in the Ocean?

Abstract

The diversity of life is one of the most striking aspects of our planet; hence knowing how many species inhabit Earth is among the most fundamental questions in science. Yet the answer to this question remains enigmatic, as efforts to sample the world's biodiversity to date have been limited and thus have precluded direct quantification of global species richness, and because indirect estimates rely on assumptions that have proven highly controversial. Here we show that the higher taxonomic classification of species (i.e., the assignment of species to phylum, class, order, family, and genus) follows a consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated. This approach was validated against well-known taxa, and when applied to all domains of life, it predicts ~8.7 million (± 1.3 million SE) eukaryotic species globally, of which ~2.2 million (± 0.18 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 suggest that some 86% of existing species on Earth and 91% of species in the ocean still await description. Renewed interest in further exploration and taxonomy is required if this significant gap in our knowledge of life on Earth is to be closed.

Full text

How Many Species Are There on Earth and in the Ocean?
Camilo Mora
1,2
*, Derek P. Tittensor
1,3,4
, Sina Adl
1
, Alastair G. B. Simpson
1
, Boris Worm
1
1Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada, 2Department of Geography, University of Hawaii, Honolulu, Hawaii, United States of America,
3United Nations Environment Programme World Conservation Monitoring Centre, Cambridge, United Kingdom, 4Microsoft Research, Cambridge, United Kingdom
Abstract
The diversity of life is one of the most striking aspects of our planet; hence knowing how many species inhabit Earth is
among the most fundamental questions in science. Yet the answer to this question remains enigmatic, as efforts to sample
the world’s biodiversity to date have been limited and thus have precluded direct quantification of global species richness,
and because indirect estimates rely on assumptions that have proven highly controversial. Here we show that the higher
taxonomic classification of species (i.e., the assignment of species to phylum, class, order, family, and genus) follows a
consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated. This
approach was validated against well-known taxa, and when applied to all domains of life, it predicts ,8.7 million (61.3
million SE) eukaryotic species globally, of which ,2.2 million (60.18 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 suggest that some
86% of existing species on Earth and 91% of species in the ocean still await description. Renewed interest in further
exploration and taxonomy is required if this significant gap in our knowledge of life on Earth is to be closed.
Citation: Mora C, Tittensor DP, Adl S, Simpson AGB, Worm B (2011) How Many Species Are There on Earth and in the Ocean? PLoS Biol 9(8): e1001127.
doi:10.1371/journal.pbio.1001127
Academic Editor: Georgina M. Mace, Imperial College London, United Kingdom
Received November 12, 2010; Accepted July 13, 2011; Published August 23, 2011
Copyright: ß2011 Mora et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding was provided by the Sloan Foundation through the Census of Marine Life Program, Future of Marine Animal Populations project. The funders
had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: moracamilo@hotmail.com
Introduction
Robert May [1] recently noted that if aliens visited our planet,
one of their first questions would be, ‘‘How many distinct life
forms—species—does your planet have?’’ He also pointed out that
we would be ‘‘embarrassed’’ by the uncertainty in our answer.
This narrative illustrates the fundamental nature of knowing how
many species there are on Earth, and our limited progress with this
research topic thus far [1–4]. Unfortunately, limited sampling of
the world’s biodiversity to date has prevented a direct quantifi-
cation 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 1). Globally, our best approximation to the total number of
species is based on the opinion of taxonomic experts, whose
estimates range between 3 and 100 million species [1]; although
these estimations likely represent the outer bounds of the total
number of species, expert-opinion approaches have been ques-
tioned due to their limited empirical basis [5] and subjectivity [5–
6] (Table 1). Other studies have used macroecological patterns and
biodiversity ratios in novel ways to improve 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 only specific groups, such as insects [9,18–19],
deep sea invertebrates [13], large organisms [6–7,10], animals [7],
fungi [20], or plants [21]. With the exception of a few extensively
studied taxa (e.g., birds [22], fishes [23]), we are still remarkably
uncertain as to how many species exist, highlighting a significant
gap in our basic knowledge of life on Earth. Here we present a
quantitative method to estimate 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
any kingdom of life (Figures 1 and 2).
Higher taxonomy data 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 estimate
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 estimate the
number of species in other locations (i.e., alpha 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 as far as we know, although
May’s aliens may disagree. Here we analyze higher taxonomic
data using a different approach by assessing patterns across all
taxonomic levels of major taxonomic groups. The existence of
predictable patterns in the higher taxonomic classification of
species allows prediction of the total number of species within
taxonomic groups and may help to better constrain our estimates
of global species richness.
Results
We compiled the full taxonomic classifications of ,1.2 million
currently valid species from several publicly accessible sources (see
Materials and Methods). Among eukaryote ‘‘kingdoms,’’ assess-
ment of the temporal accumulation curves of higher taxa (i.e., the
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cumulative number of species, genera, orders, classes, and phyla
described over time) indicated that higher taxonomic ranks are
much more completely described than lower levels, as shown by
strongly asymptoting trajectories over time ([24], Figure 1A–1F,
Figure S1). However, this is not the case for prokaryotes, where
there is little indication of reaching an asymptote at any taxonomic
level (Figure S1). For most eukaryotes, in contrast, the rate 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 (Figure 1A–1F, Figure S1). This prevents
direct extrapolation of the number of species from species-
accumulation curves [22,23] and highlights our current uncer-
tainty regarding estimates of total species richness (Figure 1F).
However, the increasing completeness of higher taxonomic ranks
could facilitate the estimation of the total number of species, if the
former 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’s Information Criterion [23] to
predict the asymptotic number of taxa of each taxonomic level
(dotted horizontal line in Figure 1A–11E; see 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 (Figure 1G, Materials and Methods). We applied this
approach to 18 taxonomic groups for which the total numbers of
species are thought to be relatively well known. We found that this
approach yields predictions of species numbers that are consistent
with inventory totals for these groups (Figure 2). 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 approach predicted that ,8.74 million
species of eukaryotes exist on Earth (Table 2). Restricting this
approach to marine taxa resulted in a prediction of 2.21 million
eukaryote species in the world’s oceans (Table 2). We also applied
the approach to prokaryotes; unfortunately, the steady pace of
description of taxa at all taxonomic ranks precluded the
calculation 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 represent
only lower bounds on the diversity in this group. Our approach
predicted a lower bound of ,10,100 species of prokaryotes, of
which ,1,320 are marine. It is important to note that for
prokaryotes, the species concept tolerates a much higher degree of
genetic dissimilarity than in most eukaryotes [26,27]; additionally,
due to horizontal gene transfers among phylogenetic clades,
species take longer to isolate in prokaryotes than in eukaryotes,
and thus the former species are much older than the latter [26,27];
as a result the number of described species of prokaryotes is small
(only ,10,000 species are currently accepted).
Assessment of Possible Limitations
We recognize a number of factors that can influence the
interpretation and robustness of the estimates derived from the
method described here. These are analyzed below.
Species definitions. An important caveat to the
interpretation 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 different conventions are not directly comparable.
For example, that prokaryotes add only 0.1% to the total number
of known species is not so much a statement about the diversity of
prokaryotes as 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 under the same
conventions, our aggregated predictions for eukaryotes and
prokaryotes should be interpreted with that caution in mind.
Changes in higher taxonomy. Increases or decreases in the
number of higher taxa will affect the raw data used in our method
and thus its estimates of the total number of species. The number
of higher taxa can change 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 19%, see Materials and Methods) revealed that
synonyms are a major problem at the species level, but much less
so at higher taxonomic levels. The percentage of taxa names
currently believed to be synonyms ranged from 17.9 (628.7 SD)
for species, to 7.38 (615.8 SD) for genera, to 5.5 (634.0 SD) for
families, to 3.72 (645.2 SD) for orders, to 1.15 (68.37 SD) for
classes, to 0.99 (67.74 SD) for phyla. These results suggest that by
not using the species-level data, our higher-taxon approach is less
sensitive to the problem of synonyms. Nevertheless, to assess the
extent to which any changes in higher taxonomy will influence our
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 current estimates are remarkably robust to
changes in higher taxonomy.
Changes in taxonomic effort. Taxonomic effort can be a
strong determinant of species discovery rates [21]. Hence the
estimated asymptotes from the temporal accumulation curves of
higher taxa (dotted horizontal line in Figure 1A–1E) might be
driven by a decline in taxonomic effort. We presume, however,
that this is not a major factor: while the discovery rate of higher
taxa is declining (black dots and red lines in Figure 3F–3J), the rate
of description of new species remains relatively constant (grey lines
in Figure 3F–3J). This suggests that the asymptotic trends among
higher taxonomic levels do not result from a lack of taxonomic
effort as there has been at least sufficient effort to describe new
Author Summary
Knowing the number of species on Earth is one of the
most basic yet elusive questions in science. Unfortunately,
obtaining an accurate number is constrained by the fact
that most species remain to be described and because
indirect attempts to answer this question have been highly
controversial. Here, we document that the taxonomic
classification of species into higher taxonomic groups
(from genera to phyla) follows a consistent pattern from
which the total number of species in any taxonomic group
can be predicted. Assessment of this pattern for all
kingdoms of life on Earth predicts ,8.7 million (61.3
million SE) species globally, of which ,2.2 million (60.18
million SE) are marine. Our results suggest that some 86%
of the species on Earth, and 91% in the ocean, still await
description. Closing this knowledge gap will require a
renewed interest in exploration and taxonomy, and a
continuing effort to catalogue existing biodiversity data in
publicly available databases.
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species at a constant rate. Secondly, although a majority (79.4%)
of experts that we polled in our taxonomic survey felt that the
number of taxonomic experts is decreasing, it was pointed out that
other factors are counteracting this trend. These included, among
others, more amateur taxonomists and phylogeneticists, new
sampling methods and molecular identification tools, increased
international collaboration, better 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 higher taxa
is unlikely to be driven by a lack of taxonomic effort.
Completeness of taxonomic inventories. To account for
yet-to-be-discovered higher taxa, our approach fitted asymptotic
regression models to the temporal accumulation curve of higher
taxa. A critical 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 accumulation curves with different levels
of completeness. 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
Table 1. Available 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
large to small species, May [7] estimated 10 to 50 million species of animals.
May [7] suggested that there was no reason to expect a simple scaling
law from large to small species. Further studies confirmed different
modes of evolution among small species [4] and inconsistent body size
frequency distributions among taxa [4].
Latitudinal gradients in species.
By extrapolation from the better sampled temperate
regions to the tropics, Raven [10] estimated 3 to 5 million species of large organisms.
May [2] questioned the assumption that temperate regions were better
sampled than tropical ones; the approach also assumed consistent
diversity gradients across taxa which is not factual [4].
Species-area relationships.
By extrapolation from the number of species in deep-sea
samples, Grassle & Maciolek [13] estimated that the world’s deep seafloor could
contain up to 10 million species.
Lambshead & Bouchet [12] questioned this estimation by showing that
high local diversity in the deep sea does not necessarily reflect high
global biodiversity given low species turnover.
Diversity ratios
Ratios between taxa.
By assuming a global 6:1 ratio of fungi to vascular
plants and that there are ,270,000 species of vascular plants,
Hawksworth [20] estimated 1.6 million 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 be 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
not always be true [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 ,0.5 million global marine species whereas the
ratio of Brachyura to total multicellular organisms in the same sampled
region would yield ,1.5 million species.
Host-specificity and spatial ratios.
Given 50,000 known species of tropical trees
and assuming a 5:1 ratio of host beetles to trees, that beetles represent 40% of
the canopy arthropods, and that the canopy has twice the species of the ground,
Erwin [9] estimated 30 million species of arthropods in the tropics.
Known to unknown ratios
.Hodkinson & Casson [18] estimated that 62.5% of the
bug (Hemiptera) species in a sampled location were unknown; by assuming that 7.5%–10% of
the global diversity of insects is bugs, they estimated between 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 ,19,800 species of marine fishes [23] and ,11,997
birds [22].
This approach is not widely applicable because it requires species
accumulation curves to approach asymptotic levels, which is only true 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 yet to be discovered is 13% to 18% [21].
This is a very recent method and the effect 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 be due to more collaborative
research [38] and the acknowledgment of technicians, field assistants,
specimen collectors, and so on as coauthors (Philippe Bouchet, personal
communication).
Analysis of expert estimations.
Estimates of ,5 million species of insects [15]
and ,200,000 marine species [14] were arrived at by compiling opinion-based
estimates from taxonomic experts. Robustness in the estimations is assumed
from the consistency of responses among different experts.
Erwin [5] labeled this approach as ‘‘non-scientific’’ due to a lack of
verification. Estimates can vary widely, even those of a single expert [5,6].
Bouchet [6] argues that expert estimations are often passed on from one
expert to another and therefore a robust estimation could be the ‘‘same
guess copied again and again’’.
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through our higher taxon approach, which suggests that our
species estimates are conservative, particularly for poorly sampled
taxa. We reason that underestimation due to this effect is severe for
prokaryotes due to the ongoing discovery of higher taxa (Figure
S1) but is likely to be modest in most eukaryote groups because the
rate of discovery of higher taxa is rapidly declining (Figure 1A–3E,
Figure S1, Figure 3F–3J).
Since higher taxonomic levels are described more completely
(Figure 1A–1E), the resulting error from incomplete inventories
should decrease while rising in the taxonomic hierarchy.
Recalculating the number of species while omitting all data from
genera yielded new estimates that were mostly within the intervals
of our original estimates (Figure S3). However, Chromista (on
Earth and in the ocean) and Fungi (in the ocean) were exceptions,
having inflated predictions without the genera data (Figure S3).
This inflation in the predicted number of species without genera
data highlights the high incompleteness of at least the genera data
in those three cases. In fact, Adl et al.’s [28] survey of expert
opinions reported that the number of described species of
chromists could be in the order of 140,000, which is nearly 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 ocean) need to be considered with
caution due to the incomplete nature of their data.
Subjectivity in the Linnaean system of classification.
Different ideas about the correct classification of species into a
taxonomic hierarchy may distort the shape of the relationships we
describe here. However, an assessment of the taxonomic hierarchy
shows a consistent pattern; we found that at any taxonomic rank,
the diversity of subordinate taxa is concentrated within a few
groups with a long tail of low-diversity groups (Figure 3P–3T).
Although we cannot refute the possibility of arbitrary decisions in
the classification of some taxa, the consistent patterns in Figure 3P–
3T imply that these decisions do not obscure the robust underlying
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 we 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 non-phylogenetic and
not widely followed among protistologists [28]. It would be
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 part by our collective curiosity about
the diversity of life on Earth and in part by the need to provide a
reference point for current and future losses of biodiversity.
Unfortunately, incomplete sampling of the world’s biodiversity
combined with a lack of robust extrapolation approaches has
Figure 1. 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 grey area its consensus standard error. (G) Relationship between the consensus asymptotic number of
higher taxa and the numerical hierarchy of each taxonomic rank. Black 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).
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yielded highly uncertain and controversial estimates of how
many species there are on Earth. In this paper, 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 approach 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 lead to even better (and likely higher) estimates of global
species richness.
Our current estimate of ,8.7 million species narrows the range
of 3 to 100 million species suggested by taxonomic experts [1]
and it suggests that after 250 years of taxonomic classification
only a small fraction of species on Earth (,14%) and in the ocean
(,9%) have been indexed in a central database (Table 2). Closing
this knowledge gap may still take a lot longer. Considering
current rates of description of eukaryote species in the last 20
years (i.e., 6,200 species per year; 6811 SD; Figure 3F–3J), the
average number of new species described per taxonomist’s career
(i.e., 24.8 species, [30]) and the estimated average cost to describe
animal species (i.e., US$48,500 per species [30]) and assuming
that these values remain constant and are general among
taxonomic groups, describing Earth’s remaining species may
take as long as 1,200 years and would require 303,000
taxonomists at an approximated cost of US$364 billion. With
extinction rates now exceeding natural background rates by a
factor of 100 to 1,000 [31], our results also suggest that this slow
advance in the description of species will lead to species becoming
extinct before we know they even existed. High rates of
biodiversity loss provide an urgent incentive to increase our
knowledge of Earth’s remaining species.
Previous studies have indicated that current catalogues of
species are biased towards conspicuous species with large
geographical ranges, body 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 sea and soil; although
their small body-size and cryptic nature suggest that many could
be found literally in our own ‘‘backyards’’ (after Hawksworth
and Rossman [33]). Though remarkable efforts and progress
have been made, a further closing of this knowledge gap will
require a renewed interest in exploration and taxonomy by both
Figure 2. Validating the higher taxon approach. We compared
the number of species estimated from the higher taxon approach
implemented here to the known number of species in relatively well-
studied taxonomic groups as derived from published sources [37]. We
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 1:1 ratio. Note that
published species numbers (y-axis values) are mostly derived from
expert approximations for well-known groups; hence there is a
possibility that those estimates are subject to biases arising from
synonyms.
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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 2,150,000 145,000
Chromista 13,033 27,500 30,500 4,859 7,400 9,640
Fungi 43,271 611,000 297,000 1,097 5,320 11,100
Plantae 215,644 298,000 8,200 8,600 16,600 9,130
Protozoa 8,118 36,400 6,690 8,118 36,400 6,690
Total 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 3,470 652 1,320 436
Total 10,860 10,100 3,630 653 1,320 436
Grand Total 1,244,360 8,750,000 1,300,000 194,409 2,210,000 182,000
Predictions for prokaryotes represent a lower bound because they do not consider undescribed higher taxa. For protozoa, the ocean database was substantially more
complete than the database for the entire Earth so we only used the former to estimate the total number of species in this taxon. All predictions were rounded to three
significant digits.
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Figure 3. Assessment of factors affecting the higher taxon approach. (A–E) To test the effects of changes in higher taxonomy, we performed
a sensitivity analysis in which the number of species was calculated after altering the number of higher taxa. We used Animalia as a test case. For each
taxonomic level, we 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 test was repeated 1,000 times and the average and 95% confidence limits of the simulations are shown as points and
dark areas, respectively. Light gray lines and boxes indicate the currently estimated number of species and its 95% prediction interval, respectively.
Our current estimation of the number of species appear robust to changes in higher taxonomy as in most cases changes in higher taxonomy led to
estimations that remained within the current estimated number of species. The results for changes in all possible combinations of taxonomic levels
are shown in Figure S2. (F–J) The yearly ratio of new higher taxa in Animalia (black points and red line) and the yearly number of new species (grey
line); this reflects the fraction of newly described species that also represent new higher taxa. The contrasting patterns in the description of new
species and new higher taxa suggest that taxonomic effort is probably not driving observed flattening of accumulation curves in higher taxonomic
levels as there is at least sufficient effort to maintain a constant description of new species. (K–O) Sensitivity analysis on the completeness of
taxonomic inventories. To assess the extent to which incomplete inventories affect the predicted consensus asymptotic values obtained from the
temporal accumulation of taxa, we performed a sensitivity analysis in which the consensus asymptotic number of taxa was calculated from curves at
different levels of completeness. We used the accumulation 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 we present only the data for Animalia; lines and test statistics are from
a regression model fitted with a power function.
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researchers and funding agencies, and a continuing effort to
catalogue existing biodiversity data 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
ocean were based on The World’s Register of Marine Species
(www.marinespecies.org, [35]). The latter database is largely
contained within the former. These databases were screened for
inconsistencies in the higher taxonomy including homonyms and
the classification of taxa into multiple clades (e.g., ensuring that all
diatom taxa were assigned to ‘‘Chromista’’ and not to ‘‘plants’’).
The Earth’s prokaryotes were analyzed independently using the
most recent classification available in the List of Prokaryotic
Names with Standing in Nomenclature database (http://www.
bacterio.cict.fr). Additional information on the year of description
of taxa was obtained from the Global Names Index database
(http://www.globalnames.org). We only used data to 2006 to
prevent artificial flattening of accumulation curves due to recent
discoveries and descriptions not yet being entered into databases.
Statistical Analysis
To account for higher taxa yet to be discovered, we used the
following approach. First, for each taxonomic rank from phylum
to genus, we fitted six asymptotic parametric regression models
(i.e., negative exponential, asymptotic, Michaelis-Menten, rational,
Chapman-Richards, and modified Weibull [23]) to the temporal
accumulation curve of higher taxa (Figure 1A–1E) and used
multimodel averaging based on the small-sample size corrected
version of Akaike’s Information Criteria (AIC
c
) to predict the
asymptotic number of taxa (dotted horizontal line in Figure 1A–
1E) [23]. Ideally data should be modeled using only the
decelerating part of the accumulation curve [22–23], however,
frequently there was no obvious breakpoint at which accumulation
curves switched from an increasing to a decelerating rate of
discovery (Figure 1A–1E). Therefore, we fitted models to data
starting at all possible years from 1758 onwards (data before 1758
were added as an intercept to prevent a spike due to Linnaeus) and
selected the model predictions if at least 10 years of data were
available and if five of the six asymptotic models converged to the
subset data. Then, the estimated multimodel asymptotes and
standard errors for each selected year were used to estimate a
consensus asymptote and its standard error. In this approach, the
multimodel asymptotes for all cut-off years selected and their
standard errors are weighted proportionally to their standard
error, thus ensuring that the uncertainty both within and among
predictions were incorporated [36].
To estimate the number of species in a taxonomic group from
its higher taxonomy, we used Least Squares Regression models to
relate the consensus asymptotic number of higher taxa against
their numerical rank, and then used the resulting regression model
to extrapolate to the species level (Figure 1G). Since data are not
strictly independent across hierarchically organized taxa, we also
used models based on Generalized Least Squares assuming
autocorrelated regression errors. Both types of models were run
with and without the inverse of the consensus estimate 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 chosen for kingdoms whereas the exponential
function for the smaller groups was used in the validation analysis
(see comparison of fits in Figure S4).
Survey of Taxonomists
We contacted 4,771 taxonomy experts with electronic mail
addresses as listed in the World Taxonomist Database (www.eti.uva.
nl/tools/wtd.php); 1,833 were faulty e-mails, hence about 2,938
experts received our request, 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. We also polled taxonomists about whether the taxonomic
effort (measured as numbers of professional taxonomists) in their
area of expertise inrecent times was increasing, decreasing, or stable.
Supporting Information
Figure S1 Completeness of the higher taxonomy of kingdoms of
life on Earth.
(DOC)
Figure S2 Sensitivity analysis due to changes in higher
taxonomy.
(DOC)
Figure S3 Assessing the effects of data incompleteness.
(DOC)
Figure S4 Comparison of the fits of the hyperexponential,
exponential, and power functions to the relationship between the
number of higher taxa and their numerical rank.
(DOC)
Acknowledgments
We thank David Stang, Ward Appeltans, the Catalogue of Life, the World
Register of Marine Species, the List of Prokaryotic Names with Standing
Nomenclature, the Global Names Index databases, the World Taxonomist
Database, and all their constituent databases and uncountable contributors
for making their data freely available. We also thank the numerous
respondents to our taxonomic survey for sharing their insights. Finally, we
are indebted to Stuart Pimm, Andrew Solow, and Catherine Muir for
helpful and constructive comments on the manuscript and to Philippe
Bouchet, Frederick Grassle, and Terry Erwin for valuable discussion.
Author Contributions
Conceived and designed the experiments: CM DPT BW. Analyzed the
data: CM DPT. Wrote the paper: CM DPT SA AGBS BW. Reviewed
higher taxonomy: CM SA AGBS.
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