How many classified species

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. 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.

PLoS Biol 9 8 : e 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. 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. 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 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 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 million species [1] ; although these estimations likely represent the outer bounds of the total number of species, expert-opinion approaches have been questioned 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. 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.

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. 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.

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. This method, however, relies on extrapolation of patterns from relatively small areas to estimate the number of species in other locations i.

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. However, this is not the case for prokaryotes, where there is little indication of reaching an asymptote at any taxonomic level Figure S1.

This prevents direct extrapolation of the number of species from species-accumulation curves [22] , [23] and highlights our current uncertainty 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. Restricting this approach to marine taxa resulted in a prediction of 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. 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. An important caveat to the interpretation of our results concerns the definition of species. Different taxonomic communities e.

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. 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.

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.

The percentage of taxa names currently believed to be synonyms ranged from 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. 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. 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.

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.

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 species at a constant rate. Secondly, although a majority 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. 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 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. 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. 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].

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. 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 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. Closing this knowledge gap may still take a lot longer. Considering current rates of description of eukaryote species in the last 20 years i. Two hundred and fifty years after Swedish botanist Carl Linnaeus devised a formal system for classifying the diversity of nature, the catalog for some classes of living things—such as mammals and birds—is nearly complete, the study says.

But the inventories for other classes are woefully sparse. For instance, only 7 percent of the predicted number of fungi—which includes mushrooms and yeasts—has been described, and less than 10 percent of the life-forms in the world's oceans has been identified. What's been discovered so far are "those things that are easy to find, that are conspicuous, that are relatively large," Worm said.

So far, some 1. To calculate the percentage of unknown species, Worm and colleagues first had to answer one of the great questions of ecology: How many species live on the Earth? Previous guesses ranged from three million all the way to a hundred million. To gain a more precise answer, the authors examined the categories into which all species are grouped. Scientists lump similar species together into a broader grouping called a genus, similar genera into a still broader category called a family, and so on, all the way up to a supercategory called a kingdom.

See photos of species classification in National Geographic magazine. There are five kingdoms: animals, plants, fungi, chromists—including one-celled plants such as diatoms—and protozoa, or one-celled organisms. Worm's team estimated the total number of genera, families, orders, classes, and phyla—a designation above class—in each kingdom. That's a relatively easy task, since the number of new examples in these categories has leveled off in recent decades.

Using complex statistics, Worm and colleagues used the number of genera, families, and so on to predict Earth's number of unknown species, and their calculations gave them a number: 8. The new study "takes a hugely clever approach, and I think it's going to turn out to be a pretty important study," said Lucas Joppa , a conservation ecologist at Microsoft Research, the research branch of the software giant.

But Dan Bebber , an ecologist at the environmental group Earthwatch Institute, said the study relies on improper statistical methods. The study team used a method called linear regression to calculate the number of Earth's species. But Bebber thinks this method is the wrong one for the data, and that the team should have used a technique known as ordinal regression. Overall, formally categorizing a new organism is a lot more complicated than discovering one, study co-author Worm said.

Scientists must compare their specimen to museum samples, analyze its DNA, and complete reams of paperwork. Most scientists "will describe dozens of species in their lifetime, if they're really lucky.

Read more: Trapdoor spider species that stay local put themselves at risk. Current estimates for the number of species on Earth range between 5. Part of the problem is that we cannot simply count the number of life forms. Many live in inaccessible habitats such as the deep sea , are too small to see, are hard to find, or live inside other living things. So, instead of counting, scientists try to estimate the total number of species by looking for patterns in biodiversity.

In the early s, the American entomologist Terry Erwin famously estimated the number of species on Earth by spraying pesticides into the canopy of tropical rainforest trees in Panama.

At least 1, species of beetle fell to the ground, of which lived only on a single tree species. Many scientists believe the 30 million number is far too high. Later estimates arrived at figures under 10 million.

In , scientists used a technique based on patterns in the number of species at each level of biological classification to arrive at a much lower prediction of about 8. But most estimates of global biodiversity overlook microorganisms such as bacteria because many of these organisms can only be identified to species level by sequencing their DNA. After compiling and analysing a database of DNA sequences from 5 million microbe species from 35, sites around the world, researchers concluded that there are a staggering 1 trillion species on Earth.

But, like previous estimates, this one relies on patterns in biodiversity, and not everyone agrees these should be applied to microorganisms. Most — and possibly all — insect species are the victim of at least one or more species of parasitic wasp.