The short answer:  about 9 million, not counting bacteria. That’s according to a new paper in PLoS Biology by Camilo Mora et al. (see also the perspective by Robert May in the same issue; both paper and perspective are free, and you should definitely read Bob’s one-page piece).

The issue of how many species inhabit our planet is a hot one: I’m often asked about it by journalists or in lectures, despite the fact that I have virtually no expertise in this area.  We know that about 1.2 million species have been described, but we also know that that’s a gross underestimate of the true number. Any time somebody samples the insects in the rain forest, for example, a large number of new species turn up, and only Ceiling Cat knows how many species of nematodes and other worms live underground.  And then there’s the real can of worms: the bacteria and archaea (see below). My usual answer to the question of “how many species?” is “probably between 5 and 50 million species.”  Well, at least I was in the right range.

Here’s the authors’ conclusion, breaking down the number of species by group; I’ll then briefly describe how they calculated these numbers (my emphasis):

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.

This means that about 1.2/8.7, or roughly 14%, of all Earth’s species have been described. Note as well that about 89% of Earth’s species (7.77/8.74) are animals, and that the vast majority of those animals are insects (see second graph below).  It was in fact Bob May who said, “To a first approximation, all animals are insects.”

How did they figure this out?  By using a two-step process.  First , they looked at “higher” taxa—phyla, classes, orders, family, and genera—and examined how the number of described taxa has increased over time, as well as the number of species. As you see from the graph below, which examines only animals (“Animalia”), every level above that of species is reaching an asymptote, while the number of species themselves continues to increase non-asymptotically.

Figure 1 from Mora et al.  The caption: “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).”

The conclusion from these observations is that we’re starting to approach a complete knowledge (the asymptote) of the numbers and nature of “higher” taxa, but haven’t yet approached a full knowledge of the number of individual species in these groups.

The authors then made a clever hypothesis: perhaps there is a quantitative relationship between the number of higher taxa (whose final numbers we’re starting to approach), and the number of species contained within these higher taxa.  If that were the case, we could use the asymptotic estimates for, say, number of families or orders, to estimate the final number of species they contain.

In fact, the authors did find such a quantitative relationship (see subfigure 1G above for a plot showing this): there appears to be a linear relationship on a log scale between taxonomic rank and number of asymptotic taxa.  From this you can extrapolate to the taxon of interest on the graph—species—and get an estimate of the number of species on Earth, in this case for animals.  But of course you can do this for other groups, too.

But is such extrapolation warranted? You can see whether it is by predicting from graph 1G above the number of species in well-studied groups (that is, groups, like birds and reptiles, in which we think we’ve already identified most of the species on Earth), and then comparing that prediction with the actual number of described species.  The graph below shows that for groups that have been pretty exhaustively surveyed for species, the prediction holds pretty well: the predicted number using the higher-taxon extrapolation is pretty close to the actual number of known species:

Knowing that one can pretty accurately predict the number of species on Earth by extrapolating from the number of known higher taxa, the authors got their figures above simply by doing this kind of extrapolation in less well known groups, assuming that the same relationship holds for them.  As I said, this is a clever idea, and it’s even endorsed by the critical Robert May in his accompanying perspective, “Why worry about how many species and their loss?”

Jonathan Eisen, a professor at the University of California at Davis, likes the paper as well, but criticizes part of it on his website, The Tree of Life, arguing (correctly, I think) that estimates for numbers of bacteria and archaea are way off because the “higher taxon” approach is invalid for those groups.  If the estimates for these microbes really does run into the “hundreds of millions,” as Eisen thinks, then that would hugely inflate the number of species on Earth:

So all seems hunky dory and pretty interesting.  That is, until we get to the bacteria and archaea.  For example, check out Table 2. . . Their approach leads to an estimate of 455 ± 160 Archaea on Earth and 1 in the ocean.  Yes, one in the ocean.  Amazing.  Completely silly too.  Bacteria are a little better.  An estimate of 9,680 ± 3,470 on Earth and 1,,320 ±436 in the oceans.  Still completely silly. . . Their estimates of ~ 10,000 or so bacteria and archaea on the planet are so completely out of touch in my opinion that this calls into question the validity of their method for bacteria and archaea at all. Now you may ask – why do I think this is out of touch. Well because reasonable estimates are more on the order or millions or hundreds of millions, not tens of thousands.

So long will it take to catalog the rest of Earth’s biodiversity, and what would it cost? Fuggedaboutit: we don’t have the time, much less the bucks. As Mora et al. note:

Considering current rates of description of eukaryote species in the last 20 years (i.e., 6,200 species per year; +811 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.

Bob May isn’t as pessimistic: he think that the bulk of Earth’s species could be discovered within a century of fairly intensive labor, although even then we’re going to miss many species since they’re rapidly going extinct.  Mora et al. suggest where that labor should be concentrated:

. . .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 researchers and funding agencies, and a continuing effort to catalogue existing biodiversity data in publicly available databases.

There are of course some potential problems with Mora et al.’s methods, and they do list them: they include uncertainty about how some taonomists define “species,” and the differential descriptive effort applied to different taxa. They also list problems with previous methods of estimating the number of species on Earth. I won’t go into this sort of detail since you can read the paper for free.

So why should we be cataloging all these species?  That’s what Bob May’s perspective is about, and he gives three reasons.  The first is because we can’t understand the evolutionary and ecological processes that create biodiversity unless we have a full understanding of the number of species.  I’m not highly convinced by this, because you can understand those processes simply by studying a smaller sample. There simply aren’t an infinite number of ways that new species can form.

The other two reasons are practical.  He argues that the number of species “underpins ecosystem services that . . humanity is dependent upon,” and that attempts to catalog biodiversity will invariably uncover some species that will be of tremendous aid to humanity.  (He uses the example of a new variety of wild rice that, when crossed to domesticated rice, improves output by 30%.  There is also the benefit for health, too: although May doesn’t mention this, a large proportion of the world’s pharmaceuticals ultimately trace back to compounds isolated from plants.)

For biologists like me, though, these economic considerations, while important, aren’t our real motivation.  We simply want to know what’s out there, for among those millions of undescribed species are millions of curious and interesting tales: things to inspire not only wonder about nature, but research and new knowledge, which is valuable for its own sake. Humans are a species permeated with curiosity, and satisfying that curiosity is a good in itself.  As I wrote in WEIT:

Each species represents millions of years of evolution, and, once gone, can never be brought back.  And each is a book containing unique stories about the past. Losing any of them means losing part of life’s history.

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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