New estimate prompts different reactions
Eight million, seven hundred thousand
species (give or take 1.3 million).
That’s the new estimate of the total number of species on Earth, based on an “innovative, validated analytical technique” that dramatically narrows the range of previous estimates (5-100 million). The study, published by PLoS Biology, prompted different reactions from two eminent biologists — Robert May and Daniel Janzen. Now read on . . .
"Why should we care about how many species are alive on earth today. . .(because) such knowledge is important for full understanding of the ecological and evolutionary processes which created, and which are struggling to maintain, the diverse biological riches we are heir to." – Robert May
It is a remarkable testament to humanity’s narcissism that we know the number of books in the US Library of Congress on 1 February 2011 was 22,194,656, but cannot tell you-to within an order-of-magnitude-how many distinct species of plants and animals we share our world with. Something like 1.5 million distinct eukaryotes have been named and recorded, but, lacking synoptic databases, even this number is uncertain owing to synonyms (the same species separately named in two or more different collections).
Part of the problem is that taxonomic effort is approximately divided among vertebrates, plants, and invertebrates, whereas plant species are roughly 10 times, and invertebrates 100 times, more numerous than vertebrates. Mammals and birds are the best known, again reflecting our narcissism: their features are akin to our own.
In this issue of PLoS Biology, Mora et al. offer an interesting new approach to estimating the total number of distinct eukaryotic species alive on earth today. They begin with an excellent survey of the wide variety of previous estimates, which give a range of different numbers in the broad interval 3 to 100 million species. I have favoured a number between 2 and 10 million, and if I had to buy a ticket in a sweepstakes, I’d have chosen 5 million.
Mora et al.’s imaginative new approach begins by looking at the hierarchy of taxonomic categories, from the details of species and genera, through orders and classes, to phyla and kingdoms. They documented the fact that for eukaryotes, the higher taxonomic categories are "much more completely described than lower levels", which in retrospect is perhaps not surprising. They also show that, within well-known taxonomic groups, the relative numbers of species assigned to phylum, class, order, family, genus, and species follow consistent patterns. If one assumes these predictable patterns also hold for less well-studied groups, the more secure information about phyla and class can be used to estimate the total number of distinct species within a given group.
In this way, Mora et al. arrive at a global total of 8.7 million eukaryotic species, with a standard error of ±1.3 million. Most are terrestrial, with 2.2 (±0.2) million being marine.
This is higher than my earlier "best guess", but I like the simplicity of this new method.
Currently, diligent field taxonomists are adding newly discovered species at the rate of very roughly 15,000 each year (when discounted for synonyms). Given that we currently recognize something like 1.5 million distinct eukaryotic species, Mora et al.’s estimated species number suggests 480 years to finish the job. It is, however, reasonable to expect that in the near future, molecular methods - "barcode taxonomy" - will greatly speed up the task of keying-out collected material, as well as resolving synonymies. But the basic field activity of collecting new material will remain a rate-limiting step. Increasing the number of people engaged on the task would obviously help, and any such increase could be made more effective-as pioneering efforts in Costa Rica and elsewhere have shown-by using "parataxonomists", local people who use rough morphological criteria to help recognise new species, in combination with taxonomic experts. All in all, my optimistic guess would be around a century to complete our assessment of the diversity of life on earth.
But this tentative assessment makes no allowance for accelerating extinction rates. As the Millennium Ecosystem Assessment emphasized, over the past century documented extinctions within well-studied groups (particularly birds and mammals) were at a rate 103±1 higher than the average extinction rate seen over the half-billion-year sweep of the fossil record. One can draw no comfort from the thought that the task of cataloguing our planet’s biological richness will be simplified by its winnowing.
Ultimately, why should we care about how many species are alive on earth today, and about how many of them are known to us? One notable Victorian physicist (I will be merciful and not name him) opined that such a quest is little more than stamp collecting. To the contrary, we increasingly recognise that such knowledge is important for full understanding of the ecological and evolutionary processes which created, and which are struggling to maintain, the diverse biological riches we are heir to. Such biodiversity is much more than beauty and wonder, important though that is. It also underpins ecosystem services that-although not counted in conventional GDP-humanity is dependent upon.
Turning from the general to the specific, I give just one among a multitude of possible concrete examples of beneficial application of taxonomic discovery. In the 1970s Yuan Longping, "the father of rice", discovered in the wild a new variety of rice, whose cross with a conventional strain led to a new variety that is 30% more efficient. This has motivated subsequent initiatives to document and protect all wild varieties of rice, which obviously can only be done if we have the appropriate taxonomic knowledge. Given the looming problems of feeding a still-growing world population, the potential benefits of ramping up such exploration are clear.
The essential fact is that, if we are to meet the challenges facing tomorrow’s world, we need a clearer understanding of how many species there are - both on land and in the even less well-studied oceans-underpinning the structure and functioning of ecosystems. Mora et al.’s interesting new approach to assessing the magnitude of this task is thus very helpful.
Robert M. May,
Zoology Department, Oxford University,
Oxford, United Kingdom
May RM (2011) Why Worry about How Many Species and Their Loss? PLoS Biol 9(8): e1001130. doi:10.1371/journal.pbio.1001130
“The species loss, and the loss of their interactions even more so, is not a tragedy because we cannot count them. It is a tragedy because they are gone, quite irrespective of whether there were 1 million, 10 million, or 30 million.” – Dan Janzen
Ten million species? The real number is easily double that, since DNA barcoding is showing us that something like a third to a half of the species that we think are one species are actually two-or-more species that merely look (or act, smell, bite, taste) like the same thing. There are six species of giraffes, three species of orcas, and two species of African elephants. The list goes on and on and on and on. Who cares? Look at a pasture filled with burros, horses, zebras and quaggas, and tell me when the usable description is “grass dotted with equines”. Malaria is carried by particular species of mosquitoes, not “mosquitoes”.
People love metrics. They are so proud of their fingers that they have to count any time they can. But tallying the number of species is really, truly a waste of time — an absurd thing to measure in most cases. It matters not a bit whether a tropical forest just cut down contained 100,000 species or 150,000 species or 300,000 species. The loss to humanity is that all those species gave us the ability to smell, see, think, feel, itch, wonder, hear and puzzle over. They moved us from primeval to the Homo sapiens we know and are; and we are busily trashing them off the planet, taking away the very things that fashioned us into being, knowing, feeling, and seeing that we are in something other than a dull white box. Humanity deserves more than alfalfa, parking lots, and comic books.
The species loss, and the loss of their interactions even more so, is not a tragedy because we cannot count them. It is a tragedy because they are gone, quite irrespective of whether there were 1 million, 10 million, or 30 million. And if you knew that there were at this moment, 27,538,258 species on planet Earth how would that help anything, anywhere, anytime? Not one bit. Turning it upside down, in order to save a significant block of species and all the wonderful (and painful) things that they do to and with each other and us, I do not need to know whether that forest contains 100,000 of them or 1,000,000 of them, or any number in between. What I need to know is what they do, how to tell one from another, where they are, and how to get a particular one to hand or eye at the moment that it happens to matter to me or to you. And what we all need to do is to get that information back into our hands and minds at the time you want it. In return, there just might be some chance of having some serious portion of that wild biodiversity (again) be welcome to at least some serious portion of society. In the face of ravenous Homo sapiens multiplying even faster than the proverbial rabbits, there is no other hope.
Please save us from the very human tendency to, in our frustration of watching and feeling our very own ancestral mold melt away, turn to counting. When the house is burning, you do not need a enumerated thermometer, you need a Fire Department.
Department of Biology,
University of Pennsylvania, USA
(Costa Rica representative on iBOL Scientific Steering Committee)