8.7 Million Species Is Not a Scientific Fact
Human beings love to classify things. We pigeonhole items into bins of our own making, for whatever the reason, to give us a feeling of having things organized and understood. Do our pigeonholes reflect categories that are “out there” in nature, or are they constructs of our own minds? Science reporters are announcing in bold print that there are “8.7 million species on Earth,” but a look at the fine print shows the error bars to be so enormous, there is more error than data. What does this imply about the scientific validity of human classification schemes?
PhysOrg announced the 8.7 million number in its summary of a paper on PLoS Biology.1 National Geographic News followed up with a headline on one of the findings by Mora et al.: “86 Percent of Earth’s Species Still Unknown?” Right off the bat, questions arise about how anyone can know the 8.7 million number with only 14 percent data. It would seem that many among the to-be-discovered organisms in the 86 percent bin might not be species at all, but members of smaller groupings like subspecies or varieties – or even of larger taxa [biological categories] like families or phyla. In the 08/20/11 story, for instance, a whole new family, genus, and species was created for Protoanguilla palau, an eel that was classified as the only member of its new taxon. Not all taxa are created equal. Some phyla like Micrognathozoa and Ciliophora have only one member (SFSU.edu) while some lower taxa, like the order Coleoptera (beetles) within the class Insecta have over 2 million species.
Other scientific classification schemes seem to carve nature at its joints more cleanly. The periodic table in chemistry is a classic example. Even there, though, the use of atomic number to distinguish groups of things can seem a bit arbitrary in some situations, because there are isotopes of some elements that are heavier than elements with a higher atomic number.
Planet classification continues to vex astronomers ever since Pluto got demoted from a “planet” to a “dwarf planet” (after some tried minor planet or plutoid). The International Astronomical Union (IAU) is the panel that places a kind of scientific imprimatur on nomenclature. Back in 2006, after much debate, they voted Pluto out of the “Planet” bin. National Geographic News reported this week, though, that “New Finds Drive Debate” about whether Pluto should be promoted back to planet. Those finds include other bodies large and small in the outer solar system (some larger than Pluto), new computer models that show objects moving around over time (moving, for instance from planetary satellite to sun-orbiter), exoplanets, and some planet-size bodies that don’t orbit stars at all.
Reporter Victoria Jaggard’s lengthy foray into the wrestling matches over planetary classification showcased the human emotions involved. “After the ruling, astronomers everywhere were besieged by complaints from everyone big and small,” planetary scientist Marc Kuchner (NASA Goddard Space Flight Center) remembered. “A planet is a very personal thing—we think of the Earth, the moon, and the other planets as part of our home, and maybe that’s why we got so upset about Pluto.” Space.com gave space to planet killer Mike Brown and planet defender Alan Stern. In the NG News article, Owen Gingerich recalled that emotions rang high partly because “It was a bureaucratic problem, as it had to do with naming rights for these kinds of things.” Does the IAU get naming rights simply because of its power and prestige? If a political dictator had the power, could he make his naming scheme the official one?
In the species wars, too, it is clear that politics gets embroiled. Commenting on the original paper by Mora et al. in PLoS Biology,2 Robert May (Oxford U) tried to argue that taxonomy is more than just stamp collecting. He asked, “Why Worry About How Many Species and Their Loss?” His answer was, “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.” The operative word is we, because May sees humans as beneficiaries of “ecosystem services” that classification helps us understand. It is difficult to see why this should be so, however, since the ecosystem is what it is, regardless of how we carve it up name-wise.
Lord May noted that we can count the number of books in the Library of Congress (at a given point in time) to eight significant figures, but cannot estimate the number of species within an order of magnitude (e.g., one million vs ten million). Part of the problem, he said, is that some species get classified into two or more different collections. Another is that taxonomists carve up organisms into equal-size bins of plants, vertebrates, and invertebrates, even though “plant species are roughly 10 times, and invertebrates 100 times, more numerous than vertebrates.” And the big mammals get noticed easier than tiny microbes living under Antarctic ice.
Mora et al. addressed the problems of subjectivity in the Linnaen classification system, but pushed ahead anyway, justifying their analytical estimate of 8.7 million species, aware that 86% of land organisms and up to 91% of marine organisms remain unknown to science. It’s a hopeless task to close that gap, they lamented, because “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.” And “With extinction rates now exceeding natural background rates by a factor of 100 to 1,000, 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. ” Yet taxonomists must press on, they argued, because “High rates of biodiversity loss provide an urgent incentive to increase our knowledge of Earth’s remaining species.”
And there’s the political rub. Laws to protect endangered species assume biologists know what species are. Mora worried in the PhysOrg article that classification is “particularly important now because a host of human activities and influences are accelerating the rate of extinctions.” But it’s much easier to discover a critter than classify it, National Geographic News explained: “Scientists must compare their specimen to museum samples, analyze its DNA, and complete reams of paperwork” to give it a place in the taxonomic tree. It’s a long process. A biologist could only classify a few dozen in a lifetime, “if they’re really lucky,” one remarked. Visualizing a crisis, with species disappearing faster than we can find them, Mora said, “With the clock of extinction now ticking faster for many species, I believe speeding the inventory of Earth's species merits high scientific and societal priority.”
1. Mora, Tittensor et al., “How Many Species Are There on Earth and in the Ocean?”, Public Library of Science Biology, 9(8): e1001127. doi:10.1371/journal.pbio.1001127 (open access).
2. Robert M. May, “Why Worry about How Many Species and Their Loss?”, Public Library of Science Biology 9(8): e1001130. doi:10.1371/journal.pbio.1001130 (open access).
An old joke applies here. A homeless person was offered a job – a very simple job – to sort potatoes into a pile of big potatoes and a pile of small potatoes. What could be an easier way to earn some whiskey dough? Before getting halfway through, though, the guy quit. Why? “The decisions,” he mumbled.
Let’s have some fun with taxonomy (a vexed issue in philosophy of science). Would it be appropriate to lump all living things into one category? Sure, if it is useful to you. You could call it “Inhabitants of the Biosphere” or some catchy name, like geota. After all, geota all share a genetic code, right? Now instead of 8.7 million things, you have one nice, tidy category. How could that be useful? Well, you’ve immediately distinguished geota from Martians and space aliens. That could be useful in some scientific discussions. But even then, you would have to worry about whether some geota migrated to Mars, as seems more plausible now, thanks to a report on PhysOrg that “Simulation shows how Earth may have seeded life on other planets.” So when NASA brings back soil samples from Mars, there will be new debates about whether any putative life forms are really geota from Earth contamination – or even whether geota originated on Mars and migrated to Earth, making the geota category moot in the first place.
The same problems exist with other classification schemes. All bodies in the solar system could be classified as sun-orbiters, if useful (after all, even moons orbit the sun in a loopy spirograph kind of way). There are no laws of classification handed down from on high. We make them up. It’s whatever your groove is, man. You can classify the elements into Earth, Air, Fire, and Water if you want to—a scheme that the Greeks found useful for centuries. In his classic cartoon Science Made Stupid, Tom Weller showed how the Greek scheme could deduce chemical compounds. Add earth to air, for instance and you get smog; fire plus water yields Tequila. Some cultures might find that knowledge useful.
In the late second millennium A.D., some human cultures have found it useful to classify living things into a hierarchy of phylum-class-order-family-genus-species, physical stuff into elements-isotopes-compounds-subatomic_particles, and orbiting bodies into planets-satellites-asteroids-comets-dust (with some new entries, such as Kuiper Belt Objects, Trans-Neptunian Objects, Near-Earth Asteroids, Plutoids, Minor Planets, Dwarf Planets, comet-like asteroids, asteroid-like comets etc.). You get into the small-potatoes dilemma when you try to classify the moons of Saturn, when bodies grade smoothly from Titan-size moons down to micron-size ring particles, or classify subatomic particles (how many muons, leptons, hadrons, quarks, and strings are there, anyway?) but for most uses, our schemes serve us well; they have led to many useful discoveries.
Conversely, you could go to the other extreme and put each object into its own bin. Chemists could have thousands of bins, with one for each chemical isotope. You could call each planetary body by its proper name without lumping it into higher groups. You could classify every individual organism of a species into its own category. We do that already in some cases, like giving names to each chimpanzee in the primate cage at the zoo. The zookeeper doesn’t want to refer to Pan troglodytes in such cases, but to Bonzo, Ponzi and Sally. The situation dictates the choice of classification.
Any situation involving human subjective judgment is going to start debates. Biological taxonomists have long endured the wars of the lumpers and splitters, those who want fewer categories, and those who want more. And don’t discount the lust for fame – the desire to discover a new species and name it yourself, or even name it for yourself. Our thought experiment about geota is just lumping taken to the extreme.
Even our best scientific classification schemes are riddled with internal flaws. Did you know there are multiple competing definitions of species? Try any one definition, and it will raise vexing questions. The common definition we learn in high school biology, the biological species concept, tries to define a species as a group in which members can produce fertile offspring. Nice, except that you’ve just ruled out fossils and the vast majority of organisms that reproduce asexually. Another recent scheme called PhyloCode tried to group living things by their evolutionary lineage. Creationists would just love that one, but even evolutionists cannot agree on ancestral lines in their mythical Tree of Life (see article on Evolution News & Views).
This excursion into philosophy of classification is trivia except when the classifiers come after your wallet. Most of us can get by in life whether or not Pluto is dubbed a planet, but the Endangered Species Act has caused landowners their livelihood. Remember the snail darter? It is human beings, with their flawed schemes, who decide what constitutes an endangered species. Reclassify snail darters as members of a larger taxon with a wider range, and presto! the landowner gets his private property rights back. What changed? The snail darter? No; the society’s decision, based on political power. All it takes for an ideologue with an agenda to stop a shopping center or factory that might create hundreds of jobs is to discover a weed or roundworm in the path of development and convince the EPA it is endangered. Classification looms large as a political issue in such cases.
This commentary focused on utility as the essence of classification. Any classification scheme is appropriate to the degree it is useful. Now, one must ask the follow-up question lurking in the shadows: useful to whom?