Image by Bettaman via Flickr
The importance of biodiversity and the loss of species as humans take over their habitats is one of the favourite themes of the environmental movement. But I have yet to see a clear exposition as to why the natural loss of species unable to cope with change is something to be opposed. The diversity of life is certainly one of the most striking aspects of our planet and it is not hard to accept that knowing how many species inhabit Earth is a fundamental question in science. In fact, without knowing this number any comments – let alone conclusions – about the danger of loss of species or the importance of bio-diversity to humanity can only be speculation.
The problem is not only that we have not identified all the eukaryote species in existence (and about 1.3 million have been classified and named) but we have no idea whether the number in existence is to be measured in millions or in hundreds of millions. About 15,000 new species are identified and catalogued each year. If Bacteria and Archaea are added to eukaryotes, the total number of species could be in the billions.
A new paper in PLoS Biology using a relatively new methodology predicts the total number of species that exist. They claim 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”. Using this approach they conclude that “there are ~8.7 million (±1.3 million SE) eukaryotic species globally, of which ~2.2 million (±0.18 million SE) are marine. …. (and) some 86% of existing species on Earth and 91% of species in the ocean still await description”.
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
But the paper is already facing objections. The methodology – some claim – actually only estmates human activity in classifying species and not the species themselves. It does seem like an extrapolation from “what has been found” to predict “all that can possibly be found” and the argument is somewhat circular and not fully convincing. It brings to mind the quotation from Donald Rumsfeld which he was castigated for but which I am finding increasingly profound:
There are known knowns; there are things we know we know.
We also know there are known unknowns; that is to say we know there are some things we do not know.
But there are also unknown unknowns – the ones we don’t know we don’t know.
I think we are still in the state of not knowing what we do not know.
“It’s astounding that we don’t know the most basic thing about life,” said Boris Worm, a marine biologist at Dalhousie University in Nova Scotia. On Tuesday, Dr. Worm, Dr. Mora and their colleagues presented the latest estimate of how many species there are, based on a new method they have developed. They estimate there are 8.7 million species on the planet, plus or minus 1.3 million.
In 1833, a British entomologist named John Obadiah Westwood made the earliest known estimate of global biodiversity by guessing how many insect species there are. He estimated how many species of insects lived on each plant species in England, and then extrapolated that figure across the whole planet. “If we say 400,000, we shall, perhaps, not be very wide of the truth,” he wrote. Today, scientists know the Westwood figure is far too low. They’ve already found more than a million insect species, and their discovery rate shows no signs of slowing down.
In 1988, Robert May, an evolutionary biologist at the University of Oxford, observed that the diversity of land animals increases as they get smaller. He reasoned that we probably have found most of the species of big animals, like mammals and birds, so he used their diversity to calculate the diversity of smaller animals. He ended up with an estimate 10 to 50 million species of land animals.
For the new estimate, the scientists came up with a method of their own, based on how taxonomists classify species. Each species belongs to a larger group called a genus, which belongs to a larger group called a family, and so on. .. In 2002, researchers at the University of Rome published a paper in which they used these higher groups to estimate the diversity of plants around Italy. There were fewer higher-level groups than lower ones at each site, like the layers of a pyramid. The scientists could estimate how many species there were at each site, much as it’s possible to estimate how big the bottom layer of a pyramid based on the rest of it. . … The scientists built a taxonomic pyramid to estimate the total number of species in well-studied groups, like mammals and birds. (They) then used it on all major groups of species, coming up with estimates of 7.7 million species of animals, for example, and 298,000 species of plants.
Terry Erwin, an entomologist at the Smithsonian Institution, think there’s a big flaw in the study. There’s no reason to assume that the diversity in little-studied groups will follow the rules of well-studied ones. “They’re measuring human activity, not biodiversity,” he said. David Pollock, an evolutionary biologist at the University of Colorado who studies fungi — a particularly understudied group — agrees. “This appears to be an incredibly ill-founded approach,” he said. There are 43,271 cataloged species of fungi, based on which Dr. Mora and his colleagues estimate there are 660,000 species of fungi on Earth. But other studies on fungus diversity suggest the number may be as high as 5.1 million species. ….
Jonathan Eisen, an expert on microbial diversity at the University of California, Davis, said he found the new paper disappointing. “This is akin to saying, ‘Dinosaurs roamed the Earth more than 500 years ago,’ ” he said. “While true, what is the point of saying it?”
At least it could be argued that there are not less than 8.7 million eukaryotic species. This is not likely to be the end of this story. Nevertheless it does at least show the scale of the problem and that the number of all living species (and not just eukaryotes) is still in the realm of the unknown.
And the importance of the disappearance of unsuccessful species and of the resulting bio-diversity on humanity is still an unknown unknown.
