by Greg Mayer

Jerry recently discussed an article in Plos Biology which tried to estimate the number of species on Earth by extrapolating from more or less known relationships between higher and lower level taxonomic diversity. Carl Zimmer has a piece in today’s Science Times, and records some dissent from the method used:

But 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.

Terry Erwin initiated quantitative efforts to estimate the number of species in 1982, when, in a brief paper, he estimated there might be 30,000,000 species of tropical arthropods. Erwin’s criticism of the Plos paper is the same as one that I and WEIT reader Mickey Mortimer registered in the comments on Jerry’s piece: taxonomic ranking is conventional (i.e. an agreement among workers on a particular taxon), and too variable among taxa to be used to reliably produce quantitative estimates like this.

Carl Zimmer used a pyramid analogy to explain the method used in the Plos paper: If you know the shape of the top of a pyramid, you can estimate the area of the base. And that’s true. But it depends on knowing how high the pyramid is, what the slope of the faces are, and that the slope is constant (at least at a suitable scale) all the way to the bottom. In the taxonomic case, the height of the pyramid is the number of ranks– this is wholly conventional, and varies among groups; the slope is the relationship between higher and lower level diversity– which we can know for well-known groups, but not poorly known groups; and that this relationship is constant (or at least functionally known) all the way down to species– which may be true in some groups, but not others. And, crucially, the height, the slope, and its constancy may differ among taxa.

The only fact of nature here is the area of the base of the pyramid (i.e. the number of species); the shape and height of the pyramid above are human constructs.

I don’t want to be too hard on Mora et al., because any method of estimating total species diversity is subject to great uncertainty. Discussing Erwin’s estimate for tropical arthropods, E.O.Wilson wrote:

Erwin’s calculations were an important step forward in the study of biodiversity. The explicit figure he arrived at initially, however, is somewhat like an upside-down pyramid balance on its point. At any step on the road to the final total of 30 millian tropical-forest arthropods, the number of species can be shifted drastically up or down by changing assumptions. If the true total is within 10 million of that number either way, it will be sheer luck.

For quite some time, I’ve told classes that the number of species on Earth is somewhere from 3 million to 30 million. I’m going to stick with that range.

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Erwin, T.L. (1982) Tropical forests: Their richness in Coleoptera and other arthropod species. The Coleopterists Bulletin, 36, 74-75. pdf

Mora, C., D.P. Tittensor, S. Adl, A.G.B. Simpson, and B. Worm. 2011. How many species are there on earth and in the ocean? Plos Biology 9(8): e1001127. pdf

Wilson, E.O. 1992. The Diversity of Life. Harvard University Press, Cambridge, Mass.