Species Uncertainties

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Science  06 Feb 2009:
Vol. 323, Issue 5915, pp. 687
DOI: 10.1126/science.1170937

Appropriately for a year celebrating Darwin, This issue of Science has a special section that surveys our current understanding of speciation processes (p. 727). The section illustrates Dobzhansky's memorable injunction that nothing in biology makes sense except in the light of evolution.

You cannot, however, even begin to make sense of biodiversity until you have some systematic sense of what is actually there. This task began, astonishingly, a full century after Newton and his predecessors, with the 10th edition of Linnaeus' De Rerum Naturae (1758). The legacy of this unhappy quirk lingers with us today. Take the U.S. Library of Congress—its Web site indicated that, as of December 2008, the library held 20,854,816 cataloged books. Yet we do not know, to within 10% or so, how many distinct eukaryotic species have been named and recorded (the species concept arguably needs to be interpreted differently for prokaryotes). This derives partly from the lack of synoptic and coordinated catalogs for many invertebrate taxa and partly from resulting unresolved synonymies: the same species separately identified and named on two or more occasions. The total number of known eukaryotic species is currently estimated as ∼1.8 to 1.9 million, but with all the synonymies removed it may be 1.6 million or fewer. Bird and mammal species are known very well and most other vertebrates reasonably well. Somewhere around 90% of plant species are probably known. But our knowledge of invertebrate species (insects, helminths, and others) is woefully inadequate. So credible estimates of the true eukaryotic species total run around 5 to 10 million, but suggestions as low as 3 million or as high as 100 million can be defended.


Given these uncertainties, we can say even less about the numbers of species likely to be threatened with extinction. Assessments by the International Union for Conservation of Nature (IUCN) suggest that ∼20% of all mammal species and 12% of all birds are seriously endangered. But our ignorance about even those insect species that have been recorded is highlighted by the fact that only 0.06% of them feature in the IUCN Red List of Threatened Species. It is nevertheless possible to estimate the rate of species extinctions over the past century as compared with average rates over the 550-million-year span of the fossil record by assuming that well-known groups, such as birds and mammals, are representative. This suggests that recent extinction rates have been 100 to 1000 times faster than average. And under further stresses from habitat destruction, alien introductions, and overexploitation, all exacerbated by climate change, a further 10-fold increase is foreseen over the coming century. This puts us on the breaking tip of a sixth great wave of mass extinctions, differing from the previous Big Five in that it is associated with our activities rather than with environmental events.

A better understanding of speciation processes, and of species themselves and their ecological roles, is not just an academic exercise. The recent Millennium Ecosystem Assessment made a comprehensive appraisal of the condition of, and trends in, the world's ecosystem services: benefits provided to humans as a result of species' interactions within ecosystems. It identified 24 categories of such services, finding that 15 are being degraded or used unsustainably, 4 have been enhanced, and the remaining 5 are unevaluatable. A separate study assessed the value of these services as being comparable with conventionally assessed gross domestic product. Most of these ecosystem services are delivered by invertebrate species, which are largely unknown to us. One consequence is that our growing understanding of how ecosystems are structured and function remains insufficient to answer the question of whether we will be able to engineer repairs to, or substitutes for, degraded ecosystem services.

Other initiatives, long overdue, are speeding up the rate at which we are cataloging the library of living species. But concomitant growth in humanity's ecological footprint, arguably to unsustainable levels, requires all these advances and more. Best of times, worst of times.

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