One of the fundamental questions of biology is the amount and full nature of biodiversity on Earth. It should be worrisome to all that 250 years after Carl Linnaeus introduced the practice of binomial nomenclature and articulated the goal of identifying all species of organisms, we still have accounted for only a tiny fraction of the whole. The number of living species thus far discovered is approximately 1.9 million.[1. A.D. Chapman et al., Numbers of Living Species in Australia and the World, 2nd ed., Toowoomba, Australia: Australian Biodiversity Services, 2009.] The true number of living eukaryotic species (all plants, animals, and microorganisms above the level of bacteria and archaeans), known plus unknown, is at least four times that number,[2. C.Mora et al., “How many species are there on Earth and in the ocean?” PLoS Biology, 9 (8): e1001127, 2011.] and likely...

Consider, for example, the fungi, with approximately 100,000 known species but by expert estimate at least 1.5 million species present in nature. Or the nematodes (roundworms), with about 25,000 species described and 500,000 believed to exist. Insects, with about a million known species, rank as the world champions of species richness, and could easily be at least three times more biodiverse than our records reflect. An instructive example is provided by the ants, which I have studied for most of my scientific career (and continue to study). These highly social insects make up about one-third of Earth’s insect biomass, and have been well studied in comparison with most other insect groups. The current number of known extant species (as of September, 2011) is 12,337, but I am confident the true number is two or even three times higher. In a 2003 study of New World species of Pheidole, the most abundant and biodiverse of ant genera, I described a total of 624 species in existing museum collections, of which 344 were new to science.[3. E.O. Wilson, Pheidole in the New World: A Hyperdiverse Ant Genus, Cambridge, MA: Harvard University Press, 2003.] Additional new species of Pheidole continue to pour in as collecting explorations of the American tropics expand. This is a typical example: the sheer amount of biodiversity waiting to be cataloged is staggering.

If we save the living environment, we will also automatically save the physical environment.

As taxonomists probe more deeply into the world of small invertebrates, protistans, and fungi, I predict we will discover numbers even larger than expected, as species are added that are very rare, seasonal, limited in distribution, or specialized for niches seldom examined. Others will be found that are “cryptic”—forming genetically separate populations, but so similar in anatomical traits conventionally used by taxonomists as to be overlooked until their DNA is sequenced. We are finding plenty of species of ants in all these categories: known from a single specimen; living in cliffside crevices or deep soil; or existing as very rare social parasites in the nests of other species.

We should not be misled by the fact that relatively big organisms, in particular the flowering plants, birds, and mammals, do appear to have been mostly discovered. They have been the subject of dedicated professional and amateur attention worldwide since the time of Linnaeus. Such, however, is not the case for the immense arrays of smaller organisms that still teem around us. The same principle is true a fortiori for bacteria and very likely archaeans. The present number of known bacterial species—populations with clearly discrete genotypes—is about 10,000; but the number occurring in a single gram of well-nourished soil is about 5,000, virtually all unknown to science. The number in 1,000 kilograms could reach into the millions. Last year’s Census of Marine Life estimated that microbes make up 90 percent of the ocean’s biomass, including as many as 20 million bacterial species. If we were then to add the viruses, the total biodiversity on Earth would soar yet again.

So—who cares? To know well the full biodiversity of Earth is not important simply to add figures to textbooks. The real purpose of science must be the original Linnaean goal: to find and take full account of each and every species of organism on Earth. The immense value of pressing toward such full knowledge is beyond dispute. Neither scientists nor the educated public have qualms about the importance of knowing in fine detail every organ, tissue, and ultimately many of the trillion cells that make up the human body. We need such full knowledge of the biosphere as well, to utilize biodiversity as a treasure house of new pharmaceuticals, physiological processes, effective clues to public health, and advances in biotechnology.[4. Sustaining Life: How Human Health Depends on Biodiversity, E. Chivian, A. Bernstein, eds., New York: Oxford University Press, 2008.] But that knowledge is needed even more urgently to understand and sustainably manage the global environment. We should be obedient to the following principle: if we save the living environment, we will also automatically save the physical environment, because the latter must be preserved in order to support the former. But if we save only the physical environment, we will ultimately lose them both.

Edward O. Wilson is University Research Professor Emeritus and Honorary Curator in Entomology of the Museum of Comparative Zoology at Harvard University.

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