How Many Species Are There?
Just four decades after James Watson and Francis Crick discovered the structure of DNA--the molecule that carries the genetic code for most living things--biologists know the number of genes, or units of DNA, in organisms ranging from yeast (6,000) to humans (100,000). They know how much information is encoded in these genes (in humans, 3 billion bits), and they can even say precisely what many of them do. But more than two centuries after Carolus Linnaeus began what might seem the most fundamental task for those who study life--classifying the world's species--biologists still cannot say how many there are. Two things on which they agree: They are nowhere even close to a complete count, and the final tally will fall somewhere between 3 million and 100 million species.
The job of cataloging Earth's species is straightforward, methodical, and slow. Taxonomists, biologists who specialize in identifying and categorizing the myriad forms of life on the planet, add an average of 13,000 species a year to the list of known organisms. At that rate, it would take centuries to complete the census. Because no central storehouse coordinates the results, even the number of species named so far--between 1.5 million and 1.8 million--is uncertain.
With roots in the 18th century, taxonomy remains a curious blend of old and new. Taxonomists, most of whom got their start as bug-collecting kids, still tromp around the countryside in muddy boots, amassing and preserving thousands of plant and animal specimens. But even with the help of modern aids like computer programs that can instantly and precisely record dozens of characteristics of an insect--body shape, size, color, even the length of a microscopic hair at the tip of a tiny leg--taxonomy is hampered by a shortage of experts who can make sense of the data. There are fewer than 200 scientists in the world, for example, who can identify tropical beetles, which some experts believe account for a third of all species on the planet. At the Smithsonian Institution's National Museum of Natural History, tens of thousands of beetle specimens (out of 12 million in the collection) have yet to be identified.
Although taxonomists continue to turn up the occasional big surprise--a previously undiscovered whale, primate, or deer, for example--they generally agree that the easiest part of the global census is done. Birds, mammals, reptiles, amphibians, and flowering plants have long captured the imaginations of naturalists and are hard to miss, and their numbers are reasonably well established. By contrast, the overwhelming majority of organisms still unknown to science are almost certainly small, even microscopic, and are to be found in the world's least-accessible habitats--beneath the ground, at the tops of tropical trees, in the deep sea, and on the backs or in the guts of other species. Insects, mites, worms, fungi, bacteria, and other tiny creatures are truly what Harvard University biologist Edward O. Wilson calls "the black hole of taxonomy." Unimaginably abundant and largely unknown, their numbers could change species totals by a factor of 10 or more.