How Many Species Exist?

The question takes on increasing significance as plants and animals vanish before scientists can even identify them

12-01-1998 // Laura Tangley

An extraterrestrial being, landing for the first time on Planet Earth, would no doubt have questions about the new world it had found. First and foremost, the visitor probably would want to know if the planet was inhabited and, if so, how many kinds of living things there are. But even if it queried all the world´s biologists, the curious traveler would not get a definitive answer to that simple question.

Scientists have made remarkable progress understanding and quantifying some aspects of life on Earth. Just four decades after James Watson and Francis Crick discovered the structure of DNA--the molecule that carries our genetic code--biologists know the number of genes, or units of DNA, in creatures ranging from yeast (6,000) to humans (100,000). They know how much information is encoded in these genes (in humans, 3 billion letters) and can say precisely how many of them work. But more than 200 years after biologists began naming and classifying the world´s plants and animals, they still do not know how many species exist. Estimates range from 3 million to 100 million or even more.

Taxonomists--biologists who specialize in identifying and classifying life on the planet--have named approximately 1.7 million species so far. Each year, about 13,000 more species are added to the list of known organisms. While scientists continue to turn up surprises--new species of whales, monkeys and deer within the past few years, for instance--a large portion of the world´s mammals, birds, reptiles, amphibians and flowering plants are named. In contrast, the lion´s share of unknown species are small, mostly microscopic organisms that live in some of Earth´s least-accessible habitats: beneath the ground, in the deep sea, in the crowns of tropical trees, and on the backs or in the guts of other creatures. Such insects, worms, mites, fungi, bacteria and other tiny life forms are what Harvard University biologist Edward O. Wilson calls "the black hole of taxonomy." Unimaginably abundant, their numbers could alter overall species totals by a factor of 10 or more.

Seriously hampering a full tally is a shortage of taxonomists to identify species, particularly in the tropics where biodiversity reaches its peak. There are fewer than 200 scientists in the world, for example, who can identify tropical beetles, a group that may ultimately account for a third of all life forms on Earth.

Even in the United States, where species are fewer and taxonomists more abundant, scientists are nowhere near reaching the end of a complete count--or even a good estimate--of species. Though they believe they have found and named the majority of North American mammals (482), birds (about 900) and flowering plants (about 19,000), "when it comes to creatures that are not big or fuzzy, what we don´t know is a percentage somewhere in the double digits," says Edward Theriot, director of the Texas Memorial Museum and president of the Association of Systematics Collections (ASC). "We don´t even know enough to tell you what we don´t know," adds George Washington University microbiologist Diana Lipscomb, an expert on protozoa who discovers new species each time she visits her Chesapeake Bay study site.

In the dark when it comes to real numbers, scientists have devised several formulas to estimate species diversity. Not surprisingly, the results vary wildly. One method--which extrapolates the ratio of temperate to tropical organisms among relatively well-known species to the rest of life on Earth--yields a figure of 3 million to 5 million species. Another based on a well-established relationship between an animal´s body size and its abundance suggests there may be 10 million species of land-dwelling animals alone.

By far the largest estimate comes from a third methodology that extrapolates the results of an intensive survey of a single site: the tops of 19 Leuhea seemannii trees in a Panamanian rain forest. Based on the number of beetle species he collected from these trees, Smithsonian Institution entomologist Terry Erwin estimates there are about 30 million species of tropical insects, a figure others have used to suggest overall species totals approaching 100 million for the entire planet.

The problem with all these estimates is that they depend on uncertain assumptions. In the case of ratios of temperate to tropical organisms, for example, scientists do not know whether the patterns they have observed among well-known groups will hold up for tiny, unstudied plants, animals and microbes. Because mathematical formulas are suspect, some experts believe that the best way to approximate species numbers is to pull together opinions from the world´s experts on every group of organisms.

Published in 1995, the United Nations´ Global Biodiversity Assessment is the most ambitious such undertaking to date. The document proposes a "working estimate" of 13.6 million species on Earth. But some field scientists, such as Erwin, dismiss this as "armchair biology."

Of course, the only way to really find out how many species inhabit the planet is to go out and count them, one by one. Spurred by concern over species extinctions, several scientists over the past decade have advocated such a complete global biological survey. To date, however, not even a single country has attempted to comprehensively survey its own biota. Now a new U.S. program called Discover Life in America--made up of some 60 U.S. government agencies, universities and other groups--is making a significant start with the launch of an "all-taxa biodiversity inventory" of species in Great Smoky Mountains National Park. The organizers´ goals are not only to identify the park´s estimated 100,000 species, but to collect data on their distribution and natural history.

Such information is vital, say biologists, who point out that even for species that are identified, we have little more than a Latin name, a collection site and a few dead specimens. Even more worrisome than our ignorance about the magnitude of life on Earth, they say, is the fact that wild species--which provide humans with food, clothing, medicine and the conditions that sustain all life--are going extinct before we know anything about them. Organizers of the Smoky Mountain biological survey estimate that the project´s first decade, which will concentrate on groups of organisms for which experts already are available, could cost between $1 million and $3 million. That may not be an insignificant sum, says ASC´s Theriot, but it´s "much less than a single´inexpensive´ fighter jet."

Laura Tangley is an associate editor for U.S. News & World Report.


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