How many kinds of bugs are there? A lot, but probably not as many as we once thought. A new estimate, based on mathematical modeling and a major bug-counting effort in New Guinea, puts the number of arthropod species in the tropics—which account for most of the animal species in the world—at about 3.7 million, way below the 30 million once suggested. The results could help scientists get a better sense of how many species are disappearing in the current human-caused mass extinction.
Ecologists' estimates of the number of tropical arthropods come from counting beetles. "Most higher organisms are arthropods, and most arthropods are insects, and most insects are beetles," says ecologist Andrew Hamilton of the University of Melbourne in Australia, author of the new study. "There's just a lot of beetles out there." About 30 years ago, entomologist Terry Erwin of the Smithsonian Institution's National Museum of Natural History in Washington, D.C., came up with an equation to estimate the number of insects in the tropics by measuring how much beetles specialize on tree species. For example, say every beetle in the world eats the leaves of two tree species. If you know how many beetles are found on one tree species and how many tree species there are, you're most of the way toward being able to count beetles. Erwin’s equation requires figures such as how many tree species are in the world, what proportion of bugs are beetles, and what proportion of beetles eat plants. In a 1982 paper, Erwin came up with 30 million tropical arthropods. Later, he came up with estimates as high as 100 million.
For the new study, published online this month in The American Naturalist, Hamilton used a technique known as uncertainty modeling to try to improve on these estimates. Although previous efforts have relied on single numbers for, say, what proportion of arthropods are not beetles, Hamilton used ranges that he found in published articles. And to come up with the number for how much beetles specialize on a species of tree—the crux of the model—he used data from a study of beetles that's been going on in New Guinea since 1994 in which the scientists census the bugs by counting them and present live insect larvae with different kinds of leaves to see what they'll munch on. Then a computer ran his model 500,000 times, picking a different value for each of the variables every time, producing a range of results. He found that the most likely number of tropical arthropods is 3.7 million, and it probably falls between 2 million and 7 million species.
Once you have the number of tropical arthropods, you've counted the biggest chunk of living things on Earth, if you leave out the bacteria. "Plants and mammals—woody things and floppy things and all that—they're just a small addition," says Hamilton. So, adding on 50,000 or so vertebrates, 400,000 plants, and about 1.3 million other organisms, that brings the total on Earth to maybe 5.5 million species. In the middle of a mass extinction, he says, it's useful to know how many species there were to start.
Evolutionary biologist Marty Condon of Cornell College in Mount Vernon, Iowa, is skeptical of the numbers. For one thing, it can be hard to tell insect species apart, and large-scale surveys often depend on species being separated on sight. She studies South American flies that can look exactly the same and can be distinguished only by genetic analysis. The only way to get better estimates of the number of tropical arthropods, she says, is to collect more data on different kinds of insects. "It seems to me that a whole lot of people are spending an awful lot of time coming up with estimates based on really not very much data," she says.
There are drawbacks to focusing so heavily on beetles and extrapolating from there, says Alfried Vogler, a molecular biologist at Imperial College London. But no one has come up with a better alternative, he says. Maybe in the future, he says, someone will figure out a way to do large-scale studies where they sample insect DNA from the environment, similar to an approach that has been used for bacteria. "That,” he says, “would be a completely different way of estimating diversity of insects."