For two years, entomologists hoisted themselves up in cranes amid the towering trees of the Panamanian tropics. They glided along treetops harnessed to a helium-filled balloon, and hiked through the moonlit jungle to set traps that use light as bait — all to come up with an informed estimate on the biodiversity of arthropods.

It took another eight years to identify the 129,494 specimens, and to extrapolate that number to come up with a global estimate of six million species. The results are published today in Science1.

“This is easily the most comprehensive survey done in one small area of tropical rainforest,” says Andrew Hamilton, an entomologist at the University of Melbourne in Australia.

Arthropods, which are distinguished from other animals by their hard, jointed exoskeletons, include insects, arachnids and crustaceans. There are more species of arthropod than of any other group, and their diversity is greatest in the tropical rainforest — so biologists scale-up the richness of rainforest populations to make global estimates.

In 1982, entomologist Terry Erwin at the Smithsonian Institution in Washington DC 'fogged', or sprayed, one species of tree in a Panamanian tropical forest with insecticide and identified the beetle species that dropped to the forest floor. Using his best estimate of the proportion of tree beetles to ground beetles, and of beetles to insects, he predicted that there were more than 30 million insect species worldwide2. Almost three decades later, Hamilton fogged plant-eating insects on several tropical trees in New Guinea and, by a similar method, reduced the global estimate to six million3.

Both studies used a subgroup of insects to predict overall numbers, whereas the new work is all-encompassing — counting all types of arthropod in larger sections of forest. “What’s phenomenal about this new study is that they cut through all of the assumptions we used,” Hamilton says. “Rather than assuming that one taxon represents another, they looked at the whole community.”

Lead author Yves Basset, an entomologist at the Smithsonian Tropical Research Institute in Panama City, and his team collected arthropods in twelve plots, each roughly the size of a tennis court, in the San Lorenzo forest in Panama. They returned with 6,144 species, hundreds of which may be new discoveries.

Plots with more tree species contained more arthropod species. The team built a model to predict arthropod diversity on the basis of the fact that for every species of tree or other vascular plant (including ferns and flowering plants), there were around 20 species of arthropod. Basset says that this model makes it easier for biologists to estimate arthropod diversity because “plants are much, much easier to survey”.

Countless little things

Based on the number of tree species in the world, the team's estimate is in keeping with Hamilton’s prediction of six million arthropod species globally. “We know only a fraction of this diversity, perhaps just one million out of six million,” Basset says. With less diversity than the 30 million that Erwin predicted, Basset says that it is possible for humans to one day discover it all.

Erwin disagrees. He says that the Panamanian tropics cannot be extrapolated to the world. Where he collects in Ecuador, insect diversity increases not only with the number of trees, but also with the number of microhabitats formed by varying compositions of tree species4.

He now refuses to put a number on global diversity. “We all make that mistake,” he says. “The problem is that the Panamanian tropics look nothing like the forests in Ecuador, and they look nothing like the forests of Borneo or the Congo.”

Despite their differences, the entomologists agree with the general pursuit of pinning down diversity. “If we continue to lose the rainforest, we will lose this diversity,” Basset says. “If one day we want to reforest, we will need to know which species are needed to decompose wood, to stabilize soil, to pollinate plants. There are countless little things, and we know almost nothing about how they keep the ecosystem alive.”

Credit: Maurice Leponce/Royal Belgian Institute of Natural Sciences