Count of arthropods in world's jungles

Count of arthropods in world’s jungles

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 1,29,494 specimens, and to extrapolate that number to come up with a global estimate of six million species. The results are published in the journal Science. “This is easily the most comprehensive survey done in one small area of tropical rain forest,” 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 rain forest, so biologists scale-up the richness of rain forest populations to make global estimates.

In 1982, entomologist Terry Erwin at the Smithsonian Institution in Washington “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 worldwide.

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 million. Lead author Yves Basset, an entomologist at the Smithsonian Tropical Research Institute in Panama City, and his team collected arthropods in 12 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. 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 species. He now refuses to put a number on global diversity. Despite their differences, entomologists agree with the general pursuit of pinning down diversity.