Panamanian Survey Puts Global Arthropod Species at 6 Million


A “blushing phantom” butterfly, Cithaerias pireta, rests briefly on a palm leaf in San Lorenzo forest. Credit: Maurice Leponce, Royal Belgian Institute of Natural Sciences

For two years in the Panamanian tropics, entomologists have been laboring in cranes, gliding amongst treetops using helium-filled balloons, hiking through the jungle at night and setting traps that used light as bait in order to come up with en estimate on the biodiversity of arthropods.

The scientists published their findings in the journal Science. It took another eight years to identify the 129,494 specimens, and to extrapolate that number to come up with a global estimate of 6 million species.


A ‘white witch moth’, Thysania agripina, perches on a hand. Credit: Yves Roisin, Université Libre de Bruxelles

This is the most comprehensive survey done in one small area of tropical rainforest, states Andrew Hamilton, an entomologist at the University of Melbourne, Australia. Arthropods have hard, jointed exoskeletons, and they include insects, arachnids and crustaceans. There are more species of arthropods than any other group and their diversity is the greatest in the tropical rainforest, so biologists scale-up the richness of rainforest populations to make a global estimates.

A previous estimate started at 30 million species in 1982 and eventually was reduced to six million species. These studies used a subgroup of insects to predict the overall numbers. However, the new work counts all types of arthropods in larger sections of forests. Rather than assuming that one taxon is representative for all taxons, the scientists 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 12 plots, each 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 trees contained more arthropod species. The team built a model to determine arthropod diversity on the basis that for every species of tree or other vascular plant, there were about 20 species of arthropods. This made things easier because plants are easier to survey than arthropods

Using the number of tree species in the world, the team estimated that there were 6 million arthropod species globally. Scientists only know a fraction of these species, perhaps 1 million out of 6 million, but believe it may be possible for humans to discover all of the arthropod species.

Other biologists disagree with this estimate, believing that the Panamanian tropics cannot be used to extrapolate a global population estimate. Since insect diversity increases with the number of trees and with the number of microhabitats formed by varying compositions of tree species, the Panamanian tropics are not representative of other forests around the world.

[via Nature]

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