Arthropods, which include insects, arachnids and crustaceans, are the most diverse group of terrestrial species on the planet. And that might be why researchers have had such a difficult time estimating their numbers, especially in tropical forests where so many arthropod species are known to thrive.
Now, Yves Basset from the Smithsonian Tropical Research Institute in Panama City, along with colleagues from around the world, report the results of a collaboration involving more than 100 researchers who sampled the San Lorenzo forest reserve in Panama—from its soil to its canopy—as part of an ongoing quest to quantify arthropod populations.
The researchers collected 129,494 arthropods representing 6144 different species from a little less than half a hectare (about 1.2 acres) of the tropical forest. They then used models to predict that approximately 25,000 arthropod species dwell in the larger, 6000-hectare (14,826-acre) reserve. But the researchers also suggest that just one hectare of forest harbors more than 60% of those 25,000 species at any given time.
It took the scientists about eight years to fully identify the species in their survey, which was published in the 14 December issue of the journal Science.
“We have a good idea of the number of large, visible species, like mammals and birds and plants,” said Andrew Sugden, international managing editor of Science, during a 12 December teleconference. “Insects and spiders are the most diverse group of animal species—and far more numerous—yet estimates of their total numbers have varied widely, especially for tropical forests. The difficulty has been with sampling, because the rainforest environment is highly complex, the forest canopy is rather inaccessible, and the creatures are so small.”
“The few temperate forests intensively studied to date often yielded no more than 3000 arthropod species within an equivalent area,” said Basset during the teleconference. “If we want to understand and conserve life on Earth, we had better then start understanding and conserving the arthropods of tropical forests.”
The large-scale study represents the first time that arthropod diversity in a tropical rainforest has been quantified with reasonable confidence, according to Tomas Roslin of the University of Helsinki in Finland, a co-author on the Science report. “Now that we know the species diversity of even a single tropical forest and how it can be uncovered, we are clearly better poised to address questions on how many species we share the planet with,” he explained.
The researchers also say that factors such as plant diversity can be powerful predictors of arthropod species in the region. For example, they say that every species of vascular plant indicates the presence of 17 arthropod species, every species of bird suggests the presence of 71 arthropod species, and for every mammal in the area there are 270 arthropod species.
“What this means is that we can make a fairly good guess at the total number of species by looking at the total number of plants,” Roslin explained. “This is excellent news because, while we have just proven that arthropod diversity rules the forest, not every study will have access to the 100 arthropod experts that we did.”
Taken together, the findings may help to guide estimates of biodiversity as well as conservation efforts in the future.