Microscopic animals that live in soils are as diverse in the tropical forests of Costa Rica as they are in the arid grasslands of Kenya, or the tundra and boreal forests of Alaska and Sweden.

That conclusion is found in research results published Oct. 18 in the Proceedings of the National Academy of Sciences.

Scientists have generally accepted that a wider range of species can be found above ground at the equator than at Earth's poles.

But this study proves for the first time that the same rules don't apply to the nematodes, mites and springtails living underground.

The team of National Science Foundation- (NSF) funded ecologists includes Tiehang Wu and Jim Garey at the University of South Florida, Diana Wall at Colorado State University, Ed Ayres now at Neon Inc. in Colorado, and Richard Bardgett at the University of Lancaster in the United Kingdom.

"Scientist E.O. Wilson noted that the key to understanding Earth's biodiversity lies in exploration of its smallest life forms," said Matt Kane, program director in NSF's Division of Environmental Biology, which funded the research.

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"Important affirmation of this idea is provided in this global study of animals, in which the significance of below-ground biodiversity is revealed."

This is the first comprehensive molecular analysis--at nearly a species resolution--of the global distribution of soil animals across a broad range of ecosystems from the tropics to the poles.

Soil samples were taken from 11 sites around the world, including tropical forest in Costa Rica, arid grassland in Kenya, warm temperate forest in New Zealand, shrub steppe of Argentina and tundra and boreal forest of Alaska and Sweden.

Through DNA testing, researchers found that each location had a diversity of soil animals, but that each ecosystem is unique with its own soil animals -- illustrating an "amazing diversity of species" that had never been discovered before, said Garey.

"On average, 96 percent of our identified soil animals were found at only a single location, suggesting that most soil animals have restricted distributions, or in other words, they are endemic," said Wall.

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"This challenges the long-held view that these smaller animals are widely distributed. However, unlike most above-ground organisms, there was no indication that latitude made a difference in soil animal diversity."

"Mites and roundworms dominate soil ecology and contribute to the breakdown and cycling of nutrients in the soil," Garey said. "These animals are essential to the proper functioning of the soil ecosystem in natural and farmlands."

The researchers also examined how the global distribution of soil animals relates to factors such as climate, soil nutrient levels and above-ground biodiversity.

Results showed that sites with greater above-ground biodiversity appeared to have lower diversity beneath in soils.

The main factors explaining this low soil animal diversity at sites with high above-ground diversity were high levels of soil inorganic nitrogen availability and lower pH compared with other sites.

Some sites with high animal biodiversity, like the Kenyan grassland site, are considered more at risk due to land use and population increase.