Science - AP

Half of Plants at Risk, Study Says

Thu Oct 31, 5:25 PM ET

By PAUL RECER, AP Science Writer

WASHINGTON (AP) - Human activities are threatening to wipe out as many as one-half of the Earth's plant species, a study suggests.

Earlier studies had estimated that only about 13 percent of all plant species are in danger of extinction. But Nigel C. A. Pitman of Duke University and Peter M. Jorgensen of the Missouri Botanical Garden in St. Louis said those estimates did not take into account the plants at risk from environmental change in the tropics where most of the world's plant species grow.

In a study appearing Friday in the journal Science, Pitman and Jorgensen determined that about 83 percent of the plant species are threatened in Ecuador, a country with a botanical richness typical of tropical countries.

Extrapolating this data to the entire world suggests that from 22 percent to 47 percent of all of the Earth's plant species are in danger of becoming extinct, Jorgensen said. The range of the estimate varies because botanists are uncertain how many plant species there are.

Estimates range from 310,000 to 422,000, Jorgensen said.

Of all the plant species, most live in the tropical belt and "in many tropical countries, our knowledge of the plant species is very sketchy," he said.

The demand for new farm land to feed a growing population in tropical countries is the biggest cause of global plant species extinction, he said.

"The natural forest is being cut down and burned and the land converted into pastures and fields for crops," said Jorgensen.

A gradual global warming (news - web sites) may aggravate the species loss, he said, because wide open, cultivated areas prevent the natural migration of plants in response to climate change.

"It (global warming) may not have had an effect yet, but may in the future," Jorgensen said. "The more fragmented the vegetation becomes, the more difficult it is for the natural environment to respond. Plants that need to move around to find a cooler place to grow can't move ... because there are farmers in the way."

Peter Raven, director of the Missouri Botanical Gardens, said the study "is a better way of estimating the number of threatened plant species" because it takes into account the high diversity of plants in the tropics.

Raven, who did not participate in the study by Jorgensen and Pitman, said that no tropical country has a solid estimate of the number of its resident plant species. As a result, earlier estimates of threatened flora fell far short of the true number, he said.