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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 may aggravate the species loss, he said, because wide open, cultivated areas prevent the natural migration of plants in response to climate change.