Many Plants' Species Close to Extinction

New calculation of threatened species gives startling result.

November 2002 by HELEN PEARSON

Nearly half of the world's plants could be close to extinction, scientists have warned. The calculation triples previous estimates.

The number of plants on the standard Red List of threatened plant species is a massive underestimate, say the botanists, because it lacks data on tropical forests. When estimates from here are taken into account, the fraction of species under threat spirals from 13% to between 22% and 47%.

Monitoring the environments most at risk would cost only US$100 per species per year - $12.1 million in total - says Nigel Pitman of Duke University in Durham, North Carolina, one of the report's authors.

"We may be on the edge of a mass extinction of plants," says Pitman. "We'd like to see a major investment for the world's threatened flora."

The figures are startling, and probably in the right ballpark, says botanist Michael Nee of the New York Botanical Garden. Razing tropical forests for farming is thought to be a prime cause of species annihilation. "There are too many people raping the landscape," says Nee.

Red or dead

Species get put on the World Conservation Union (IUCN) Red List when they are formally identified as being close to extinction. But this excludes unidentified or poorly studied plants.

"There are thousands of plants in the tropics that deserve red-listing but no one's got around to checking if they qualify," says Pitman.

Working in tropical countries, Pitman and his colleague Peter Jorgensen found that the number of species unique to each country is a rough guide to the number that is threatened.

Ecuador, for example, has 4,000 species that are found nowhere else. Nearly 3,500 are under threat, because they often grow in small regions, where a landslide or fire can wipe them out.

To find the global proportion of plants under threat, Pitman and Jorgensen pooled the numbers of species unique to each country. The exact number is hard to pin down because estimates of the number of plant species range between 310,000 and 422,000.

"It's an interesting attempt to connect the dots of our picture of global plant extinctions," comments ecologist Hal Mooney of Stanford University in California. "The numbers they calculate should add to growing concern about irreversible species loss."



References

Pitman, N.A. & Jorgensen, P.M. Estimating the size of the world's threatened flora. Science, 298, 989, (2002).

Nature Science

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