- Out of 4,000 plant species surveyed, 22 percent were classed as threatened in a recent study.
- The loss of plants could have potentially catastrophic effects for life on Earth.
- Human-induced habitat loss is the main problem.
More than a fifth of the world's plant species faces the threat of extinction, a trend with potentially catastrophic effects for life on Earth, according to research released on Wednesday.
But a separate study cautioned that extinction of mammals had been overestimated and suggested some mammal species thought to have been wiped out may yet be rediscovered.
Stephen Hopper, director of the Royal Botanic Gardens in Kew, London, said the report on plant loss was the most accurate mapping yet of the threat to the planet's estimated 380,000 plant species.
"This study confirms what we already suspected, that plants are under threat and the main cause is human-induced habitat loss," Hopper said at the launch of the so-called Sampled Red List Index.
The study, carried out by Kew with the Natural History Museum in London and the International Union for the Conservation of Nature (IUCN), set a "major baseline" for future conservation efforts, he said.
"We cannot sit back and watch plant species disappear -- plants are the basis of all life on Earth, providing clean air, water, food and fuel. All animal and bird life depends on them, and so do we," Hopper added.
The study comes ahead of a meeting in Nagoya, Japan, from October 18 to 29, where members of the UN's Biodiversity Convention will set new targets to save endangered wildlife.
Craig Hilton-Taylor of the IUCN said he hoped the Nagoya meeting would set the goal of preventing the extinction of any known threatened species by 2020.
"We want to make sure that plants will not be forgotten," he said.
In their study, researchers assessed almost 4,000 species, of which 22 percent were classed as threatened, especially in tropical rain forest.
Plants were more threatened than birds, as threatened as mammals and less threatened than amphibians or corals, it said. Gymnosperms, the plant group including fir trees, were the most threatened.
The greatest peril came from man-induced habitat loss, mostly the conversion of natural habitats for crops or livestock. Human activity accounted for 81 percent of threats, said Kew researcher Neil Brummitt.
Meanwhile, a study by two Australian authors said Tuesday that fewer mammal species than believed may be extinct, especially those animals threatened by habitat loss.
Diana Fisher and Simon Blomberg of the University of Queensland said they had identified 187 mammals that have been "missing" since 1500, 67 species of which had subsequently been found again. Their paper was published in Proceedings of the Royal Society B: Biological Sciences a journal of Britain's de-facto academy of science.
"Extinction is difficult to detect," the study said. "Species with long gaps in their sighting records, that might be considered possibly extinct, are often rediscovered."
Mammals hit by habitat loss were "much more likely to be misclassified as extinct" than those affected by introduced predators and diseases or by overhunting.... Hence impacts of habitat loss on extinction have likely been overestimated, especially relative to introduced species."
The authors said efforts to hunt extinct mammals should be diverted away from often fruitless attempts to rediscover "charismatic" species such as the thylacine, a stripy, carnivorous marsupial, the last known example of which died in 1936 in Tasmania.
Last week, conservationists announced that two species of African frog and a Mexican salamander feared to have become extinct last century had been found again after teams explored remote places, sometimes at great risk to themselves.