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 September 29.
But a separate study cautioned that extinction of mammals had been overestimated and suggested sGardens 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.
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.
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.