Up to half of the world's plants could become extinct this century, according to a new analysis of the true number of endangered species.

Existing studies suggest that about 13 per cent of the known plants in the world are threatened but the true figure could be far higher, says a study published today in the journal Science.

Nigel Pitman of Duke University in Durham, North Carolina, and Peter Jorgensen, of the Missouri Botanical Garden in St Louis, calculate that at least 22 per cent of species should be on the Red List of the world's threatened plants.

The figure of 13 per cent is a "serious underestimate" because it was based largely on what is known about threatened species in the temperate regions of the world – such as Europe and North America – but it is in the Tropics where plant diversity is at its richest and most vulnerable, they say.

"The results suggest that as many as half of the world's plant species may qualify as threatened with extinction under the World Conservation Union classification scheme," the scientists write. "Comprehensive Red Lists for plants are available for only a scattering of tropical countries, making it difficult to assess the true scale of the global conservation crisis for plants."

The two scientists looked at the number of plants that are endemic to nearly 200 countries and used this as a basis for estimating how many are threatened by such things as habitat loss and deforestation.

They say that the concentration of endemic plants can be directly linked to the proportion that would be expected to be threatened with extinction. Globally, this means that between 22 per cent and 47 per cent of plants could become extinct in the foreseeable future. In some countries the figure could be as high as 80 per cent or more of its endemic plants.

There are thought to be from 310,000 to 422,000 species of plants in the world but Peter Crane, director of the Royal Botanic Gardens at Kew, said fewer than one in 20 had been formally assessed for their conservation status. "For some plant groups which have recently been studied by specialists at Kew, at least 50 per cent of the Brazilian species are considered to be threatened," he said.

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