Published online 28 September 2010 | Nature | doi:10.1038/news.2010.499

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Habitat loss is the biggest hazard to plant biodiversity.

Among the gymnosperms, the most at-risk plant group, over 75% of cycad species are threatened with extinction. RBG Kew

More than 20% of the world's 380,000 plant species are at risk of extinction, making plants more threatened than birds, according to the first global analysis of the status of plant biodiversity.

The risk assessment, called the Sampled Red List Index for Plants, was conducted by plant scientists at the Royal Botanic Gardens, Kew, UK, and is published today. It finds that gymnosperms — or seed-bearing plants — including conifers and cycads, are the most at-risk group of plants, with more than 75% of cycad species currently threatened with extinction.

Habitat loss, resulting from the conversion of natural habitats for agriculture and livestock grazing, is the biggest threat to plants' survival, the study concludes.

Existing indicators of biodiversity — such as the Red List of Threatened Species of the International Union for Conservation of Nature (IUCN) — focus mainly on vertebrates, and have assessed only a handful of plants, says Eimear Nic Lughadha, a plant scientist at Kew and a lead researcher on the plant risk assessment.

"By bringing plants into the equation we hope to get a broader and more accurate picture of what is happening to the world's biodiversity," says Nic Lughadha.

Stephen Harris, curator of the Oxford University Herbaria, UK, agrees that conservation efforts have been one-sided. "When people think about what needs conserving, they rarely think about plants," he says.

Harris says he is not surprised that the study found cycads to be the most endangered group. "They have a very small population size, and very limited distribution."

Species head count

The researchers assessed a total of 7,000 plants species — including bryophytes (mosses and liverworts) and legumes (peas and beans) — from the five major plant groups. Examining more than 1 million herbarium specimens, the researchers looked at when and where a specimen was picked. This information was then fed into a computer-based geographical information system to produce a map of the different locations, or distribution, of the species worldwide, and also the size of the area or areas that they cover. Other information, such as habitat loss, collected from scientific literature and expert consultation, was also fed into the system to show whether the areas inhabited by particular species are under threat.

The study uses IUCN criteria to classify species as threatened. For example, a species is classified as 'critically endangered' if the area that it occupies covers less than 10 square kilometres and if the species is known to exist at only a single location.

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The researchers plan to reassess the threat status to plants in 2015. Using the data published today as a baseline, they will be able to gauge whether the risk to plants is growing with time.

Stephen Hopper, director of the Royal Botanic Gardens, says that the assessment will help countries to measure progress towards new targets to halt loss of the world's biodiversity by 2020, which are due to be agreed when the countries that are signatories to the Convention on Biological Diversity meet in Nagoya, Japan, in October.

"The 2020 biodiversity target that will be discussed in Nagoya is ambitious, but in a time of increasing loss of biodiversity it is entirely appropriate to scale up our efforts," says Hooper. "We need a renewed commitment to care for biodiversity."