Estimating the size of the world's threatened flora.

The most commonly cited figure for the fraction of the global flora threatened with extinction—13%—is known to be a serious underestimate, because it does not include a reliable tally of species at risk in the tropical latitudes where most of the world’s plants grow (1, 2). Here we estimate the missing tropical data from global patterns of plant endemism. 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 (IUCN) classification scheme. 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