Current estimates put 13 percent of global flora at risk of extinction. But this number does not include a reliable count of plants from tropical regions, where the majority of the world's plants grow. To eliminate some of that uncertainty, Nigel C. A. Pitman of Duke University and Peter M. Jørgensen of the Missouri Botanical Garden reviewed data from 189 countries and found that the number of threatened species in a particular region could be reliably inferred from the number of plants native to the area. Although the total number of plants worldwide remains unknown (estimates range from 310,000 to 422,000 species), their calculations show that between 94,000 and 144,000 species are at risk of dying out. The authors also undertook a diversity project in Ecuador, where they analyzed the status of nearly 4,000 plant species. Based on this work, the scientists posit that a complete catalogue of the world's endangered plants could be assembled for around $100 per species annually, or about $12 million for all of the world's biodiversity hotspots.