Species extinction is a permanent event that we as a conservation community strive to prevent. As we progress through the Anthropocene the extinction rates of plants and animals are expected to increase, and some biodiversity hotspots are projected to become centers of extinction. Though speculation is common about increased extinction rates in the future, we have yet to quantify the current extinction rates of plants and many animals. This study represents the first effort to quantify the extinction rate of the vascular flora of North America, north of Mexico. We compiled our data on potentially extinct species by querying plant conservation databases, searching the literature, and vetting the resulting list against a large group of botanical experts from across North America. Because taxonomic opinion can vary widely amongst experts, we developed an Index of Taxonomic Uncertainty (ITU). The ITU scale ranges from A to F, with an A rank indicating unanimous taxonomic recognition and an F rank indicating taxonomic recognition by a single author. The ITU allowed us to evaluate extinction rates under standardized taxonomic considerations.
Our working list suggest approximately 120 plants are globally historic or extinct from our study area, since European settlement. Eighty-four plants are from western North America and 35 are from eastern North America. The disparity between western and eastern North America extinction rates may be a result of survey effort before widespread settlement. The majority of extinct plants were single site endemics and occurred in areas not recognized as a biodiversity hotspot. Given the paucity of plant surveys in many areas of North America, particularly prior to European settlement, the actual extinction rate of vascular plants is undoubtedly much higher than this study indicates. The number of plants that went extinct before being documented by science is impossible to quantify, but it is plausible that hundreds of single site endemics went extinct before they were described. The fact that many extinct plants occurred in habitats not recognized as biodiversity hotspots has significant implications for current conservation efforts. If limited conservation resources focus only on biodiversity hotspots, it is likely that the extinction of single site endemics will continue into the future. We recommend further research, particularly taxonomic and field, on single site endemics to ensure their protection into the future.