Publications

Extinction rates are expected to increase during the Anthropocene. Current extinction rates of plantsand many animals remain unknown. We quantified extinctions among the vascular flora of the continental UnitedStates and Canada since European settlement. We compiled data on apparently extinct species by querying plantconservation databases, searching the literature, and vetting the resulting list with botanical experts. Becausetaxonomic opinion varies widely, we developed an index of taxonomic uncertainty (ITU). The ITU ranges fromA to F, with A indicating unanimous taxonomic recognition and F indicating taxonomic recognition by only asingle author. The ITU allowed us to rigorously evaluate extinction rates. Our data suggest that 51 species and 14infraspecific taxa, representing 33 families and 49 genera of vascular plants, have become extinct in our study areasince European settlement. Seven of these taxa exist in cultivation but are extinct in the wild. Most extinctionsoccurred in the west, but this outcome may reflect the timing of botanical exploration relative to settlement.Sixty-four percent of extinct plants were single-site endemics, and many occurred outside recognized biodiversityhotspots. Given the paucity of plant surveys in many areas, particularly prior to European settlement, the actualextinction rate of vascular plants is undoubtedly much higher than indicated here.