Estimating the Minimum Population Size of Dawson’s Hookless Cactus (Sclerocactus dawsoniae)

BLM Technical Note 460

In 2021, the Bureau of Land Management assessed the minimum population size of the federally threatened plant Colorado hookless cactus (Sclerocactus glaucus), resulting in an estimate of approximately 104,000 individual plants. Since that time, genetic investigation revealed that the northeastern distribution of Colorado hookless cactus, known colloquially as the “northern” or “De Beque” population, represents a distinct species—described as Dawson’s hookless cactus (Sclerocactus dawsoniae). This technical note describes the results of a sampling-based procedure to estimate the minimum extant population size of Dawson’s hookless cactus. Between April 2021 and May 2023, 50 mapped habitat areas were assessed, of which 28 areas were suitable for sampling. The other 22 areas were rejected due to an insufficient number of plants found within the mapped area. Based on the resulting sample, it was determined that the estimated minimum population size of Dawson’s hookless cactus is 17,362 individual plants.