Mentzelia oligosperma Nutt. (Stickleaf, Loasaceae): Distribution and Abundance in Illinois

We conducted field searches for populations of Mentzelia oligosperma Nutt. (stickleaf) throughout the known range of this species in Illinois during the growing seasons of 2010-2019. A species common to the prairies and plains of western North America, it is habitat-specific and rarely observed in Illinois. Populations are confined mostly to narrow limestone cliff ledges along the bluffs of the Mississippi River in Adams, Calhoun, Monroe, Pike, and Randolph counties on the western border of the state. We identified 18 potential stickleaf sites based upon herbaria searches and interviews with biologists. Subsequent field investigations determined the presence of 15 extant sites, including two new locations at Hull in Pike County and the Fults Hill Prairie Annex in Monroe County. Five historic populations are extirpated. The state total of 498 plants represents populations ranging from six to 85 individuals at 15 sites, with most locations having fewer than 20 individuals. Stickleaf is currently listed as endangered by the Illinois Endangered Species Protection Board due to its restricted habitat and low population densities.