To the unnamed road grader operator, resident of Rangeley CO, who on May 13, 1992, idled his machine atop Calamity Ridge and jumped down to admonish us, as we collected insects from P. obcordata flowers, to use care in dealing with those plants with the small yellow flowers because “they grow nowhere else in the world but Rio Blanco Co.”: We dub thee Sir Dudley, Knight of the Twinpod, and thank you belatedly for your challenge, for your concern.

Physaria obcordata is a rare endemic of western Colorado. Plants are self-incompatible; they cannot reproduce sexually unless pollinators move pollen between flowers of different genets. We found no evidence that: seed production was limited by inadequate pollination or that pollinations between near neighbor plants (inbreeding depression) or between distant plants (outbreeding depression) resulted in reduced fruit or seed set. Examples are given of the potentially devastating effect of grazing on fruit and seed production. Preliminary evidence suggests that P. obcordata can hybridize with its common congener, P. acutifolia. Only crosses with P. obcordata as pollen recipient and P. acutifolia as pollen donor seemed fertile. Primary flower-visitors of P. obcordata are ground-nesting native bees in the families Andrenidae and Halictidae. Most species visiting the flowers of P. obcordata are generalists. Only two bee species were likely mustard specialists. The only non-bee visitor of any import was a dipteran, Gonia (Tachinidae). Predictions of the distances flown by common bee visitors were made using the equations of Greenleaf et al. (2007). No bee is likely to travel more than 1 km from its nesting site to visit P. obcordata flowers and most are likely to fly distances that are significantly less. The implications of flight range estimates for gene flow between Physaria populations and for pollinator protection are discussed. Several recommendations are made for conservation of P. obcordata and its attendant bees.