Pollinators complicate conservation of a Piceance Basin endemic, Physaria obcordata (Cruciferae)

Authors: TEPEDINO, VINCENT - Utah State University; BOWLIN, WILLIAM - Utah State University; Griswold, Terry

Submitted to: Natural Areas Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2011
Publication Date: 4/1/2012
Citation: Tepedino, V.J., Bowlin, W.R., Griswold, T.L. 2012. Pollinators complicate conservation of a Piceance Basin endemic, Physaria obcordata (Cruciferae). Natural Areas Journal. 32(2): 140-148.

Interpretive Summary: Physaria obcordata, is a rare endemic plant of the Piceance Basin, Colorado, an area undergoing rapid oil and gas development and with prospects for substantial oil shale recovery in the future. The species has been listed as Threatened under the U. S. Endangered Species Act. This study was conducted to uncover information about the reproduction of P. obcordata and to make recommendations for its conservation. We found that plants are incapable of reproducing sexually unless insects move pollen between flowers of different plants, i.e., the plants are self-incompatible. At the time of the study (1992), there were sufficient numbers of pollinators visiting the flowers to produce a full compliment of fruits and seeds. We found no evidence of reduced fruit or seed set resulting from pollinations between near neighbor plants (inbreeding depression) or between distant plants (outbreeding depression). We did find evidence of the potentially devastating effect of cattle grazing on fruit and seed production. There was a preliminary indication that P. obcordata is capable of hybridizing with its common congener, P. acutifolia. Primary pollinators of P. obcordata were ground-nesting native bees in the families Andrenidae and Halictidae. Most species that frequent the flowers of P. obcordata also visit a variety of other flower types. Only two bee species may restrict their visits to the flowers of crucifers. The only non-bee pollinator of any importance was a fly, Gonia (Tachinidae). No bee species is likely to travel more than one km from the nesting site to visit P. obcordata flowers and most are likely to fly distances that are significantly less than one km. The implications of these flight range estimates for gene flow between Physaria populations and for pollinator protection is discussed. Several recommendations are made for conservation of P. obcordata and its attendant bees.

Technical Abstract: Physaria obcordata, a rare endemic plant of the Piceance Basin, Colorado, was found to be incapable of reproducing sexually unless flowers were attended by pollinators. The plants are self-incompatible; pollen must be moved between flowers of different plants by an extraneous agent to effect fertilization. We found no evidence that sexual reproduction of plants was limited by inadequate pollination. Nor was there evidence of reduced fruit or seed set resulting from pollinations between near neighbor plants (inbreeding depression) or between distant plants (outbreeding depression). Examples of the potentially devastating effect of grazing on fruit and seed production are given. There was preliminary evidence that P. obcordata is capable of hybridizing with its common congener, P. acutifolia. Only crosses in which P. obcordata acted as pollen recipient and P. acutifolia as the pollen donor seemed fertile. Flower-visitors and likely pollinators of P. obcordata were identified. Primary pollinators were ground-nesting native bees in the families Andrenidae and Halictidae. Most species that frequent the flowers of P. obcordata are generalists which also visit a variety of other flower types. Only two bee species were possible crucifer specialists. The only non-bee pollinator of any importance was a dipteran, Gonia (Tachinidae). Predictions of the distances flown by common bee pollinators are made using recently published equations (Greenleaf et al. 2007). No bee is likely to travel more than one km from the nesting site to visit P. obcordata flowers and most are likely to fly distances that are significantly less than one km. The implications of these flight range estimates for gene flow between Physaria populations and for pollinator protection is discussed. Several recommendations are made for conservation of P. obcordata and its attendant bees.