Comments: Berberis canadensis (and the majority of barberries) is an alternate host for the black stem rust of wheat, oats, rye barley, and various wild and cultivated grasses (Weakley 1993, Steffey 1985, Rudolf 1974, Steyermark 1963). The U.S. Department of Agriculture and state agriculture offices initiated a comprehensive barberry eradication program in the past to elimiate black stem rust. As a result, numerous populations of this species were destroyed (Weakley 1993, Wiegman 1993, Homoya 1992, Steyermark 1963).
Loss of primary habitat has also played a significant role in the demise of this species. The elimination of the natural fire regime has resulted in the succession of savanna and open woodland habitats into closed-canopy woodlands. Only in sites with extremely shallow soils or areas that are frequently mowed or cleared does B. canadensis persist in any significant populations. Since settlement, much of the available habitat has been destroyed, converted to cultivated fields, land development, and urbanization (Weakley 1993). These threats remain for extant populations.
Grazing is a potential threat to extant populations (Smith 1992, Ludwig 1993). Grazing may serve to maintain the open character of woodlands, but its effects on B. canadensis plants are unknown. In some areas, the plants appear to be grazed by cattle (Smith 1992). Soil compaction and disturbance may negatively impact individual plants and populations.
Competition from exotic plant species (such as Lonicera tatarica and Rhamnus cathartica) is a threat to populations (Ludwig 1993). These species can form dense stands and eliminate ground layer herbaceous and other shrub species, including B. canadensis. Excessive shading and canopy closure in woodlands may be a factor in reducing seed production in the species, as has been noted in Missouri (Smith 1992).