Effects of introduced bison on wetlands of the kaibab plateau, Arizona

The plains bison (Bison bison bison) is an American icon and an animal of conservation priority, but it is also a large ungulate that can have significant impacts on plant communities. A herd of plains bison re ently colonized the portion of the Kaibab Plateau, Arizona, administered by Grand Canyon National Park (GRCA). This herd is descended from animals introduced to the region in an early cattle-bison breeding experiment, and genetic tests demonstrate a high level of cattle gene introgression. The Arizona Game and Fish Department now manages the herd as a valued wildlife game species on Forest Service land in the neighboring House Rock Val ey. The GRCA managers consider bison to be nonnative and are concerned that bison degrade park resources, including sensitive springs, seeps, and pond habitats. Uncertainty regarding these effects has been a point of disagreement and conflict in interagency discussions. We quantified the effects of bison on spring and pond wetland vegetation within GRCA and found that as evidence of bison use increased, vegetative cover decreased by 70-90%, vegetative height across functional groups decreased to 25% of that in low- or nonuse sites, and bare soil increased to 40-50%. Plant responses monitored within and outside two bison exclosures mirrored these effects within two years of establishment The GRCA will need to decide whether these effects are beyond the acceptable range of impacts for the national park and therefore require management actions to mitigate bison impacts. We advocate the continuation of an ongoing collaborative process, based on strong and transparent science, to find mutually amenable management approaches and solutions for the managing agencies.