Grassland vegetation diversity responses to the reintroduction of Wood Bison (Bison bison athabascae) in Aishihik, Yukon

• Author / Creator

  Lori Denise Schroeder

• In the southwest Yukon, the rain shadow effect of the Coast Mountains produces xeric south-facing slopes that support some of the most northerly grasslands in North America. These dry and nutrient poor forb-dominated grassland pockets within the vast matrix of boreal forest support distinctive plant communities that contain several rare and endemic plant species, as well as disjunct populations of prairie species such as Prairie Smoke (Geum triflorum) that are considered rare in the Yukon although common to the south. The unique flora of these grasslands, coupled with the fact that they occupy less than 1% of the landscape, makes them of high conservation concern. Wood bison (Bison bison athabascae) were reintroduced into southwest Yukon in the late 1980’s as part of national recovery efforts. The Aishihik wood bison population has subsequently increased from 34 in 1988 to 1,470 individuals in 2016, prompting concerns about how they may reshape the landscape, influence other wildlife, affect traditional uses of the land, and alter these unique and rare grasslands, considered by some to be relicts of ice age steppe vegetation. Thus, there is a need to understand the potential impact of reintroduced wood bison on these native grassland communities, which do have a long-term history of grazing by large mammals.

  I documented the floristic diversity of native grasslands in the Aishihik area of northwest Canada, and evaluated how current bison grazing may alter this diversity. Specific objectives were to: 1) determine whether the diversity of these grasslands has changed over the last 30+ years following wood bison reintroduction, and 2) relate grassland diversity responses to bison grazing intensity. I hypothesized that there would be little change in diversity at low levels of bison grazing because the convergent pressures of aridity and a long history of grazing have preconditioned these grasslands to tolerate a certain amount of grazing. With increased grazing intensity, I hypothesized that diversity would decrease. Contrary to expectations, I found that richness and all other measures of diversity increased with greater bison presence, as did both forb and graminoid abundance, though not all groups were affected equally. Small upland Carex species increased most dramatically (i.e. species that are known to be indicators of overgrazing in the prairies), but not at the cost of forbs, which were the drivers of diversity in this system. Similarly, rare species (predominantly forbs) showed no signs of decline. Another grazer on these grasslands, the Arctic ground squirrel (Urocitellus parryii), was also positively associated with graminoid abundance.

  The grasslands of the southwest Yukon co-evolved with a large number of mega-herbivores north of the ice sheets, in a harsh, often cold and dry, environment. The current findings suggest they may be so disturbance and aridity adapted that their diversity may be more limited by too little grazing or excess moisture, than by the reintroduction of wood bison. Supporting this hypothesis, plant community diversity decreased with increasing precipitation in these grasslands. Climate change, and the predicted increase in temperature and precipitation in this region of the Yukon, may pose a greater threat to plant diversity on these grasslands than herbivory. Results of this study have implications for reconciling wood bison and grassland conservation initiatives in the region.

• Subjects / Keywords

  • bison

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-sn8x-6h21

• License

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