Evaluating the potential impacts of reintroduced plains bison (Bison bison bison) contained in a soft-release pasture in Banff National Park

Abstract

Plains bison (Bison bison bison) have been absent from Banff National Park (BNP) since before its establishment, primarily due to over-hunting in the 19th century. Reintroducing bison to BNP is an important step to restore the full diversity of species and natural processes in the Park’s ecosystem. The reintroduction of a dominant ungulate is expected to have a significant impact on the ecosystem through grazing and physical disturbance. My research assessed what impact reintroduced bison had in a 18 ha soft-release pasture system. I monitored the impact bison had on vegetation using a before-after control impact (BACI) experimental design. I also examined a) habitat selection b) diet and c) forage selection of bison. Data were collected in 2016 and 2017, before and after the bison were reintroduced. My results showed bison selected burned forest (Manly’s selection ratio,  = 2.1) and grassland ( = 1.6) landcover types. Bison strongly avoided the Panther river ( = 0.1), shrubland ( = 0.4) and gravel riparian ( = 0.4, CI = 0.3-0.6) and to a less extent the winter pasture ( = 0.7) and vegetated riparian ( = 0.8, CI = 0.7-0.9). Grass dominated bison diet in the summer (92%) which corresponded with strong selection for this vegetation type ( = 1.9). Forbs and shrubs represented minor components of their diet (<1% & 7% respectively) and consequently were avoided (forb,  = 0.03; shrub, ( = 0.2). Bison browsing consisted primarily of Salix spp. Multivariate analysis of variance (MANOVA) on forage classes and herbaceous species confirmed that bison had strong impacts on vegetation through herbivory and physical impacts (forage class, F 1, 54 = 0.1, p = 0.04; herbaceous species F 1, 54 = 2.7, p = 0.01). Grass cover decreased most significantly in grassland, which also contained cover of Festuca spp. a dominant grass found in bison diet (46.6%, Table 6). Burned forest contained substantial forb cover and biomass, and both decreased significantly in this landcover type however forbs were a minor component of bison diet. My results support Parks Canada’s ecological motivations for bison restoration, however because impacts to shrub were minimal, Parks Canada’s desire for bison to reduce shrub encroachment and restore grassland vegetation might not be realized, especially when the bison population is small. Ongoing monitoring of ecosystem effects is essential for determining if bison in BNP reach ecologically effective densities and fulfill their ecological function.