Resource Selection Function-Adjusted Carrying Capacity Informs Bison Conservation Management in the Imperilled Mixed Grassland Ecosystem

Abstract

In Canada, plains bison (Bison bison bison) was assessed by The Committee on the Status of Endangered Wildlife in Canada as threatened. While bison are no longer at risk of demographic extinction, conservation programs remain challenged by the rarity of large populations and most bison are found in small, isolated, and confined herds. In this context, proper assessment of ecological carrying capacity is critical to inform habitat management and conservation efforts for species recovery. Although estimated food-limited carrying capacity is influenced by forage availability, forage requirements, and offtake proportion, it should also consider habitat selection by animals, especially inside confined settings to help avoid overgrazing. To support bison management in Grasslands National Park Canada, we integrated remote sensing, geographic information systems, and resource selection functions (RSFs) to examine variables that were potentially associated with bison habitat selection and return a comprehensive estimate of bison carrying capacity. Relevant variables were then integrated with estimates of forage availability using remote sensing and extrapolated to the bison containment scale. Factors of relevance to the RSFs include vegetation landscape units, slope, distance to water, fence, and road. In particular, bison selected for upland and sloped grasslands, which were characterized by the highest forage availability (1 064.5 kg ha−1 and 1 238.5 kg ha−1), while avoiding water in both growing and dormant seasons. The top-performing RSFs models in growing and dormant seasons were assessed using k-fold cross validation and achieved good predictive capacity (Spearman rank correlation [rs] ≥ 0.83, P < 0.01). Application of traditional clipping biomass samples and remote sensing derived variables is helpful in estimating annual forage quantity for bison (R2 = 0.75, P < 0.05). When accounting for bison resource selection, our model resulted in a carrying capacity estimate of about 0.0424 bison ha−1 or 764−770 bison (each requiring 12.2 kg forage/d), compared with 0.0587 bison ha−1 or 1 062 animals estimated when considering only the availability of forage. Such potential to increase carrying capacity to more than 1 000 individuals stresses the importance of testing management tools to shape bison grazing and increase forage utilization across a greater proportion of the 18 000-ha containment area. By improving our understanding of the interaction of this species with the mixed-grass prairie ecosystem, the integration of RSFs with estimates of carrying capacity can help inform conservation management of bison and multispecies at-risk habitat.