Co-occurrence of reintroduced and resident ungulates on a shared winter range in northwestern Canada

For interspecific competition to occur species must use the same limited resources (e.g. food, habitat) and overlap in time and space. Bison (Bison bison) were reintroduced to southwestern Yukon, Canada, where they are sympatric with resident caribou (Rangifer tarandus), moose (Alces americanus) and thinhorn sheep (Ovis dalli). Concerns from local communities regarding potential competition between reintroduced bison and resident ungulates prompted us to test their spatial distribution for co-occurrence. In late-winter, we conducted multiple aerial surveys (n = 1-5) of 12.2 km2 cells (n = 779) to develop a presence-absence matrix (4 species x 779 cells) of the spatial distribution of ungulates. Randomization procedures were used to conduct a null model analysis of co-occurrence. Community-wide null model analysis indicated that community members were neither segregated nor aggregated; rather, their spatial distribution was random because they did not differ from simulated null communities (n = 50,000). Similar analyses conducted on sub-matrices for each species pair also did not find evidence for segregation or aggregation among any species pairs except caribou and sheep, who were spatially segregated. We conclude that the overall potential for competition between reintroduced bison and resident ungulates during late-winter is low, based on spatial overlap. However, further investigations on the potential for competition among other niche dimension axes (e.g. food, habitat use) are recommended. Even though bison are reintroduced, these species had interacted for thousands of years and have probably co-evolved mechanisms to partition resources and co-exist on a shared landscape.