Drivers of spatial dynamics of a large ungulate (American Bison, Bos bison ): forage productivity or site fidelity?

Optimal foraging theory predicts that animals should reduce energy expenditure and maximize fitness by establishing home ranges in areas with abundant, high-quality forage. However, ungulates may not select optimal foraging habitat when behavior is driven by site fidelity. On Vermejo Park Ranch, New Mexico, managers translocated American Bison (hereafter, bison) to high elevations to reduce historically heavy grazing in lower elevations and to distribute the ecological effects of grazing across the broader landscape. We hypothesized that: (i) home range size is driven by forage abundance and quality; but that (ii) site fidelity can result in suboptimal selection of home ranges in areas with lower forage abundance and quality. We estimated space use and resource selection of free-roaming GPS-collared bison occupying lower elevation sites of historical use and bison translocated to higher elevations to evaluate drivers of home range size and selection. Home range size was strongly negatively influenced by forage abundance. However, without translocation, bison exhibited fidelity to areas of historical use and selected seemingly suboptimal home ranges with lower forage biomass. Conversely, bison that established home ranges following translocation to higher elevations selected home ranges with greater forage biomass. Greater forage biomass enabled bison to use smaller home ranges, increasing spatial efficiency and cohesion. Bison on Vermejo that were not translocated by managers established home ranges at lower elevations despite the availability of higher quantity and quality (nitrogen yield) of forage at higher elevations. Thus, home range selection appeared to be guided by site fidelity rather than foraging optimally across the larger landscape. For managed ungulates, translocation may facilitate access to superior resources and eventually begin to restore knowledge of migratory routes and seasonally abundant forage.