Feeding Relationships Among Bison, Pronghorn, and Prairie Dogs: An Experimental Analysis

Bison (Bison bison), pronghorn (Antilocapra americana), and prairie dogs (Cynomys ludovicianus) form a grazing association on the northern United States mixed—grass prairie. Marked similarities in diet and spatial use between bison and prairie dogs on dog town edges and between pronghorn and prairie dogs on town centers create a high potential for interspecific interactions. Observational data were deemed insufficient to detect and properly characterize such interactions, and an experimental analysis was undertaken for this purpose. Two sets of exclosures, built on centers and edges of active and poisoned prairie dog towns, and on uncolonized prairie sites, were used to evaluate foraging relationships among bison, pronghorn, and prairie dogs feeding within and adjacent to exclosures and in control areas. Bison foraging groups were largest, nearest neighbor distances least, and foraging efficiency (bite: step ratios) highest among bison foraging in town edge control areas of the active towns, suggesting that these were the best areas for bison foraging. Prairie dogs showed smaller nearest neighbor distances and higher bite: step ratios when foraging in these areas than when foraging within adjacent exclosures, suggesting that bison foraging on town edges improved prairie dog foraging there. These results indicate a mutually positive relationship between bison and prairie dogs on town edges. Nearest neighbor distances and bite: step ratios of prairie dogs on town centers (within and adjacent to exclosures) suggested that pronghorn foraging did not appreciably affect prairie dog foraging. However, comparisons of pronghorn foraging between active and poisoned town centers showed nearest neighbor distances always smaller and bite: step ratios often higher on the poisoned town centers, suggesting that the presence of prairie dogs on town centers may have slightly degraded pronghorn foraging there. This indicates a complex relationship on town centers between pronghorn and prairie dogs, which was neutral for prairie dogs but slightly negative for pronghorn under the conditions of the study. Results of vegetation analyses within exclosures and in control areas showed significant enhancement of shoot nitrogen in plants growing on areas subject to heavy grazing pressure, such as active dog towns and control areas on dog towns, and may partially explain improvements in prairie dog and bison foraging there.