Bison Selection of Prairie Dog Colonies on Shortgrass Steppe

Abstract

American bison (Bison bison L.) preferentially use black-tailed prairie dog (Cynomys ludovicianus L.) colonies over uncolonized range in the mixed-grass prairie of North America. To assess bison use of prairie dog colonies in a different ecosystem, the shortgrass steppe, this study was conducted at the Vermejo Park Ranch, New Mexico. Driving surveys were conducted in summer 2007 to determine the number of bison on and off prairie dog colonies in 2 pastures. Prairie dogs occupied 25.5% and 48.5% of the 2 pastures surveyed. Bison of both sexes used prairie dog colonies more than expected compared to a scenario of bison using colonies based on colony availability (P < 0.001). With the exception of bulls in one pasture, bison observed off-colony were more likely to be grazing than bison observed oncolony (P < 0.04). When selection of prairie dog colonies was assessed for only those bison observed grazing, bulls in both pastures and cows in one pasture used prairie dog colonies more than expected based on availability (P ≤ 0.001). Forage quality was superior on prairie dog colonies, with crude protein higher (P < 0.001) and acid detergent fiber lower (P < 0.001) in blue grama (Bouteloua gracilis [Willd. ex Kunth] Lag. ex Griffiths) collected on-colony than offcolony. Further studies under a variety of conditions are still needed, but selection for prairie dog colonies by bison at Vermejo, combined with findings from previous studies, suggests that during the growing season, bison and cattle (Bos taurus L.) might select prairie dog colonies over uncolonized range in both the northern and southern extents of North American grasslands.