Group-size-dependent association between food profitability, predation risk and distribution of free-ranging bison

Predation risk and competition can impose foraging costs, whereas social information acquired from conspecifics may lead to foraging gains. By altering cost–benefit trade-offs of foraging, variations in group size and predation risk can influence plant selection by herbivores. This influence may vary seasonally, depending on how foraging constraints vary throughout the year. Empirical evidence of these combined effects remains limited, especially in natural settings. We evaluated the spatial association between wheat sedge, Carex atherodes, and bison, Bison bison, foraging under predation risk during summer and winter. To maximize their energy intake rate, bison should feed on Carex atherodes. We found that the strength of selection for foraging sites with C. atherodes decreased with increasing risk of wolf, Canis lupus, encounter in winter, but not in summer. Bison faced greater risk in winter than in summer. Selection for C. atherodes was further influenced by group size. Larger bison groups displayed stronger selection for C. atherodes in winter but weaker selection in summer. Seasonal variations in group-size effects can be explained by changes in the relative costs and benefits of social foraging. Bison groups are much larger in summer than winter, implying potentially stronger competition among bison foraging on C. atherodes in summer. Carex atherodes is more inconspicuous in winter than in summer, thereby increasing the value of social information during winter months. We suggest that predation risk and spatial heterogeneity of highly profitable food influence the foraging decisions of bison differently in summer and winter because of seasonal differences in cost–benefit trade-offs.