Time allocation of bison in meadow patches driven by potential energy gains and group size dynamics

Abstract

Understanding the behavioural mechanisms involved in broad‐scale spatial organisation of grazing herbivores requires uncovering the factors controlling foraging decisions, such as patch residency time. Foraging theory specifies that rate maximizers must simultaneously consider both the optimal residency time in a food patch and the optimal diet. Specifically, resource depletion or spatial variation in food type availability should not influence food choice, but only patch residency time. Few studies, however, have tested these principles together, and none on free‐ranging large herbivores. We evaluated the combined effects of forage characteristics, predation risk, and group size on residency time by free‐ranging bisonBison bisonin summer. We hypothesized that residency time in meadows would increase with the availability ofCarex atherodes, a highly profitable plant species maximizing energy intake rate, but decrease with sward complexity (i.e. plant species composition and structure) within foraging stations. We also anticipated that predation risk and group size would influence the relationship between vegetation characteristics and residency time. Residency times were measured in 44 sites using cameras located at meadow edges. We determined that residency time in meadows varied with meadow area, group size, biomass ofC. atherodesavailable on the area, and proportion ofC. atherodeswithin foraging stations. We found that the likelihood of departure decreased with an increase in the total biomass ofC. atherodesavailable over the meadow, an effect attenuated by an increase in group size. Residency time in meadows was also influenced by plant species composition, with higher accessibility ofC. atherodeswithin feeding stations increasing residency time. We found little evidence, however, that sward structure and predation risk influenced residency time. Overall, our study demonstrated that the search for rapid energy gains, together with the constraints imposed by group living, can explain time allocation in habitat patches by large gregarious herbivores.