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 bison Bison bison in summer. We hypothesized that residency time in meadows would increase with the availability of Carex 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 of C. atherodes available on the area, and proportion of C. atherodes within foraging stations. We found that the likelihood of departure decreased with an increase in the total biomass of C. atherodes available 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 of C. atherodes within 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.

Spatial Heterogeneity in the Strength of Plant-Herbivore Interactions under Predation Risk: The Tale of Bison Foraging in Wolf Country

“…Bison left larger meadows when there were more areas of high-quality vegetation with no previous foraging activities than in small meadows. This observation is consistent with previous reports that bison consume only a small proportion of C. atherodes before leaving an area [74]. The proportion of meadows comprised of craters should not necessarily vary with meadow area, but with the energy gain function, which is likely to increase rather linearly over time for large mammalian herbivores [82], [83].…”

“…The fact that larger meadows were characterized by a lower proportion of craters than small meadows likely indicates that bison stopped foraging before their instantaneous intake rate of digestible energy dropped to the long-term expected rate. This early departure from meadows has already been suggested by Fortin et al [84] and Courant and Fortin [74], and would be consistent with the predictions of a shell game [43], [44].…”

“…For example, bison should have to spend more time searching for profitable patches in poor versus rich meadows, experience stronger competitive interactions for the few rich patches available in poor meadows, and spend less time in poor meadows. Consistently, bison remain longer each time they enter meadows offering more of the highly profitable slough sedge ( C. atherodes ) [49], [74] and spend more total time in these meadows over a season [52].…”

A “death trap” in the landscape of fear

“…Despite of the preferential use of forest gaps by deer in BPF (Kuijper et al 2009), they cannot support long-term occupation by ungulates due to their small size. This contrast to heterogeneous landscapes with both forest and large open spaces, where prey aggregates for considerable amount of time in open habitat (Courant and Fortin 2012;Harvey and Fortin 2013). In our study area, deer use the forest gaps only for short periods of time, with each patch visited less than 2 min, meaning that most of the time they are present in closed forest habitat (Kuijper et al 2009).…”

Vigilance behaviour of the year-round territorial vicuña (Vicugna vicugna) outside the breeding season: Influence of group size, social factors and distance to a water source

“…It is possible that the social pressure exerted during foraging by the simultaneous presence of several groups in the vega, could be such that animals even increase their harvest rate as competitor density increases (Shrader et al, 2007;Courant and Fortin, 2012). In contrast to the seasonal waterholes found in other arid environments where animals go only to drink (Crosmary et al, 2012), the vegas in the Puna environments receive a ground-water supply all year round.…”