Foraging decisions of bison for rapid energy gains can explain the relative risk to neighboring plants in complex swards

Abstract

Herbivores commonly base their foraging decisions not only on the intrinsic characteristics of plants, but also on the attributes of neighboring species. Although herbivores commonly orient their food choices toward the maximization of energy intake, the impact of such choices on neighboring plants remains largely unexplored. We evaluated whether foraging decisions by herbivores aiming at a rapid intake of digestible energy could explain multiple neighboring effects in complex swards. Specifically, we assessed how spatial patterns of occurrence of Carex atherodes, a highly profitable sedge species, could control the risk of bison (Bison bison) herbivory for seven other plant species. The foraging behavior of 70 free-ranging bison was evaluated in their natural environment during summer, and then related to plant characteristics. We used this information to estimate the instantaneous intake rate of digestible energy at individual feeding stations. We found that neighbor contrast defense and associational susceptibility can both be explained by simple foraging rules of energy maximization. Energy gains were higher when C. atherodes was consumed while avoiding the species for which we detected neighbor contrast defense. The lower intake rate associated with their consumption was due to an increase in handling time caused by their small size relative to C. atherodes. Bison also had higher energy gains by consuming instead of avoiding the plant species that experienced associational susceptibility. Because most of these plants were at least as tall as C. atherodes, their presence increased the heterogeneity of the grazed stratum. Avoiding their consumption increased handling time thereby reducing the instantaneous rate of energy intake. Overall, we found that bison adjust their fine-scale foraging decisions to vertical and horizontal sward structures in a way that maximizes their energy intake rate. Energy maximization principles thus provide a valuable framework to evaluate a broad-range of neighboring effects for prey faced with generalist consumers.

Risk perception by endangered European bison Bison bonasus is context (condition) dependent

“…Nutrition quality was the primary determinant of hay consumption when resources were scarce at the end of winter (supporting hypothesis 4), however there was no evidence of compensatory drinking to cope with poorer quality forage (refuting hypothesis 5), although water from snow may be taken despite the energetic costs associated with this during cold periods. Distance from home range centre affected haystack selection and consumption, but opposite that predicted based on optimality theory of minimising energy expenditure (refuting hypothesis 6), illustrating that bison were so short of forage within the forest that they would travel further and then consume more once they selected an optimal patch (Courant and Fortin 2010;Babin et al 2011;Kowalczyk et al 2011). From spring to summer, European bison remain within forested habitats in Białowie_ za.…”

“…In contrast, less fibrous forage will be processed faster and will yield a higher digestible content and energy ingestion (i.e. lower GUDs) (Courant and Fortin 2010;Babin et al 2011).…”

Plant Trait Assembly Affects Superiority of Grazer's Foraging Strategies in Species-Rich Grasslands

“…forage that the animals select when given a minimum of physical constraints. In heterogeneous grasslands, a herbivore's selection of the best quality components is impeded by their low abundance [2] and/or complex spatial distribution [3], [4]. Especially in species-rich grasslands herbivores must solve two opposing problems: obtaining maximum quality and sufficient quantity (see review by Hejcmanová & Mládek [5]).…”

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

“…Sward complexity can influence the profitability of a given plant species by influencing bite mass and, thereby, handling time at feeding stations (Burlison et al 1991, Laca et al 1994, Hambäck and Beckerman 2003, Searle et al 2005b, Shipley 2007). The slower intake rate of digestible energythat herbivores generally experience when feeding in complex than in simple swards (Wilmshurst et al 2000, Courant and Fortin 2010) should translate into shorter residency times in heterogeneous than in homogeneous food patches.…”