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

Abstract

Spatial heterogeneity in the strength of trophic interactions is a fundamental property of food web spatial dynamics. The feeding effort of herbivores should reflect adaptive decisions that only become rewarding when foraging gains exceed 1) the metabolic costs, 2) the missed opportunity costs of not foraging elsewhere, and 3) the foraging costs of anti-predator behaviour. Two aspects of these costs remain largely unexplored: the link between the strength of plant-herbivore interactions and the spatial scale of food-quality assessment, and the predator-prey spatial game. We modeled the foraging effort of free-ranging plains bison (Bison bison bison) in winter, within a mosaic of discrete meadows. Spatial patterns of bison herbivory were largely driven by a search for high net energy gains and, to a lesser degree, by the spatial game with grey wolves (Canis lupus). Bison decreased local feeding effort with increasing metabolic and missed opportunity costs. Bison herbivory was most consistent with a broad-scale assessment of food patch quality, i.e., bison grazed more intensively in patches with a low missed opportunity cost relative to other patches available in the landscape. Bison and wolves had a higher probability of using the same meadows than expected randomly. This co-occurrence indicates wolves are ahead in the spatial game they play with bison. Wolves influenced bison foraging at fine scale, as bison tended to consume less biomass at each feeding station when in meadows where the risk of a wolf's arrival was relatively high. Also, bison left more high-quality vegetation in large than small meadows. This behavior does not maximize their energy intake rate, but is consistent with bison playing a shell game with wolves. Our assessment of bison foraging in a natural setting clarifies the complex nature of plant-herbivore interactions under predation risk, and reveals how spatial patterns in herbivory emerge from multi-scale landscape heterogeneity.

Functional responses in animal movement explain spatial heterogeneity in animal–habitat relationships

“…As such, the anti‐predator movement tactics of large herbivores are not restricted to the shell games typically observed in the predator–prey spatial games of large species (e.g. Laundré ; Harvey & Fortin ; Courbin et al . ).…”

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).…”

Complex tactics in a dynamic large herbivore–carnivore spatiotemporal game

“…Other potential prey for wolves in PANP included elk Cervus canadensis , white‐tailed deer Odocoileus virginianus , mule deer Odocoileus hemionus , moose Alces alces and beaver Castor canadensis . While direct predation on adult bison appears to be generally infrequent in PANP (Harvey and Fortin ), investigation of wolf kill sites identified by clusters of GPS locations (Webb et al ) revealed a minimum of 8 adult bison killed by wolves during the late winter of –2014. Moreover, several bison in PANP show signs of altercation with wolves (e.g.…”

“…Moreover, several bison in PANP show signs of altercation with wolves (e.g. missing tails, Harvey and Fortin ). Wolves are also reported to preferentially prey upon bison calves (Carbyn and Trottier , Metz et al ), )a pattern which should trigger attacks on bison groups from spring to autumn (Carbyn and Trottier ).…”

“…Most empirical work has focused on the point of view of prey (Moll et al 2017), hindering our comprehension of the game since the tactics of one player cannot be fully appreciated without consideration of the tactics of the opponent (Lima , Sih ). The fear of predators can be a dominant factor driving prey behavior and distribution (Brown et al 1999, Laundré et al 2001, Clinchy et al ). For instance, nymphs of the grasshopper Melanoplus femurrubrum responded in the same manner to nursery web spiders Pisurina mira that had their mouthparts glued together than to unaltered spiders capable of killing their prey (Schmitz et al 1997).…”