Effects of elk group size on predation by wolves

Abstract

Wolf-prey research has focused on single-prey systems in North America dominated by moose (Alces alces) or white-tailed deer (Odocoileus virginianus). Elk (Cervus elaphus) are social ungulates and the main prey item of wolves (Canis lupus) in Banff National Park (BNP), Alberta. Grouping behaviour may affect the functional response of predators by changing how predators encounter and kill prey. We studied wolf predation on elk in BNP during the winters of 1997-1998 and 1998-1999 and tested how elk group size affected the availability of and encounter rates with elk groups and attack success of wolves. Wolves encountered larger elk groups than expected based on availability, and killed more elk from large groups than expected based on numbers of encounters. Elk group size increased with elk density in BNP. Increased rates of encounter with and success of attacking large elk groups, and the positive group size -density relationship may be a mechanism for density-dependent predation. We developed a predation-risk model to test the prediction that grouping will benefit individual elk, given this predation regime. Elk appeared to adopt two different strategies to minimize predation risk: living in small herds that were rarely encountered by wolves or living in large herds that reduced their predation risk through dilution.Résumé : La recherche sur les associations entre les loups et leurs proies en Amérique du Nord s'intéresse principalement aux systèmes à une seule proie, surtout l'orignal (Alces alces) ou le cerf de Virginie (Odocoileus virginianus). Le wapiti (Cervus elaphus) est un ongulé social qui constitue la proie principale du loup gris (Canis lupus) dans le parc national de Banff (BNP) en Alberta. Le comportement de regroupement peut affecter la réponse fonctionnelle des pré-dateurs en changeant la façon dont les prédateurs s'attaquent aux proies et les tuent. Nous avons étudié la prédation exercée par les loups sur les wapitis dans le parc de Banff au cours des hivers de 1997-1998 et de 1998-1999 et examiné comment la taille du groupe affecte la disponibilité des proies, la fréquence des rencontres et le succès des attaques des loups. Les loups rencontrent des groupes de wapitis plus grands que ne le permettrait de croire leur disponibilité et tuent plus de wapitis dans les grands groupes que ne le laisserait croire la fréquence des rencontres. La taille des groupes de wapitis augmente avec leur densité dans le parc. La fréquence accrue des rencontres et le succès plus grand des attaques en présence de grands groupes de wapitis, de même que la corrélation positive entre la densité et la taille des groupes, sont peut-être des mécanismes qui favorisent une prédation dépendante de la densité. Nous avons conçu un modèle de risque de prédation pour vérifier la prédiction qui veut que, chez les wapitis, le regroupement avantage les individus dans un tel régime de prédation. Les wapitis semblent adopter deux stratégies pour minimiser les risques de prédation : la vie dans de petits groupes qui ont rarement à fair...

Spatial Patterning of Prey at Reproduction to Reduce Predation Risk: What Drives Dispersion from Groups?

“…2). These results are similar to empirical findings of encounter rates between wolves and elk groups where intermediate group sizes of elk had higher encounter rates with wolves than individuals or large groups (Hebblewhite and Pletscher 2002). Together, these results suggest that at small to intermediate group sizes, group detectability effects have a stronger influence on encounter rates than the minimizing effect associated with increasing group size (Ioannou et al 2011).…”

“…access to food; parasite avoidance) may influence social behaviour in animals Ruxton 2002, Beauchamp 2014), here we focus on temporal trade-offs to sociality in terms of predation risk. In certain situations, the dispersion of individuals may be advantageous over group-living: for example, if groups are more detectable than individuals (Hebblewhite and Pletscher 2002;Ioannou and Krause 2008), if aggregation leads to area-restricted search behaviour from predators (Tinbergen et al 1967;Scharf et al 2011) or if the number of individuals predated per encounter is greater than one (Treisman 1975).…”

“…Note that this hypothesis runs counter to recent research suggesting that increasing prey aggregation leads to lowered visual detection by predators due to increasing distances between predator and prey as prey aggregate into larger, but fewer, groups (Ioannu et al 2011). In forested environments, however, vision is limited and predator-prey encounters may be driven more by olfaction and/or audition, which may result in a positive relationship between prey group size and detection by predators (Hebblewhite and Pletscher 2002). The group detectability hypothesis predicts that dispersed individuals would have a higher LOS than grouped caribou and the magnitude of this difference would be driven by differential detectability based on group size.…”

Prey selection and dietary response by wolves in a high-density multi-species ungulate community

“…The effect of group size and distribution pattern of prey species on encounter rate could have strengthened the selection of <1-year-old boar. Several studies have reported encounter rates to be higher for larger groups of prey (Hebblewhite and Pletscher 2002;Creel and JAJr 2005), probably because of their increased detectability. In the Casentinesi Forests, wild boars live in larger groups than the other ungulate species, particularly the roe deer.…”

Spatial Epidemiology of Chronic Wasting Disease in Wisconsin White-Tailed Deer

“…Because female white-tailed deer associate within matrilineal social groups while males tend to be alone or in small groups outside the breeding period (Nixon et al, 1991), these social groups may not change in size with deer density. However, changes in population density or size (Borowski, 2000;Hebblewhite and Pletscher, 2002) may affect the frequency and intensity of interactions among deer within or among social groups (Hirth, 1977;Nixon et al, 1991;Grenier et al, 1999;Kie and Bowyer, 1999).…”