Giving-up densities and foraging behaviour indicate possible effects of shrub encroachment on arctic ground squirrels

Abstract

Behavioural responses of prey to predation risk are often mediated by vegetation structure. Erect woody shrubs are increasing in many arctic and alpine environments, and this change in habitat structure has the potential to alter perception of risk and fear for foraging herbivores. To assess the role of shrub cover in determining behavioural responses to foraging under predation risk, we used measurements of giving-up density (GUD) and video recordings of vigilance behaviour of individual Arctic ground squirrels, Urocitellus parryii, across an alpine tundra to shrub ecotone. Dense shrub habitat was associated with higher GUDs, implying that foraging costs were higher in dense shrub than in less shrub-dominated habitats. Foraging strategies differed between habitats, with a negative relationship between visibility and GUD in high-visibility, open-tundra habitats and a positive relationship between visibility and GUD in low-visibility, shrub-dominated habitats, which may be indicative of alternative foraging strategies in different habitats. Squirrels initially made a high investment in vigilance, which was independent of patch residence time. This presumably increases the costs of quitting patches earlier. In shrub-dominated habitat, erect bipedal forms of vigilance were also observed more frequently. Our results indicate that shrub encroachment into northern and alpine tundra may impose costs on foraging arctic ground squirrels and alter foraging strategies.