Density-Dependent Foraging Behaviors on Sympatric Winter Ranges in a Partially Migratory Elk Population

Evelyn Merrill,Holger Bohm,Hans Martin,Madeline Trottier,Scott Eggeman,Joshua Killeen,Mark Hebblewhite,Jed Pettit,Jodi Berg

doi:10.3389/fevo.2020.00269

Abstract

Many large herbivore populations are partially migratory, in which the population is comprised of both non-migratory (resident) and migratory individuals. Density-dependence contributes to regulating the dynamics of partially migratory populations by altering habitat selection, vital rates, or rates of behavioral switching between migratory tactics. Studies of mechanisms leading to these shifts have focused mainly on their behavior on summer range, overlooking the potential for density-dependent effects during winter that may influence decisions to migrate. We hypothesized that competition for food and safety from wolf predation risk on winter ranges would differentially affect habitat selection, movements, and grouping behavior of migrant and resident female North American elk (Cervus canadensis) on their sympatric winter range. We used GPS locations from 155 elk-winters from 92 adult female elk at Ya Ha Tinda, Alberta, Canada, over a 14-year period when the elk population declined by ~70% to test our hypotheses. Elk showed consistently strong selection for areas of high forage biomass that corresponded to longer residence times and shorter return times to areas of high forage biomass. The strength of the selection diminished at high elk population size as did the extent to which elk traded off forage for safety from wolf predation risk. Elk increased movement rates and extended return times only to the riskiest areas. Median group size and mean sociality among elk increased at low population size, with resident elk groups being larger and more cohesive than migrant groups. Similar density-dependent responses by migrant and resident female elk on sympatric winter range indicate resident elk do not alter foraging behaviors to compensate for exposure to low nutritional resources in summer, implicating seasonal differences in nutrition are not mediated by winter densities in this system. We discuss the implications of competition on winter ranges for the maintenance of partial migration in ungulates in montane systems.

Highlights

Density dependence plays a central role in understanding a variety of ecological processes including population dynamics and community organization
We evaluated the assumption that migrant and resident elk make similar trade-offs between foraging opportunities and safety from wolves on the sympatric winter range of the Ya Ha Tinda adjacent to Banff National Park (BNP) (Figure 1) in response to ∼70% population decline over a 14-year period (Hebblewhite et al, 2018)
To test our hypotheses that elk selection changed with elk population size, we modeled the integrated Step Selection Analysis (iSSA)-derived coefficients of individual elk for forage biomass, predation risk, and their interaction separately using migratory tactic and annual elk abundance of elk on the Ya Ha Tinda winter range

Summary

Introduction

Density dependence plays a central role in understanding a variety of ecological processes including population dynamics and community organization. Density influences habitat selection and movements by animals to determine species distributions at both small and large scales (Owen-Smith et al, 2010; Almeida et al, 2015). Partial migration, where individuals follow resident or migratory tactics of movement (Dingle and Drake, 2007), is hypothesized to result from density-dependent trade-offs between costs and benefits influenced by phenotype, individual state, or the behavior of conspecifics in the population (Berg et al, 2019). Migratory and resident tactics may be maintained by differential density-dependent regulation of vital rates that must demographically balance each other over the long term (Kaitala et al, 1993; Hebblewhite and Merrill, 2011). How density-dependent changes in behaviors of large herbivores might contribute to switching and more broadly the maintenance of partial migration has not been well-studied

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