Mark Hebblewhite

Linking predation risk and forage to ungulate population dynamics

Many ungulate populations are partially migratory, where some individuals migrate and some do not. The success of either migratory strategy is the result of differential forage selection and predation risk-avoidance. In this dissertation, I examine how multi-scale resource selection by migrant and resident elk (Cervus elaphus) differentially influence demography. I studied the partially migratory Ya Ha Tinda elk population, which winters in the province of Alberta and migrates in summer to Banff National Park. I reviewed population trends from 1970-2005, and found the migrant proportion declined from 95% to 60%. To examine the role of differential forage selection, I built a spatially and temporally-explicit forage model using field and remote sensing data to predict available forage biomass and quality to elk. By selecting intermediate forage biomass in phenologically delayed areas, migrants had 5% higher forage digestibility than residents. Next, I developed a spatially and temporally explicit predation risk model for wolves (Canis lupus), elk’s main predator. Predation risk at the summer range scale was 70% lower for migrants compared to residents. Yet, despite riskier summer ranges, resident elk adopted fine-scale foraging and anti-predator strategies within these ranges to reduce the risk they experienced to only 15% higher than that of migrants. Furthermore, predation risk experienced by migrants during migratory periods was 52% higher than residents. Differences in resource selection translated to demographic differences between strategies in Leslie-matrix models. Bottom-up effects manifested in higher pregnancy rates and female calf weights for migrants. Yet top-down effects of predation by wolves and grizzly bears (Ursus arctos) were greater for migrants, who experienced lower adult and calf survival. Resident adult and calf survival was higher despite poorer forage because of fine-scale risk-avoidance combined with a behavioural response to live in larger group sizes in summer. The ratio of migrant to resident population growth rates from Leslie-matrix models matched the observed rate of migratory decline from 1970-2005. I conclude that given current conditions, notwithstanding potential density-dependent processes, the proportion of migrants in this population will continue to decline as a result of top-down limitation despite the significant benefits of higher forage quality from migration.