Demographic balancing of migrant and resident elk in a partially migratory population through forage-predation tradeoffs

Partial migration is widespread in ungulates, yet few studies have assessed demographic mechanisms for how these alternative strategies are maintained in populations. Over the past two decades the number of resident individuals of the Ya Ha Tinda elk herd near Banff National Park has been increasing proportionally despite an overall population decline. We compared demographic rates of migrant and resident elk to test for demographic mechanisms partial migration. We determined adult female survival for 132 elk, pregnancy rates for 150 female elk, and elk calf survival for 79 calves. Population vital rates were combined in Leslie-matrix models to estimate demographic fitness, which we defined as the migration strategy-specific population growth rate. We also tested for differences in factors influencing risk of mortality between migratory strategies for adult females using Cox-proportional hazards regression and time-varying covariates of exposure to forage biomass, wolf predation risk, and group size. Despite higher pregnancy rates and winter calf weights associated with higher forage quality, survival of migrant adult females and calves were lower than resident elk. Resident elk traded high quality food to reduce predation risk by selecting areas close to human activity, and by living in group sizes 20% larger than migrants. Thus, residents experienced higher adult female survival and calf survival, but lower pregnancy and calf weights. Cause-specific mortality of migrants was dominated by wolf and grizzly bear mortality, whereas resident mortality was dominated by human hunting. Demographic differences translated into slightly higher (23%), but non-significant, resident population growth rate compared to migrant elk, suggesting demographic balancing between resident strategies during our study. Despite statistical equivalence, our results are also consistent with slow long-term declines in migrants because of high predation because of higher wolf-caused mortality in migrants. These results emphasize that migrants and residents will make different tradeoffs between forage and risk may affect the demographic balance of partially migratory populations, which may explain recent declines in migratory behavior in many ungulate populations around the world.