Predation risk drives long-term shifts in migratory behavior and demography in a large herbivore population

Migration is an adaptive life-history strategy that helps individuals across taxa maximize fitness by obtaining forage and avoiding predation risk. The mechanisms driving migratory changes are poorly understood, and links between migratory behavior, space use, and demographic consequences are rare. Here, we use a nearly 20-year record of individual-based monitoring of elk (Cervus canadensis) to test hypotheses for changing patterns of migration in a large herbivore, elk (Cervus canadensis), in and adjacent to a large protected area in Banff National Park (BNP), Canada. We test whether bottom-up (forage quality) or top-down (predation risk) factors explained trends in i) the proportion of individuals using 5 different migratory tactics, ii) differences in survival rates of migratory tactics during migration and whilst on summer ranges, iii) cause-specific mortality by wolves and grizzly bears, and iv) population abundance. We found dramatic shifts in migration consistent with behavioral plasticity in individual choice of annual migratory routes. Shifts were inconsistent with exposure to the bottom-up benefits of migration. Instead, exposure to landscape gradients in predation risk caused by exploitation outside the protected area drove migratory shifts. Carnivore exploitation outside the protected area led to higher survival rates for female elk remaining resident or migrating outside the protected area. Cause-specific mortality aligned with exposure to predation risk along migratory routes and summer ranges. Wolf predation risk was higher on migratory routes than summer ranges of montane-migrant tactics, but wolf predation risk traded-off with heightened risk from grizzly bears on summer ranges. A novel eastern migrant tactic emerged following a large forest fire that enhanced forage in an area with lower predation risk outside of the protected area. The changes in migratory behavior translated to population abundance, where abundance of the montane-migratory tactics declined over time. The presence of diverse migratory life histories maintained a higher total population abundance than would have been the case with one migratory tactic in the population. Our study demonstrates the complex ways in which migratory populations change over time through behavioral plasticity and associated demographic consequences because of individuals balancing predation risk and forage trade-offs.