Density-independent predation affects migrants and residents equally in a declining partially migratory elk population
Mark Hebblewhite, Daniel R. Eacker, Scott Eggeman, Holger Bohm, Evelyn Merrill
Migration is expected to benefit individuals through exposure to higher quality forage and reducing predation rates more than non-migratory conspecifics. Previous studies of partially migratory ungulates (with migrant and resident individuals) have focused on bottom–up factors regulating resident and migrant segments, yet differential predation between strategies could also be a density-dependent regulatory mechanism. Our study tested for density-dependence in mortality, as well as mechanisms of bottom–up or top–down regulation in the resident and migrant portions of the partially migratory Ya Ha Tinda elk population. We tested for density dependence in adult female and juvenile survival rates, and then discriminated between predator- and food-regulation hypotheses by testing for density-dependence amongst mortality causes for adult female elk. Notably, the population declined almost 70% from near previously published estimates of carrying capacity over 10 years, providing ideal conditions to test for density dependence. In contrast to predictions, we found only weak support for density dependence in adult survival and juvenile survival. We also found few differences between migrant and resident elk in adult or juvenile survival, though juvenile survival differences were biologically significant. Predation by humans and grizzly bears was density dependent, but similar between migratory strategies. Predation by wolves was the leading known cause of mortality, yet remained constant with declining elk density equally for both migrant and resident elk, indicating wolf predation was density-independent. Instead of being strongly regulated by food or predation, instead, we found adult female survival was driven by density-independent predation and climatic factors. The few differences between migratory strategies suggest equivalent fitness payoffs for migrants and residents. This population is being limited by density-independent predation leading to declines of both migratory strategies. Our results challenge classical predator–prey theory, and call for better integration between predator–prey and migration theory.
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