Partial migration of elk in a heterogeneous landscape

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Migration is one of nature's most iconic processes. Understanding the maintenance of partial migration, where only a portion of the population migrates, has caught the recent attention of population ecologists, but also conservationists because of the dramatic changes in migratory systems facing climate change and a growing human foot print. Understanding the maintenance of partial migration fundamentally requires long-term demographic monitoring to distinguish among hypotheses for its maintenance, but long-term studies on partially migratory ungulates are exceedingly rare. In the next 5 years, my research team extends our 13-year demographic study of a partially migratory elk (Cervus elaphus) population near Banff National Park, Alberta. We build on our past research comparing behavior, space use, and demography among migratory segments addressing 3 new questions. First, recent theory indicates spatial and temporal variation in environmental conditions strongly affects individual fitness in ways that may have opposing effects at the population level. We ask to what extent can local spatial variation in habitat conditions off-set stochastic variation in the onset, rate of change, and duration of growing season (phenological) trends that drive forage quality at the large-scale. This is an area not well studied in the foraging ecology of large herbivores. Second, we expand our focus on the top-down effects from wolves to predator communities and ask how large carnivore interactions influence the spatiotemporal patterns in predation risk for elk during 3 periods (calving, late summer and winter) that impose different movement and nutritional constraints on herbivores. Third, scant data from studies on mammals indicate switching among migratory strategies is rare, pointing to them being fixed. Genetic data from elk in this system do not support this, and after 13 years switching now averages 16% (3-25%) per year, implicating a bigger role than we originally argued. We now hypothesize that partial migration is maintained by density-dependent fitness balancing and switching. To evaluate this hypothesis we first need to determine whether intrinsic (age, learning) or extrinsic factors (predation risk, forage quality) influence switching. We plan to build spatial and aspatial demographic models to evaluate the mechanisms behind partial migration and illustrate how these approaches can help devise local management and global strategies for conserving migratory populations under environmental change.