Shifts in strategy: Calving and calf survival in a partially migratory elk population

• Author / Creator

  Berg, Jodi Elizabeth

• Partial migration, a phenomenon wherein only one fraction of a population migrates, is taxonomically widespread. While well-studied in birds and fish, partial migration in large herbivores only recently has come into the spotlight due to migratory ungulates’ global loss. In this dissertation, I summarized both individual- and population-level mechanisms for partial migration in ungulates, showing how density-dependent and -independent factors, alone or together, can combine with genetic variation and individual differences to maintain both migrants and residents within a population. Evidence suggested that partially migratory behavior is a state-dependent response of individuals, but data that empirically demonstrate which factors determine the relative costs and benefits to using migratory versus resident tactics are wanting. I sought to fill this void by monitoring elk (Cervus canadensis) that used 3 migratory tactics (resident, western high-elevation migrant, eastern low-elevation migrant) in the eastern slopes of the Rocky Mountains, Alberta. The Ya Ha Tinda elk population declined by almost 70% over the last several decades, and the migratory patterns shifted towards residency and a relatively new, low-elevation migration. During two time periods (2002 – 2006, 2013 – 2016), I assessed whether differences in selection for forage and avoidance of predation risk during calving might contribute to shifts in the number of individuals following each of the migratory tactics across time. I employed a machine-learning algorithm to predict dates of parturition based on GPS movements of elk equipped with vaginal implants (n = 60) and found predictions were within 1.43 ± 0.85 days of the known date. I applied the model to an additional 58 GPS-collared elk without vaginal implants and found parturition occurred 8 May – 11 July with median parturition dates differing among migratory tactics in 2013 – 2016 and residents shifting towards an earlier date. Using the 26 days that elk remained relatively localized in their post-parturition movements, I compared habitat characteristics of calf-rearing areas to 10 similar-sized areas centered on random locations from summer for the same individual in a latent selection framework. All elk, regardless of migratory tactic, consistently selected for forage resources during calving more than during summer, with limited evidence for trading off forage due to predation risk. Selection for forage exposed western migrants to high risk of bear (Ursus spp.) predation, residents to high risk of wolf (Canis lupus) predation, and eastern migrants to low bear and wolf predation because they avoided risk by using areas of high human activity. Patterns of spatial use during calving were consistent with the recent decline in western migrants and increase in eastern migrants, implying that conditions on calving areas contributed to these changes. I next estimated timing and causes of neonatal juvenile mortality, and the spatiotemporal scales for which biological and environmental factors were related to risk of mortality for elk calves. During 2013 – 2016, I captured 94 neonates, monitored risk of mortality to neonatal elk to 90 days of age, and assigned cause of death based on field evidence. The overall mortality rate for calves from all causes to 90 days of life was 0.61 (95% CI: 0.52, 0.70). Mortality hazard peaked at ~3 – 7 days of life and remained high in the first 20 – 30 days of life when 90% of deaths occurred. The overall mortality rate for calves of resident females to 90 days of life (0.69, 95% CI: 0.13, 0.99) was over 80% higher than for calves of eastern migrants (0.37, CI: 0.03, 0.97). Of 57 mortalities, I attributed 29 deaths to bears (51.7%), 7 to cougars (Puma concolor, 11.7%), 4 to wolves (6.7%), 8 to unknown predators (15.0%), and 9 to other causes (15.0%). Daily risk of mortality was most influenced by available forage biomass and low predation risk associated with human infrastructure on the summer range. Differential exposure to environmental factors on summer ranges resulted in variable predator-specific mortality and higher calf survival of eastern migrants, consistent with the shift in the number of elk migrating onto industrial forest lands. This study is one of the few that documents factors contributing to emerging migratory behavior in maintaining a partially migratory population.

• Subjects / Keywords

  • elk
  • partial migration
  • selection
  • survival

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-y2h3-b976

• License

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