Density‐independent predation affects migrants and residents equally in a declining partially migratory elk population

Abstract

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, 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.

Density-Dependent Foraging Behaviors on Sympatric Winter Ranges in a Partially Migratory Elk Population

“…We expected stronger selection responses by elk to predation risk because wolves are the leading cause of mortality for adult females in this system (Hebblewhite et al, 2018). Several factors may contribute to the moderate response.…”

“…As the population of elk declined, we were surprized that the median group size increased rather than decreased (McLellan et al, 2010). Because wolf predation on elk is not density dependent in this system and wolf abundance did not decline (Hebblewhite et al, 2018), per capita predation risk from wolves may have increased as elk abundance declined leading to stronger gregariousness. Maintaining relatively large groups under high predation risk reflects an optimal group size in terms of tradeoffs in group foraging constraints, reducing predator encounters, and maximizing dilution effects (Focardi and Pecchioli, 2005).…”

“…During this study, the partially migratory elk population declined from ∼1,400 animals in 2002-2003 to ∼450 in 2015-2016. The leading cause of the decline was high predation by gray wolves (Canis lupus) on adult females (Hebblewhite et al, 2006(Hebblewhite et al, , 2018, combined with high predation by grizzly (Ursus arctos) and black bears (Ursus americanus) on neonatal elk (Berg, 2019). Switching among migratory tactics occurs but is relatively low (15%/yr; Eggeman et al, 2016).…”

“…We predicted elk would exhibit density-dependent habitat selection, but we expected the extent of response to be less for residents because they needed to compensate for low forage quality in summer (Hebblewhite and Merrill, 2009). Because residents maintained larger, more cohesive groups than migrants (Robinson et al, 2010;Hebblewhite et al, 2018), we also expected the strength of selection for high-forage areas by residents would be associated with longer residence times and high return times. In contrast, because migrant elk maintained less coherent groups that were less familiar with risk from predators including humans (Robinson and Merrill, 2013), we predicted greater trade-offs between high-forage areas and safety and increased movements in risky areas.…”

“…In this paper, we evaluated the assumption that migrant and resident elk make similar trade-offs between foraging opportunities and safety from wolves on the sympatric winter range of the Ya Ha Tinda adjacent to Banff National Park (BNP) (Figure 1) in response to ∼70% population decline over a 14-year period (Hebblewhite et al, 2018). Previously, we have reported migrant elk are exposed to higher forage quality on summer ranges consistent with the forage maturation hypothesis , and indicating migrants may enter winter in better condition.…”