Large herbivores in a partially migratory population search for the ideal free home

Abstract

Migration is a tactic used across taxa to access resources in temporally heterogenous landscapes. Populations that migrate can attain higher abundances because such movements allow access to higher quality resources, or reduction in predation risk resulting in increased fitness. However, most migratory species occur in partially migratory populations, a mix of migratory and nonmigratory individuals. It is thought that the portion of migrants in a partial migration population is maintained either through (1) a population-level evolutionary stable state where counteracting density-dependent vital rates act on migrants and residents to balance fitness or (2) conditional migration, where the propensity to migrate is influenced by the individual's state. However, in many respects, migration is also a form of habitat selection and the proportion of migrants and residents may be the result of density-dependent habitat selection. Here, we test whether the theory of Ideal Free Distribution (IFD) can explain the coexistence of different migratory tactics in a partially migratory population. IFD predicts individuals exhibit density-dependent vital rates and select different migratory tactics to maximize individual fitness resulting in equal fitness (λ) between tactics. We tested the predictions of IFD in a partially migratory elk population that declined by 70% with 19 years of demographic data and migratory tactic switching rates from >300 individuals. We found evidence of density dependence for resident pregnancy and adult female survival providing a fitness incentive to switch tactics. Despite differences in vital rates between migratory tactics, mean λ (fitness) was equal. However, as predicted by the IFD, individuals switched tactics toward those of higher fitness. Our analysis reveals that partial migration may be driven by tactic selection that follows the ideal free distribution. These findings reinforce that migration across taxa may be a polymorphic behavior in large herbivores where migratory tactic selection is determined by differential costs and benefits, mediated by density dependence.

Summer elk calf survival in a partially migratory population

“…These results confirm that the differences we observed in neonate survival to 90 days of life translate to annual calf survival, which is not always the case for juvenile ungulate survival (Hurley et al 2014, Eacker et al 2017). Further, Martin et al (2021) showed adult female elk were switching between migratory tactics in a manner consistent with choosing the best tactic based on reproductive success. Our results emphasize the important differences in reproductive benefits between different tactics of migratory ungulates and point to the necessity of focusing on differential reproductive success between migratory tactics in future studies.…”

“…Differential mortality of calves between migratory tactics of a population supports the advantage of migrating (Fryxell and Sinclair 1988, Fryxell et al 1988) and points to the role that predation can have on shifting the balance between growth rates of migratory tactics due to differential recruitment. Using an integrated population model for this same elk population, Martin et al (2021) estimated annual calf survival of 3 different migratory tactics, the residents and eastern migrants we report here, and also western migrants. Results similarly showed lower annual calf survival for residents, intermediate survival for western migrants, and slightly higher survival for eastern migrants.…”

“…Results similarly showed lower annual calf survival for residents, intermediate survival for western migrants, and slightly higher survival for eastern migrants. Their analysis indicated that higher juvenile survival in the eastern migrants appears to be contributing to the shifting migratory behavior in the Ya Ha Tinda elk population that began over a decade ago, and is more important than individuals switching between migratory tactics (Eggeman et al 2016, Martin et al 2021). These results confirm that the differences we observed in neonate survival to 90 days of life translate to annual calf survival, which is not always the case for juvenile ungulate survival (Hurley et al 2014, Eacker et al 2017).…”

“…We assessed differences between resident and migrant elk in terms of adult female body condition and timing of birth, calf elk survivorship and cause‐specific mortality, and factors influencing calf mortality risk to help explain the shifting migration dynamics in the Ya Ha Tinda elk population. We focused on neonate calf survival to address the change in migratory tactics because recent estimates of adult survival rates were similar among eastern migrant (i.e., elk migrating to low‐elevation summer ranges east of the Ya Ha Tinda) and resident elk (Hebblewhite et al 2018, Martin et al 2021). Based on the recent proportional increase in migrants, we first hypothesized that migrant elk would exhibit higher adult female body fat and higher calf birth mass, thus resulting in higher calf survival, than resident elk because of earlier exposure to plant growth and forage resources associated with a greater percentage of burns and cut forests (Berg et al 2021).…”

Life history trade-offs in glucocorticoid-mediated habitat selection

“…In partially migratory populations, for example, habitat selection phenotypes become spatially segregated when migrants move to seasonal ranges and residents remain on the same ranges year-round. Despite their spatial separation, both residents and migrants maintain similar energy balance (Bonnet-Lebrun, Somveille, Rodrigues, & Manica, 2020) and overall fitness (Martin, Hebblewhite, & Merrill, 2022) even while glucocorticoid production differs by migratory strategy (Fudickar, Schmidt, Hau, Quetting, & Partecke, 2013). Thus, partial migration may be one example of adaptive result of feedback between habitat selection and glucocorticoid production.…”