COS 161-2 - Testing the Phenological Mismatch Hypothesis in a Partially Migratory Large Herbivore Population

Abstract: Migrations are critical ecological processes around the globe. Migratory behavior is thought to have evolved to maximize access to high quality forage and minimize predation. But changes in climate are affecting forage availability, which could alter the benefits of migration. Reduced forage quality due to climate change may reduce reproduction in migrating ungulates, but the specific mechanisms remain elusive. Climate change could affect migration and demography through phenological mismatch. Phenological mismatch is defined as a divergence between the timing of behaviors - such as migration or reproduction - with peak forage resource availability. The long-term Ya Ha Tinda elk (Cervus canadensis) monitoring project in Banff National Park, Canada provides an opportunity to test for phenological mismatch in ungulates. From 2001-2022, we captured 350 adult female elk and monitored individual migration and reproduction using GPS and VHF collars. We first used Google Earth Engine to assess trends in the start date of the growing season and snow cover from 2000 – 2022 using MODIS NDVI (MOD13Q1, 250m²). Next, we classified each of ~ 300 individual elk-years to a migrant or resident tactic, and estimated migration date, using net squared displacement. We then predicted parturition date for female elk using the MoveApps workflow, and cross-validated parturition predictions with 60 pregnant females with known parturition dates. The phenological mismatch hypothesis predicts that divergence in the correlation of phenological events like start of spring and migration, and parturition will reduce reproductive success. We used these 3 measures - start of spring, migration date, parturition date - to test this hypothesis on measured calf survival to ~ 100 days using mark-resight data. Preliminary analyses demonstrate that timing of the start of spring and snowmelt has advanced in Banff, setting the stage for the potential for phenological mismatch. Timing of migration appears plastic, however, because we found individual elk migration dates appeared to closely match timing of start of spring - with some tactic-specific differences. We found less variation in timing of parturition, however, consistent with it being less plastic. Finally, we report some preliminary evidence that the greater the phenological mismatch between timing of spring and timing of migration, the lower the probability of a calf surviving to day 100. Our unique long-term study provides valuable insights into the challenge of determining the mechanisms in which climate change may impact wildlife populations.