Climate change is influencing the timing of different events for plants, called their phenology. For example, certain plants might start to green-up earlier in the spring, or senesce earlier in the fall. This in turn changes the timing of the availability of food sources for herbivores. Additionally, climate change influences the amount and duration of snow cover. With warmer temperatures, on average there are fewer days with snow on the ground in some areas, and drought events will mean snow cover may become more unpredictable.
Like other ungulate species, elk migrate to access food for grazing, and to avoid snow. Because food availability and snow cover are sensitive to climate change, it is reasonable to predict that the phenology of elk migrations has shifted over time. There are about 20,000 elk in Yellowstone, and they are the primary ungulate in the ecosystem. Elk are important herbivores in the Yellowstone system, and an important prey species for predators; therefore, it is particularly important to understand how they are responding to climate change and how their migration patterns are impacted.
A newly published study used telemetry data from nine Yellowstone elk herds, which included 414 individuals, from 2001 to 2017. These elk migrate throughout the region to access resources that are seasonal and unstable in a particular location throughout the year. Their goals were to determine what drives the migration patterns of elk, and how has the timing of these migrations been impacted by climate change in the region.
As for what drives migration patterns, the researchers found that several factors played a role, but which factor was important changed throughout the year. They found that elk left their winter-ranges in response to a trade-off between current and potential future grazing conditions. In the summer, elk moved to higher elevations and snowmelt determined when they would arrive in their summer ranges. Snow accumulation and exposure to hunting served as indicators for when elk would depart from summer ranges and make their way to winter ranges.
Importantly, the timing of all these factors that drive elk migration have changed. Snowmelt, snow accumulation, and spring green-up dates all changed over the course of the study. This lead the timing of migration to shift over the 16 year study period. By the end of the study, elk were arriving in their winter grounds 50 days earlier than the beginning of the study. However, changes were variable across the study area, and different herds experienced different rates and directions of change. This could impact herbivory, predator-prey, disease, and other dynamics in complex ways that will be hard to predict.
Rickbeil, GJM, Merkle, JA, Anderson, G, et al. 2019. Plasticity in elk migration timing is a response to changing environmental conditions. Global Change Biology 25: 2368–2381.