Plasticity in elk migration timing is a response to changing environmental conditions

Abstract

Migration is an effective behavioral strategy for prolonging access to seasonal resources and may be a resilient strategy for ungulates experiencing changing climatic conditions. In the Greater Yellowstone Ecosystem (GYE), elk are the primary ungulate, with approximately 20,000 individuals migrating to exploit seasonal gradients in forage while also avoiding energetically costly snow conditions. How climate‐induced changes in plant phenology and snow accumulation are influencing elk migration timing is unknown. We present the most complete record of elk migration across the GYE, spanning 9 herds and 414 individuals from 2001 to 2017, to evaluate the drivers of migration timing and test for temporal shifts. The timing of elk departure from winter range involved a trade‐off between current and anticipated forage conditions, while snow melt governed summer range arrival date. Timing of elk departure from summer range and arrival on winter range were both influenced by snow accumulation and exposure to hunting. At the GYE scale, spring and fall migration timing changed through time, most notably with winter range arrival dates becoming almost 50 days later since 2001. Predicted herd‐level changes in migration timing largely agreed with observed GYE‐wide changes—except for predicted winter range arrival dates which did not reflect the magnitude of change detected in the elk telemetry data. Snow melt, snow accumulation, and spring green‐up dates all changed through time, with different herds experiencing different rates and directions of change. We conclude that elk migration is plastic, is a direct response to environmental cues, and that these environmental cues are not changing in a consistent manner across the GYE. The impacts of changing elk migration timing on predator–prey dynamics, carnivore–livestock conflict, disease ecology, and harvest management across the GYE are likely to be significant and complex.

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

“…In the face of widespread decline of migratory behavior in ungulates, there is growing emphasis in understanding the behavioral mechanisms that maintain partial migration to facilitate conservation efforts (Nicholson et al, 1997;White et al, 2007;Rickbeil et al, 2019;Sawyer et al, 2019). Partial migration is thought to be maintained by density-dependent fitness balancing between strategies at the population-level or by switching at the individual level (Lundberg, 1988;Kaitala et al, 1993).…”

Conserving transboundary wildlife migrations: recent insights from the Greater Yellowstone Ecosystem

“…Aggregated GPS data show how numerous major herds move annually between high‐elevation summer ranges in core areas and low‐elevation winter ranges near GYE frontiers (Figure ; Rickbeil et al . ). These year‐round ranges encompass an area approximately five times the size of Yellowstone National Park (YNP; Figure ).…”

“…However, technological limitations and imposing terrain constrained more detailed research until recently (eg White et al 2010;Middleton et al 2013;Cole et al 2015). Aggregated GPS data show how numerous major herds move annually between high-elevation summer ranges in core areas and low-elevation winter ranges near GYE frontiers (Figure 2; Rickbeil et al 2019). These year-round ranges encompass an area approximately five times the size of Yellowstone National Park (YNP; Figure 2).…”

What are the effects of climate variability and change on ungulate life-histories, population dynamics, and migration in North America? A systematic map protocol

“…Climatic variation occurs across multiple spatial and temporal scales, and understanding the impacts of both short-and long-term changes will provide valuable information to wildlife and land managers. Climate is an important driver of ungulate life-history characteristics, population dynamics, and migratory behaviors and changes in climate can directly or indirectly affect the growth, development, fecundity, dispersal, demographic trends, and long-term viability of populations [9,13] as well as the timing and locations of migratory movements [14,15]. Here, we use the term "climate variability" to refer to interannual or interdecadal fluctuations in temperature and precipitation, and the term "climate change" to refer to persistent, multidecadal deviations from long-term averages [16].…”

“…The effects of changes in the timing of spring green-up and winter severity, two key drivers of ungulate migration in North America, have also been documented. Elk in the Greater Yellowstone Ecosystem delayed departure from winter range habitat when spring green-up occurred later [15], mule deer in the Sierra Nevada migrated earlier in years with earlier green-up and low snow depth [14], and mule deer in Wyoming altered their use of stopover sites based on changes in plant phenology [26]. Lastly, there have been efforts to project the potential future impacts of climate change on ungulates.…”