Long-distance mammal migrations face many new pressures globally. In North America, land management practices, development, and changing environmental conditions have contributed to the decline of migratory behavior in multiple mammal species, including elk, the most numerous ungulate in the Greater Yellowstone Ecosystem (GYE). Here, we examine (1) how migration timing changed from 2001 to 2015 in nine herds of partially migratory elk across the GYE, (2) what the environmental determinants of migration timing are in spring and fall, and (3) how these environmental determinants have changed since 2001 for each herd. Due to regional observations of drought and warming, we hypothesize that elk will migrate earlier in spring and later in fall. We expect vegetation green-up and snowmelt to be important environmental factors governing spring migration and snow accumulation and plant senescence to be linked to fall migration. Lastly, we expect snow melt and vegetation green-up to advance throughout the study period and snow accumulation and plant senescence to occur later. We test this using the most comprehensive dataset assembled on elk migration in the GYE, representing 285 spring migrations and 215 fall migrations linked to estimates of snow cover and plant phenology while controlling for hunting and supplemental feeding.
Results/Conclusions
In spring, elk departed winter ranges earlier but spent more time in migration corridors, resulting in no significant change in arrival date on summer range. In fall, elk departed summer range later and spent more time in migration corridors resulting in later arrival dates on winter range. Snow melt and green-up were the environmental factors most strongly linked to spring migration initiation, while snow melt and the date of maximum instantaneous rate of green-up were associated with arrival date on summer range. Plant senescence and snow accumulation were associated with fall migration initiation while snow accumulation was linked to arrival date on winter range. On average, from 2001 to 2017 the timing of snow melt and spring green-up advanced with more rapid advances occurring at higher elevations; plant senescence occurred later in the year with higher elevations again displaying more rapid changes while snow accumulation also occurred later. These results demonstrate that across the GYE (1) migration timing of elk is changing at a significant rate, (2) environmental conditions can explain much of the variance in migration timing, and (3) these environmental factors are changing in a way which is consistent with changing elk migration timing and changing climatic conditions.