#Study reveals impacts of climate change on migrating mule deer

When drought reshuffles the green-up of habitats that mule deer migrate across, it dramatically shortens the annual foraging bonanza they rely on.

That is the main finding of a new University of Wyoming study, which shows the benefits of migration are likely to decrease for mule deer and other migratory herbivores as drought becomes more common due to ongoing climate change.

Drought reduces the availability of key food resources by shortening the duration of spring green-up—and altering the progression of the “green wave” across the landscape.
The study was conducted by a team of researchers working with lead author Ellen Aikens, a 2019 graduate of the Wyoming Cooperative Fish and Wildlife Research Unit at UW. The paper was published this week in Global Change Biology, a leading journal documenting the biological effects of global change.
“This research shows that climate change can alter the underlying distribution of food resources by compressing the time when optimal forage is available, which reduces the benefit of migration,” Aikens says. “This work highlights an emerging threat to migratory mule deer and likely many other migratory species.”
Aikens’ analysis combined 19 years of drought data going back to 2001, with a 2013-15 GPS dataset of mule deer migrations in the Wyoming Range.
In a wet year, the study found that mule deer have access to newly sprouted springtime plants during an extended period, up to 120 days. That’s a full four months when snow is melting, and runoff is saturating the soil and causing forage plants such as sticky purple geranium to emerge.
Deer get a significant portion of their forage benefit for the entire year by following this green wave of plants, which, in wet years, progresses in an orderly fashion from low-elevation winter ranges to summer ranges in the high mountains.
Previous work by Aikens has shown that mule deer are experts at “surfing the green wave” across the landscape. Their movements allow them to always be in the right place at the right time to consume plants at their peak green-up, when they are protein-rich and easy to digest.