From the Field

Mapping the Migrations of the Greater Yellowstone

July 13, 2016

Mule deer crossing pine creek. Photo © Joe Riis

There is an extraordinary effort underway to map long-distance mammal — especially elk, deer, moose and pronghorn — migrations in and out of the Greater Yellowstone ecosystem.

At the recent “Invisible Boundaries” exhibit opening on migrations at National Geographic Headquarters, ecologist Arthur Middleton explained how migrations are the arteries of this landscape and this mapping effort represents no less than understanding the pulse of one of the few remaining largely intact ecosystems in North America.

Understanding the basic ecology of the large herbivores that make these journeys is key to understanding why they migrate.

Mule deer are a good example of this process. Mule deer spend the short days of winter within the sagebrush expanses of the basins and foothills surrounding the high mountains. Subsiding winter and lengthening days in April marks the beginning of “green-up”, where grasses and forbs (herbaceous flowering plants) begin to emerge from the soil. Research scientist Matt Kauffman and his colleagues have studied how migrating mule deer track the green-up, foraging for grasses along an elevational gradient at exactly the time when they are most nutritious. They call this “surfing the green wave.”

By the end of June, they will have arrived on their high elevation summer range, and will remain there to graze until fall when they will reverse the process. These migrations vary in length, but some are quite long — the recently discovered Red Desert to Hoback migration is more than 150 miles one-way.


Similar to other sagebrush dependent species in the American West, mule deer populations have been declining in recent decades. The reasons aren’t fully understood, but it is clear that human activities play a critical role.

Increasingly, the landscapes that support these incredible mammal migrations are becoming more difficult to navigate and many migration corridors have already been lost (Berger 2004). Habitats are fragmented by roads, well pads, and turbines associated with energy development. Roadways experience increasing traffic levels, and private lands are increasingly at risk of residential development.

Helicopter brings in a mule deer for a tracking collar. Photo © Scott Copeland
Helicopter brings in a mule deer for a tracking collar. Photo © Scott Copeland

Although migrating animals can typically move through disturbed habitats, so long as routes are not severed by game-proof fencing, recent research indicates that such disturbances alter the behavior of migrating animals (Beckmann et al. 2012, Lendrum et al. 2013, Sawyer et al. 2013, Seidler et al. 2014). In a study before and after energy development in south-central Wyoming, deer migrated faster and stopped over to forage less often following development (Sawyer et al. 2013).

In recent years, researchers at the University of Wyoming and Wyoming Game and Fish Department have been applying new GPS collar technology to track migrating wildlife. With its abundant herds of elk, mule deer and pronghorn, western Wyoming and the Greater Yellowstone region have been at the epicenter of this effort.

There was a critical gap, however, in our understanding of mule deer populations along the eastern slope of the Greater Yellowstone. So this past March, The Nature Conservancy partnered with the Wyoming Migration Initiative and the Wyoming Game and Fish Department to collar 90 deer from south of Lander all the way to north of Cody, Wyoming. The Eastern Yellowstone Mule Deer Migration Project was born.

GPS collar. Photo © Scott Copeland
GPS collar. Photo © Scott Copeland

With the aid of real-time satellite collars, we are already gaining a new understanding of where the Eastern Yellowstone mule deer migrate and these data will be critical in helping us conserve them. Cody herds, for example, have migrated distances of over 90 miles where they summer in the heart of Yellowstone and Grand Teton National Park. In the southern Winds herd, deer have migrated along the Sweetwater River passing directly through TNC’s Sweetwater River Preserve. Our partners at WMI post migration maps weekly on Facebook so that anyone can follow along. They have also launched a crowdfunding campaign to help raise money for the collars.

Concerns have been recently raised about the negative effects of collaring species both on the animals themselves and a broader concern of de-wilding nature. These are valid concerns. In the Eastern Yellowstone study, two hundred mule deer will be collared for two years, after which the collars will automatically drop off, to document the migrations of tens of thousands — and many more into the future. Certainly every scientist engaged in collaring bears a responsibility to ensure that the benefits to the species are significant enough to justify the downsides. In our case, the data generated by this work will be critical to targeting conservation efforts and securing funds.

Fall on the Snake River. Photo © Scott Copeland
Fall on the Snake River. Photo © Scott Copeland

Long-distance mammal migrations are a global treasure. Scientifically, these migrations represent a critical piece of a functioning ecosystem. But, more than that, these migrations offer hope for sustaining a piece of wild earth, the value of which was put so eloquently by Wallace Stegner years ago “…We simply need that wild country available to us, even if we never do more than drive to its edge and look in. For it can be a means of reassuring ourselves of our sanity as creatures, a part of the geography of hope.”

All of us drawn to these migrations and dedicated to conserving them believe that with focused attention and sustained effort these migrations will continue to exist long into the future, ensuring both functioning ecosystems and demonstrating that we can indeed learn to balance the needs of both people and nature.

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1 comment

  1. Any possibility of getting some high tech idealists to develop RFID tags which will adhere to skin/hair/fur for one full molting period, so that tags could be “sprayed” onto deer, coyotes, etc., each springtime and then tracked until the next spring, when the affixing can be repeated? Two years would be better, but I suspect, without data, that most animal molt, or shed their horns/antlers, etc, on an annual basis. Animals which keep horns for multiple years (big horn sheep?) could be tagged on the horns with tags which will last, that is, broadcast, for several years. Nature does this with thorny seeds which adhere to hair and fur, waiting to sprout until the after the hair is shed. We would probably prefer some kind of glue, since many super sticky glues that are, as far as we know, not harmful when ingested in small quantities, are already available. Think of confetti, or “fairy dust” from Peter Pan.