Jerod Merkle remembers helping to capture a collared mule deer in Wyoming’s Red Desert a few years ago when the ground was bare of snow. A wildlife ecologist and associate professor at the University of Wyoming, Merkle was part of a team gathering biological samples and retrieving GPS collar data, which track where an animal moves on the landscape.
Biologists typically capture large game in winter because it’s safer for the animal — snow slows wildlife down and they aren’t overly stressed by hot temperatures. But on this particular day, Merkle recalls that temperatures were in the 50s. It was December.
“More and more, we’re doing captures and there’s no snow on the ground,” Merkle said. “It’s like springtime.”
As climate change leads to higher temperatures across the Greater Yellowstone Ecosystem, rising heat is becoming a more frequent environmental stressor for wildlife. The data Merkle helped collect that day would later be used in a new study published in the scientific journal Ecosphere last October, which seeks to answer the question, “How are animals coping with really hot days?”
Eight years in the making, the study examines GPS data for nine large mammal species — bison, bighorn sheep, mule deer, pronghorn, moose, mountain goats, elk, cougars, and wolves — to better understand how animals are responding to increasing temperatures and if they have the potential to adapt to a warming climate.
The research team, overseen by Merkle at the University of Wyoming, focused on where animals moved during the summer months using data collected between 2001 and 2019. The resulting paper offers one of the largest multi-species comparative studies of its kind.
In addition to tracking so many large mammals, the 20-author study is unique due to a complex, collaborative effort that made it possible. Both federal and state agencies contributed data to the project, enabling scientists to analyze wildlife populations throughout Wyoming, as well as in Idaho, Montana and Colorado.
The study’s results point to the value of a permeable landscape free of human-made barriers in helping large mammals adapt to climate change, and bring added focus to Greater Yellowstone as a place where investing in connectivity matters.
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Outside of Alaska, scientists say Greater Yellowstone is the only location where a study like this could occur.
“It’s really the only place in the Lower 48 where we have these intact assemblages of large mammal species,” said Justine Becker, co-lead author of the study and now an assistant professor in Montana State University’s ecology department. “What we wanted to do was look at as many of these large mammal species as possible in a relatively similar environmental context.”
What Becker and the research team found was that all nine species adjusted their behavior when it got hot outside. Animals moved into places where they could escape the summer heat, like north-facing slopes, shaded areas, or higher elevations.
Becker wasn’t surprised by the results, but says it’s encouraging to see that large mammals can find ways to cool down, especially because they are slower to evolve than smaller animals.
“One of the biggest concerns that we have with large mammal populations is that because they are so large bodied, they tend to have slow life histories, so longer generation times,” Becker said. “It’s harder for them to adapt evolutionarily through natural selection processes … when the pace of [environmental] change is really rapid.”
The study also revealed that animal behavior varies between both individuals and broader species groups. Rather than consistently selecting one or two specific habitats as a refuge, animals made different choices about the best way to avoid hotter temperatures.
This points to the value of having options, Becker says. If large mammals have multiple habitat types to choose from, they have the best chance at adapting to environmental stressors. The catch is that these animals often face physical barriers that prevent them from getting there.
“It’s not just about these animals having options in their environments, but having the ability to access them,” Becker said. “And that really means that they need to be able to move freely across the landscape.”
Permeability matters especially in environments like the Wyoming prairie, where animals like pronghorn may need to travel farther distances to find relief from the heat. Study results showed that pronghorn in Wyoming’s Shirley Basin — a high elevation sagebrush steppe and grassland ecosystem with little tree cover — are making the biggest behavioral adjustments of all nine mammal species to reach cooler areas, compared to those in more mountainous or forested habitats.
Merkle, who runs a lab at the University of Wyoming focused on movement ecology and conservation, sees a clear link between the study’s results and the value of improving connectivity.
“The positive thing is that these species and populations have a lot of tools in their toolbox to deal with heat,” Merkle said. “Because individuals are doing different things, that means we have to continue helping them have access to a diversity of habitats. That could be through connectivity-type measures or keeping the landscape open through fencing or crossing structures over highways. All of that is going to help them.”
The importance of connectivity is not a new concept, but Becker says this study presents it from a different angle: large mammals need to move not only to migrate or find food, but also to adjust to stressful environmental conditions.
Cathy Whitlock, a paleoecologist and regents professor emerita of Earth sciences at MSU who was not involved in the study, says the results highlight why Greater Yellowstone is such a critical area for wildlife connectivity.
“I think the conclusions of the study are spot on, that having a diverse and heterogenous landscape is important,” Whitlock said. “And that’s why places like Yellowstone are so important, because they’re still kind of a functioning ecosystem.”
Whitlock, who spent decades studying the region’s ecology and is a lead author of both the 2017 Montana Climate Assessment and the 2021 Greater Yellowstone Climate Assessment, also agrees that connectivity is key if species are going to adapt to a warming planet.
“You have to give animals the space to move so that they can find suitable habitat,” she said. “Greater Yellowstone is one very large island in a landscape of protected areas in the western U.S., so as species have to move to higher latitudes or higher elevations, these islands of protected areas are very important … They are stepping stones, and the challenge for mammals is going to be to get there.”
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Beneath the rigor and breadth of this nine-species study lies a web of collaborative relationships.
State and federal agencies across Greater Yellowstone, including the Bureau of Land Management, National Park Service, U.S. Geological Survey, Wyoming Game and Fish Department, and Idaho Department of Fish and Game, as well as statistical consulting firm Western Ecosystems Technology Inc. and the University of Wyoming all contributed to the project. The collaboration enabled scientists to use data from 17 different study populations in their final analysis.
“You just don’t see studies on these types of charismatic large mammals that are this taxonomically broad and this spatially complete,” said Embere Hall, co-author and supervisor for WGFD’s science, research and analytical support unit. “And that’s all due to strong partnerships.”
Building trust and reciprocity with land managers is a core value of Merkle’s research lab. In the case of this paper, the team used data sets originally collected with different objectives in mind. But when applied to a new research question, scientists were able to generate ecological insight that all parties were curious about, pulling new knowledge out of old data.
“Greater Yellowstone is one very large island in a landscape of protected areas in the western U.S., so as species have to move to higher latitudes or higher elevations, these islands of protected areas are very important.”
cathy whitlock, paleoecologist, regents professor emerita of Earth sciences, Montana State University
In thinking about why this collaboration was fruitful, Merkle credits three things: the lab’s ability to process large amounts of data, cultivating relationships over time, and being willing to step outside of individual silos.
“You have to be thinking about questions and ideas that are value-added for the manager on the ground,” Merkle said. “Having all of those points, the data science, the partnerships, and the understanding of science and management and conservation all in one is pretty unique. You just don’t see that too often in academia.”
On the land management side, Hall echoed a similar sentiment.
“Sometimes we silo ourselves, right? We think that, because we’re agency X, we’re the most important or have the final say,” Hall said. “I think we were all able to lower our guard a little bit because we knew that the answers we could get out of this work would be so valuable and it was worth stepping outside of our respective hats.”
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On the ground, this research is helping to support funding requests for connectivity projects.
Jill Randall, big game migration coordinator for Wyoming Game and Fish, sees the large mammal study as contributing to a growing body of evidence that backs up the department’s priorities.
“Having data helps us,” she said. “It’s cumulative, it adds to the argument. It’s further supporting decisions that are moving toward accommodating wildlife in this changing landscape.”
Randall plans to weave the study’s results into upcoming grant applications for two different wildlife-crossing projects. The first, informally known as the Dubois project, aims to reduce wildlife-vehicle collisions along a 25-mile section of U.S. Highway 26 and improve connectivity between the Wind River Range and the Grand Tetons. The second, Halleck Ridge, would add an overpass structure to allow wildlife to safely cross Interstate 80.
Since its completion in 2012, Wyoming Department of Transportation estimates that wildlife-vehicle collisions have been reduced by over 80 percent.
Several other wildlife-crossing projects have already been completed or are in process in Wyoming, including Trapper’s Point, Dry Piney, and the most recent Kemmerer South project, which is supported by a $24 million grant through the federal Wildlife Crossing Pilot Program. Because the pilot program expires this year, Randall is applying for other grants through the Federal Highway Administration, as well as soliciting private foundations, to fund WGFD’s future connectivity work.
Randall knows firsthand the value of wildlife-crossing infrastructure. She remembers scheduling her day to avoid Trapper’s Point after dark, which she had to drive through twice a day to get to and from town.
“No joke, it used to be a death gauntlet to drive through there,” she said. “It was so unsafe … and the [wildlife] population impacts were horrible.”
Trapper’s Point is located outside of Pinedale near the “Path of the Pronghorn,” a 6,000-year-old migration route, and includes two overpasses and six underpasses plus 12 miles of wildlife fencing. Since its completion in 2012, Wyoming Department of Transportation estimates that wildlife-vehicle collisions have been reduced by over 80 percent.
Randall says the large mammal study can help land managers feel more confident, armed with a high-level view of what is needed on the landscape to benefit the health of many different species, not just a few.
“We can account for more variables when you have all those different data sets involved,” she said. “You can take a lot of the assumptions out. It’s not just one herd, one landscape that you’re trying to apply broadly.”
Knowing that these animals can adapt, Randall says, can help unify people around a shared vision for what projects will make a real impact.
“In the world of landscape connectivity, broken is generally irreversible,” she said. “In Wyoming: less people, more connected landscapes, therefore more wildlife. We’re trying to fight tooth and nail to keep what we have instead of trying to reestablish things that are broken.”
