For large mammals such as elk, mule deer, and pronghorn, migration is a normal seasonal event. Twice per year, herds move according to the weather and the availability of food, following the spring green-up to their summer range in the mountains, then back to the lowlands in the fall to avoid the harshest winter weather. Migration is necessary to acquire resources (food, water, shelter, mates) and avoid risk (predators, drought, pests). But we now know that it also contributes to the overall health of western ecosystems, as herds redistribute resources, energy, nutrients, and genes along their path.
Natural and anthropogenic features of the landscape influence how animals move through their seasonal and home ranges. Roads and urban development, for example, fragment the landscape, creating obstacles and barriers that make it more difficult for ungulates and other large mammals to navigate. Some recent research examines how the encroachment of human development is affecting the behavior of these species in Utah, and what can be done about it.
In a 2023 MS thesis, Levi Watkins of Brigham Young University (BYU) examined elk (Cervus canadensis) migration in central and eastern Utah by capturing and placing GPS collars on them. By analyzing their movements in relation to environmental factors and landscape barriers such as railroads, highways, and other human development, which he mapped using GIS software, he was able to conclude that these features act as significant barriers to elk movement. Highways in particular, especially those with a speed limit of 65+ mph and traffic of greater than 1,000 vehicles per day, altered elk migration behavior in ways that forced them to go around or seek an alternate route. Watkins concluded that state wildlife agencies should consider these barriers in their management unit plans for elk in the state.
A second MS thesis by Ronan Hart (BYU) looked at the effect of anthropogenic linear features such as roads, trails, pipelines, and fences on the migration behavior of mule deer (Odocoileus hemionus) and pronghorn (Antilocarpa americana). He too collared individuals and tracked their movements in relation to mapped features, finding that the space use of both species is affected by the density of linear features. The home range size of pronghorn and mule deer, for instance, suffers from increasing road density, especially in winter. Pronghorn are more sensitive to fences, which impede anti-predation behavior (they slowly crawl under, whereas mule deer jump over). He also found that pronghorn showed a stronger response to increasing traffic than mule deer, and avoided crossing even minor roads during summer daytime.
Other human infrastructure, such as oil and gas development, can also have profound effects on mammal migrations. A 2022 study led by Sam Chambers of the US Geological Survey examined the additional energy expenditure required for mule deer to traverse or avoid oil and gas fields at twelve sites on the Colorado Plateau in Utah. Using a model of caloric expenditure based on bodyweight, the team found that average caloric output increased by 70% for deer to avoid oil and gas sites, regardless of terrain. This increase can have a significant impact on the health of individuals and populations, which sometimes barely get by on the food that is available to them. Often, they do not have energy to spare, and must compromise in other ways, such as decreasing the overall distance they travel. These considerations, the authors note, should be factored into decisions over where to build new projects.
These studies underscore the need to better understand not only the structural and behavioral components of animal migrations, but also how to better preserve their natural movement across the land. It is well known, for example, that protected areas such as national parks, forests, and monuments protect biodiversity from habitat loss and ecological change. As climate change pushes animals into new areas in search of resources, they will need ways to overcome habitat fragmentation to get there. There is an immediate need for regional and continental-scale networks of protected lands that allow for mammals to disperse, migrate, and adapt.
A 2023 study led by William Newmark, a vertebrate zoologist at the Natural History Museum of Utah, looked at the utility of two proposed large-scale habitat connectivity corridors to preserve and enhance medium and large mammal populations in the West. They found that linking known migration routes and protected areas in the Yellowstone-Glacier and Rainier-Cascades areas would greatly increase mammal persistence by enhancing movement and dispersal ability and increasing population size. Another effort called the Yellowstone to Yukon Initiative (Y2Y) seeks to connect lands along 2,100 miles of the Rocky Mountains all the way to northern Canada. It is possible that such an effort could be expanded to include the entire spine of the North and South American continents. Grand Staircase-Escalante National Monument lies within this continuum of protected lands, and will continue to be integral to habitat connectivity efforts in the West.
As part of my role with GSEP, I sit on the Utah Wildlife Connectivity Working Group, a collaborative of scientists, agency staff, and nonprofit organizations dedicated to creating wildlife connectivity and promoting roadway safety. A recent report by the Pew Charitable Trusts (a member of the Working Group) outlined the benefits, needs, and funding opportunities for wildlife connectivity projects across the West. In March, Utah Governor Spencer Cox signed the state budget, which allocated $20 million for wildlife crossing projects in the state. “This appropriation marks one of the largest investments a state has ever made in wildlife crossings,” said Wildlands Network Western Program Director Michael Dax. The group will continue to study, strategize, and advocate for wildlife-friendly corridors in the state.
[Photo credit Stephen Leonardi/Unsplash]