Each year, thousands of migratory mule deer and pronghorn antelope journey northwest from their winter homes in the Green River Basin, a grassland valley in western Wyoming, to their summer homes in the mountainous landscape near Grand Teton National Park.
But to reach their destination, these ungulates must successfully navigate the more than 6,000 kilometers (3,728 miles) of fencing that crisscrosses the region. That’s enough distance to span nearly twice the length of the U.S.-Mexico border.
In a new study, wildlife biologists at the University of California, Berkeley, combined GPS location data of tagged mule deer and pronghorn with satellite imagery of fences to find out just how often these animals encounter fences, and what happens when they do. The results, published on Jan. 7 in the Journal of Applied Ecology, help pinpoint which fences pose the biggest barrier to ungulates trying to access their ideal habitat.
Along with the study, the team is also publishing a software package that will help wildlife managers around the world quickly analyze GPS tracking data to identify fences and other barriers that might be impeding the vital movements of animals.
“We need fences — they help keep livestock safe, can help keep livestock and wildlife separate, and mark property boundaries,” said Arthur Middleton, an assistant professor of wildlife management and policy at UC Berkeley and senior author of the paper. “So, the question becomes, how do you identify which fences are really important, and which are problematic from a wildlife standpoint, and then seek some way to mitigate the impacts?”
Fences don’t always pose an insurmountable barrier to wildlife, and different species find different ways to get around them. Mule deer are willing to jump over fences that are low enough. Pronghorn antelope, however, are reluctant to jump over fences and instead must seek out areas where they can move underneath.
Wenjing Xu, a Ph.D. student at UC Berkeley and lead author of the paper, took these different behaviors into account when creating the software package that compares animal tracking data with fence maps. The program can categorize different types of behaviors that animals might engage in when they encounter a fence, such as quickly crossing over the fence, pacing back and forth along the fence, or turning around and walking away from the fence.
To understand how fences are impacting mule deer and pronghorn, Xu started by painstakingly comparing fencing maps from the federal Bureau of Land Management and the U.S. Forest Service with satellite imagery, adding in fences that were not included in the government surveys. When all the fences were accounted for, Xu was surprised at the sheer amount of fencing in the region.
“The total length of fences is really, really striking, especially with what we know about the different types of wide-ranging animals that live in that area,” Xu said.
Xu then compared these maps to GPS tracking data that collected locations every two hours for 24 tagged female mule deer and 24 pronghorn antelope.
Each year, mule deer encountered fences an average of 119 times, Xu found. Pronghorn antelope encountered fences at more than twice that rate, about 248 times per year. About 40% of these fence encounters resulted in a change in the animals’ behavior.