In response to a 2009 study detailing pronghorn antelope migration patterns south of the Pioneer Mountains, the National Park Service has been creating more animal-friendly fences across the northern boundary of Craters of the Moon National Monument.
The three-year study was conducted by the Lava Lake Institute for Science & Conservation, which is connected to Lava Lake Land & Livestock, a large sheep-ranching operation adjacent to the monument near Carey.
The study showed that several hundred pronghorn spend their summers in the Pioneer foothills, then migrate east along the northern edge of the monument every fall on their way to lands surrounding the Idaho National Laboratory east of Arco, where they gather with pronghorn arriving from other areas. From there, they disperse to even farther locales, some crossing the Continental Divide to winter in southwestern Montana.
The migration pattern is reversed in the spring, though the study did not determine whether individual animals return to the same summering areas.
“Those animals a month ago may have been 160 miles away,” institute President Brian Bean said. “Landscape-scale conservation is needed to protect this keystone species.”
Bean said tracking of radio-collared animals showed the migration corridor narrows to only about 20 yards along the northern part of Craters of the Moon. He said obstacles there could be disastrous for the local pronghorn population.
John Apel, integrated resource program manager at the monument, said motion-triggered cameras showed almost 500 pronghorn passing through that corridor in the spring.
“We knew that pronghorn migrated through, but [until the study] it wasn’t known how many or that they followed a pretty well-defined route,” he said.
Pronghorn are built for speed—up to 55 mph for short distances—and, after cheetahs, are considered the second-fastest animals on earth. However, that built-to-run body structure does not allow them to jump high, and, unlike deer, they usually duck under fences rather than jump over.
Apel said the Park Service is undertaking a project to modify eight miles of fencing along the northern border of the monument, adjacent to grazing allotments on BLM land. He said the allotments used to be grazed by sheep, and the fences included a lower mesh component to keep the sheep in. He said only cattle are grazed there now, and the mesh is no longer needed.
Apel said the new 40-inch-high fencing consists of a smooth bottom wire to allow pronghorn to pass under and a smooth top wire to allow deer to jump over, with barbed wire in between. He said 50-foot sections of the fence, placed at intervals, can be dropped to create open corridors. Those can be used in the spring before cattle are run on the allotment.
He said the fence is also being marked with white tabs to alert the pronghorn of its presence. Bean said that when they’re running fast, the animals sometimes don’t see fences and can die after colliding with them. Apel said the tabs also benefit sage grouse.
Apel said about four miles of the old fence has been replaced, and the goal is to complete the project by 2016.
Bean said the 2009 study showed that fences are a widespread problem for pronghorn. He said radio-collar data showed animals migrating as far as Interstate 15 in southern Montana, but turning back when they were blocked by fences along the highway. He said the same problem exists along I-15 near Dubois, Idaho, and along U.S. Highway 93 in northern Nevada.
Bean said a similar study of pronghorn that spend summers in Jackson, Wyo., showed that two of seven migration corridors there had been abandoned due to fences.
He suggested that fencing projects such as the one being undertaken at Craters be considered for other areas.
“Things like this can make sure that these large, wide-ranging ungulates can do these migrations for another thousand years,” he said.