Wyoming Pronghorn Bitter Creek Migration Routes

The Bitter Creek pronghorn herd inhabits south-central Wyoming, primarily to the south of Interstate 80. This herd unit includes the Great Divide Basin in the Red Desert Basin, characterized primarily by arid to semiarid sagebrush-steppe and harsh winters. Although the herd is generally remote, human development, such as oil and natural gas infrastructure, cattle and sheep ranching, and surface mining, affect pronghorn movement. Additionally, Interstate 80 functions as a large barrier to pronghorn movement, which motivated a GPS-collaring study (2017–20) to better understand the effect of Interstate 80 on pronghorn. Most pronghorn in this herd are residents, but some individuals have intermittent migrations or seasonal forays to the south in the winter (fig. 33). These migrations do not occur every year and seemingly depend on winter severity (in other words, facultative migrations). Such movements do not necessarily occur between distinct seasonal ranges, so boundaries between seasonal ranges are unclear. Pronghorn can migrate more than 75 mi (121 km), as far south as Sunbeam, Colorado, and the herd spends most of the year near Interstate 80. The GPS-collared pronghorn with the farthest recorded migration remained within 6.2 mi (10 km) of Interstate 80 for 85 percent of its collar life (3.5 years; B. Robb, University of Wyoming, written commun., 2024). However, the high traffic volume of Interstate 80, net-wired right-of-way fencing, and the Union Pacific railroad that partly parallels the interstate deter most pronghorn from crossing the interstate. Only one migratory pronghorn successfully crossed Interstate 80, near mile marker 161, which indicated significant effects of the interstate as a barrier on the Bitter Creek herd. Fencing around oil and natural gas infrastructure also creates barriers to movement and poses a direct mortality risk. These mapping layers show the location of the migration routes for pronghorn (Antilocapra americana) in the Bitter Creek population in Wyoming. They were developed from 29 migration sequences collected from a sample size of 6 animals comprising GPS locations collected every ~2 hours.

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