America’s pronghorn migration faces human obstacles

When you think of the planet’s greatest migrations, perhaps you think of the annual trek of the wildebeest through Africa’s Mara ecosystem, or the salty trails of the sperm whales, oceanic giants who feed in the waters of the frigid poles but mate in the warm tropics. Maybe you imagine the four generations it takes for Monarch butterflies to complete just a single migration cycle from their wintering sites in Mexico northward towards Canada and back.

But there is another, perhaps lesser-known epic migration taking place through the American Midwest. While the monarchs are flapping their way across the continent eating up all the milkweed they can find, the pronghorn move through Montana, Idaho, and Wyoming. The Greater Yellowstone Ecosystem’s “Path of the Pronghorn” is one of the last of America’s terrestrial migrations, but it’s being threatened by natural gas-related development, along with the familiar problems of fencing and roads.

To see how the increasing specter of natural gas development might affect the pronghorns’ 170-mile migration pathway, researcher Renee G. Seidler of the Wildlife Conservation Society, together with colleagues from Princeton University, the University of Montana, and the Idaho Department of Fish and Game, outfitted 250 female pronghorn with GPS collars during the winters of 2005 through 2009.

For five consecutive springtime migrations, the researchers were able to identify the paths and stopping points for the herds of pronghorn antelopes as they moved from the colder north to the more agreeable south. When they combined the GPS data with detailed satellite imagery and two gas fields in development, they were able to see which anthropogenic factors were most important in affecting the pronghorns’ migration. If the ungulates are unable – or unwilling – to migrate, then they might wind up being excluded from their northernmost range entirely, limited to just their southern territory in and around Grand Teton National Park.

Typically, pronghorn select their stopover sites for their nutritional benefit. That’s important, since they spend more than three quarters of their time during the spring migration at stopover sites. These are sites with optimal, high-quality plants to eat. But the researchers discovered that their stopover sites were instead determined by potentially life-threatening impediments.

For example, those pronghorn that migrated along US route 191 through Antelope Alley had to contend with high traffic on the highway, impenetrable fences, and land development. At one point they were restricted to a bottleneck just 1500 meters wide. Prior to crossing the highway, they chose stopover sites that did not have optimal food. They entirely avoided shrub habitat, their main source of nutrition, during their approach to the highway, and began eating well again only after the crossing.

By contrast, those pronghorn that migrated along US route 189 through Trapper’s Point experienced lower traffic, and wildlife-friendly fences through which the animals could pass. This pathway was also farther away from natural gas development. As a result, these animals were able to forage for high quality foods throughout their migration.

These findings carry three important implications with them. One is that management plans that only account for traditional variables like the nutritional qualify of plants will lead to insufficient or unsuccessful outcomes, because they won’t include the effects of anthropogenic factors on pronghorn behavior. “Highways with dense traffic and housing developments might present obvious obstacles in need of mitigation,” said Seidler in an official statement, “but in some areas, addressing threats requires more than simple observation and intuition.”

The second is that under- or over-passes should be constructed to facilitate better movement of animals across highways. It isn’t just that highways make collisions more likely, which is the obvious problem. The deeper issue, which is far subtler, is that the stress of crossing a highway actually modifies the animals’ behavior. Even if they survive the crossing, they might wind up with less energy or more susceptible to illness or injury because of poorer nutritional intake.

Certainly stress is a part of life; another ungulate migration, the wildebeests’, involves crossing the Mara river and dealing with the toothy jaws of African crocodiles. But that’s a stressor that the wildebeest have evolved to deal with. Human-related stressors, however, are ones for which the pronghorn are simply unprepared.

The third is that impenetrable fencing ought to be replaced with more wildlife-friendly varieties. If the pronghorn are unable to diffuse across such barriers with ease, then they risk genetic isolation and possible inbreeding.

As state and federal governments consider proposals for natural gas development in the area, officials will be best served by a better, more thorough understanding of the factors that influence the pronghorns’ ability to journey through their habitat. Only by combining protected areas with wildlife connectivity strategies – highway overpasses and friendly fencing – will we be able to ensure the future migration of North America’s fastest land mammal. – Jason G. Goldman | 01 October 2014

Source: Seidler R.G., Joel Berger, Scott Bergen & Jon P. Beckmann (2014). Identifying Impediments to Long-Distance Mammal Migrations, Conservation Biology, DOI: http://dx.doi.org/10.1111/cobi.12376

Header image: Pronghorn antelope in the upper Green River Basin, by Jeff Burrell/Wildlife Conservation Society.

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