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Wildlife migration is a natural phenomena that has been occurring for eons. Thousands of animals move across the landscape to find food or water, or to escape changes in weather like snow or drought. Think elephants, caribou, gazelle, wildebeest, or even closer to home- bears, elk, and pronghorn. Pronghorn and elk migrate between summer and winter ranges; grizzly bears travel from berry patches in valleys to white bark pine groves atop mountains; young wolverines set out from their maternal home range to find a territory of their own.
Recently this necessary, old as god ecological process has been interrupted by (drumroll here) humans. Historically, some migratory species have been over-hunted to the point of extinction. Others have been dramatically reduced in number, and then contained, to be found only in intensely-managed wildlife preserves or zoos. Think Bison. Or Bighorn sheep isolated on the Kofa National Wildlife Refuge in Arizona....
And in the past few decades, the last remaining free-ranging herds of wild animals in the West have been facing more and more threats. Roads, railroads, fences, dams, agricultural fields, housing developments, and most recently- energy development are some of the major threats to migratory routes. The oil and gas fields near Pinedale, Wyoming are turning the migratory routes into an obstacle course for pronghorn. This ongoing large-scale habitat conversion threatens to stop migration all together if we don't do something.
Their inability to jump fences makes matters worse for pronghorn. Although pronghorn seldom jump fences, they can duck under certain types—the Wyoming Game and Fish Department recommends fences with an unbarbed bottom strand at least 16 inches above the ground—but few fences are put up to accommodate pronghorn. Fences that parallel highways cut deep into pronghorn habitat.
In Wyoming Wildlife Conservation Society has assembled one of the largest and longest-term data sets on pronghorn. As winter regresses in Wyoming’s plains, buttes and dunes, thousands of pronghorn move north across the Red Desert and the Green River Basin to the Gros Ventre Mountains and as far as Grand Teton National Park, a migratory route that can extend almost 170 miles. Joel Berger has been studying the pronghorn's movement both with modern GPS technology and through documenting pre-historic evidence of their movements. Such data are becoming ever more crucial as natural gas development burgeons in the Green River Basin and the Red Desert, putting the squeeze on pronghorn migratory routes.
Photo caption: The migration corridor between GTNP and winter ranges in the upper Green River basin of Wyoming. Dots reflect ca 11450 points of 10 colour coded adult female pronghorn. Insets (a–c) reflect geographical bottlenecks. Map published in Connecting the dots: an invariant migration corridor links the Holocene to the present, Berger et al 2006Because an entire population accesses a national park (Grand Teton) by passage through bottlenecks as narrow as 121 meters, any obstruction to movement will ultimately result in the extinction of this population. The most endangered bottleneck, Trappers Point, is a few miles west of Pinedale, where twice yearly several thousand migrating pronghorn funnel through a mile-wide slot between the rivers—a bottleneck that has been effectively narrowed to just a half mile by houses, dirt roads, fences, a livestock holding pen and U.S. Highway 191.
The hope lies in protecting wildlife corridors that connect these animals to their seasonal habitats.
At the 2009 Joint USGS-SCB conference in Flagstaff, Joel Berger will present on what it takes to conserve a migration corridor.
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