Humans Put Squeeze on Ancient Pronghorn Migration

Nicholas Bakalar
for National Geographic News

July 19, 2006

Pronghorn—antelopelike animals native to the United
States—have followed the same migration routes through Wyoming's
Greater Yellowstone region for more than 6,000 years.

The animals' 100-mile (160-kilometer) seasonal journey is the longest land-mammal migration in the continental United States and is second only to the Arctic caribou's trek for long-distance migration in the Western Hemisphere.

But now the pronghorn's ancient routes between calving and wintering grounds are in danger from human development, and the future of Yellowstone's pronghorn herd is uncertain (pronghorn photo and profile).

Joel Berger, a senior scientist with the New York-based Wildlife Conservation Society, recently led a study of changes in the pronghorn's migration routes.

Using global positioning systems, Berger and colleagues tracked the movements of migrating pronghorn. They also estimated the maximum width of geographical bottlenecks along the animals' paths.

Their results, published in the latest issue of the journal Biology Letters, show that six of the eight historical pronghorn routes have vanished, and bottlenecks along the remaining routes have narrowed.

"The issue is critical," Berger said. "Any alteration in the bottlenecks through which pronghorn travel will only have serious negative effects and further squeeze the passage zones."

What's more, the scientists found that when the bottlenecks are blocked, the animals don't seek alternate routes—they just stop migrating.

Experts fear that if further development closes off these migration paths, it will interfere with the pronghorn's life cycle, eventually causing the species to disappear.

Narrowing Paths

Long-distance migrations are being disrupted all over the world, mainly due to explosive human population growth.

In Africa the great wildebeest migrations have ended within the last 40 years, the study authors write, partly due to fenced-in farms blocking their way.

In Mongolia railroad lines prevent gazelle migration, while North American highways act as a barrier for brown bears, and hydroelectric dams in Canada stymie woodland caribou.

For pronghorn, winter migrations take about two to three days with the animals traveling 30 miles (48 kilometers) in a day.

Spring migrations follow the melting snow and can take as long as a month for all the animals to finish the journey.

The pronghorn at one time had eight different migration routes that allowed them to travel from Grand Teton National Park to just north of Yellowstone National Park (take a virtual tour of Yellowstone).

Now only two of these routes remain, and both paths include bottlenecks that vary from 328 to 2,000 feet (100 to 610 meters) wide.

One of these, the bottleneck at Trapper's Point, was once 6,562 feet (2,000 meters) wide before development slimmed it down.

Oil Spread

Takehito Ito is a scientist at the Arid Land Research Center at Tottori University in Japan who was not involved in the study.

Narrow bottlenecks "are obviously the priority areas for conservation of the pronghorn population," Ito said. "But the area is outside of the national park."

This study, he continued, "shows the inadequacy of current conservation strategies and the necessity for larger protected areas."

The spread of the oil and gas industries in the region is a looming conservation issue, WCS's Berger says. (Read about the natural gas boom in the U.S. West in National Geographic magazine [July 2005].)

"The rate of housing increases at the southern terminus of the migration route is in direct proportion to the number of gas wells that are being drilled," Berger said.

"With more people, there is more habitat loss, and unless some safeguards are put in place, Wyoming's wildlife will lose."

But the problem is not difficult to solve, Berger says: People need to protect and conserve the land along the pronghorn's path.

"When migration routes are not predictable, it's less clear what land to protect," he said.

"But in the case of the pronghorn it's easy, since the route is invariant and apparently has been that way for some 6,000 years."

Editor's Note: Joel Berger has received a number of research grants from the National Geographic Society. National Geographic News is part of the Society.

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