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CODY, Wyo. — Over the past six years, biologists studying a herd of pronghorn known for its arduous fall migration have tracked the herd’s population and where the animals are spending their winter.

What they’re finding has ecologists concerned that the world-famous “Path of the Pronghorn” may be in jeopardy, a result of habitat fragmentation and a growing number of obstacles interfering with their journey.

“At this point, we haven’t seen any differences in population,” said Jon Beckmann, a leading scientist for the Wildlife Conservation Society’s North America Program. “But we are starting to see the pronghorn abandon their winter range. They’re beginning to shift their use.”

Beckmann is conducting one of several studies focused on the pronghorn and their long migration through the Greater Yellowstone Ecosystem. He said oil and gas development in the herd’s winter range is quickly reducing the habitat upon which the animals depend for survival.

He’s particularly concerned about gas development in the Pinedale Anticline Project Area, or PAPA, located in the Upper Green River Basin. Well pad construction in the area has reduced available habitat by 23 percent since 2006, while road construction has reduced it by another 5 percent over the same time, according to a 2009 report.

The decline in habitat, Beckman said, has reduced the area’s mule deer population by as much as 47 percent. Unless the pace of gas development is slowed, he said, a decline in pronghorn may not be far behind, and their famed migration out of Grand Teton National Park could be disrupted.

“Places where we used to have pronghorn in high numbers are shifting to the periphery of these gas fields,” Beckmann said. “Pretty soon, we’ll start seeing the demographic effects of that.

“We’re trying to get the Bureau of Land Management and these energy companies to understand that, as they’re fragmenting this habitat, it’s having an effect on the wildlife.”

More development

A supplemental environmental impact study released by the Bureau of Land Management in 2009 suggests that gas development in the PAPA area will probably continue through 2023.

The work includes the construction of new well pads, the expansion of existing well pads and the construction of new roads and pipelines. With the infrastructure in place, and with a 40-year production life, the drilling is expected to last through 2065.

With the development set to move forward and become a part of the landscape for the next half-century, the BLM has launched a study of its own to monitor the gas field’s effects on the wildlife population.

Therese Hartman, with the BLM field office in Pinedale, said that while the study is only in its second year, it also has recorded a decline in the area’s mule deer population.

But the data on pronghorn, she said, remains inconclusive, and it doesn’t yet meet the “mitigation matrix” set by the BLM.

Changing the trends

“We are seeing that the pronghorn are using the habitat differently, but what that means we don’t know yet,” Hartman said. “Mule deer have already met that threshold, and now we’re looking for ways to mitigate that and change that trend.”

The mitigation matrix developed by the BLM was designed to respond to “emerging and undesirable changes” in wildlife population numbers around the gas field. In the PAPA area, a matrix has been established for mule deer, pronghorn and sage grouse.

For mule deer and pronghorn, the BLM said, mitigation efforts kick in when scientists document a 15 percent population decline in any given year. For grouse, once recommended for listing as an endangered species, mitigation begins when scientists observe a 30 percent decline in the number of active leks, sites where males strut for the attention of females.

In January, the BLM further clarified its rule, saying that habitat loss alone can’t trigger any mitigation action. Only changes in population can trigger action, the ruling said.

“With pronghorn, we don’t know what we don’t know yet,” Hartman said. “We’re just in the beginning of that study.”

The Path of the Pronghorn remains the second-longest terrestrial migration of any animal in the Western Hemisphere, second only to the caribou of Alaska.

It’s a journey that begins in the sagebrush flats around Grand Teton National Park. As winter approaches, the herd moves east over rugged terrain, seeking a windswept thrust of land in the Upper Green River Basin.

As the crow flies, it’s about 120 miles, though the route is much longer on the ground. Archaeological evidence suggests they’ve been taking this route for more than 6,000 years.

Steve Cain, senior wildlife biologist for Grand Teton National Park, is working on a separate study that’s also looking for possible changes in the area’s pronghorn population and migration patterns.

The study, which includes the National Park Service, Wyoming Game and Fish and the Wildlife Conservation Society, is only in its second year, following up on an earlier study that took place back in 2004.

“From a National Park Service perspective, we’re very concerned about the maintenance of that migration corridor and the permeability of it for pronghorn,” Cain said. “It’s essential to the long-term existence of pronghorn in Grand Teton and the Jackson area.”

Cain’s study, “Path of the Pronghorn: Understanding Migration, Population Dynamics and Predator-Prey Systems in a Complex Environment,” is younger than the study conducted by Beckmann.

About 2004, Cain said, they began tracking 10 pronghorn from Grand Teton with GPS technology. The current cycle is tracking 30 animals, looking for trends in a larger population sample.

“So far, we haven’t noticed any obvious deleterious effects on their migration,” Cain said. “Pronghorn play an important ecological role in Grand Teton. They’re a sagebrush grassland animal, and they fill a niche in that area that’s not really filled by any other particular species.”

While several studies about the pronghorn are going on, biologists agree that the nature of the Upper Green River Basin makes it prized winter range, free of snow with plenty of forage for the animals to survive the season.

Pronghorn lose body mass all winter as it is, Beckmann said, and, while they’re able to slow the loss, pushing the animals to another range could disrupt nature’s balance.

“If you put them in a situation where they’re in 2 feet of snow all winter, they’ll burn that many more calories trying to dig and eat,” Beckman said. “That will hasten their decline in body mass, and it could lead to a population decline.”

While the Green River mesa provides good winter range, the geology underfoot also makes it mineral-rich. Natural gas sits locked in shale buried 13,000 feet below ground, and it’s now accessible to new drilling techniques.

The high interest in the PAPA area has already turned it into one of the fastest growing gas fields in the country. Well densities have increased over the years, and the BLM has already approved more for the future.

The PAPA area covers 198,000 acres, though just 6 percent of that, or about 12,000 acres, is proposed for continued development, the BLM said.

Gas production could be as high as 21 trillion cubic feet. It’s enough, energy companies say, to heat 12.5 million homes for 20 years. Wyoming stands to gain $8 million in royalties.

As Beckmann notes, the state is at a crossroads when trying to manage its vast petroleum reserves, world-class wildlife and the habitat on which it depends for survival. Decisions made today could have long-lasting consequences, he said.

“Mule deer have already been documented to have crossed that threshold, and the population is starting to collapse,” Beckmann said. “Pronghorn are a little more mobile and it takes longer for the impacts to show up, but we’re starting to see the precursors.”

Contact Martin Kidston at or 307-527-7250.