Struggling Wyoming pronghorn subject of major study

— January 7, 2014

They might not have the esteem of the grizzly bear, or the intrigue of the elusive big horn sheep, but pronghorn antelope are one of Wyoming’s iconic animals. They represent the sagebrush steppe ecosystem and lure hunters into the state. In southwestern Wyoming the animals are in trouble, said Tony Mong, a senior wildlife biologist with the Wyoming Game and Fish Department.

One of the traits that make the pronghorn so intriguing is its ability to adapt to harsh environments, living off sagebrush when other foods dwindle, and traveling long distances, said Jeffrey Beck, associate professor of wildlife habitat restoration ecology at the University of Wyoming.

Yet the resilient animals aren’t thriving like they have in the past. Beck and Mong are involved in a three-year study to understand why and to help guide management to help the herds.

Pronghorn populations fluctuate. Because most does have twins each year, with good weather and moisture, a herd can rebound quickly after years of high mortality. But that hasn’t been the case for pronghorn in the Bitter Creek and Baggs herds, which haven’t rebounded in 20 years. In the early 1990s there were as many 25,000 pronghorn antelope in the Bitter Creek Herd, Beck said. The population is now 8,000 to 10,000 — about half the management objective for the area, according to Mong.

The Baggs herd had about 14,000 animals in the early 1990s, Beck said. It now has about 6,000 to 8,000 animals, below the management objective of about 10,000 pronghorn, Mong said.

The herds experienced high losses in the particularly harsh winter of 2007-2008. High mortality wasn’t a surprise, the issue is that the herds should be rebounding from that winter by now, but haven’t.

The question is simply why, Mong said.

The study aiming to answer that question is a collaboration between the University of Wyoming, the Wyoming Game and Fish Department and the Bureau of Land Management. It’s believed to be one of the largest studies ever conducted on pronghorn. Researchers hope to tease out the impacts of environmental conditions and oil and gas development, Beck said.

Impacts of oil and gas development on pronghorn is poorly understood, according to Erik Molvar, wildlife biologist with WildEarth Guardians. Unlike sage grouse, and even more recently mule deer and elk, there are few completed studies looking specifically at pronghorn in oil and gas development areas. Molvar was not familiar with the ongoing study of the Baggs and Bitter Creek herds, but said it could help fulfill an important need for information. The radio collars can show how close the pronghorn come to developed areas, before they start avoiding habitat they would otherwise use, he said. In the early 2000s, a comprehensive model covering land from the Greater Yellowstone Ecosystem into Colorado and Utah showed winter habitat for mule deer and elk. Along the Atlantic Rim and in the Baggs area were the most important and endangered linkage for the entire ecosystem.

“This is a key area to examine wildlife migration and those migration corridors,” Molvar said.

In 2007 the Bureau of Land Management approved a large coal bed methane field in the Atlantic Rim area. That, and other oil and gas development, are having some impact on deer and elk, and likely pronghorn populations, too, Molvar said.

To tease out whether or not its oil and gas development, or environmental factors like drought, hurting the pronghorn populations,  its important the Baggs and Bitter Creek populations are compared to a herd in a similar ecosystem — but without the oil and gas development. In this case, researchers are looking to a separate herd in the Red Desert where there isn’t development. It also will look at how the Baggs and Bitter Creek herds respond to development-related infrastructure and try to pinpoint crucial winter range in south-central Wyoming, as well as determine if fencing and other movement barriers impede migration and habitat selection of the animals.

The goal of the study is to better understand what pressures are impacting herd health and then allow agencies to better manage for the population, whether it’s through permitting energy development in ways that make it more harmonious for the animals or managing hunting quotas and seasons.

Biologists captured 130 adult female pronghorn in mid-November using net guns and a helicopter. They were transported through the air to a mobile work station where each underwent tests for pregnancy, body condition and disease, as well as stress which can be measured through fecal samples. The animals were then fitted with collars to track movement.

The pronghorn also will be frequently monitored from the air.

Energy development is one of several factors scientists are looking at in hopes of pinpointing what is hindering the herds’ growth. Competition with other animals could also impact the herd. There’s a wild horse herd that also competes with resources with the pronghorn in the area, Mong said.
Several energy companies are helping fund the study, Beck said.

A graduate student is currently compiling the initial data gathered from the capture of the animals. This summer scientists will monitor the animals from the air and on the ground, counting fawns and mortalities and evaluating how the animals spend time — foraging, versus resting, versus staying vigilant.

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