A new study from the U.S. Geological Survey and the University of Wyoming shows that industrial energy development is negatively affecting mule deer in the West. In many western states, the health of migratory muley herds depends on their ability to move freely across large swaths of land to track the spring green-up — a phenomenon scientists call “green-wave surfing.” By timing their migration to coincide with fresh spring growth, the deer take advantage of the most nutritious forage available as they’re recovering from months of subsisting on sparse winter forage. 

The 14-year study examined what happens when oil and gas drilling takes place across the deer’s long-established migration routes. Researchers specifically examined the construction of coalbed-methane wells—a form of natural gas drilling—in the migration corridor of a mule deer herd southwest of Rawlins, Wyoming.

Using remote satellite imagery to examine how well the deer “surfed” along the corridor, researchers found that as drilling increased, the deer began to “hold up” when they approached the wells, likely stymied by the increased noise, traffic, and human presence. The herd did not appear to grow accustomed to activity, and small projects were just as disruptive as large projects. While the animals were eventually able to move past the wells and continue their journey, the delays were enough to throw off the timing of the migration. As a result, mule deer were eating plants after their nutritional value had passed its peak. Feeding on this “stale” forage meant that the animals missed out on the migration’s key function: gaining maximum nutrition. 

“Mule deer are known for how precisely they match their movements with spring green-up, so this result was particularly striking,” said Ellen Aikens, lead author and assistant leader of the USGS South Dakota Cooperative Fish and Wildlife Research Unit, in a press release. “The gas wells caused them to let the best food of the year slip away from them.”

The study results were published on October 6 in the science journal Nature Ecology and Evolution. The authors say that migration maps need to be factored into future development decisions.

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“This study makes clear that when development continues to erode the value of corridors, it could have long-term effects on these migrations. But it points to conservation solutions as well,” says co-author Matt Kauffman of the USGS Wyoming Cooperative Fish and Wildlife Research Unit at the University of Wyoming. “Once migration corridors have been mapped, development can be planned to minimize disruptions to migrating herds, whether in Wyoming, the American West, or wherever landscapes are rapidly changing.”

Mapping migration corridors has been a focus of many federal and state initiatives in the past five years, Kauffman says, including in Wyoming. Mule deer numbers have been declining across the West for years, and Wyoming is no exception. According to a 2018 report from the Wyoming Mule Deer Initiative, the state’s mule deer population peaked at about 578,000 in 1991. By 2016 numbers had declined more than 30 percent to 396,000 animals. Although mule deer populations are naturally susceptible to boom and bust cycles caused by a wide range of factors, scientists believe that habitat conditions play an outsize role in limiting their numbers. “Activities disturbing even a small portion of a herd’s seasonal range can have major population-level consequences,” reads the report.