Study finds Jackson bighorn sheep die-off when population hits 500

The Wyoming Game and Fish Department (WGFD) and the University of Wyoming (UW) launched a special study that has found a pattern within the Jackson bighorn sheep herd: once the population hits about 500, an “all ages pneumonia die-off” occurs, according to Buckrail. The finding is alarming, especially since the last helicopter survey tallied about 500 bighorn sheep.

“This is what’s worrying to us,” said Aly Courtemanch, a WGFD wildlife biologist. “It seems that whenever we get to 500 sheep, it’s followed by these [population] crashes.”

Typically, when bighorn sheep come out of their summer range, they have high fat reserves that they can live off of during the winter months. Between 2015 and 2019, body fat levels were “consistently high,” according to Courtemanch; however, over the last three years, biologists have discovered a “noticeable drop.”

“As this herd is growing, once we get around 500 sheep, we’re seeing this corresponding drop in body fat,” said Courtemanch. “That’s what we would expect for a population that’s reaching or exceeding its carrying capacity on the landscape. If history repeats itself, we’re setting ourselves up for another pneumonia crash.”

Pregnancy rates have also dropped to 40%, which is significantly lower than the 80% to 90% of years past.

“That’s shockingly low for an ungulate species,” said Courtemanch. “That’s another sign something is going on.”

When the bighorn sheep population “exceeds the habitat’s carrying capacity,” the result is reduced nutrition and “corresponding weakened body conditions” that make the animals “more susceptible to pneumonia,” according to Buckrail. To solve this, researchers suggest improving habitat through prescribed burns, mowing sagebrush and controlling invasive plants; however, these things are not allowed in wilderness areas where bighorn sheep spend the “majority of their time.” 

Other ways to manage the population is by keeping the herd at objective to avoid this level of habitat stress.

“Hunting is one of our main tools for managing populations, so we’re constantly trying to avoid these big increases in population so that we don’t have the big crash,” said Mark Gocke, WGFD public information specialist.

The WGFD/UW study has been on-going since 2015 and is the first of this caliber with regard to Rocky Mountain bighorn sheep.