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Jackson, Wyo. • The population of non-native mountain goats is growing prolifically in the Teton Range in Wyoming, while the number of native bighorn sheep is in noticeable decline, a biologist says.
Counting bighorn sheep from a helicopter over the past three years, Wyoming Game and Fish Department biologist Aly Courtemanch has tallied no more than 57 bighorns in the Tetons, a considerable drop from counts of 96 in 2008 and 81 in 2010.
"We've always said we believe the population was stable at about 100 to 125 sheep, but it seems like these recent counts indicate that it's dipped," Courtemanch said.
Factoring in sheep that she missed from the helicopter, Courtemanch figures there's likely 80 or so sheep inhabiting the Tetons.
Meanwhile, an aerial count found 43 mountain goats — an exotic species introduced by Idaho decades ago to be hunted.
"The fact that we saw almost as many goats as we saw sheep is concerning," Courtemanch told the Jackson Hole News & Guide (http://bit.ly/2oxzt2g ).
Bighorn sheep and mountain goats inhabit the West, including Colorado, Montana and Idaho, but mountain goats generally live in different terrain than bighorn sheep.
However, in the Tetons, one goat was seen within a couple hundred yards of the sheep, Courtemanch said.
The closeness is worrisome for managers because Teton Range goats have tested positive for strains of bacterial pathogens that can be deadly in bighorn, triggering potentially catastrophic pneumonia outbreaks.
Transmission among the two species has not been documented, nor is their relationship to one another well understood, at least in the Tetons.
Grand Teton National Park has been working on a management plan to rid the Tetons of mountain goats, but wildlife biologist Sarah Dewey said she couldn't say right now when the plan might be completed.
Courtemanch said more research in the years ahead will reveal whether invading mountain goats are partly to blame for the bighorn decline.
"We don't know if the drop in sheep numbers we're seeing is a direct effect of the mountain goats being there," said Courtemanch, who studied the herd for her University of Wyoming master's thesis. "There's a lot of pressures on that sheep population, and mountain goats might be one of those."