Why there's new hope for Sierra Nevada bighorn sheep to make a comeback

Scientists say the population of endangered Sierra Nevada bighorn sheep appears to have stabilized after suffering devastating losses during the record-breaking winter of 2022-2023.

Tom Stephenson, who leads the Sierra Nevada Bighorn Sheep Recovery Program, said the number of ewes, or female sheep, counted this summer is about the same as the previous summer.

Plus, he said, the abundant summer forage helped produce a “good crop” of new lambs — almost 100. And at least some ewes, which are critical for population growth, were spotted in herds that researchers previously feared might have been wiped out.

“Very small numbers,” Stephenson said of those herds. “Nevertheless, even if these populations hang on with a small number of animals, there’s a chance they can rebuild.”

Stephenson estimates the current total population of Sierra Nevada bighorn sheep at between 380 and 400 animals.

An average winter is good for bighorn

Sierra Nevada bighorn sheep are well-adapted to snowy winters at high altitudes, Stephenson said. But several above-average snow years over the past decade have seriously hampered the species’ recovery.

In the summer of 2023, researchers found that half of the bighorn sheep that were being tracked had died in the previous winter. Most were victims of avalanches, starvation, or mountain lions.

Even with new lambs born that summer, the population dropped by 40%.

But this past winter, the snowpack was average, which “for bighorn sheep isn’t bad,” Stephenson said. “They’re well adapted to a good amount of snow.”

Still, overall the population of Sierra Nevada bighorn is about half what it was at the peak of the recovery program, in 2016.

How did Sierra Nevada bighorn become endangered?

Before European settlers arrived in the 1700s and 1800s, thousands of bighorn sheep are thought to have occupied the Sierra Nevada, from the Yosemite region south to Mount Whitney and the high slopes of Sequoia National Park.

Domestic sheep brought by Europeans passed on disease to the native bighorns, which had no immunity. By the mid-1990s, there were only about 100 Sierra Nevada bighorn sheep left.

Conservationists ramped up efforts to save the species, and they were listed as endangered by the state and federal government in 1999.

How the bighorn could recover

Two major factors affecting the Sierra Nevada bighorn recovery are climate and predation by mountain lions. As the climate changes, the Sierra Nevada is expected to experience less snow, on average, but also more intense storms.

Both extremes are problematic for the sheep: too much snow means it’s harder for the animals to find food and more likely they’ll get trapped in avalanches, or move to lower elevations where they’re more vulnerable to becoming a mountain lion’s meal.

Too little snow produces less fodder in the summer to feed new lambs and fatten up their parents to withstand the next winter.

Rather than fret about the unpredictable weather, Stephenson said he’s more focused on the other threat — mountain lions. “When it comes to lion management, we do have an ability to influence that,” he said.

In the early years of the recovery program, mountain lions known to target bighorn sheep were regularly euthanized. That approach became politically unpopular — mountain lions are beloved in California — and the program has since shifted toward relocating lions that prey on bighorns to areas outside of the recovery zones.

But the process of getting permission to relocate a mountain lion can be cumbersome, Stephenson said. “If we could take those actions as soon as we see a threat, that would be a huge benefit to the [bighorn] population,” he said.