California's Big Snow Year Decimated Endangered Sierra Nevada Bighorn Sheep

Half of the endangered Sierra Nevada bighorn sheep tracked by scientists died during last season’s record-breaking winter, according to researchers interviewed by LAist.

Some sheep got trapped in avalanches, some died of starvation, and some were killed by mountain lions when the sheep were forced to move to lower elevations to look for food.

The population is now estimated at 360 sheep, a 40% decline from a year ago, according to Tom Stephenson, who heads the California Department of Fish and Wildlife's Sierra Nevada Bighorn Sheep Recovery Program. That estimate includes new lambs that were born in the spring.

Some herds, including two of the three living in Yosemite National Park, have been mostly or perhaps entirely wiped out.

During their summer field surveys, researchers found just one live ewe, the term for adult female bighorns, from Yosemite's Mount Gibbs herd. Last year there were 20.

They found no live ewes from Yosemite's Cathedral Range herd, which they say leaves little chance the herd can recover naturally.

"It's frustrating," Stephenson said. "I've spent a lot of my career trying to get this animal to recovery. And when you think you have it figured out, you'll get a curveball thrown at you like a winter as extreme as the one we just had."

The extent of the devastation is evident in the field notes recorded since the beginning of 2023 and posted on the recovery program's website:

• Taboose Creek herd, Feb. 1: "All collared ewes are believed to be dead from heavy snows this winter."
• Mount Williamson herd, Feb. 23: "4 bighorn have been killed by lion predation since January, and as many as 6 others may have died due to heavy snows this winter."
• Sawmill Canyon herd, April 16: "7 bighorn are known to have been killed by lions in Sawmill Canyon since January, including a ewe that was captured 4 days prior to lion predation."
• Laurel Creek herd, May 27: "All collared animals are believed to have died this past winter and spring during heavy snows."
• Big Arroyo herd, May 27: "All collared animals are believed to have died this winter, during heavy snows."

State and federal wildlife officials have managed Sierra bighorn herds since the 1970s in an effort to re-establish a healthy population. The sheep were listed as an endangered species in 1999.

(

Courtesy: Sierra Nevada Bighorn Sheep Recovery Program

/

CA Department of Fish & Wildlife

)

Other Sierra Nevada herds fared better, including Yosemite's Mount Warren herd. In the Mount Baxter herd, which is the largest, near the eastern Sierra town of Independence, researchers observed 75 sheep over the summer.

Researchers try to keep tracking collars on about one-third of all female ewes in order to gauge a herd's health and ability to reproduce. They generally collar fewer rams. Half of the collared animals died over the winter.

The bighorn recovery team also does winter and summer surveys of the areas occupied by the 14 bighorn herds they monitor, but some sheep territory is difficult to access. Researchers say it’s possible some sheep in the hardest-hit herds survived and simply couldn't be found.

"I don't want to let go of this idea that there still might be sheep out there," said Sarah Stock, a Yosemite National Park wildlife ecologist, referring to the decimated Cathedral Range herd. "But I am also realistic at the same time."

Brief history of the Sierra Nevada bighorn

The Sierra Nevada bighorn broke off from their desert bighorn cousins to form a distinct subspecies some 600,000 years ago, according to John Wehausen, who has been studying the Sierra sheep for close to 50 years. Wehausen now heads the Sierra Nevada Bighorn Sheep Foundation.

Sierra Nevada bighorn sheep often feed in meadows at the bottom of rocky slopes and snowfields. Then, they'll work their way back up the slopes to spend the night at high elevations among the boulders.

(

Bernd Zeugswetter

/

LAist

)

Up until European settlers came 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.

But European settlement was devastating for the sheep, mostly because imported domestic sheep passed on bacteria to the native bighorns that caused fatal respiratory diseases. The bighorns had no immunity.

That led to many decades of decline. 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.

Since then, state and federal wildlife officials have tried to carefully manage the herds. They’ve transported ewes and rams from healthy herds to augment smaller ones, and started new herds in other parts of the sheep's historic range.

Bighorn sheep once occupied a large swath of the Sierra Nevada, from the Yosemite region south to Mount Whitney and the high slopes of Sequoia National Park.

(

Courtesy: Sierra Nevada Bighorn Sheep Recovery Program

/

CA Department of Fish & Wildlife

)

In March 2015, for example, in a celebrated event, 10 ewes and three rams were airlifted to Yosemite's Cathedral Range to start a herd there. (Stock, the Yosemite wildlife ecologist, gave a passionate TEDx Talk about the release in 2016.)

At that time, in 2016, the Sierra Nevada bighorn population numbered over 600 animals — healthy enough for Stephenson to think the animal might be downlisted from endangered to threatened within the next five years.

Now, that seems like a distant goal.

In a recent talk for the Yosemite Forum lecture series, Wehausen noted that after 37 years of trying to re-establish a healthy bighorn sheep population in the Yosemite area, "we are exactly where we started," he said. "It seems like it's a juncture and we should ask some hard questions about what we're doing here."

Why did the bighorn have such a tough winter?

The naturalist John Muir wrote in 1894 that bighorn sheep "ranks highest among the animal mountaineers of the Sierra."

Wheeler Ridge, where one of the bighorn herds lives, during the heavy 2022-2023 winter.

(

Bernd Zeugswetter

/

LAist

)

The padding on their hooves grips surfaces like a rock climbing shoe. Their ultra-powerful legs can propel them up near-vertical slopes at high speed. Their keen eyesight helps them spot food and predators across the vast expanses of the Sierra.

Their bodies are equipped to spend winters in freezing conditions at elevations above 11,000 feet. The species has survived at least three ice ages (Wehausen says new evidence shows they've survived six).

So why was this winter so tough on them?

First off, the amount of snow was exceptional, at least in recent history. "Last winter was the largest in 23 years," said Karl Rittger, a research scientist at the University of Colorado's Institute of Arctic and Alpine Research.

A total of 11 atmospheric rivers hit California, dumping two to four times the average precipitation in the central and southern Sierra Nevada mountains, according to the institute's data.

Researchers had documented heavy losses of bighorn in the winter of 2016-2017 and, to a lesser extent, 2018-2019. But deaths this year were the highest since species recovery efforts began.

Stephenson said the Sierra Nevada had likely experienced storms of similar intensity at some point in history, and the bighorn species persisted. But because the population is now so small, it's difficult to sustain major losses.

"This is a good example of how much effort needs to go into recovering an endangered species once a population declines," he said.

A smaller population also means less genetic diversity that might help sheep weather future storms, said Dani Berger, a Ph.D student at Utah State University who studies the way Sierra Nevada bighorns handle snow.

"This is a game of small numbers and in small numbers you're vulnerable to things like unexpected changes in the environment," she said.

Sierra Nevada bighorn sheep rely on their excellent eyesight to spot approaching predators and sources of fodder.

(

Bernd Zeugswetter

/

LAist

)

Migration routes disrupted

Another potential reason the Sierra bighorns fared so poorly this year is because they lack generational knowledge of migration routes that might've helped them find food and escape avalanche-prone areas. Some of the herds that experienced the biggest losses this year were relatively new to their area — within the last 10 years — either because they moved there on their own or because researchers transported them there.

"Those historic migration patterns would have given them many more opportunities to move to lower elevations where they would be much less impacted by these severe winters," Stephenson said.

Still, Berger noted, in many parts of the Sierra, the winter was so snowy, including at lower elevations, that the sheep had no good options.

"If you stay up high, you might die in an avalanche and might not have food. Or you can come down to lower elevations and risk being eaten by a mountain lion," Berger said. "It's hard to say which is the bigger threat, but it seems this winter, snow was definitely the bigger threat."

Mountain lions threaten recovery

In late August, the Los Angeles Times reported on the Sierra Nevada Bighorn Sheep Recovery Program’s failed effort several years ago to relocate two male mountain lions to a desert mountain range to prevent them from preying on the bighorn sheep.

One of the lions had killed more than 10 Sierra bighorns, Stephenson said, "so we really couldn't just let it remain within the recovery area and continue to eat bighorn sheep."

Mating season for bighorn sheep takes place in the fall. Rams compete to mate with the females, including by slamming their heads together.

(

Bernd Zeugswetter

/

LAist

)

The hope, Stephenson said, was that moving the lions to a mountain range in the Mojave National Preserve — with a large stretch of desert between them and the Sierra Nevada — would prevent the lions from trying to return.

But they did try. One starved to death in the desert while the other was so emaciated that it had to be euthanized. Mountain lion advocates, referenced in the L.A. Times piece, said the effort caused the animals unnecessary suffering.

After the L.A. Times and several other media outlets published articles, Stephenson said he got threats from angered readers and calls for his resignation.

Stephenson said he felt the story failed to explain the dire straits faced by Sierra bighorn and his program's longer-term efforts to manage the directly competing interests of the sheep and lions.

When the Sierra bighorns were listed as endangered, mountain lions were recognized as a major threat to their recovery. In the early years of the recovery program, mountain lions known to target bighorn sheep were regularly euthanized — sometimes several of them each year, he said.

Stephenson said the program has since shifted toward relocating mountain lions to areas where they have abundant deer and other prey. They've learned that it's much easier to relocate female lions than males "because [males] have such a strong homing instinct and they want to return," he said.

"We're obviously trying to run this recovery program as responsibly as possible and we aren't trying to eliminate lions from the eastern Sierra," Stephenson said. He added that the local mountain lion population "is still incredibly healthy."

Historically, grizzly bears and wolves that once lived in the Sierra Nevada might have kept the mountain lion population in check.

"We know that wolves preyed on mountain lions," Stock said. "So our predator-prey system is kind of out of whack. And that could be part of the issue that we're seeing play out right now."

Wehausen sees a glimmer of hope in a new pack of gray wolves that made an appearance this summer in Sequoia National Forest.

What climate change might mean for Sierra bighorn

The upside of all that winter precipitation was plentiful summer forage for the surviving sheep and their new lambs. LAist joined researchers on a recent hike to look for bighorn on the backside of Wheeler Ridge, northwest of Bishop. A group of around 10 sheep — ewes, rams and yearlings — nibbled grass at the bottom of a snowfield.

Two of the rams occasionally butted heads, a sign that mating season is starting.

Tom Stephenson has headed up the Sierra bighorn recovery program since 2008.

(

Bernd Zeugswetter

/

LAist

)

Some of the sheep wore tracking collars with a colored tag to make them easier to spot among the tan and beige boulders that provide near perfect camouflage. Almost all had bulging bellies.

"They're really fat," Stephenson said, peering through binoculars. "They look superb."

Whether that translates to healthier herd numbers next year might depend on whether the sheep face another harsh winter.

As the climate changes, the Sierra Nevada is expected to experience less snow, on average, but also more intense storms. If temperatures are warmer, more water makes its way into the atmosphere through a process called evapotranspiration, Rittger, the snow expert, explained.

"There's more water in the atmosphere for it to sort of drop onto the Sierra Nevada so you can get these big, extreme storms and we just happened to get a bunch of those last year," Rittger said.

More intense storms could be especially hard on herds that live in the typically snowiest parts of the Sierra Nevada and where it's harder to find paths down and out of the snow. This includes the northern herds around Yosemite.

Wehausen told LAist that during his decades of work trying to recover bighorn sheep, he's "been surprised at lots of things that slapped us in the face," including last season’s devastating winter.

He has come to think that saving the species will require much longer-term thinking — figuring out how best to help them withstand the still-uncertain future climate.

"They're really a magnificent animal living in a magnificent mountain range," Wehausen said. "I think it's worth the effort to get them to the next glacial period."