shadows in the sagebrush
Mountain Lions, Mustangs, and the Myth of the Predatorless Problem
Amidst the tranquil expanse of California's Mono Lake Basin, a burgeoning conflict unfolds between the influx of wild horses, land use, and the delicate ecological balance. Our team's recent assignment to this area reveals a common complex interplay of natural and human-influenced dynamics.
The wild horses have been observed to be moving west into the Basin and toward Mono Lake — in fact, a number of them have taken shelter at the ecologically unique lake since this past winter. Where the mustangs are believed to originate is a BLM-designated wild horse habitat called the Montgomery Pass Herd Management area, almost 50,000 acres that straddle California and Nevada due east of where greater numbers are currently roaming.
The present concern as stated by the Bureau of Land Management and Inyo National Forest is that the wild horses are out of their designated area, overpopulated, and causing damage to the landscape.
The refrain about overpopulation and destruction from federal agencies isn’t unique. But this herd is. Unlike most in BLM-managed areas, now confronts its first government-led removal in decades—a testament to the wonder of their own natural ecosystem, but also a harbinger of challenges ahead. While they’ve historically evaded helicopter roundups and bait trap removals, they haven’t — until recently — evaded mountain lions.
“For many years, 20% to 30% of the foals were being taken [by mountain lions],” Professor John Turner said in an interview for the SF Chronicle about the Montgomery Pass wild horse influx.
Today, mountain lion populations are in decline across the Western United States, a multifaceted issue involving various factors such as urbanization, high-speed roadways, human-wildlife conflicts, permitted hunting in some states, and livestock ranching across public lands.
In the shadow of human expansion, these silent predators of the west, continue their age-old nightly hunt. They’re opportunists, killing the food source available to them, whether large ungulates in the backcountry or household pets left unattended in suburbs. With them, the mountain lions bring ecological balance, or threat, depending on perspective. Their natural role is increasingly entangled with human-induced changes to the landscape.
“Wild horses have no natural predators.”
You can do a quick internet search and you’ll find the top results lead you to this statement on just about any official government website. It’s a pervasive narrative now so entrenched in conservation conversation, so influential in environmental policy, that few headlines (or journalists writing headlines) seem to question it.
And while some scientific research has concluded that mountain lions do not — at least in recent history — have a significant impact on wild horse populations, the significance between their potential impact from their current impact on prey species cannot be disentangled from human-induced impacts on the cougars themselves. This begs consideration for a reframe: Is it that wild horses have no natural predators, or is it that our society and government have hushed the ecosystem-balancing power of mountain lions to prioritize sprawl, convenience, and commercial interests?
Of the data showing mountain lion predation as a method of significant wild horse population control, Turner co-authored a pivotal study conducted in the Montgomery Pass Wild Horse Territory (MPWHT), along the California-Nevada border, which provides critical insights into the dynamics between mountain lions and wild horses. Spanning 11 years (1987-1997), the study focused on the numbers, productivity, and survivorship of the feral horse population in the MPWHT, primarily situated in pinyon-juniper woodlands.
The study conclusively found that the resident mountain lion population significantly influenced the number of horses in the MPWHT, primarily through the predation of foals.
Influence of predation by mountain lions: Key Findings and Data Points
Population Dynamics: The adult horse population in the MPWHT averaged 150 individuals, with a noticeable decline observed throughout the study period.
Mountain Lion Predation: Despite mule deer being their primary prey, mountain lions extensively preyed on horse foals in the MPWHT. The average annual foal mortality due to mountain lion predation was 13.5, representing 41.5% of the foals produced.
Lion Population Changes: The mountain lion population within the study area fluctuated, starting at 4 to 5 from 1987 through 1991, peaking at 8 in 1992, and then gradually declining through 1996, reducing to 3 lions in 1997.
Correlation with Lion Numbers: The increase in foal survival in the latter part of the study, especially in 1997, correlated with a substantial decrease in the number of lions.
Recent Evidence backs prior research
Southeast of the Mono Basin, a small herd of wild horses roam Arizona’s Tonto National Forest at the Salt River. The herd population is effectively managed by the PZP vaccine, providing humane fertility control for wild mares. But for the viability of the herd, not every mare is vaccinated or boosted every year, which means a couple of foals are born to the Salt River herd most years. In early spring of this year, a five-month-old foal went missing. Mirabelle, as she was called Mirabelle by Salt River Wild Horse Management Group, was soon after found deceased. The SRWHMG later confirmed by clear evidence that she was killed by a mountain lion.
“We were not quite sure if we still had mountain lions anymore, but now we know for sure,” they wrote to their community online.
Findings of a 2021 study on mountain lions
Additional studies provide evidence that mountain lion predation on wild horses has been a significant ecological interaction in the western United States since Turner’s research in the late 1990’s. In a study out of the University of Nevada in Reno from 2009-2012, researchers monitored 21 (GPS)-collared cougars in the western Great Basin and eastern Sierra Nevada from 2009–2012 to see what they were hunting to eat.
“Taken together, our data suggest that cougars may be an effective predator of feral horses and that some of our previous assumptions about this relationship should be reevaluated and integrated into management and planning,” the research team concluded.
Key Findings from the University of Nevada, Reno
Wild horses accounted for nearly 60% of the diets of cougars in the Great Basin (13 horses recorded).
Ten cougars regularly consumed horses, and horses were the most abundant prey in the diet of 8 additional individuals in the Great Basin.
Cougars ate an average of one horse almost every two weeks in the Great Basin.
Mule deer comprised of the vast majority of the diet for cougars in the eastern Sierra Nevada reference group.
According to the study, “the probability of predation on horses compared to other prey species increased with elevation, horse density, and decreasing density of mule deer on the landscape, and was more likely to occur in sagebrush (Artemesia spp.) than in pinyon (Pinus monophylla)–juniper (Juniperus osteosperma) forests.”
Elevation and the change in landscape could be a factor in the long-running relationship between mountain lions and the Montgomery Pass wild horses.
“We’ve had a change in the vegetation conditions,” Steve Abele told us about the area the designated areas wild horses are migrating from. Abele is a seasoned wildlife biologist tasked over the last decade with sage grouse preservation in the Mono Basin.
“We’ve moved away from sagebrush into more of a Pinyon-Juniper woodland. It’s a mix of pinyon pine and western juniper. Those trees are more thirsty than sagebrush, and they start to affect the water supply to meadow systems. So now the horses are moving out of there and west into the Mono Basin, looking for what they want.”
At the time of this publication, Wild Narrative Project is in cooperative communication with Inyo National Forest, digging into any data available about changes in mountain lion activity in the National Forest and surrounding public land. But the information we’ve asked isn’t readily available to the public. We were asked for more time for the agency to provide responses on the “highly-sensitive topic”.
The highly sensitive topic: Public lands ranching
According to the American Wild Horse Campaign, a wild horse preservation organization, mountain lion populations are kept unnaturally low in areas where wild horses are designated to roam because federal agencies permit the killing of mountain lions for preying on livestock that graze the same areas.
The Montgomery Pass Wild Horse Management Area and the public lands due west where the horses have migrated are no exception, as a significant portion of the public acreage in the area is shared with livestock grazing seasonally on government-permitted allotments.
The California Department of Fish and Wildlife Services will issue a permit to kill a mountain lion that they’ve determined is responsible for killing domestic animals, including livestock and pets.
truth to chew on
As we delve deeper into the dynamics near Montgomery Pass, the evidence — from both scientific study and boots on the ground — shreds any notion that wild horses have no natural predators. The evidence reminds us of our modern, human-centric impact on this ancient and ongoing dance between predator and prey.
Just a week ago, in fact, the Salt River Wild Horse Management Group in Arizona confirmed news that one of the newer Salt River wild foals recently perished due to a mountain lion.
On the group’s Instagram announcement of the foal’s death, a follower asked, “...do they kill them just to kill or actually for food?”
The SRWHMG replied, “They don’t kill just to kill. They are trying to survive and the wild horses are part of the circle of life.”
Sources:
https://wildlife.onlinelibrary.wiley.com/doi/abs/10.1002/jwmg.22087
https://mountainlion.org/about-mountain-lions/frequently-asked-questions/
https://mountainlion.org/2020/12/02/defining-depredation-in-california/
https://www.blm.gov/programs/wild-horse-and-burro/herd-management/herd-management-areas/nevada/montgomery-pass