Southern California’s mountain lions face long-term survival challenges

Mountain lions are pretty, elusive, and prone to the occasional fatal attack on humans. But those in the Santa Monica Mountains of Southern California are also hemmed in by 10-lane freeways and sprawling suburban and rural development which keep them in their place. That isolation, scientists predict, will promote inbreeding and a loss of genetic diversity, meaning one day those lions may be gone.

Are the Mountain Lions in Santa Monica Gone?

In the case of the Santa Monica lions, “gone” has a more technical term: “genetic extinction vortex.” This term, coined in 1986, can be used to describe factors such as environmental, pollution, and habitat loss which result in increasingly smaller populations of a species and the loss of genetic diversity. Without genetic variation, a species cannot respond to changes and other variables, meaning that even while population numbers may seem healthy, extinction can be a real threat.

One 2016 study described the monitoring of more than 50 mountain lions in the Santa Monica Mountains and found that in a 14-year stretch, only one lion crossed US 101 to travel from the Simi Hills to the Santa Monica Mountains, and only two crossed the 101 in the opposite direction. That’s likely insufficient to maintain genetic diversity, and in this case, the migration had negligible effect—researchers found that the lone male who crossed US 101 into the Santa Monica Mountains wound up mating with two of his daughters and a granddaughter.

“As it stands, the genetic diversity of the Santa Monica Mountains population is among the lowest ever documented for the species,” the National Park Service noted. “Only the Florida panther population experienced lower genetic diversity, and nearly went extinct as a result.”

(Note: Panthers and mountain lions are the same species, although their size and appearance differences are the result of climate, geography, and diet.)

Researchers went on to model the persistence of lions in the Santa Monica Mountains if this isolation persists and put the chances of extinction at nearly 100% within 50 years.

Even without isolation, the mountan lion's species small size would also endanger it. The National Park Service estimates that just a few untimely mountain lion deaths—say, from car accidents or the accidental ingestion of rodenticide—could “disrupt breeding and lead the population into a downward spiral.”

On the other hand, just a small boost in connectivity between the Santa Monica Mountains lion population and those in adjoining areas would reverse this genetic depression.

“Making it possible for even just one mountain lion from elsewhere to join the population every two years, would bring extinction risk down dramatically to just 2.4%,” the park service noted. “The ability of young mountain lions to disperse to other areas would further help by reducing the chances of inbreeding.”

While even single-animal transplants can help sustain genetic diversity, it’s rarely a one-and-done proposition. As an article in Nature posited, “while translocations may introduce diversity, sustaining diversity in small and isolated populations will require either repeated translocations or restoration of landscape connectivity.”

The effects of genetic decline are being seen not only in California but everywhere that mountain lions live, according to a 2018 study. In that study, researchers “found strong geographical structure and signatures of severe inbreeding in all North America populations. Tracts of homozygosity were rarely shared among populations, suggesting that assisted gene flow would restore local genetic diversity.”

Mountain Lions in North and South America

Mountain lions once roamed over almost the entirety of North and South America, but their habitat in North America has since shrunk to occupy generally mountainous land generally west of the Continental Divide—with one anomalous colony, in southern Florida.

The Florida panther, which was declared endangered in 1967, had been reduced to a population of as little as 30 adults when wildlife officials hatched a plan to release eight female cougars from Texas in 1995, with another Texas cougar brought in every six years afterward. The slow influx, researchers hypothesized, would restore genetic variability without “significant alteration to its basic genetic makeup which may be adapted to local environmental conditions.”

It helped, but the genetic diversity problem has not been quashed, notes Genetic Engineering & Biotechnology News.

“The translocation of Texas cougars had boosted genetic diversity and health of Florida panthers,” the site noted. “The sequences also brought new insights: even after mixing in the Texas DNA, the Florida population remains closer to the genetic brink than previously thought.”

Genomic sequencing is helping biologists, land managers, and other wildlife officials better deal with at-risk animals and make more intelligent species and land management decisions.

“Now we can make more informed decisions,” noted Warren E. Johnson, PhD, a molecular ecologist affiliated with the Smithsonian Institution, in Genetic Engineering and Biotechnology News. “In the past, we made decisions based on limited genetic information.”