Penned in by Freeways, L.A. Mountain Lions Are Becoming Isolated—and Aggressive

Mountain lions in the Santa Monica Mountains are killing each other and inbreeding at alarming rates.

Image
Flickr/National Park Service

An ugly family feud is unfolding within Los Angeles County that has nothing to do with the Kardashians. In fact, it concerns one of the city’s most interesting animal species.

Up in the picturesque Santa Monica Mountains just north of the city, the region’s mountain lion population is acting in ways that run counter to their survival. Male mountain lions are killing their spouses. Playful behavior among young cubs has turned violent. And mountain lion families are inbreeding at elevated rates— which can lead to offspring with cardiovascular and mental deficiencies. The social deterioration of L.A.’s mountain lion population, however, is not a natural progression. It appears to be a grim reaction to ever-encroaching human development.

A new study published by California’s National Park Service indicates genetic diversity is abnormally low among those living south of the 101 Freeway, one of the city’s busiest. Generally speaking, mountain lions are a highly mobile species. It’s not uncommon for the animals to defy state, or even regional boundaries in their pursuit to reproduce. So the lack of geographical diversity in the population in the Santa Monica Mountains is raising red flags. L.A.’s highway network, the study concludes, has essentially become an inescapable fence—dooming the city’s mountain lions to behave irrationally.

“It has taken time to document what the impacts of encirclement have been,” says Art E. Eck, a former superintendent with the National Park Service. According to Eck, now a preservation activist, debates were raging just 20 years ago about whether mountain lions even lived in the Santa Monica Mountains. A few would be spotted from time to time, Eck says, but many believed they were simply traveling through the area. But the mountain range indeed housed scores of mountain lions. Since their existence was largely unknown, however, the potential effects of highway development on the animals was “a real distant thought,” Eck says. Only now are the costs of their highway-driven isolation coming to light.

The colored dots represent areas where mountain lions live near the 101. (National Park Service)

In 2002, a research team from the National Park Service began a 10-year study on the migration of California mountain lions. At its conclusion, they estimated the genetic diversity of each mountain lion population. According to the report published today by the journal Current Biology, the genetic diversity of the Santa Monica mountain lions was at one point the second-lowest ever recorded in North America.  Even more shocking was the low diversity rate they reflected when compared to other mountain lion populations living near L.A.

The main reason? The Santa Monica mountain lions must cross the 101 if they wish to intermingle with other populations and breed—a nearly impossible feat given the 175,000 vehicles that travel the freeway each day.

Free to migrate, mountain lion populations living north of 101 are more genetically diverse than those in the Santa Monica Mountains. (National Park Service)

As the figure above shows, mountain lions live both north and south of the 101. The Santa Monica Mountain breed, however, is surrounded by mobility deterrents. The freeway is an obvious death trap for them. Move south and they end up at the Pacific Ocean. And to the west is busy interstate 405. This isolation has resulted in an estimated genetic diversity rate of 0.31, according to the study. Mountain lions living north of Freeway 101 on the other hand, in the Simi Hills west of L.A., have a genetic diversity of 0.48—the estimated average rate for all California mountain lions. Unable to mate outside of the Santa Monica Mountains, that population is forced into an atypical lifestyle dominated by their homogeneity. And the destabilizing effects are becoming apparent.

Under normal circumstances, young male lions leave their communities 12 to 18 months after their birth, says Seth Riley, one of the researchers behind the study. But the 101 is forcing them into a Malthusian state of survival. The male mountain lions are plentiful and agitated, but the spoils available are limited.

“It’s typically advantageous for male lions to fight, kill, or run off other males. But in a normal situation, every male disperses,” says Riley. Due to the isolating effect of the freeway, however, “what ends up happening is you have these interactions that shouldn’t be happening—fathers are killing their sons and vice versa,” he says.

In rare circumstances, a few mountain lions have successfully migrated out of the Santa Monica Mountains. Two years ago, one successfully crossed the freeway and ultimately settled in Griffith Park in the Hollywood Hills. Named P22 by researchers, the migrant mountain lion became a local hero. Twitter accounts and Facebook fan pages have popped up in his honor.

On the surface, P22’s escape from the Santa Monica Mountain may be read as an evolutionary success amid urban encroachment. Nevertheless, unless a lady mountain lion moves to the Hollywood Hills, P22 will be unable to reproduce.

The Santa Monica mountains lions remain wedged between a rock and a hard place: To stay put or meet P22's lonely fate? Both scenarios point to a difficult future.  

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