The most amazing thing about Los Angeles’ mountain lions may be that they exist at all in a sprawling city crisscrossed by freeways. Now a new study finds those highways have trapped a group of California’s top predators in the Santa Monica Mountains, putting their long-term survival in doubt.
“Freeways in the area are almost complete barriers to mountain lion movement,” according to the paper, which was published in the journal Current Biology.
Seth Riley of the National Park Service and his colleagues radio-tracked 26 mountain lions in the Santa Monica Mountains and five of the animals in the adjacent Simi Hills and Santa Susana Mountains between 2002 and 2012. DNA analysis showed that the Santa Monica mountain lions are suffering from low genetic diversity as a result of inbreeding and the inability of juveniles to move into new territories. “Not a single subadult mountain lion has successfully dispersed out of the Santa Monica Mountains,” Riley wrote.
Familiarity is breeding extreme contempt: One male killed its children, its brother, and a mate. Altogether, seven mountain lions died from what biologists call “intraspecific strife.” Others died in collisions with cars; police shot another one.
“We haven’t seen genetic defects yet,” Riley, a wildlife ecologist, said in phone interview. “If there really was no exchange of genetic material, it would be bad news in the long run. Eventually, you would see inbreeding effects and eventually lose the population.”
One adult mountain lion did escape from the 255-square-mile Santa Monica Mountains, making his way to the Hollywood Hills, where he lived in “the smallest annual range ever reported for an adult male.” Surrounded by roads and human development, the mountain lion designated as P22 led a lonely life, never finding a mate.
In February 2009, a young male called P12 braved the 101 freeway and crossed into the Santa Monica Mountains. He mated, and the genetic diversity of the local population, until then the second-lowest in North America, spiked.
So how can L.A.’s lions be saved?
In short, build more freeways.
Mountain lion freeways.
The researchers found that mountain lions were able to move between the Santa Susana Mountains and the Simi Hills by using a tunnel under a smaller freeway that was surrounded by vegetation. Radio-collared mountain lions crossed that highway 23 times, according to the study, which was conducted in collaboration with the University of California, Los Angeles, and the University of California, Davis.
Riley said the park service has been in discussions over the years with Caltrans, the state transportation agency, about building a tunnel under the 101 freeway or an overpass above it to allow mountain lions to travel between the Santa Monica Mountains and the Simi Hills and Santa Susana Mountains.
“Migration events between populations may not have to be frequent to maintain genetic diversity, and we have observed that one successful migrant can have a significant impact, especially in mountain lions, where individual males can have high reproductive success,” the study states. “In highly developed areas, the conservation of natural habitat on both sides of freeways and effective corridors across them or translocations may be necessary if large carnivores are to persist in proximity to the megacities of the future.”
If worse comes to worst, Riley said, wildlife officials would have to move the animals in and out of the Santa Monica Mountains to maintain genetic diversity.
"Our hope is not to do that intense type of manipulation," he said. "But we certainly would do that if we got desperate enough."