For years, a male mountain lion ruled the Santa Monica Mountains in southern California. Dubbed P1 by scientists, he feasted on deer and sired 22 cubs. But P1's kingdom was surrounded by freeways and suburbs, and troubles arose. The details of inbreeding and murderous conflict are revealed online this week in Current Biology. Charting the ups and downs of P1 and his kin, scientists have discovered a steep decline in genetic diversity. Eventually, they say, planners will need to create a corridor to connect this small group of predators to other populations of mountain lions—if they want the population to survive.
It's rare to have a large carnivore living in a megacity. Like other top predators, mountain lions (Puma concolor) need a large range—about 1100 square kilometers for a healthy population to persist. That makes life for mountain lions in the Santa Monica Mountains, which span 660 square kilometers, a bit cramped. In the second half of the 20th century, the habitat became cut off from mountains to the north. The study is "dramatically documenting impacts of isolation by highways," says Paul Beier of Northern Arizona University in Flagstaff, who has studied mountain lions in the Santa Ana Mountains, to the east.
The isolation of the Santa Monica population was probably gradual, starting with the construction of the Ventura Freeway in the 1950s. Housing developments on either side of the freeway reduced the amount of habitat that the cats could easily travel through. When researchers started studying coyotes and bobcats in the Santa Monica Mountains National Recreation Area in 1996, they weren't even certain mountain lions still existed in the park.
In 2002, a remote camera snapped a picture of P1. "It’s amazing that we even have a large carnivore," says Seth Riley, a wildlife ecologist with the National Park Service (NPS). Riley and other NPS staff captured P1 and other mountain lions by setting foot snares (the latest versions send a text message when tripped) and fitted them with GPS collars. Evolutionary biologist Robert Wayne of the University of California, Los Angeles, and colleagues analyzed DNA from samples of blood, as well as fur and scat left on trails. The DNA revealed low genetic diversity, compared with other populations, although there aren't any apparent physical problems. Among large carnivores in North America, only Florida panthers are in worse shape.
The genetic analysis also revealed that P1 was the top cat. For years, he alone mated with two adult females in the park. The problem was the lack of room for his offspring. Juvenile mountain lions must disperse from their birthplace, because their fathers monopolize the females. As far as researchers know, only one juvenile has escaped. In February 2012, researchers received a photo of a young male in Griffin Park, a 26-square-kilometer urban refuge to the east. After he was trapped, genetic analysis suggested he had come from the Santa Monica Mountains, traveling about 30 kilometers and crossing Interstate 405 and the Hollywood Freeway. "Now he’s stuck in this tiny dead-end home range with no breeding opportunities," Riley says.
Back in the Santa Monica Mountains, P1 had snapped. He killed one of his sons, and, for no apparent reason, a daughter as well. Then, even more inexplicably, he killed one of his own mates. "It doesn’t seem like a good strategy," says Riley, who is baffled. In 2009, the researchers discovered another worrying development: P1 had bred with one of his daughter, reducing genetic diversity.
The good news is that one mountain lion has crossed into the Santa Monica Mountains, although it may have spelled the end for P1. In 2008, NPS caught a young male, which they named P12, just north of the Ventura Freeway. They gave him a tracking collar, and in February 2009, they saw that he crossed the freeway in the middle of the night. (It's not clear that P12 dashed across the freeway; he may have taken the one underpass.) Not long after that, P1's collar was discovered near a ranch. The researchers noticed rocks nearby, with blood from an unidentified panther, so the collar may have come off in a fight. He hasn't been seen since. By that time, P1 was old and it's possible that P12 subsequently killed him, Riley says.
P12 brought fresh blood to the population. He had eight offspring, passing on genes from the north. Unfortunately, P12 has also mated with one of his daughters. The lesson is that just a single immigrant can rapidly improve the gene pool, but that further inbreeding will erode that progress just as quickly. "The idea is not novel, but the dramatic confirmation is a wake-up call," Beier told Science in an e-mail.
It's not clear how long the mountain lions can persist by themselves in the park. A postdoc has joined Riley and Wayne to analyze the viability of the population. Ultimately, these mountain lions will need better access to other habitat and individuals. (Moving mountain lions into the park is a last-ditch option, Riley says.) After a young mountain lion was killed by traffic, the California Department of Transportation added new fencing to funnel animals toward the road that crosses under the freeway, but Riley says it's not a good passage for mountain lions. "In the long run, we need a tunnel or an overpass to be effective."