For humans, the Santa Monica Mountains can seem like a green escape from the concrete and traffic that go with urban living.
For mountain lions, they’re more like a giant zoo with deadly freeways for a cage.
A new study published on Thursday in the journal "Current Biology" shows that the nearly complete isolation of mountain lions in the Santa Monica Mountains has led to a host of negative consequences for the population.
“We see a number of things going on that we think are unusual and are associated with the fact that the population is constrained like this,” said Seth Riley, a wildlife ecologist for the National Park Service at the Santa Monica Mountains National Recreation Area.
Riley and other researchers ran genetic analyses on 42 lions in the mountains and found that the population had among the lowest genetic variation in the Western United States.
The scientists were also able build a family tree for the resident pumas using the data. Much of tree has doubled branches, where inbreeding has occurred. The mountains only provide enough habitat to support one or two male mountain lions.
“Individual adult males can really dominate the reproduction in the population,” Riley said.
All young males and half of young females normally disperse out of their birth ranges. The Santa Monica Mountains population is unable to, and as a result, a higher proportion of animals die in conflicts with other lions. The study shows that of seven known instances of intraspecific killings, five were carried out by male lions against their brothers, offspring or former mates.
“All of those things really don’t seem to make much sense in an evolutionary perspective. The whole goal is to pass your genes on to other generations,” Riley said.
Individual behaviors dominate
The study shows that the behavioral quirks of individuals can have a significant impact on such a small population.
Only one young lion has crossed into the Santa Monica Mountains in a decade and successfully reproduced. Analyses show that the male lion, designated P12, brought an injection of fresh genetic material into the area.
“The fact that P12 crossed and then became a dominant male, and then he transmitted all of this genetic material from the north into the Santa Monica Mountains, and so he really transformed the genetic structure of the population,” Riley said.
The benefit has been somewhat lessened as P12 has since mated with one of his daughters.
Riley said that the case of P12 shows that even occasional instances of lions crossing into the mountains could provide a way for a small population to continue existing there.
“It doesn’t have to be a lot. One per year, say, that ended up being a viable animal could probably be enough for a species like mountain lions, that have relatively low density, and one animal could make a big difference,” Riley said.
Even infrequent crossings may be a tall order, though. No lion has been known to successfully cross out of the Santa Monica Mountains.
Even the famed mountain lion P22 that likely crossed two busy freeways and is currently living in Griffith Park cannot be considered a success. He has the smallest range ever reported for an adult male.
“I guess he made it out of the Santa Monica Mountains, but not to a viable home range with any reproductive opportunities,” Riley said.
The new study suggests that direct human intervention could have a large benefit for the Santa Monica population of cougars. Those interventions include building a wildlife corridor across the 101 Freeway and physically relocating mountain across the freeway.
Last month, Caltrans applied for $2 million in federal funding to develop plans for a wildlife bridge over the 101 Freeway at Liberty Canyon.
Relocating lions is a deeply controversial measure, since introducing a foreign lion into an area is bound to spark conflicts. It was done for the Florida panther, when inbreeding threatened the subspecies’ reproductive success.
Riley said that though the National Park Service prefers to let natural processes occur, officials are considering all options to keep the local puma population healthy.
“It’s something that we hope not to have to get to, but it is something that they had to do in Florida to keep the Florida panthers going,” Riley said. “At some point, if you think that human-caused problems are significant enough, then maybe you have to take more direct management action.”