Freeways, rat poison affecting bobcats' genetic diversity in LA

Freeways and other human activity and urban development have restricted genetic diversity among populations of bobcats in and around the Santa Monica Mountains, a new study reveals.

The study published earlier this year in the journal “Evolutionary Applications” shows that the 101 and 405 freeways act as barriers to the movements of bobcats and their genes.

“We knew from some previous work that 101 was a barrier. Now we’ve seen that the 405 is also a significant barrier, not just to movement but also to gene flow,” said Seth Riley, a wildlife ecologist with the National Park Service and an author on the paper.

The freeways have effectively divided bobcats into three genetically distinct groups.

• The group with the largest contiguous area lies west of the 405 Freeway and extends to Point Mugu.
• The 101 Freeway separates it from another population to the north.
• The population with the smallest geographical parcel resides east of the 405, in the Hollywood Hills and Griffith Park.

Bobcats in the Santa Monica Mountains have been studied for nearly 20 years. That continuous stretch of research into wildlife near urban areas has afforded a better understanding of trends within the population of the wide-ranging carnivore.

Notably, researchers were able to observe the impacts of a devastating bout of mange that ran from 2002-2005. Mange is caused by mites, which are believed to persist in the environment but only cause disease when other factors impact the host's immunity. 

The disease, which in this case has been linked to rodenticide exposure, was so powerful that at its peak, it killed about 70 percent of monitored bobcats.

That culling led to more genetic restriction within the fragmented populations. The study showed that an epizootic — a widespread outbreak of disease within an animal population — can cause a genetic bottleneck within a single generation of bobcats.

It also showed, however, that natural selection works to keep variability within some genes.  Riley said that genes relating to animal immunity were kept at higher levels of diversity.

“We’re seeing evidence of natural selection where, because these genes are important for immune function, diversity is maintained even though the population drops down to very few individuals,” Riley said. ”This is the first opportunity we’ve had to really look in more detail at genes that matter, genes that actually are important for the biology and the health of the animals.”

Riley said that the finding was only possible because the bobcat population has been studied for so long.

“Especially in terms of things like how the population is changing over time, and what are some of the effects of things like freeways and disease and contaminants, you just would never see those things if you were not able to do such a long-term study,” Riley said.

Riley said that the population does appear to be recovering from the outbreak and that some areas that lost all bobcats are now host to reproducing females.

Still, he said that continuing usage of rodenticides means that another outbreak of disease could further restrict the bobcats’ genetic diversity. 

“Depending on how much rodenticide use there is, the disease epizootic could come back at some point,” Riley said. “There have been some changes in policy relative to the use of the contaminants, so we’ll see how much of an effect those have.”