When a mountain lion in Los Angeles dies, it's most likely at the hands (or paws) of another mountain lion. When the otherwise solitary cats must contend with a cramped habitat hemmed in by freeways, aggressive, often deathly, interactions are a certainty. But the second most frequent cause of death for mountain lions in Los Angeles is cars. And that's not just true for mountain lions. Just about every species, both native and introduced, that calls California home turns up as roadkill from time to time.
For the last seven years, more than 1,000 volunteers have submitted more than 50,000 observations of roadkill to something called the California Roadkill Observation System, also appropriately known as CROS. The largest data collection effort in the world devoted to roadkill, CROS is run by the Road Ecology Center at UC Davis, which is itself managed by a biologist named Fraser Shilling. It might seem like a fairly macabre pastime, but Shilling points out that while the volunteers who contribute to the database come from a wide range of backgrounds, all have something in common. "They think that collecting information about harm that people cause the environment can help reduce the harm."
Wildlife collisions aren't just bad for the animals of course. When an animal – especially a large one – enters a road or highway, it poses a hazard to drivers, who might crash into the animal or who might swerve and get in an accident anyway. In addition to the property damage from these accidents, there's also the threat of injury or death.
According to one of Shilling's recent analyses, there were nearly 6,000 car accidents that involved wildlife in California between February of 2015 and February of 2016. According to his calculations, they came at a cost of some $225 million dollars. More than nine of every ten incidents involved deer, with coyotes, black bears, mountain lions, and others filling in the rest.
While just about every road, street, and highway winds up with the carcasses of dead bugs, birds, reptiles, amphibians, or mammals on them (though CROS only tracks vertebrates, not insects or other invertebrates), some spots are worse than others. Thanks to the massive amounts of data that Shilling and his colleagues have amassed, they can use powerful statistical modeling techniques to pinpoint roadkill hotspots. These are places that have a disproportionate number of dead animals compared to other stretches of highway.
For 2016, those hotspots included the area where Routes 1 and 101 meet north of the Golden Gate Bridge, for example, two stretches of Interstate 280 near San Mateo and Palo Alto, the coastline near Santa Rosa, and so on.
There are strategies for mitigating those hotspots. In some places, wildlife bridges like the one planned for Liberty Canyon in Agoura Hills are best. But these come with a hefty price tag, and aren't necessarily viable options everywhere or for every species. All told, the Liberty Canyon crossing could cost as much as $100 million dollars.
The most common strategy in California involves building tunnels or culverts underneath roadways. There's just one problem. "We find that most wildlife do not want to go through tunnels," says Shilling. Only a select few species are willing to use them, he explains. So while there may indeed be deer or bears on both sides of a highway, they could remain genetically isolated from each other. "Wildlife will approach roads and will die on the surface, which challenges the idea of building these wildlife tunnels," he adds.
And then, of course, the animals have to actually find those crossings. Larger mammals like coyotes or bobcats have large enough ranges that they could find the over- and under-passes we've built to provide them safe transit pathways. But smaller mammals, along with all the reptiles and amphibians, don't move around all that much. Finding a safe crossing is more a question of luck than intelligence.
There is another strategy though for minimizing the impact of roads on wildlife. When wild animals become roadkill, it's usually because of speed. A car barrels down a highway so fast that by the time an animal has registered the threat it's too late. Or a driver cruises along so rapidly that by the time he or she spots an animal, there isn't time to brake safely.
"The bottom line is that one of the cheapest solutions for roadkill is having reasonable speed limits and enforcing them," says Shilling. "In most places it's tried, people are responsive and there are way fewer collisions."
While Los Angeles sees plenty of roadkill, there are lots more data available in CROS for Northern California and the Central Valley, making identifying hotspots in the Southland more of a challenge. Indeed, CROS only covers some 5 to 10 percent of California's roadways.
Still, the scant data that are available paint a bloody picture. Much of LA's share of the 405 is covered in roadkill data, as are State Routes 27 and 23 between the 101 and the ocean. That should serve as no surprise, since both roads cut straight through the Santa Monica Mountains NRA. Another local roadkill cluster occurs along the 2 and 210 just south of the Angeles National Forest. And in Orange County, the 74 and 241 are death traps for animals, likely because they're so close to a wide expanse of green space in the Santa Anas. Apparently, animals living along the edge of these protected areas occasionally venture too far into the developed landscape and quickly get ground into the pavement.
To enhance the database, Shilling is embarking on a grand mission to sample every road in the state, and he's planning and working with an army of volunteer cyclists to do it. "Literally every mile of every road in California, four times a year," he says. In the meantime, we can all do our part. As with most other citizen science efforts, all you need is a GPS-enabled smartphone with a camera. You can easily enter your data on the CROS website.