Here's How to Design a Wildlife Crossing That Wildlife Will Actually Use

Roll down your windows while cruising down the 101 through Agoura today, and you'll see a few one or two story buildings, brown rolling hills, and depending on the season, some green trees. But if everything goes according to plan, then by 2021 you'll be able to see a massive bridge arching over the highway, connecting restored habitat on either side of one of the nation's busiest roadways. The wildlife crossing at Liberty Canyon, when completed, promises to be the world's largest urban wildlife crossing.

Hailed as the only hope for the long-term survival of the mountain lion population in the Santa Monica Mountains, such a connector would not only allow young mountain lions born in the Santa Monicas the chance to get out before being killed by one of the more dominant, older males, but would also allow newcomers into the mountains, bringing much needed genetic diversity. Indeed, in 2013 a mountain lion from the San Gabriels survived crossing more than a dozen lanes of traffic in the greater LA area – including the 5, the 118, and the 101, but was stopped short by a 10 foot tall concrete retaining wall topped by a chain link fence. The cats can jump high, but not quite that high. With nowhere to go, the cougar was forced to turn around…and was almost immediately hit by a car.

Indeed, sometimes it's not the roads themselves that are problematic, but other constructed elements, like walls and fences. "Just because an area is paved doesn't mean that wildlife can't cross over it," says Frazier Haney, Conservation Director for the Mojave Desert Land Trust. But with roads come these other hazards.

As conservation biology has matured as a field of inquiry, researchers and activists have come to understand that protecting isolated landscapes in parks and preserves isn't enough. Wildlife need to move around, and animals like mountain lions or bison need safe passage to traverse hundreds of square miles. (Two adult male cougars live today in the Santa Monica Mountains, and that's actually a little crowded for them.) Not only do wild animals require high-quality intact patches of habitat; they also need the ability to travel between them, lest those habitat patches turn into islands.

Mountain lions aren't the only critters that would benefit from greater connectivity in the heavily urbanized Southland. Just about everything from butterflies to lizards to bobcats would. There's just one catch: building a connector isn't enough. The animals have to actually use it.

That's where architect Clark Stevens, Executive Officer for the Resource Conservation District of the Santa Monica Mountains, comes in. Designing an effective wildlife crossing requires that he implement not just expertise in engineering, but also in geology, botany, wildlife biology, and animal behavior.

The first step, according to Stevens, was choosing the right spot, and there were at least two choices. "One cuts through Ventura County along the 23, but that's not ideal because it's open," he explains. "There's not a lot of good cover, it's mostly agricultural." To the east, however, was the Liberty Canyon site, which was promising for two reasons.

First, data from wildlife cameras and collared animals showed that they were already using the area. It's the spot where the male mountain lion was killed in 2013, and it's also the spot where, in 2015, a female successfully emigrated out of the Santa Monicas.  "They're moving in there," says Stevens. "They're interested."

Second is the quality of the habitat on either side of the freeway. "The eastern connector runs [from the Santa Monicas] up through the Las Virgenes Open Space, a big patch of good habitat in the Santa Susana Field Lab, the Chatsworth-Simi Divide, and to the 118 where there's an existing crossing," says Stevens. Once there, an animal would be in the Santa Susana Mountains, just one step away from the cougars' promised land: Los Padres National Forest.

That linkage is relatively wide. An effective wildlife corridor isn't just a narrow strip of green between parks. "Really what we're talking about is linkages where you can feed and breed," says Stevens. Animals moving between protected habitats aren't simply on a conveyor belt.

And most animals aren't mountain lions. Lizards, snakes, squirrels, turtles, insects, and small birds all have different connectivity requirements than the big cats. A side-blotched lizard could spend its whole life on the crossing itself. But if there are enough lizards on the crossing, they could effectively transfer genes between the two sides.

The next consideration in designing a crossing is giving animals as many choices as possible. "It's a game of odds," Stevens says. If there are more ways to utilize the crossing – such as tunnels – then more animals will survive it. Some animals, especially prey animals like deer, will more likely opt to cross on top of the bridge. Predators like bobcats would be less scared by a long, dark tunnel.

Vegetation matters too. Long before the freeway was built, a creek ran through the canyon. Stevens plans on re-creating that riparian corridor. "You can create a scent trail that's creek-like with mulefat and some other plants," he says. Recreating the original habitat as much as possible will allow animals to approach the freeway as safely as possible. Combined with the strategic placement of wildlife-proof fences (which extend several feet underground to block burrowing critters) animals can be funneled away from other, dangerous parts of the freeway.

Then there's the geometry. Places where it is thought that lions have attempted crossing are places with both plentiful vegetative cover and a high vantage point, which allows the cats visual access to the opposite side. That's why Stevens plans to precisely slope the approaches on either side of the road to allow the lions an unobstructed view across.

In some sense, each wildlife crossing has to be uniquely designed, taking into account the particulars of the local landscape and the species targeted. But there are still patterns that apply more broadly. Bighorn sheep, for example, are hemmed in by freeways in the Mojave Desert, and long dark tunnels represent a tremendous risk for them. But that doesn't mean underpasses can't be designed to be sheep-friendly. "If sheep can see a lot of daylight on the other side of the undercrossing, then they're much more likely to use [it]," says MDLT's Haney.

So while animals can't read street signs, habitat connectors like the one proposed for Liberty Canyon can in fact be designed to increase the likelihood that they'll be used. Like people, animals each have their own particular personalities. But even they are not immune to the subtle psychological influences of clever architectural design.