The Bay Area Critical Linkages map from the Conservation Lands Network shows critical habitat linkages (in solid green) and highway barriers to wildlife movement (in red). Image courtesy

Animals are on the move. Whether we are aware of their movements or not, wildlife migrates by day and by night to hunt, mate, forage, and flee danger. We have a wealth of open space here in the Bay Area, and that helps to provide animals room to roam. But those open spaces are often bisected by our built environments, the deadliest of which is roads. One way to protect the movement — and the very lives — of animals is by protecting and providing safe wildlife corridors.

One of the first steps in planning wildlife corridors is to better understand the migration patterns of animals, and that is not always an easy task. Maps can help us understand, but first the data has to be collected from a collared animal, citizen scientist platform, genetic analysis, or even roadkill. “All these things can be mapped,” said Dan Rademacher, director of GreenInfo Network, a nonprofit that supports public interest groups through information technology and mapmaking. “Maps help us to understand complicated spatial patterns because they distill complexity into a visual picture that can be taken in at a glance. They can also help us see what is special about an animal through a mapped abstraction of their movement over time.”

Consider the mountain lions in the Santa Cruz Mountains, a range and a bioregion that runs from the San Francisco Peninsula to the Central Coast. Biologists estimate that there are from 30 to 60 mountain lions that live and roam in this span. The males cover territories up to about 50 miles, while females tend to keep to smaller areas. According to Julie Andersen, a resource specialist with the Midpeninsula Regional Open Space District, every road that runs east to west over the Santa Cruz mountains creates a deadly obstacle for mountain lions and other animals. About one mountain lion a year is killed on California State Route 17, a four-lane highway that runs from San Jose to Santa Cruz. With its heavy commuter traffic, steep terrain, blind curves, and animals on the move, it is one of the most dangerous highways in the state.

It is also a formidable roadblock to animals that would otherwise move across protected lands on both sides of the highway.

An extremely helpful way to see the tracks of a mountain lion’s home range is with the Puma Tracker, an interactive map by the Santa Cruz Puma Project. You can click on a mountain lion that was been collared and watch its movement over time. By using this tool you can see how some lions approach SR-17 repeatedly, but turn around as if they have reached a wall. You can also see that several have gotten across. “They are the lucky ones,” said Andersen. “Most of them don’t make it.”

Help is on the way for these lions trying to cross the road. The first wildlife corridor ever to be built on SR-17 will break ground in 2021. The Land Trust of Santa Cruz County, the county Regional Transportation Commission, and Caltrans are building a tunnel below the highway at Laurel Curve three miles south of Summit Road. The $12 million project is slated to be finished by 2022, according to the Santa Cruz Sentinel.

Another crossing being planned is south of Los Gatos near the northern tip of the Lexington Reservoir. The Midpeninsula Regional Open Space District’s Highway 17 Wildlife and Trail Crossing is entering the environmental review process. When this $31 to $40 million project is complete it will include separate crossings for wildlife and people. The wildlife corridor — which will also be used by bobcats, deer, coyotes, and racoons — will tunnel below the roadway, while the pedestrian bridge will cross over in a different location. Both will connect the protected open spaces on either side. Two options for each crossing are being considered during the environmental review process, and when that has been completed in 2022 or early 2023, preferred options will be selected.

Biologists fear that without providing mountain lions with their traditional roaming range, the lions will lose their genetic diversity, and that could lead to the population’s collapse. “That’s what the lions in Southern California are facing,” said Andersen. “There’s inbreeding and fighting between males because they have nowhere to disperse that isn’t occupied.” The lions’ plight is serious enough that the California Fish and Game Commission is currently considering a petition to protect mountain lions in some regions under the California Endangered Species Act, including those in the Santa Cruz Mountains.

“None of these crossings alone will solve the problem, but when you start to have more connections in more places, you have widespread connectivity,” said Andersen. “That’s what provides a long-term, sustainable future for the wildlife, while allowing us to have a wide-ranging highway system that connects us without impacting wildlife.”

Corridors that allow the movements of animals come in different shapes and sizes: The East Bay Regional Park District has protected breeding California newts for over 20 years by closing South Park Drive in Tilden Park during the rainy season; the River Otter Ecology Project works with agencies to enable otters to make the crossings they need to access foraging and nesting areas; and Sonoma Land Trust launched the Sonoma Valley Wildlife Corridor Project through which they protect land and implement other strategies such as monitoring, meeting with private landowners, and capturing movement with remote cameras.

A wildlife corridor of another kind is taking shape in the 17,000-acre Yolo Bypass Wildlife Area. Situated just east of Davis along I-80, this recreational destination lies within the Yolo Bypass, a 59,000-acre floodway that protects the state capitol and other communities when the Sacramento River reaches capacity. The wildlife area is a working landscape with rice and grazing leases, and is home to 200 species of birds and an array of other animals.

The bypass doesn’t have a lot of cover or shelter for the animals, and when the water starts to rise, they need to move. To provide safe passage for them to escape the slow-moving floods, the Yolo County Resource Conservation District is working on a pair of corridors, according to YCRCD Director Heather Nichols. Shielded with new vegetation, the corridors will serve coyotes, mule deer, foxes, and racoons, and improve habitat for migratory songbirds such as golden-crowned sparrows.

Two separate corridors will provide about five miles of cover. Staff and community members have already planted native grasses, gum plant, coyote brush, American dogwood, and narrow-leaf milkwood along the corridors. One corridor will be on the ground, and the other will follow the historic Lisbon Trestle, which was once part of a rail system connecting Oakland and Sacramento. The trestle corridor can be likened to New York’s Highline Park, but for wildlife.

Our understanding of wildlife movement has grown with technology and data tracking, and we can use that knowledge to keep wildlife on the move. The more corridors we can protect the better it will be for biodiversity and genetic diversity. And yet, no matter how much we learn about animals and their movements, they will retain a sense of mystery. “There’s still the unknown right under our noses, with animals moving through our cities and parks and wild spaces in ways that we can never completely understand,” said Rademacher. “There will never be a complete map of every animal movement all the time, and that’s a good thing.”

The goal of corridors is to protect animal populations, but there is also something in it for us. Just thinking about animals moving across the landscape sparks our imagination and adds to the awe of life.

Aleta George covers open space for the Monitor.