By Katharine Gammon, The Guardian
A wildlife crossing across the 101 freeway will connect two parts of the Santa Monica mountains for animals.
Above the whirring of 300,000 cars each day on Los Angeles’s 101 freeway, an ambitious project is taking shape. The Wallis Annenberg wildlife crossing is the largest wildlife bridge in the world at 210ft long and 174ft wide, and this week it’s had help taking shape: soil.
“This is the soul of the project,” says Beth Pratt, the regional executive director, California, at the National Wildlife Federation, who has worked on making the crossing become a reality over the last 13 years. She says she’s seen many milestones, like the 26m pounds of concrete poured to create the structure, but this one is special.
“To be able to put my hand in that soil and toss it on and know that we’ll be putting milkweed plants that will flourish for monarch butterflies, or picturing the first mountain lion paw print on that soil,” she says, fills her with hope. “It is wonderful to watch this habitat take shape.”
The plot is a native wildlife habitat that connects two parts of the Santa Monica mountain range, with the hopes of saving creatures – from the famous local mountain lions, down to frogs and insects – from being crushed by cars on one of the nation’s busiest roadways.
With nearly an acre of local plants on either side and thick vegetated sound walls 12ft high to dampen light and noise for nocturnal animals as they slip across, it’s an unprecedented feat of engineering. Imagination, too.
The project began in 2022 through a public-private partnership that brought together many organizations to cover the $92m in costs, according to Caltrans, the state transportation department. Research shows that wildlife crossings save money because it limits animal interactions with vehicles.
“As soil gets placed over the bridge, we’re one step closer to reconnecting wildlife with habitat that’s been divided for generations,” said California’s governor, Gavin Newsom, in a statement. “We’re not only making habitats whole again, we’re making our roads safer.”
Most wildlife bridges and underpasses around the world are made of cement and steel. But this one is designed to seamlessly glide into the environment on both sides – as well as send a message to the humans who pass under it each day. After all, it was humans who sliced the habitat into pieces, segmenting wildlife into small islands with lines of concrete and asphalt. Now, humans can be part of the solution.
The detail in the project reflects years of expert consultants in research and design. The soil itself is engineered – a combination of light rocks, horticultural subsoil and topsoil, enriched with compost. Then it gets some additions to match the biology of local soils, so the bridge can match the environment around it. That means cultivating mycorrhizal fungi, bacteria and other microorganisms, and then reintroducing those to the soil profile into the plants.
Doing so creates a “living and breathing portion of the soil layer that supports the plant material”, says Robert Rock, a landscape architect with Living Futures in Chicago who led the design.
Covering the entire surface of the crossing will require approximately 6,000 cubic yards of soil and will take several weeks to complete. After the soil is in place, workers will plant 5,000 native plants, including sages, buckwheat, milkweed, sunflowers, deerweed, penstemon, toyon and laurel sumac. As long as the weather cooperates, officials say that they can begin planting in May.
The project has its own native plant nursery, funded by the National Wildlife Federation, which has shepherded the hand collection of more than 1.1m seeds of 30 different hyper-local species from the five miles around the site, growing plants in 1-gallon pots. Rock says there are thousands of different plants in the nursery – but the next phase will include 10 times that many.
When it comes to pressures from fires or rain events, Rock says that leaning into nature is the best way to handle a changing climate. By using native plants – and keeping out invasives such as mustard – the area will be more resilient to any climate onslaughts. When the project is completed, stormwater from the structure will be piped back into the landscape.
The project aims to complete its work by the end of 2026.
The crossing is not just a structure – it is also a symbol of the connection between humans and their wildlife, even in one of the most urban areas in the country. “It’s not just a bridge that cars drive over,” says Pratt. “This is a real extension of the Santa Monica mountains ecosystem. And to me, that’s hopeful – even a freeway is redeemable.”