In the experimental world of animal infrastructure


But Banff’s wild crosswalks, like most, suffer from some sort of Horseless Transport Syndrome, their designs limited by existing infrastructure. Tunnels are often poorly adapted culverts, the (usually concrete) tubes that carry water under roads. And flyovers are generally borrowed from roads in bulk — they’re built as if they’re going to bear the weight of an 18-wheeler, then “top-dressed” with foliage, Lister says.


A scattering of experiments begins to rethink this model. One is the Wallis Annenberg Wildlife Crossing, the $90 million wildlife bridge under construction north of Los Angeles. Designed by architect Robert Rock, it eschews the vaulted arch of older bridges in favor of a vast flat expanse that requires only one column to support it between mountains and over a highway traversed by an estimated 300,000 cars every day. It’s the “poster child for innovation,” says Renee Callahan, executive director of ARC Solutions, a group investigating how to build better bridges for wildlife. “It’s literally designed for species from mountain lions to mule deer to deer mouse,” Callahan says. “They’re designing it all the way down — to literally the mycorrhizal layer, in terms of the soil, to make sure the soil itself has the fungal network that can support the native vegetation.”

There are many unknowns when construction will begin, not least how different species will react to the sheer number of vehicles passing underneath. The National Park Service will monitor activity on the bridge, as well as DNA profiles of animals on either side of the highway. Many are watching to see what will happen to the mountain lion population in the area. Over time, inbreeding has led to genetic abnormalities, such as a telltale kink in the tails of local cats. The agency predicted the population would die out within decades without a crossing.

In the US, the infrastructure bill’s $350 million lags far behind what will be needed to address the fragmentation caused by the country’s 4 million miles of public roads. But there are a handful of innovations that could fail the cost-benefit analysis by building crosswalks at a lower cost or in places where this was previously not feasible.

Animal bridges are currently only built when there is protected land on either side of the road, as the typical cost of building a concrete bridge is hard to justify in a place one could develop in a few years. Lighter, cheaper, modular systems could be used in places where the future is less secure, Huijser explains: “If the adjacent lands become unsuitable for wildlife, we take it apart and you can move it.”

A candidate material for such modular systems is precast concrete. There is also excitement about fiber reinforced polymer (FRP), a material that is less dense than concrete that is made of structural fibers set in resin. FRP has been used to build pedestrian and bicycle bridges in Europe and a quick and easy nature bridge in Rhenen, just south of the Gooi in the Netherlands. Currently, the Federal Highway Administration does not allow its use in traffic infrastructure in the US, but there is a growing demand for change. “These are barriers that mainly have to do with policy and governance. They’re not about science and they’re not about technology,” Lister says.

“They know that the last thing anyone wants is to have a big structure, with a lot of publicity, built — and then it doesn’t work.”

Darryl Jones

Designers like Lister and innovators like Callahan are outspoken advocates of building bridges for wildlife across the country. Road ecologists and natural scientists, on the other hand, remain more cautious. “They are hypercritical because they know that the last thing anyone wants is to have a big structure, with a lot of publicity, built – and then it doesn’t work. Because everybody comes out of the woodwork and says, ‘See! Waste of time! Complete nonsense!’” says Jones.

But today, even cautious types want to see more built. While we may not have done enough research to have all the answers, it would be dangerous to take that as a signal to stop, Huijser says. He calls such exaggerated caution a “type II error– a false negative. In this age of mass extinction, it’s like the house is burning down and our solution so far has been to spray it a few times with a water gun. To conclude that water is not the answer would be a mistake.

Despite the challenges in Ede and elsewhere, says Van der Grift, the answer is learning by building. We still need to invest in the real work of tagging, installing trail cams and doing DNA testing and long-term population monitoring, he stresses. But we need to build more crosses first – and the evidence we have so far says we need to build big and bold. “You have to realize that you can hardly do too much,” he says. “You do what you think is necessary, study it, and then, nine times out of ten, you’ll see, ‘Oh, I should have done more.’ But there’s no point waiting until you realize that.”

Matthew Ponsford is a freelance reporter based in London.