In the mid-2000s, toads were meeting a gruesome end near Ede, an old, leafy town in the middle of the Netherlands. Local residents came to the rescue. For a few weeks each spring, the town erected a set of temporary fences along a kilometer or so of road, in an area where the animals crossed over from their winter habitat in the south to three breeding ponds in the north. When the toads hit the barrier, they’d hop sideways for a few meters until they dropped into a bucket, one of 36 pitfall traps that lined the fence. Every day, volunteers would diligently carry the toads to the other side and send them on their way. It was a crude, somewhat laborious way of mitigating the hardship of being an amphibian in a world built for humans. But it was a lifeline that Ede residents were happy to provide for their warty neighbors—which, like so many other species worldwide, have suffered difficulties feeding, breeding, and migrating as their familiar landscape is carved apart by human infrastructure. 

What followed has taken on the air of a cautionary fable among a small international community of ecologists and ecological designers. A few years in, Ede decided to swap its ad hoc screens for permanent barriers and replace the three dozen buckets with a pair of wildlife tunnels passing under the road. For ecologist Edgar van der Grift and other scientists monitoring the change, it was clear that the underpasses were popular. Many toads hopped happily toward their breeding ponds—even finding occasion to copulate mid-journey, a 2019 study notes. But when the researchers studied the effect that this new infrastructure was having on the toad population, they were alarmed by the results. “We saw a crash,” says van der Grift, one of the world’s leading experts in wildlife crossing structures. “In five, six years, the population went down from over 10,000 individuals to less than 1,000.” In the years since, van der Grift has persuaded Ede to add a third tunnel, in a heavily frequented spot along the road. But discussions are still ongoing about how to reverse Ede’s dwindling numbers.


For advocates of wildlife crossings, any such sign of failure inevitably sets alarm bells ringing far and wide. Countries have started to invest big in these bridges and tunnels. President Biden’s November infrastructure bill allocated a landmark $350 million investment in animal crossings across the US, where some estimate roughly 1 million vertebrate animals die each day. In April, the National Wildlife Federation broke ground on a pioneering urban bridge—a $90 million custom-designed acre of “wilderness” that will float across 10 lanes of the US 101 freeway, linking two islands of mountain lion habitat north of Los Angeles. Early adopters Canada and the Netherlands are already home to decades-old networks of road-spanning projects, with arcs of chaotic forest reaching over highways. Australia, Brazil, China, and South Africa are following suit, hoping they can avoid the fate of seeing natural habitats sliced into sickly, disjointed fragments.

Around the world, cities are building a huge variety of structures intended to mitigate the impacts of urbanization and roadbuilding on wildlife. The list includes green roofs, tree-lined skyscrapers, living seawalls, artificial wetlands, and all manner of shelters and “hibernacula,” including 3D-printed hempcrete birdboxes for endangered owls in Melbourne and gigantic bat caves constructed like earthen igloos in the Texas hills.

But the data on how effective these approaches are remains patchy and unclear. That is true even for wildlife crossings, the best-studied and most heavily funded example of such animal infrastructure. Though road ecologists know these crossings can play a vital role in reducing roadkill, the story of their impact on wildlife conservation is still being told. This question is only growing more urgent: to meet the UN Sustainable Development Goals by 2040, a projected $97 trillion “tsunami” of new roads, railways, pipelines, and power lines will be needed, which would in effect double human infrastructure from 2012 levels, according to the World Wildlife Fund. That would put even more pressure on global biodiversity; one-sixth of all species at risk of extinction are threatened by human infrastructure development.

Wildlife crossings certainly look like success stories. Every day, remote-sensing cameras beam back images of animals taking advantage of them. There are the eager pioneers, like roe deer and foxes, which cross even before construction is completed. There are shy holdouts, like gray wolves or grizzly bears, which might take generations to become users. At Singapore’s Mandai Wildlife Bridge, a total of 70

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By: Matthew Ponsford
Title: Inside the experimental world of animal infrastructure
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Published Date: Thu, 16 Jun 2022 09:00:00 +0000

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