Inside the experimental world of animal infrastructure

Inside the experimental world of animal infrastructure

In the mid-2000s, toads were meeting a gruesome end near Ede, an old, leafy town in the middle of the Netherlands. Local residents stepped in to save the animals. The town constructed temporary fences along a kilometer of road each spring for a few weeks. This was the area where the animals crossed from their winter habitat in south to three breeding areas 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. Volunteers would tirelessly transport the toads from one side to the other and then send them on their journey. It was a simple, but laborious way to alleviate the hardships of being an amphibian living in a world designed for humans. It was a lifeline for Ede residents, who were happy to provide for their warty neighbours–which, as many other species, have had difficulties feeding, breeding and migrating as their natural habitat is being destroyed by human infrastructure.

What followed was a cautionary tale among a small international group of ecologists and ecological designers. Ede decided to replace its temporary screens with permanent barriers after a few years and install a pair of tunnels for wildlife under the road. Edgar van der Grift, an ecologist, and other scientists who were monitoring the changes saw that underpasses were very popular. Many toads hopped happily toward their breeding ponds–even finding occasion to copulate mid-journey, a 2019 study notes. The researchers were alarmed at the effects of this new infrastructure on the toad population. Van der Grift, a world-renowned expert in wildlife crossing structures, said that they saw a crash. “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. However, discussions continue about how to reverse Ede’s declining numbers.


For advocates of wildlife crossings, any such sign of failure inevitably sets alarm bells ringing far and wide. These tunnels and bridges have been financed by many countries. 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. Canada and the Netherlands have long-standing networks of road-spanning projects that span decades. These include arcs of chaotic forests that reach over highways. Australia, Brazil and South Africa are following their lead, hoping to avoid seeing natural habitats cut into disjointed pieces.

Cities all over the globe are creating a wide range of structures to reduce the negative effects of urbanization and roadbuilding. This includes green roofs, tree-lined skyscrapers and artificial wetlands. There are also shelters and “hibernacula” such as 3D-printed hempcrete birdboxes for endangered birds in Melbourne and huge bat caves that look like earthen igloos in Texas hills.

But the data on how effective these approaches are remains patchy and unclear. This is true even for wildlife crossings which are the most well-studied and heavily funded example of such infrastructure. Although road ecologists are well aware that these crossings can help reduce roadkill, the story of their impact 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 are certainly success stories. Remote-sensing cameras capture images of animals using them every day. There are the pioneers like roe deer or foxes who cross even before construction is complete. Some shy holdouts like gray wolves and grizzly bears may take generations to become users. At Singapore’s Mandai Wildlife Bridge, a total of 70 species–including pangolins, sambar deer, long-tailed macaques, fruit bats, and red jungle fowl (a close relative of the domestic chicken)–have crossed the road.

“Ten seconds after they’re open, there’s animals using them,” says Darryl Jones, the author of A Clouded Leopard in the Middle of the Road, which tells the stories of deadly highways and lifesaving crossings from Brisbane, Australia, to Alberta, Canada. “The big question now is–and that is a valid question–So, what? Does that actually make a difference?”

It takes less than half an hour by train to get from Amsterdam to the Gooi, a region of historic villages and medieval fortified settlements that’s home to the Dutch TV industry. This short ride takes you through one of the most intricately engineered landscapes on Earth: over railroad girders, across shipping channels, past windmills (both ancient stone structures as well as modern tubular steel turbines) and across vast fields created by the polderdike system, which transformed the country’s natural marshy areas into productive farmland.

For those who are interested in learning more about wildlife crossings, the Gooi can be a good place. The region boasts one of the world’s densest collections of such infrastructure, with four bridges, two major underpasses, and a network of tunnels for badgers, amphibians, and reptiles, all within about 10 minutes’ drive from the quaint local capital of Hilversum.

As van der Grift walks along the deck of one of these bridges, he points out fox droppings, blue herons, and trails where groups of roe deer walk in single file, retracing their footsteps each day. The six-lane highway below, which is hidden by the raised banks and “berms” on either end, can be seen from this point. Despite efforts to muffle the noise with the local beech and maple leaves, it is still quite audible. The crossing is dotted with ponds that are close enough for toads and their friends to hop from one pond to the other.

The Netherlands built its first wildlife bridges to stop deer from becoming roadkill. But in the 1990s, the country began to shift to a more holistic ecological mindset, using bridges to link fragments of protected areas. In 2005 the Dutch parliament made such “defragmentation” a long-term nationwide policy, known as MJPO in Dutch. This program marked a shift towards a conservation-driven agenda that prioritized helping all species, including reptiles, bats and butterflies, to move across the altered landscape. In modeling carried out for the Dutch transport ministry, van der Grift identified 215 bottlenecks where species struggled to pass, flagging them as places where crossings could make the greatest difference. Today there are 70 wildlife bridges in the country and more than 2,000 other structures, such as badger tunnels, rope bridges between trees, and aquatic underpasses. Van der Grift is a gentle giant with a dry humor who spent two decades trying to stop animals’ seemingly insatiable desire for safety under cars. Animals can’t or won’t cross a road. Van der Grift states that it prevents animals from crossing and from coming in contact with one another. These effects are not limited to roadkill. If the barrier is large enough, it can make whole animal populations less viable and more susceptible to inbreeding or decline.

Wildlife crosses can reduce this barrier effect, it is thought, making the road more open for a variety of species. He explains that few studies have been able conclusively prove this to be true.

toad crossing concept


The devil is in the details. For example, Ede had plenty of amphibian traffic through the toad tunnels. But there were not enough structures–just two tunnels initially, hundreds of meters apart, rather than the 10 or more that scientists had recommended. Marcel Huijser is a well-known US road ecologist and long-time friend of van der Grift. He says that many toads who move along the barrier want to cross the road to reach the breeding pond. However, they did not encounter any crossing structures early enough.

Unexpected impacts and side effects crop up at nearly every crossing. Although often considered a cheaper option, underpasses are less popular among many species and are rarely used for butterflies. Many aquatic mammals won’t swim into tunnels where they can’t see the other end. However, they can be persuaded to walk through the tunnel if a narrow ledge is constructed above the waterline. One bridge in the Gooi was taken over by a large buck, who had a territorial effect beyond the rutting season. He acts as a gatekeeper, allowing males to cross but preventing them from accessing most females.

Van der Grift is on the bridge, tracking slow worms and armless and legless reptiles. They wobble forward like clumsy serpents. The genetic profiles of the two populations on the west and east sides of the highway were different decades after they were split by road and railways. When the bridge opened in 2016, he hoped to see the two populations start to mix. DNA testing confirms that they are. He says, “We can see that the genetic patterns of these populations are becoming closer to one another.” “So there is exchange.” But the ultimate goal is to have self-sustaining, healthy and viable animal populations. It’s not clear if the defragmentation efforts are achieving that. Van der Grift claims that he and his colleagues drafted a plan to conduct a nationwide empirical evaluation on the MJPO program about a decade ago. However, it was never funded. The MJPO has since been discontinued and replaced by other defragmentation programs. These studies are often considered too expensive. Silviu Petrovan, a University of Cambridge zoologist, says that it can take decades to separate the signal from noise. Some animals, like amphibians, naturally have population numbers that vary greatly from year to year, he says, meaning totals can zigzag “due to reasons that have nothing to do with your mediation.”

“We are now at the stage where the data is coming in–it’s really coming through.”

Darryl Jones

One animal that seems to have benefited from Dutch defragmentation policies is the badger. In the 1980s there were fewer than 1,200 of them nationwide. Their numbers have tripled since the country started building “badger pipes” under-road. Van der Grift’s team has produced models that strongly suggest that tunnels have a positive impact on population viability. He says that no scientific study has been done to support this claim. This would require decades of population monitoring.

The Netherlands is not alone in its limited assessment of the impact of wildlife crossings. Petrovan states that even though we do enough research, we don’t do the necessary amount of funding, do the research long enough, and have enough control settings to be able conclude that we achieved our objectives. He says that many cross-projects don’t even get to the point where they can clearly define the goals they want to achieve.

Jones strikes an optimistic tone. He says, “We are now at a stage where the data has started to come in–it’s really happening.” He’s particularly encouraged by the ability to do genetic testing: “We’ve got very profoundly useful and effective ways to assess this stuff.”

Historically, in the US, conservation has not been the point of animal crossings. Wildlife bridges are primarily used to protect traffic. There have been two dozen overpasses constructed at key areas for migrating deer or elk. The highway has many victims, but endangered species of smaller animals barely make it into the list. “Amphibians? Reptiles? Please …” van der Grift summarizes how such concerns are often dismissed.

Most studies of US crossings have tracked their impact on road collisions and insurance claims. There, they excel: “When sited correctly, with appropriate fencing, to the target species, we know what wildlife crossings work well over 90% of the time,” says Nina-Marie Lister, who leads the Ecological Design Lab at Ryerson University in Toronto. “They avoid 90 to 95% of wildlife vehicle collisions. This is an amazing number in science.

animal crossing


Wildlife underpass crossing culvert  for animals under a motorway in the Netherlands



animal crossing


Clockwise from top left: California’s mountain lions, Kenya’s elephants, Singapore’s pangolins, and the Netherlands’ amphibians are among the focal points of wildlife crossing developments.

The reduction in property damage and human injury can be significant. In the mid-2010s, for example, a project on State Highway 9 in Grand County, Colorado, added two wildlife bridges, five large arch underpasses, and 10.4 miles of wildlife fencing at a cost of $10 million. The result was an 89% reduction in roadkill. The Center for Large Landscape Conservation, a nonprofit working on ecological connectivity, projected that the crossings would pay for themselves in approximately 22 years, less than a third of the structures’ planned 75-year life span.

But if the goal is to stop animals being hit by cars, a bridge is unnecessary. Petrovan, who studies wildlife crossings for Conservation Evidence (a database of scientific findings on conservation actions), says that a fence could be put up to stop them going. It makes us feel better because we see fewer people being killed. He says that it doesn’t give any benefits to the population.

Huijser says the US has been less inclined than his native Netherlands–and “almost anywhere else I’ve worked”–to think about conservation as a goal of crossings. This is changing. The Infrastructure Investment and Jobs Act, which was signed into law in November and has allocated $350 million for wildlife crossings for the next five years, provides new federal funding for projects and research to reduce wildlife-vehicle collisions as well as connect fragmented areas of habitat. Although that amount is just 0.3% of the bill’s $110 billion budget for roads, road ecologists have hailed it as a landmark investment. Rob Ament, senior conservationist at The Center for Large Landscape Conservation, said that there is now a publicly funded method to build crossings that are conservation-oriented, even though collision reduction remains our primary goal. Wildlife crossings will no longer be competing with potholes for scarce tax funds because they are now funded exclusively. Ament states that he believes it is a significant step forward. He says that the bill recognizes the need to design infrastructure with both the needs of people and the movement of goods and people in mind. “And finally, we’re doing that.”

But what to build? North America’s most influential examples of crossings lie along the Rocky Mountain Front in Canada. The Trans-Canada Highway divides the area, which is home to the largest number of large mammals on the continent. At Banff National Park, a set of 44 wildlife crossings (six overpasses and 38 underpasses) have been built to bridge the gap, creating a linked-up system used by a wide range of species including elk, cougars, and coyotes, as well as rarer animals such as red fox, grizzly bears, wolves, wolverines, snakes, beavers, and lynx.

But Banff’s wildlife crossings, like most, suffer from a sort of Horseless Carriage Syndrome, their designs circumscribed by existing infrastructure. Tunnels are often little-adapted, concrete tubes that carry water under roads. And overpasses have generally been borrowed wholesale from roadways–they are built as if they are going to carry the weight of an 18-wheeler and then “top-dressed” with foliage, Lister says.

nest infrastructure concept


A scattering of experiments are starting 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 avoids the humped arch of older bridges in favor of a vast flat expanse that needs just one column to support it between mountains and across a highway traversed each day by an estimated 300,000 cars. Renee Callahan is the executive director of ARC Solutions which studies how to build better wildlife bridges. Callahan states that it is designed for all species, including mountain lions, mule deer, and deer mice. “They’re designing the entire thing down–literally the mycorrhizal layers, in terms soil, to ensure that the soil has the fungal network that supports the native vegetation.”

There are many unknowns during construction, not to mention how different species will react under the vehicle traffic. The National Park Service will monitor activity on the bridge and DNA profiles of animals along either side of the freeway. Many are anxious to see what happens to the mountain lion population in the area. Inbreeding has caused genetic abnormalities over time, such as a kink in the tails of local cats. The agency predicted that the population would disappear within a few decades without any crossing.

Across the US, the infrastructure bill’s $350 million falls far short of what will be needed to address the fragmentation created by the country’s 4 million miles of public roads. There are a few innovations that could change the cost-benefit equation by allowing crossings to go up at lower costs or in places not possible before.

Animal Bridges are not built where there is protected land. This is because the cost of building a concrete bridge on a site that might be developed in a few years would be prohibitive. Huijser says modular systems that are lighter and more affordable could be used in areas with less secure futures.

One candidate material for such modular system is precast concrete. There’s also excitement about fiber-reinforced polymer (FRP), a material less dense than concrete that is made from structural fibers set in resin. FRP has been used in Europe to build foot- and bike bridges and a quick-and easy wildlife bridge in Rhenen. It is located just south of the Gooi, in the Netherlands. The Federal Highway Administration currently prohibits FRP from being used in US traffic infrastructure. However, there is growing demand for its use. These are barriers that are primarily about policy and governance. Lister says that they are not about science or technology.

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

Darryl Jones

Designers like Lister and innovators like Callahan are vocal proponents of building wildlife bridges across the country. On the other hand, wildlife scientists and road ecologists are more cautious. They are hypercritical because they know that it is not what anyone wants for a large structure to be built with lots of publicity. Because everyone will be able to see the structure and then say, “See!” It’s a waste of time! Complete crap!'” Jones says.

But today, even cautious people want to see more construction. Huijser states that even though we may not have done enough research to know all the answers, it would still be dangerous to consider this a warning sign to stop. He calls such over-cautiousness a “type II error“–a false negative. It is almost as if the house is on fire. Our solution has been to spray water at it a few more times. It would be a mistake to conclude that water is not the solution.


Despite the challenges in Ede and elsewhere, van der Grift says, the answer is learning while building. He emphasizes that we still need to do the hard work of tagging, installing trail cameras, DNA testing, and long-term population monitoring. We must first build more crossings. The evidence so far suggests that we should build bold and big. He says, “You must realize that you almost can’t do too much.” “You do what you think is necessary, study it, and then, nine out of 10 times, you will see, ‘Oh, I should have done more.’ But there’s no point in waiting until you have figured that out.”

Matthew Ponsford is a freelance reporter based in London.

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