cross-posted from: https://lemmygrad.ml/post/11712543

Banner image: The Common wombat (Vombatus ursinus) frequents the western side of Cawleys Bridge, and it is hoped the bridge will reinforce the wombat population in Royal National Park. Image courtesy of Simone Cottrell / DCCEEW.

Above an Australian highway, a bridge reconnects wilderness for quolls, koalas and other animals

• A new wildlife overpass that spans a major highway south of Sydney is reconnecting habitat between Heathcote National Park and Royal National Park, helping animals safely cross one of Australia’s busiest road corridors.
• The retrofitted bridge includes features for a wide range of species, from rope crossings for gliding marsupials to vegetated pathways for ground-dwelling animals such as wombats, echidnas and amphibians.
• Ecologists say reconnecting fragmented habitat is increasingly important as roads, urban expansion, extreme weather events and climate-driven bushfires isolate wildlife populations and reduce genetic diversity.
• Research from Australia and elsewhere shows that wildlife crossings can significantly reduce animal deaths and help species move, forage and breed, but only when these structures are carefully designed around animals’ behavior and habitat needs.

SYDNEY, Australia. At dusk on the edge of the bush in Australia’s Heathcote National Park, a spotted-tailed quoll lowers its tawny head to the ground, pink nose twitching. The dense forest, the scent of damp earth and eucalyptus leaf litter gives way, abruptly, to heat and a chemical tang. Ahead: open space. Noise. Light.

A car zooms past, loud and fast. It doesn’t slow down. None of the vehicles do. It’s unlikely any driver going 110 kilometers per hour (68 miles per hour) would notice the brown, cat-sized quoll, camouflaged with white spots that beautifully blend into its native bush home.

Forty thousand vehicles a day move along this stretch of the M1 Princes Motorway — four lanes of fast-moving traffic that slice between Heathcote National Park on one side and Royal National Park on the other.

This is the primary route from Sydney to industrial centers in the southern part of the state of New South Wales, and there’s heavy truck traffic.

The quoll (Dasyurus maculatus) waits at the highway’s edge for a break that doesn’t come.

Headlights streak. Engines roar. The air pulses with pressure and speed. Crossing here isn’t just dangerous — it’s nearly impossible. The highway might as well be a canyon.

And yet, on the other side of the road lies something essential: new territory that includes more of the bird eggs and the rabbits that quolls eat, and mates with more varied DNA, both essential for long-term survival.

For decades, quolls, wallabies, deer, koalas and other animals have died trying to cross this highway. More than 200 have perished in just five years, but this number only includes larger species because smaller animals are hard to count.

But now, just above the traffic, a new, safe path is taking shape.

A koala (Phascolarctos cinereus) in eucalyptus tree on the Woronora Plateau, west of Cawleys Bridge. Image courtesy of DCCEEW.

A bridge across a river (of cars)

On a late summer afternoon in early March, I stand on the nearly completed Cawleys Bridge, a wildlife overpass-to-be that stretches above the roadway. It’s an ordinary structure that’s being transformed into something far more ambitious. Below, cars flash by and the sound is a constant, oceanic whoosh. Around me, the bridge smells of fresh soil — dark, mineral-rich earth recently spread across its surface.

Heavy machinery moves slowly back and forth, laying the final layers of habitat. State transport workers position massive tree trunks into place — logs arranged as architecture to form a new kind of ecosystem, one that’s designed to serve a wide variety of native species.

While the structure’s combination of features is new, its design is based on previously built wildlife bridges and the research proving that animals use them.

The science will continue. Motion-sensing cameras are already installed at entry points and along the crossing, ready to document which animals venture across the remodeled bridge, and how many.

Early monitoring, before the retrofit, showed just how inhospitable the bare bridge had been before modification. “When we monitored it, in [Australian] winter, nothing was using it,” said Kylie Madden, an ebullient ecologist with the New South Wales Environment and Heritage agency. “In summer, we did get a few crossings of these goannas [lizards], and we had one ringtail possum. But it was such an unfriendly situation.”

Now, the previously animal-unfriendly, plain-concrete bridge — once used only for road-maintenance vehicles — has been transformed into habitat that feels more like home. And its design allows for a huge variety of species to cross.

Ecologist Bob Crombie and DCCEEW ecologist Kylie Madden, who worked to make Cawleys Bridge a reality for years, met in person for the first time on the bridge. Image courtesy of DCCEEW / Alex Pike.

All animals considered: The scaly, the diminutive and the arboreal

High above the ground, thick draped ropes stretch across open air, intended for arboreal marsupials like sugar gliders (Petaurus breviceps) and common ringtail possums (Pseudocheirus peregrinus) — species that prefer to move through the forest on tree branches well above the forest floor. “There’s absolutely no way a sugar glider will make it across that road without connection,” Madden said.

Below the graceful ropes, there’s a wooden pathway for animals that typically like to travel just above the ground, including reptiles and koalas.

At the base, soil and native plantings will form a continuous vegetated corridor for ground dwellers: wombats (Vombatus ursinus), echidnas (T__achyglossus aculeatus), amphibians and insects. They’ll be funneled toward the bridge and away from the road by two long “wings” of fencing that run perpendicular to the bridge.

“We’re trying to make this functional for everything,” Madden said, leading me over to soft mats of coral fern that are habitat for the many species living at the road’s edge. “There are endangered species, like the red crowned toadlet [Pseudophryne australis] within just 10 meters [33 feet] of this bridge.” Madden crouched down to check for the tiny toads under the ferns next to a roped-off patch of ground marked with bright orange flags. “But they are never crossing without this structure.”

These tiny amphibians, no bigger than a thumb, live among the damp understory. Without cover, without continuity, the road is either a dead end or a death trap for them.

DCCEEW’s Kylie Madden and Bob Crombie, a retired Royal National Park ranger and ecologist, pored over a hand-drawn map from 1978. It identified the connections between Heathcote National Park to the west and Garrawarra State Conservation Area to the east, which leads into Royal National Park. Image courtesy of DCCEEW / Alex Pike.

Uniting two iconic national parks

Experts say there’s an urgent need for these crossings. Royal National Park is the world’s second-oldest park, opened in 1879. But despite its size — 150 square kilometers (58 square miles) — it’s increasingly isolated. To the east lies the Pacific Ocean; to the west, a vast and continuous expanse of bushland stretching across the Woronora Plateau, toward the Eastern Highlands that separate inland Australia from the coast. Between them: the highway.

“It’s iconic,” Madden said of Royal. “But what it’s not connected to — because of the M1 — is this vast tract of wilderness.”

Even highly mobile animals struggle when habitat is fragmented by infrastructure. Madden said wombats are almost extinct within the national park, while still relatively common on the other side of the road.

Koalas (Phascolarctos cinereus) were once present in the park, but they’ve all but disappeared. “There’s habitat there,” Madden said, “but almost no koalas.” Some populations remain on the eastern side. But without safe passage, they can’t recolonize the park — especially after a fire like the one that decimated Royal National Park in 1994.

That blaze was catastrophic, burning through more than 90% of the forest. In the decades since then, bushfires have increased in frequency, intensity and size, driven by hotter and drier conditions induced by climate change, according to CSIRO, Australia’s federal science agency. Experts say the park is likely to burn again.

“We need to really make sure our reserves are connected,” Madden said, “for all species … to make them as resilient as possible for the future.”

This map of Cawleys Bridge and Princes Motorway shows how the bridge links Heathcote National Park to the Garrawarra State Conservation Area, which leads into Royal National Park. Image courtesy of DCCEEW.

Reconnecting fractured landscapes is key to the future health of wildlife. In a study outlining the need to reconnect nature, biologist Stuart Pimm and colleagues wrote that, “Even when natural habitats remain, they often come in fragments too small or isolated to sustain viable populations.”

When animals can move safely throughout their entire range, they avoid inbreeding: There’s a larger choice of mates, which increases genetic diversity. This bestows stronger defense to fight off disease and the ability to adapt to environmental changes, including climate change.

It also means that if there’s a bushfire in one area, animals can escape, either settling elsewhere or rebuilding populations in burned areas afterward.

But without connection, these animals might as well be living on islands.

An aerial shot documented Cawleys Bridge before the final bridge “furniture” and plants were installed. Image courtesy of Transport for NSW.

Making it happen

The idea for Cawleys Bridge emerged from years of growing awareness, pressure and collaboration. Bob Crombie, now a retired ranger at Royal National Park, originally raised the idea in 1974. It was then taken up by a local branch of the Country Women’s Association and the National Parks Association. In 2021, the Sutherland Shire Environment Centre joined the coalition, and they began working with Transport NSW, the state’s transportation agency. Public concern over the plight of koalas helped push the project forward.

“In 2022, the idea of retrofitting Cawleys Bridge to reconnect Heathcote and Royal National Park came up in discussions,” Sally Webb, an official at Transport NSW, told Mongabay in an email

Now, she said, there are regular meetings between the transportation and environment agencies “to discuss how we can contribute to implementing the NSW Koala Strategy.” This government initiative, launched in 2021, centers on protecting koalas and their habitats.

By late 2023, the wildlife bridge had secured internal approval from four government agencies. The NSW Koala Strategy funded the bridge conversion, including A$800,000 (nearly $600,00) for construction and maintenance and A$75,000 (about $54,000) for monitoring. The result is the hybrid structure that stands today, combining multiple crossing types into a single span: fencing to direct animals, ramps, ground cover, climbing structures and aerial rope systems.

Across New South Wales, such crossings are still rare. Cawleys is just the second wildlife bridge in the Sydney area, which is a biodiversity hotspot. It’s the only one to link disconnected landscape between national parks.

There are 10 wildlife crossings along Australia’s heavily populated east coast, and Transport NSW has more than 25,000 records of animals using them. Most are underpasses, which are far less costly to build or retrofit than bridges.

Data from the agency’s Fauna Connectivity Database show that across 29 road projects nationwide, at least 67 species use crossings, including threatened animals like koalas and potted-tailed quolls, as well as more common ones, like emus (Dromaius novaehollandiae).

For ecologists, the lesson is clear: Animals will use these structures — but only if they feel safe.

Bob Crombie, retired ecologist and former NPWS park ranger, on Cawleys Bridge, which he first envisioned retrofitting for wildlife passage back in 1974. Image courtesy of DCCEEW / Alex Pike.

A proven connection

Animal-friendly crossings directly address the significant impact human development has on native species.

“One of the primary drivers of global landscape fragmentation is road construction,” said Brendan Taylor, a wildlife ecologist and author of the book The Evolution of Wildlife Crossings in Eastern Australia. He likened it to “casting a net over the landscape, with each road separating formerly connected habitats.”

That idea came from Road Ecology: Science and Solutions, a game-changing 2003 book by Richard T.T. Forman and other noted ecologists. “That book was part of the growing movement to better understand and assess the impact of roads on wildlife populations,” Taylor said. Wildlife crossings, whether bridges or underpasses, are attempts to cut holes in that net, he added, in order to “perforate the roaded corridor,” allowing animals to move, forage and breed.

When combined with fencing that keeps animals off the roadway and directs them toward a crossing, extensive research shows that these structures greatly reduce hazardous accidents and animal deaths.

Thousands have been built throughout Europe, with 600 in the Netherlands alone, and more than 1,000 in the U.S. and Canada. In the Brazilian Amazon, there’s now a primate canopy bridge. Kenya has constructed underpasses for elephants, and Singapore has built green bridges for various mammals. Crossings and culverts are being retrofitted into existing roads, and in many places, new roadways consider wildlife movement from the design stage, like the Delhi-Mumbai Expressway.

Connectivity is critical for the survival of many species. But crossings aren’t one-size-fits-all. The infrastructure needs to address the movements of different species and their preferences. That requires habitat-specific research.

Conservation biologist Ross Goldingay has spent years studying animals including Australia’s eastern pygmy possum (Cercartetus nanus), a small, tree-dwelling marsupial. Despite its diminutive size — about 12 centimeters (5 inches) from head to tail — pygmy possums can travel up to 500 meters (1,640 feet) in a single night. Their movements benefit the landscape as they feed on pollen and nectar and pollinate plants and trees. Like many forest-dwelling and prey species, they’re reluctant to move through open spaces.

The tiny and widely loved eastern pygmy possum is one of the smallest pouched animals. The Royal National Park, to the east of the bridge, is a hotspot for these miniature marsupials. Image courtesy of Kerri-Lee Harris / DCCEEW.

“If you want to try and maintain connectivity, you need to assist these animals. So how do you get them over a road?” Goldingay asked.

For the past three decades, his research has focused on exactly that. He’s conducted multiyear surveys to figure out what design elements are needed for animals to use crossings. He learned that installing tall wooden poles helps gliding possums cross gaps in the forest, including roads. In monitoring underpasses along the coast, he saw that a wide range of mammals regularly use them.

“Landscaping really matters,” Goldingay said. He’s found that pygmy possums, like other bush creatures, seek out plants for cover. His research has demonstrated that “where you’ve got the vegetation coming up … you’ll get more animals passing through.” Without it, crossings can fail.

His research also confirmed that rope bridges can ensure that arboreal animals such as squirrel gliders (Petaurus norfolcensis) have crossings that suit their habits: They can’t use underpasses. Animals that like to travel off the ground but not in the treetops — including koalas and the endangered broad-headed snake (Hoplocephalus bungaroides) — need another type of crossing.

Going over the road is just one solution. Some animals are going under, using existing culverts that were constructed when roads were built, usually to channel rainwater underneath. In one study for Transport NSW, Taylor and his colleagues counted 36 species, from frogs to wallabies, that used even simple drainage pipes, just 1 m (3 ft) wide, to cross beneath roadways.

Now, new “fauna sensitive” road construction guidelines in NSW include larger culverts, when appropriate, offering a cost-effective alternative to more expensive bridges.

A spotted-tailed quoll (Dasyurus maculatus). Image by John Harrison. CC BY-SA 3.0

Looking ahead

Back on the bridge in the long light of late April, which is early autumn here in New South Wales, the dump trucks and transport workers are gone. Aside from the steady thrum of cars below, all is quiet. Work on Cawleys Bridge is complete and the final pieces of a living corridor have fallen into place.

While the traffic below remains unchanged — fast, loud and relentless — above, there is a carefully constructed thread linking two vast landscapes.

It will take time, but studies show that animals find and use these bridges, adding them to their memory banks and teaching their young. “Animals learn to use these pathways over time,” said Madden, the ecologist, “and that’s what a passage across a dangerous roadway is” — a pathway.

For spotted-tail quolls, koalas, tiny sugar-glider possums and so many other animals, a bridge like this may be the difference between turning back from — or dying on — a “canyon of a road” or crossing into new territory.