Safe passage for native wildlife using highway underpasses

In the first long-term study of underpasses in Australia, published in the journal Ecology and Evolution, the underpasses, located on the NSW Mid North Coast, were shown to help the movement of many wildlife species across landscapes on either side of highways.

The study also allayed concerns that predators, particularly introduced feral pests, use underpasses as a prey-trap, at least by the observations at these locations.

  Video transcript

Wildlife cameras detected more than 4,800 medium-to-large mammals and goannas using highway underpasses located at Port Macquarie and Grafton over a two-year period.

Species such as eastern grey kangaroos, swamp wallabies, red-necked wallabies, red-necked pademelons and lace monitors crossed some underpasses more than once per week.

Rufous bettongs and echidnas crossed individual underpasses every two to four weeks.

Red-necked pademelon with baby (Credit Goldingay, Taylor, Parkyn).

“More than 4800 detections were made; that number was quite astounding,” said lead researcher Associate Professor Ross Goldingay.

“These crossing rates suggests animals used the underpasses to forage on both side of the freeways.

“This research provides compelling evidence that highway upgrades in Australia need not threaten wildlife populations if road underpasses are installed. The underpasses are installed in conjunction with mesh fences (often with a floppy top to stop koalas climbing over) that line the highways to keep animals off the road and direct them to the underpasses to cross.”

A combined wildlife-drainage underpass at Grafton NSW (Credit Goldingay, Taylor, Parkyn).

However, Associate Professor Goldingay cautioned: “We should not use apparent underpass effectiveness to justify expanding our road networks where they do not belong.

“Australia’s wildlife species are increasingly threatened with extinction by habitat clearing and fragmentation. One leading cause of this is the expansion of our road network, particularly the upgrade and duplication of major highways.

“Underpasses are a useful generic tool to enable wildlife to move across landscapes with roads. But not all ground-dwelling species of wildlife will find underpasses to their liking but so far many do.”

The predators detected at the underpasses were the introduced red fox (pest), feral cat (pest) and dingo.

“We did not see increased activity at the underpasses, allaying concerns of the ‘prey-trap hypothesis’ – whereby predators might easily pick off unsuspecting animals funnelled into the confined space of an underpass,” Associate Professor Goldingay said.

“Only the fox was detected frequently enough to be a potential concern. However, its activity coincided less than expected with the activity of the mammals most at risk to it. In fact, potential prey may actually avoid using the underpasses when foxes are about.”

A dedicated wildlife underpass at Port Macquarie NSW (Credit: Goldingay, Taylor, Parkyn).

The researchers studied 12 road underpasses – five under the Oxley Highway at Port Macquarie; and seven under the Pacific Highway south of Grafton – over two to three years, comparing camera trap detections of animals at underpasses with those at nearby forest sites.

Read Good news: highway underpasses for wildlife actually work published in The Conversation.

Study details

‘Use of road underpasses by mammals and a monitor lizard in eastern Australia and consideration of the prey-trap hypothesis (2022)’ by Ross L Goldingay, David Rohweder, Brendan D Taylor, Jonathan L Parkyn.

Published in Ecology and Evolution

DOI: https://onlinelibrary.wiley.com/doi/10.1002/ece3.9075