Australian wildlife is increasingly threatened with extinction. One of the main drivers of this is habitat clearance and fragmentation. A related factor is the expansion of our road network, in particular the upgrading and duplication of our highways.
Governments are widening our major highways in the interest of road safety and traffic flow. But the main roads become obstacles to the movement of wildlife, as well as places where cars can hit and kill many animals.
Our new research explores whether highway underpasses help animals cross roads safely. We wanted to know if the animals actually used underground passages and if they had any hidden dangers funneling the animals through a confined space, making it easier for predators.
What did we find?
To find out if the underpasses are working, we used wildlife cameras to monitor 12 underpasses for over two years in northeastern New South Wales. Five under the Oxley Highway at Port Macquarie and seven under the Pacific Highway south of Grafton.
What we found was quite amazing. Many more animals than expected were using the underpass. We detected over 4,800 medium to large mammals and goannas, while smaller species such as snakes and rodents also used the underground passages, but were less reliably detected by our cameras.
Species such as eastern gray kangaroos, swamp wallabies, red-necked wallabies, red-necked pademelons, and lace monitors passed through some underground passages more than once a week. Rufous Bettongs and echidnas traveled through individual underground passages every two to four weeks. These passage rates suggest that animals use underground passages for food on both sides of the highways.
We were particularly interested in knowing if the endangered koala would use the underground passages. They did, occasionally. We found that they did not avoid underground passages, as they were rarely detected in the nearby forest.
Do fenced underpasses reduce or eliminate wildlife roadkill? Anecdotal evidence for our study road in Port Macquarie suggests that road fatality rates were very low. Only four roadkills (two eastern gray kangaroos, a red-necked wallaby and a brush-tailed opossum) have been reported to the local animal welfare group for this stretch of road over a three-year period encompassing our study. However, we should not be complacent. Roadside fences develop holes and need to be repaired to retain their value.
Do underground passages attract predators?
Many people believe that underground passages increase predator risk. This idea – known as the “prey trap hypothesis” – suggests that predators will be drawn to places where they can easily capture unsuspecting animals funneled into the confined space of an underground passage.
We have detected red foxes, wild cats and dingoes using these underground passages. But of these, only foxes were detected frequently enough to be of potential concern.
We tested the prey trap hypothesis by testing three predictions. If the hypothesis were correct, foxes should be more frequent in underpasses than in forest, foxes should concentrate their activity in underpasses where potential prey is more abundant, and the timing of use of underpasses by foxes and potential prey should coincide.
What we observed did not match those predictions. At Port Macquarie, foxes were detected at three underpasses, while they were absent from two. Of the three underground passages used by foxes, one particularly favored by foxes was not favored by bandicoots and pademelons, potential prey.
We expected to detect foxes close to prey detection time. But on average, there was a gap of more than three hours between detecting foxes and bandicoots or pademelons, and more than four hours between foxes and wallabies. We also found that foxes were detected less often on nights when potential prey was using the underpasses.
These observations suggest that potential prey may avoid underground passages when foxes are nearby.
What conclusions can we draw from this?
Underpasses are a useful tool for allowing wildlife to move through landscapes with roads. Not all ground-dwelling wildlife will find underground passages to their liking, but many do.
Could we upgrade highway underpasses with very high road fatality rates? It depends. The conditions must be met. For an underpass to be installed, the road must be sufficiently elevated. You also need roadside fencing to prevent animals from taking the shortest but most dangerous route across the highway. These fences do not work if there are intersections, freeway ramps or driveways.
What we need is to prioritize areas where underpasses are possible, where endangered species exist and where roadkill rates are high. It is very expensive to upgrade underpasses on existing roads, so we need to focus on priority areas.
In the future, it may be possible to use cattle grid type structures to keep animals like koalas from going around fences and onto roads. These are currently being tested.
To understand how to make underpasses even more efficient, we need more publicly available research. Currently, much of the monitoring relies on expensive but unpublished reports from consultants.
Underpasses are not a panacea for wildlife impacts. And we shouldn’t use their efficiency as a justification for running highways through pristine areas. They are a tool to minimize the impacts of road projects that enjoy broad community support.
Overpasses and underpasses for migrating animals can reduce collisions with automobiles
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