Helping the squirrel glider to cross the road

Squirrel gliders on the edges of the busy Hume highway north of Melbourne have been given their own overpass to reduce the number of deaths and to enable populations separated by the freeway to meet, mix and mate.

The first of three rope bridges and three ‘glider poles’ has been installed across the Hume freeway at Violet Town between Seymour and Benalla, 200 km north of Melbourne to allow squirrel gliders, possums and other native animals to pass safely across the road.

rope bridge
The rope bridge stretching across the Hume freeway. Squirrel gliders and other nocturnal marsupials will use it to cross the road safely

The bridge and glider poles are the first of their kind in Victoria, and the culmination of a three-year research program into the effect that roads and traffic have on wildlife.They are mainly being installed fmainly or the benefit of the squirrel glider, a species threatened with extinction in Victoria and NSW. The animal moves by gliding from tree to tree. Where there are large gaps in tree cover, such as roads, it is unable to cross and individuals or populations can become isolated.

Researchers from the Australian Research Centre for Urban Ecology (ARCUE) based at the Royal Botanic Gardens, and from Monash University, the University of Melbourne and VicRoads have concentrated on finding out how, where and why animals cross the busy freeway.

“The world’s ecologists are watching this program closely because we have a good understanding about what has happened before an animal-only overpass has been built,” Dr Rodney van der Ree, an Ecologist at ARCUE and Honorary Fellow at the School of Botany, University of Melbourne, said.

“It is crucial to encourage movement across roads and among squirrel glider populations so that they can access food, shelter and mates,” said Dr van der Ree.

“What appears to be happening, is that those squirrel gliders who are on opposite sides of the highway are effectively different groups. Despite being just 100 metres apart, they might as well be one kilometre or 10 kilometres apart. They have become cut-off from the rest of their group. We are fearful that this could lead to local extinction.”

He said the project “puts Australia at the forefront of international efforts” to determine how effective structures, such as overpasses, culverts and pipes really are in enabling animals to move more easily.
Monash University postgraduate researcher Silvana Cesarini captured and released 315 squirrel gliders on a 80 kilometre stretch of the freeway in the area between August 2005 and December 2006.

Cesarini said squirrel gliders were chosen for this project because they are endangered in Victoria (the southern extreme of their range is around Seymour), which makes them a valuable species to investigate, “and because they are a model species for researching the barrier effect of roads: any gap in the canopy that is greater than the maximum distance achievable by gliding can result in isolation.

“For other native marsupials (common brushtail possums, common ringtail possums, koalas etc.) the freeway can be crossed on the ground, which most probably would have more implications for mortality than for isolation. Squirrel gliders are an ideal species for broadening our knowledge of habitat fragmentation and its consequences.”

The gliders were caught in metal mesh cages, nailed 3-4m high on trees and baited with a mixture of honey, peanut butter and oats. All the animals were individually marked with an ear tattoo. Animals caught at sites where bridges will be constructed were also tagged with microchips. Sixty-four gliders were fitted with single stage tuned loop radio-transmitters mounted on neck collars. Each collar emits a radio signal at a specific frequency which can be picked up by a receiver. They will be tracked at sites along the Hume Freeway as well as further from the freeway to provide a comparison.

“It is important that animals have the opportunity of crossing partial barriers like a freeway so that groups don’t become isolated,” she said. “Isolation is negative especially in terms of local extinctions. Small patches of habitat that can sustain a group of squirrel gliders, for example, might become vacant. If connectivity is good, gliders from surrounding habitat can reach the vacant vegetation and recolonise it. If that patch is too isolated though, it would not be recolonised.”

Connectivity is also considered important to maintain high levels of genetic variability, due to dispersing animals successfully breeding in the new territory, which is advantageous when adaptation to environmental changes is required. “The obvious example here is climate change, which will probably put to the test many species and populations that do not have enough genetic variability to adapt to new climatic conditions,” she said.

“Another scenario is one where the available habitat on one side of the road is the minimum necessary for a group of gliders to persist. If some of that vegetation is cleared for whatever reason, the resources available to that group of gliders would no longer be enough. If they could cross the road, they would be able to reach more resources and therefore be able to persist.”

Cesarini said the presence of mature trees in the median strip makes a substantial difference in the ability of gliders to cross the freeway. “Tall trees in the median allow the glide distance to be basically cut in half. Conversely, where trees are absent in the centre median, only one male out of all the animals tracked was located on opposite sides of the freeway. The distance between trees at that site is approximately 70m and I can’t rule out that he might have just run along the road.”

According to the findings, 70 per cent of the gliders tracked at sites with trees in the median strip were observed crossing the freeway compared with 10 per cent crossing where there were no trees.
It’s not known how many squirrel gliders get killed trying to cross the road. “We don’t know this yet, and it is a very hard parameter to estimate,” Cesarini said. “It is likely that gliders get killed as they glide over the freeway, so rather than getting hit on the road surface (where they would more easily be seen), they probably get thrown to the side of the road (where they are very difficult to spot).

“We have only collected few glider road kills; however, an honours student, Sarah McCall, has looked at the survival of gliders along the freeway as compared to small country roads, and found the survival at freeway sites to be approximately one third of that at sites far from the freeway.

The bridges and poles should help to reduce the threats to survival of the squirrel glider populations in the area.

“We have found that squirrel gliders are still quite widespread across the region, despite the level of clearing and number of roads” said Ms Cesarini.”However we must work to reduce the threats to survival”
Other animals likely to use the rope bridge include the brushtail and ringtail possum, yellow-footed antechinus and the brush-tailed phascogale.

building bridge
VicRoad workers installing the rope bridge across the Hume freeway at Violet Town, 23 km south-west of Benalla

Similar bridges have been set up in NSW on the Pacific highway at Karuah for gliders, ringtail and brushtail possums, and in North Queensland for rainforest possums. Many similar structures are used across the world for various species, including monkeys and squirrels.

©Sue Cartledge

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