Wildlife take to the 'high' road
Why did the possum cross the highway? Because it can do so safely - at least that is what an Australian study into the impact of wildlife rope bridges shows.
Dr Rodney van der Ree, a senior ecologist with the Australian Research Centre for Urban Ecology, says his study shows native Australian animals are using wildlife bridges to cross highways.
It also highlights the importance of creating corridors for wildlife across major roads as a way of preventing local extinctions of native species.
Van der Ree says wildlife bridges are becoming more common across Australia.
However, this study is the first in which animals' movements were tracked before the wildlife bridge was erected.
"We've been able to measure the extent of the problem before the bridge went up," he says.
Vulnerable to disease
His team radio-tracked the endangered squirrel glider, brushtail and ringtail possums and the small mouse-like marsupial antechinus for six months ahead of the rope bridge being built across the Hume Highway near Benalla, in Victoria.
It found that the squirrel glider, which moves by gliding from tree to tree, only crossed if there was a tree in the median strip of the highway.
The brushtail possum never crossed and only two or three antechinus made it across the road.
"What ends up happening is that lots of small populations become extinct locally because they are unable to recolonise," he says.
Eight weeks after the Benalla rope bridge was installed, van der Ree placed cameras on each end of the bridge to record animal movements.
He says early results show the animals have acclimatised to the bridge.
Since June last year they have recorded more than 50 crossings of ringtail possums and almost as many partial crossings, seven partial crossings of brushtail possums and four partial crossings by squirrel gliders.
"These early findings are impressive as they show that native animals have acclimatised to the 70 metre rope bridge and are using it to cross the highway to find food, shelter and mates," says van der Ree.
He says the squirrel glider results are particularly positive as they are faced with the threat of extinction in Victoria and New South Wales.
The team will continue to monitor movements for another few years to determine whether population sizes on either side of the highway change and what motivates the animals to use the bridge.
He says climate change will also increase the importance of similar structures as animals will need to move from lowlands to highlands as the temperature increases.
"Ultimately the measure of success is whether the population size on each side of the road increases," he says.
"This is not just about crossing the road, this is about reducing the risk of extinctions."