I always knew that the wildlife crossing structures along the Hume Highway were a bit of a mystery to some people. But until a recent news article by Tim the Yowie Man I hadn’t realised just how hot the rumour mill was running. I’m hoping this post will help set the record straight and prevent even wilder speculation (some people suggested they were a scientific experiment in alien communication!) So let’s go right back to the beginning – where all good stories start.
Arboreal animals, like possums, squirrels and monkeys, are finely adapted to life in the tree tops. Their feet are almost permanently poised to grab onto branches, strong claws sink into thick bark, and curly, ‘prehensile’ tails give extra grip in tricky situations. These animals are so good at moving from tree to tree that most species rarely come to the ground – this is where roads (and cars) can become a problem.
In south-east Australia, there is lots of concern over the impacts of roads on threatened species like the squirrel glider. Squirrel gliders are exceptional little aeronauts and regularly glide across 30–40 metre gaps between trees. Some can even reach 60 or 70 metres if they launch from a high enough tree and the conditions are right. But the longer the glide, the lower they land, and gliders attempting to glide over wide roads may end up colliding with traffic or even landing on the ground. Neither of these places are ideal. A squirrel glider running along the ground is like you or I trying to run with both feet loosely tied together – you might scramble from A to B, but you won’t outrun a car. So there’s potential for wide roads to have barrier or mortality impacts on squirrel glider populations.
To look into this more closely, a team of researchers from the Australian Research Centre for Urban Ecology, University of Melbourne and Monash University (in collaboration with VicRoads and an ARC Linkage Grant) investigated the impacts of a four-lane divided highway, the Hume Freeway, on squirrel gliders in north-east Victoria. Between 2005 and 2007, surveys were conducted to monitor glider populations and track their movements across the road. Firstly, they found that where there were no tall trees in the centre median, the gap across the Freeway was too wide (>50 metres) for squirrel gliders to cross (van der Ree et al 2010). Secondly, they found that squirrel gliders living next to the Hume Freeway had lower survival rates compared to squirrel glider populations living further away (McCall et al 2010). Not much good news for freeway-side squirrel gliders.
But based on all this research came some action. In 2007 rope bridges and glider poles were installed to help squirrel gliders move safely across the road. Rope bridges (also called rope ladders or canopy bridges) are exactly what they sound like – a bridge made of rope that links habitat trees on either side of the freeway. Glider poles are tall wooden poles that can be placed in the roadsides and median as surrogate trees, allowing gliders to safely cross the road in a few short glides. These structures were installed at five sites (2 canopy bridges, 3 glider poles) where research showed the road was causing problems for squirrel gliders. Similar structures were also used in the recent upgrade of the Hume Highway in New South Wales in 2009.
So, that’s what they are and that’s what they’re there for. But that’s not the question you all want to know, is it? What you want to know is, do they work? Me too. And this is where my research comes in – figuring out just how successful these structures have been. I’m still working on getting all the answers and will give more details on the monitoring side of my research in future posts (there are just too many cool details to squeeze into one short post!). I will answer one question now though – yes! animals are using these structures to cross the Hume Freeway and at this point, I think a picture is worth a thousand words.
van der Ree, R., Cesarini, S., Sunnucks, P., Moore, J.L., Taylor, A., 2010. Large gaps in canopy reduce road crossing by a gliding mammal. Ecology and Society 15.