Wildlife can have a tough time crossing roads. Noisy, fast vehicles and wide, open gaps in habitat make it an uninviting and risky venture. This means some animals are cut off from food, shelter or loving company on the other side of the road, young have trouble dispersing to find new territories and populations might become small and genetically isolated.
On the Hume Freeway in north-east Victoria, though, specially designed structures are making life a lot easier for squirrel gliders. Rope ladders bridge the gap between trees on either side of the freeway and wooden “glider poles” in the centre median and roadsides replace missing trees, helping these small, threatened marsupials cross safely.
Early research tracking the nightly movements of almost 50 gliders, showed that they used tall trees in the freeway median as a stepping stone, and crossed in a few short glides. But animals couldn’t cross where the gap across the freeway was wider than 50m (squirrel gliders glide an average of 30–40m).
On top of that, the average lifespan of gliders living near the freeway was estimated to be only about one year, compared with two to four years for those living further away. This reduced survival is most likely caused by roadkill at the freeway – in fact, one of the gliders we tracked was killed while crossing the road.
Based on this research, crossing structures were installed in 2007 at five sites where the freeway was a barrier to glider movement. The aim was for rope bridges and glider poles to provide safe passage for gliders across the freeway, re-connecting habitat and reducing roadkill - not to mention baffling passing motorists.
This is where my research project comes in, monitoring glider populations to figure out just how well these structures really work.
Over the past five years we’ve used motion-triggered cameras to spy on animals that use the crossing structures. It began slowly, with only a few gliders tentatively inspecting the structures during the first two years.
Since then, both the rope bridges and glider poles have become popular, with squirrel gliders crossing more than 2000 times. We’ve also detected common brushtail possums, common ringtail possums, sugar gliders, brush-tailed phascogales, and even a goanna using the structures to cross the freeway.
We also repeated the original tracking study to compare squirrel glider movements before and after the structures were installed. Within four years of installation, rope bridges and glider poles re-established glider movement across the freeway, whereas sites without crossing structures remained a barrier. However, even with crossing structures installed, gliders are still around half as likely to cross the freeway as they are to cross quiet, narrow roads, meaning the barrier effect of the freeway is only partially repaired.
Still, a partial fix may be good enough if it allows squirrel glider populations to persist. We’re investigating changes in population size, survival rates and gene flow to see if these have improved since the structures were installed. Only then can we know if the rope bridges and glider poles are successful.
Gliders aren’t the only animals getting the crossing structure treatment. Millions of dollars are spent on culverts, tunnels, and land-bridges all over the world, helping everything from turtles to elephants get safely across roads. Evaluating the success of these efforts is critical, because if we’re relying on these structures to mitigate road impacts on threatened species, then we need to be very sure that they work.