With the thesis finally vanquished (more on that in a later post), it’s high time I put together a bit of a summary of what I found. Thankfully I had the opportunity to write an article for the Ecological Society of Australia’s Bulletin which does exactly that, so I’ll repost it here. Make sure you check out the rest of the ESA Bulletin (here). It’s a cracker of an issue if you’re interested in anything to do with urban ecology.
So, with no further ado, here’s a bit of an overview of my field, my study and some very cool Australian mammals.
Roads, (huh) what are they good for?
People depend on roads. We use them to get to work, to move goods around the country or to take an iconic road trip. That’s probably why in Australia we have over 800 thousand kilometres of roads, driven by more than 15 million vehicles. But our need to travel can take a toll on the environment, especially our native wildlife.
The spread of roads across Australia. The areas in dark green are more than 1 km from the nearest road. The white areas are not. (credit Google Earth Engine)
The most obvious issue is roadkill. We’ve all seen the carcasses that litter the roadsides and maybe we’ve even been unlucky enough to hit an animal ourselves. Most motorists wouldn’t realise that the roadkill problem reaches far beyond the unlucky individual. When many animals are killed it can drain the local population, reducing numbers to a point where species can become locally extinct.
Then there are effects that aren’t so obvious. Some animals avoid walking out on to the road’s harsh, unfamiliar surface or shy away from the noise and open space. Others can’t physically make it across the gap a road creates in their habitat. This means they’re trapped on one side of the road, unable to reach food, shelter or mates on the opposite side.
I work in the field of road ecology, where we try and understand the environmental impacts of roads, trains and other linear things and find ways to put a stop to all of this carnage. When it comes to wildlife, we often build wildlife crossing structures – bridges over roads or tunnels under them – to allow animals to cross safely on their own. But how well do these structures work? I aimed to answer this question for one of our threatened mammals, the squirrel glider.
A threatened species in a threatening landscape
The squirrel glider is a small gliding marsupial that is threatened with extinction in the south-eastern parts of its range. They move by gliding from tree to tree, with an average distance of 30–40 m. Gliders depend on mature woodland with big old hollow-bearing trees. Unfortunately, in my study landscape of north-east Victoria, all that’s really left of the pre-European woodland occurs in linear strips, mostly along roadsides. So to conserve this species we need to make sure that they can survive and flourish in roadside habitats.
The only woodland that’s left is along roadsides, paddocks and waterways.
One such roadside is the Hume, a four-lane interstate freeway. It’s travelled by about 10,000 vehicles per day, 25% of which occurs at night when native mammals like the squirrel glider are most active. The freeway ranges from 50–100 m wide and has been this way since it was upgraded around 40 years ago.
Those of you who are great at maths will have noticed that the width of the freeway is generally further than a squirrel glider can glide. Some researchers got to work investigating what sort of impact this had on squirrel glider populations. Through radiotracking studies they discovered that where treeless gap across the freeway was wider than 50 m, glider movement was heavily restricted (only 1 out of 50 crossed). Researchers also conducted mark-recapture surveys over 2.5 years and found that the survival rates of squirrel gliders living next to the freeway were 60% lower than those living further away. Gliders living along the freeway were facing a few challenges.
Safe crossings for high-flying mammals
That’s quite a gap for a small animal!
As a solution, two types of crossing structure were installed along the Freeway in 2007 – canopy bridges and glider poles. Canopy bridges are a kind of rope ladder, allowing animals to scurry across above the traffic. Glider poles are tall, wooden poles placed in the roadsides and centre median, and act as ‘stepping-stones’ for gliders to cross in a few short glides. Given that prior research
had highlighted two key problems – barrier and survival effect – I wanted to see if crossing structures would increase movement across the road and improve the survival rates of squirrel gliders that lived alongside the freeway.
Spying and stalking
A rope bridge over the Hume Freeway in Victoria (left) and glider poles on the Hume Freeway in New South Wales (right)
The first thing I looked at was animal movement, installing motion-triggered cameras to see if squirrel gliders would actually use canopy bridges and glider poles to cross the freeway. It took about two years before squirrel gliders got used to the structures and started crossing them regularly. In the seven years since, we’ve detected 1000’s of crossings. Gliders weren’t the only species making the most of the crossing structures
. We also spotted brushtail possums, ringtail possums, sugar gliders, brush tailed phascogales and even a lace monitor making the trek across the freeway.
Squirrel Glider on a rope bridge over the Hume Freeway in north-east Victoria.
I was also able to look at movements in a little bit more detail. There were microchip scanners installed on the bridges so that if an already tagged animal went across, we’d know who it was. This showed me that several different individuals crossed the canopy bridges, some of them multiple times each night to reach habitat on both sides of the road. I worked with a masters student Meli Carmody, who repeated that earlier radiotracking study
to investigate how road crossing behaviour had changed after the structures were installed. It turned out that where canopy bridges or glider poles had been installed, squirrel gliders were now able to cross the freeway at sites that had previously been a barrier to movement. At sites that were left with no structures, the freeway remained a barrier. This shows us that regular movements across the Hume Freeway wouldn’t be possible without crossing structures in place.
Our monitoring set up, complete with camera, microchip scanner and solar panels.
All this crossing activity was great, but did it result in gene flow? Were the animals that crossed the road “getting lucky” on the other side? To find out, mark-recapture surveys were conducted along the freeway to obtain tissue samples from squirrel gliders before and after the structures were installed. I was then able to use their genetic data to conduct a parentage analysis – identifying paternal and maternal relationships between individuals that lived on opposite sides of the freeway.
The analysis revealed that at sites where crossing structures were present, offspring occurred on the opposite side of the freeway to one or both of their parents. Somebody had to cross the road for this to happen! By cross-checking the identity of these family members with data from the cameras and microchip scanners, I was able to confirm that they had used the canopy bridges to cross the freeway. This reproductive success is an important component of gene flow and the effectiveness of crossing structures. Overall, I found that installing a crossing structure resulted in detectable improvements to gene flow within just five years.
A slide from one of my presentations, showing how family members occur on opposite sides of the freeway where a rope bridge is present.
Surviving in the danger zone
Finally, how did the crossing structures influence survival rates? The earlier mark-recapture study had identified a negative effect of the freeway on glider survival. I repeated that study, surveying for an additional five years after the structures were installed to see if survival rates had improved.
The results were surprising. It turns out that many of the animals that were missing and presumed dead at the end of the first study, were actually alive, and detected during later surveys in the second study. So the longer sampling period gave us a better estimate of survival rates because we had more of an opportunity to recapture previously tagged animals.
In light of all the other information, this finding actually makes a bit more sense. Radiotracking showed that gliders rarely crossed the road at sites where there were no crossing structures. If they’re not crossing the road, they can’t get hit by cars and so we wouldn’t expect survival to be reduced.
Monitoring effort matters
There’s more work to do, but ultimately my research suggests that crossing structures successfully reduce the effect of the road on squirrel gliders. I’ve showed that canopy bridges and glider poles can allow squirrel gliders to regularly cross the Hume Freeway, giving access to habitat on both sides as well as facilitating genetic exchange. But what I’ve really tried to show is that by using a comprehensive monitoring program we can be much more confident about how effectively crossing structures mitigate the negative impacts of roads on wildlife. Having information on movement, gene flow and survival collected before and after structures are installed was critical to getting the full picture.
When new roads are built, crossing structures are often installed as part of the environmental conditions of approval – granting permission for a road to potentially cause damage, on the assumption that a wildlife bridge or eco-tunnel will do the trick. If it turns out that these structures don’t work as well as we expected, not only are we wasting money on ineffective conservation measures, but we won’t know if we should be trying other things instead, or even seriously reconsidering where we allow roads to be built. Scientifically robust monitoring programs are the only way to truly understand these issues and make smart decisions for conservation.
As human populations spread, very few landscapes will remain road-free. Roads already cut through our national parks, conservation reserves and protected wildlife corridors. We must find ways of getting where we need to go without spoiling our natural heritage.
Soanes K, Carmody Lobo M, Vesk PA, McCarthy MA, Moore JL, and van der Ree, R. (2013) Movement re-established but not restored: inferring the effectiveness of road-crossing mitigation by monitoring use. Biological Conservation. 159: 434–441.
McCall S, McCarthy MA, van der Ree R, Harper MJ, Cesarini S and Soanes K (2010) Evidence that a highway reduces apparent survival rates of Squirrel Gliders. Ecology and Society 15.
van der Ree R, Cesarini S, Sunnucks P, Moore JL, Taylor A (2010) Large gaps in canopy reduce road crossing by a gliding mammal. Ecology and Society 15.