Bridging the great divide: road crossings and their value for wildlife

By Kylie Soanes (This article was first published in the April 2013 issue of Decision Point, The Monthly Magazine of the Environmental Decisions Group)

People need roads – whether it’s to travel across the country or nick down to the local shops. Unfortunately, roads – especially big ones – come with a high cost for many species of wildlife. For many animals, roads are big, noisy, sometimes lethal barriers. They cut through patches of habitat and restrict movement, dispersal and gene flow. They essentially have all the impacts of habitat fragmentation with the added risk of being flattened by a freight truck.

Road ecology is the study of the environmental impacts of roads and the ways we can try to mitigate these impacts. I see it as a bit like playing the 1980’s arcade game ‘Frogger’. There are a limited number of lives – our wildlife population – and we need to move animals safely across the road to access habitat, food and mates on the other side. If too many are killed by traffic, or don’t cross the road at all, we lose too many lives and the population goes extinct. Game over.

One of the most common methods of mitigating these impacts is to build crossing structures. These might be bridges that go over roads, or tunnels that pass under them. The purpose of these structures is to help wildlife cross safely.

Box 1 Easy snacks for wise predators?

Gliders are prey for large nocturnal raptors like barking or powerful owls. One concern is that an owl could sit and wait near a crossing structure for their next easy meal to walk across – they’re supposed to be wise after all. It’s something we keep an eye out for, but so far we’ve had no evidence of owl predation near the structures. Unfortunately, these owls are usually also threatened species and in a lot of places there aren’t many left to cause too much damage. If it were to become an issue we could install predator shields and refuge sites on the structures to give gliders and possums extra protection. It only becomes a problem, however, when the number of gliders eaten by owls while crossing the structure is more than the number of gliders that would be killed by traffic if the structure wasn’t there.

Bridging the gap for arboreal mammals

Not all roads are impassable to all species; but the bigger the road, the harder it is to cross. The squirrel glider, for example, is a small threatened marsupial in south-east Australia that has no problem crossing small roads. It’s great at crossing gaps in tree cover and can easily glide 30 – 40 m between trees. Unfortunately, major roads like the Hume Freeway (linking Melbourne to Sydney) are 50 – 100 m wide, and this presents a major problem for glider populations.

“To justify the cost of these efforts we need to know that mitigation is producing results”

Road ecologists working on squirrel gliders have used radio-telemetry to track the nightly movements of almost 50 gliders and found that while squirrel gliders could easily cross the short gap over quiet, single lane roads (control sites), they didn’t cross the 50 m gap across the freeway (van der Ree et al. 2010). However, at some freeway sites, squirrel gliders used tall trees present in the centre median as a ‘stepping stone’ and crossed in a few short glides. The freeway also reduced population survival rates, with mark-recapture revealing the survival rate of squirrel gliders living near the freeway was 60% lower than populations living at control sites further away (McCall et al 2010). The most likely cause is roadkill, as animals attempting longer glides across the freeway stray into the path of traffic.

Squirrel Glider on a rope bridge over the Hume Freeway in north-east Victoria.

As a result of this research, crossing structures were installed in 2007 at five sites where the Hume Freeway was a barrier to glider movement. These structures included glider poles and canopy bridges. The poles are tall wooden stakes, resembling oversized telegraph poles, in the centre median. They act as surrogate trees to reduce the gap across the freeway. Canopy bridges are long rope ladders strung between trees on either side of the freeway. They provide a structure for the gliders (and other wildlife) to climb across. These structures aimed to help squirrel gliders to cross safely, providing connectivity, reducing roadkill and ultimately enabling viable roadside populations.

Connectivity restored?

What we needed to know next was how well these structures worked and the first thing we investigated was animal movement. We installed motion-triggered cameras on the canopy bridges and glider poles to see which species would go across and how often.

Things started slowly – we detected only five squirrel glider crossings during the first two years. However, thanks to those brave pioneers, gliders eventually adapted to the structures and over five years of monitoring we’ve detected more than 2000 crossings (Soanes et al 2013). Other regular visitors include common brushtail possums, common ringtail possums, brush-tailed phascogales and sugar gliders. Even a goanna has had a go.

We also repeated the previous radio-telemetry study, allowing us to compare glider movements before and after mitigation at control sites and impact sites. Canopy bridges, glider poles and vegetated medians increased the probability that squirrel gliders would cross the freeway, while unmitigated sites remained a barrier to movement (Soanes et al 2013). However, no mitigation effort increased the probability that a glider would cross to the same level as control sites, those narrow, quiet roads. So, while movement was re-established, it was not restored.

Box 2 How effective a solution?

Mitigating the impacts of roads on wildlife is a major problem all around the world. Scientists working on this issue have even carved out a new discipline – road ecology. Millions of dollars are being spent on crossing structures helping everything from turtles to elephants safely cross roads. Many road agencies now rely on crossing structures to mitigate the impacts of construction projects on threatened species and meet regulatory requirements.

However, for the most part, we simply don’t know how well these mitigation measures really work because monitoring programs evaluating population-level impacts are rarely conducted. Long-term studies using before-after-control-impact population monitoring are required to truly evaluate the impacts of mitigation on population persistence. (see Decision Point 64)

It’s important to comprehensively evaluate these projects to ensure that successful strategies are widely adopted and unsuccessful ones are not repeated. If we’re going to rely on these structures for conservation, we need to be very sure that they work.

What’s next?

We now know that squirrel gliders need vegetated medians, canopy bridges or glider poles to cross major roads. We also know that even with these measures in place, the impact of the freeway on movement is only partially mitigated. What we need to know next is: Is this reduced movement enough to maintain viable populations and gene flow? If so, is it more cost-effective to install canopy bridges, glider poles or simply leave tall trees in the centre median?

We’ll answer these questions using before-after-control-impact monitoring. We’ll investigate changes in population size, survival rates, breeding patterns and gene flow because it’s not enough to know that animals use crossing structures. To justify the cost of these efforts we need to know that mitigation is producing results significantly better than no mitigation. If none of the methods effectively reduce impacts of roads on glider populations, we’ll need to look into alternatives.

It’s unrealistic to expect we will stop using these major roads (or that the building of roads will not increase), but it’s unacceptable that our need for road transport comes at the expense of continued species extinction. Mitigating the impacts of major roads on squirrel gliders is but the first step in the much bigger process of creating wildlife-friendly landscapes.


McCall SC, MA McCarthy, R van der Ree, MJ Harper, S Cesarini & K Soanes (2010). Evidence that a highway reduces apparent survival rates of squirrel gliders. Ecology and Society 15.

Soanes K, M Carmody Lobo, PA Vesk, MA McCarthy, JL Moore & R van der Ree (2013). Movement re-established but not restored: Inferring the effectiveness of road-crossing mitigation for a gliding mammal by monitoring use. Biological Conservation 159, 434-441.

van der Ree R, S Cesarini, P Sunnucks, JL Moore, A Taylor (2010). Large gaps in canopy reduce road crossing by a gliding mammal. Ecology and Society 15.

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One Response to Bridging the great divide: road crossings and their value for wildlife

  1. Pingback: Rope bridge helps Australian wildlife cross dangerous road | Dear Kitty. Some blog

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