Move over, chickens! How can koalas more safely cross roads?

Most of us can’t live without roads. Until we humans evolve flight (or invent scalable rocket packs) we’re going to have to get used to moving around the world stuck to the ground, at least for traveling short distances, and we generally use roads to get us from point A to point B. But while they make our own lives a bit more convenient, roads are bad news for terrestrial wildlife. Those critters, like us, are also stuck to the ground.

Roads directly affect wildlife in a few different ways. For one thing, building roads usually requires the destruction, if not the intense modification, of natural habitats. As a result, animal behavior is often affected. Roads form barriers, which fragment natural habitats, and when animals try to cross roads, they can be fatally injured from vehicle collisions. Developing road networks also indirectly impact wildlife populations because they allow humans increased access to parts of the habitats that were previously inaccessible. Humans bring with them things like litter, noise, and disease.

Here’s the problem: the number of vehicles on roads is increasing. In many cases, the solution to the traffic problem is to build more roads, or widen existing roads. And while that might alleviate the hassle for our species, it only makes things worse for wildlife. Most research on the effects of road network design on wildlife populations has quantified impacts of currently existing roadways and the effects of small-scale mitigation measures, like fencing or overpasses. What’s needed is an evaluation of the implications of wholly different road network designs. For example, is it better to have a few wide roads or many narrow ones? That’s what Jonathan R. Rhodes and colleagues, from the University of Queensland, in Brisbane, Australia, have attempted to discern. They reported their findings in PLoS ONE.

Rhodes applied a set of mathematical models to real-world data on koala population structure and movement patterns taken from the Port Stephens Local Government Area in New South Wales, Australia. The area, some 150km north of Sydney, is home to one of the state’s more significant koala populations. While koalas are adept at climbing trees, they can only move between trees by walking on the ground. And that includes crossing roads. That’s why vehicle collisions are the most frequent cause of death for koalas there, though they also suffer from dog attacks, disease, and brushfires. More generally, they’re affected by habitat loss and fragmentation due to agriculture, mining operations, and urbanization.

koala roads Move over, chickens! How can koalas more safely cross roads?

The study area’s location in Australia, the estimated distribution of koala habitat, and the estimated average daily traffic volume on major roads.

More vehicles, the researchers reasoned, could be accommodated in two ways: by widening existing roads to allow for a greater volume of traffic, or by building new roads, creating a more dense network. Both could result in increased animal mortality, but for different reasons. Under the first solution, more animals would die because they would be more likely to be struck by a vehicle while crossing a road. Under the second solution, more animals would die because they’d be more likely to encounter a road while moving through their territories.

“In the vast majority of cases, we found that increasing road density elevated mortality rates more rapidly than did increasing traffic volume on existing roads,” Rhodes says. “Our studies indicate that strategies that focus on the creation of new roads are likely to be more harmful to wildlife than those that build capacity within an existing network.” So fewer, wider roads are better for koalas than more roads overall. They also found that males were more susceptible to vehicle-related injuries than females, simply because they tend to have larger home ranges and move more on average, especially during breeding season. That makes them more likely to need to cross roads.

The researchers are quick to note that policymakers have to account for more than one wildlife species when making infrastructure decisions. While this model could be applied to other species with similar movement patterns, it can’t account for other species’ road avoidance and driver visibility. Koalas, after all, are fairly small; ungulates tend to be far more visible and easily avoided. And, of course, alternative road network design ought to be used in combination with other efforts to mitigate vehicle-animal collisions, like speed reduction measures, fences, overpasses, underpasses, lighting, signs, and more.

When a “more optimal” solution simply means “fewer fatalities,” it may seem like a callous sort of mathematical exercise. But this is the sort of information that is useful to policymakers when faced with the specter of the ever-increasing number of cars, a problem that does not appear to be going away anytime soon. – Jason G. Goldman | 26 March 2014

Source: Rhodes J.R., Lunney D., Callaghan J., McAlpine C.A. & Ouzounis C.A. (2014). A Few Large Roads or Many Small Ones? How to Accommodate Growth in Vehicle Numbers to Minimise Impacts on Wildlife, PLoS ONE, 9 (3) e91093. DOI:

Images: shutterstock.com, Rhodes et al., (2014).

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