A few large roads or many small ones? How to accommodate growth in vehicle numbers to minimise impacts on wildlife
Rhodes, JR, Lunney, D, Callaghan, J & McAlpine, CA 2014, PLoS ONE, vol. 9, no. 3, e91093.
This study determined how koala mortality rates would be impacted by either increasing traffic volume on existing roads, or increasing the density of the road network in Port Stephens, New South Wales. This was done using simulation models of koala movement in combination with a model of the risk of mortality when crossing a road. The authors found that the effect of increased road network density on koala mortality was higher than the effect of increased traffic density on existing roads, although both effects caused the mortality of koalas to rise. The only time that increased traffic volume had a higher impact on koala mortality than increased road density was when road density was extremely low and traffic density was extremely high.
For this case study, an approach called ‘elasticities’ was used. This approach allows the user to quantify the proportional change in one variable in response to a proportional change in another variable. It was found that the elasticity of road density was approximately 50% higher than that of traffic volume. It was also found that male koalas were more susceptible to road fatalities than female koalas and this was primarily due to their larger home ranges and greater travelling distances. The authors acknowledged some uncertainty in their model’s parameter estimates but found consistent results under different simulations; that mortality was more elastic to road density than it was to traffic volume.
Some factors that were not incorporated into the model but could have an effect on koala mortality include avoidance behaviour of koalas towards roads, and visibility of a koala on the road which could result in a driver swerving to miss the animal. These factors could increase the probability of survival for koalas on roads. Furthermore, other strategies such as animal road crossings, fences, signs and lighting are currently in use on roads to help reduce mortality rates. Therefore, reducing the density of roads is not the only option available to planners to help minimise road fatalities.
The main implication from this study is that in order to minimise koala fatalities (and those of other mobile animals) in the future, upgrading existing roads to support higher traffic volumes will have a lesser impact on koalas than building new ones to reduce traffic volume on existing roads. The location of future roads is also critical, and choosing to build new roads away from critical koala habitat will help to minimise fatalities.
Summarised by Miranda Rew-Duffy
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