What can be learnt from case studies? Dr Rodney van der Ree, Part 4

Roads and other types of linear infrastructure dissect most types of landscapes throughout the world. Road networks are continuing to expand, as new roads are constructed or existing roads are widened to accommodate an increasingly mobile society. The ecological impact of roads and traffic are potentially profound as the road-effect zone (after Forman et al.1997) may extend for many hundreds or thousands of metres either side of the road. Roads can have both positive and negative effects. For example, roads necessarily result in the loss of habitat, can cause the degradation of adjacent habitat and act as a barrier or filter to the movement of fauna. Other effects include noise and light pollution affecting wildlife and air and water-borne chemical pollution affecting plants and aquatic systems. On the positive side, vegetation along roadsides often represent the only native habitat in highly cleared landscapes, and thus may have significance for the conservation of biodiversity and maintenance of landscape processes. The significant challenge for management and conservation is to identify and quantify the extent to which roads and traffic disrupt and modify ecological processes and construct systems that minimise the negative consequences. The extreme cost to build a road in terms of dollars and potential environmental impact demands that we get it right.

In this talk I will summarise the effects of linear infrastructure on wildlife species under three broad categories, proposed by Jaeger et al (2005). These are species that: (i) avoid the road surface; (ii) avoid traffic emissions and disturbance (e.g. light, noise, chemical emissions); (iii) avoid vehicles - the ability (or inability) of the animal to move out of the way of oncoming vehicles; and (iv) those that are attracted to the road. The use of these four categories is useful to characterise species responses because it allows us to identify the problem and consider suitable and logical approaches for mitigation.

Species that avoid the road typically have low-rates of mortality due to collision with vehicles because they rarely venture onto the road. For these species, barrier effects will be high and wildlife crossing structures may not help because the animals do not approach the road and will never reach the crossing structure. Road avoidance also encompasses the situation where the linear infrastructure is fenced or designed in such a way that it represents a complete barrier to movement. For example, a steep cutting or embankment may be a sufficient deterrence to create a barrier before the road is even reached. Habitat area and quality is reduced when animals avoid habitat adjacent to the road due to traffic emissions or disturbance. The size of this area will increase as traffic volume and/or traffic speed increases. Species that are able to avoid cars are those that are willing to attempt to cross the road and are able to do so without collision. Some species are attracted to the road (e.g. basking by reptiles) or the resources available on the roadside (e.g. carrion or increased grass growth), which, depending on their ability to avoid cars, may result in either negative or positive consequences.

Clearly the type and severity of road impacts on wildlife is dependent on the characteristics of the species, the landscape, road design and traffic conditions. The combinations of the different responses will require different types of mitigation. However, there are still significant knowledge gaps that must be filled to achieve successful mitigation and a sustainable road network. These include further study of population processes such as demography, dispersal, spatial patterning and comparing these with contiguous habitats to determine road effects and ultimately assess population viability. A potentially important area of research are interactions and exchanges between road systems and adjacent habitats, including the spatial extent and ecological impacts of the movement of biota, of altered hydrology, flows of sediments and particles, and noise effects. Finally, an understanding of landscape-level effects of road systems, including properties of different network structures and the effects of road density on ecosystem processes, is also required.

References

Jaeger J. A. G., J. Bowman, J. Brennan, L. Fahrig, D. Bert, J. Bouchard, N. Charbonneau, K. Frank, B. Gruber & K. T. von Toschanowitz. (2005) Predicting when animal populations are at risk from roads: an interactive model of road avoidance behavior. Ecol. Model. 185, 329-48.

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Science & Technology