Road building amply illustrates the kind of difficult trade-offs between social, economic and environmental interests that are inherent in sustainable development. While there is broad – if not universal – agreement that access to roads benefits formerly isolated communities both economically and socially, it is also well recognized that they come at the cost of local biodiversity, both directly and indirectly through environmental damage following the arrival of new settlements and associated clearance of native vegetation. From a sustainability point of view, the challenge is to avoid or minimize the environmental impacts while still providing the socio-economic benefits.
Road building affects biodiversity in a variety of ways, not all of them fully understood, or even known. The most obvious ways are direct habitat loss, the risk of animals being hit by vehicles using the road, and air pollution. Added to these are fragmentation of habitats, with roads acting as a barrier to dispersal and foraging by animal communities; and easier access for hunting, logging and other potentially unsustainable human activities. Taken together, these effects can result in the increasing dominance of species that proliferate along forest edges and in more disturbed habitats, and a concomitant decline species that depend on interior habitats.
A new paper published in the Proceedings of the Royal Society B: Biological Sciences reports on the results of a recent study of biodiversity impacts in the eastern Brazilian Amazon, an area where major expansion of road networks looks set to continue, radically transforming vast areas of relatively undisturbed rainforest. To do this, the researchers compared the number and mix of forest birds in 18 small hydrological catchments with the distribution of road networks in 2000 and 2008, using a new metric called roadless volume (RV).
RV has been recently developed to assess the environmental impacts of road networks in the United States. Rather than simply measure the area not under tarmac, RV gives a higher value (i.e. a higher RV) to habitats depending on how far they are from all roads. An important contribution of this research was to test whether RV can, in reality, predict biodiversity changes and thus contribute to more sustainable road network planning.
For four months in 2010-2011, a team of field researchers from the Sustainable Amazon Network conducted the most comprehensive survey to date of forest birds in the municipalities of Santarém and Belterra, in Brazil’s Amazonian Pará state. Based on the number of birds of different species heard or sighted, they were able to measure how the number and mix of forest species differed among catchments characterized by different levels of road development across the region.
Even though roads are not a physical barrier to most bird species, the results were clear: biodiversity had suffered from road building. In driving habitat loss and fragmentation, overexploitation of forest resources, and disrupting patterns of movement, roads can lead to the extinction of local species while also isolating sub-populations of the remaining species and making them more vulnerable.
Even more interestingly, RV was a better predictor of biodiversity impacts than direct habitat loss, indicating that it can help to make better predictions of habitat changes such as the proliferation of ‘edge’ environments, and associated changes in forest structure microclimates. In general, areas with higher RV also appear less vulnerable to selective logging, hunting, fires and roadkill.
These findings of the study reinforce those of Laurance et al. in a global analysis on strategies for road building to minimize environmental damage that was published in Nature magazine last month. This analysis offers a template for proactively zoning and prioritizing roads during the most explosive era of road expansion in human history.
‘One clear message from our research for road planners in the Amazon is that it’s not just how many roads you build, but where. If you want to safeguard the Amazon’s world-famous biodiversity, you need to leave at least some habitats far from roads,’ says SEI Research Fellow Toby Gardner, one of the researchers on the study.
The study was funded by the UK and Brazilian governments, The Nature Conservancy, Microsoft Research, the Grantham Institute for Climate Change and the European Research Council.
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