THE EUROPEAN TREE FROG HAD a problem. The dense network of roads built to serve the people of the Zeeuws Vlaanderen region of the Netherlands had thoroughly carved up its habitat. Each of the shreds that remained held too few frogs to create a thriving population. The roads isolated the fragments so effectively that even purpose-built frog underpasses wouldn’t solve the problem. As a last resort, conservationists opted to create new habitats to compensate for those lost to road development – but where?
The plight of the European tree frog is a familiar one. Engineers around the world have grown accustomed to dealing with the environmental fallout from roads that accidentally block animals’ movement, disturb wetland drainage or act as a beachhead for invasion of alien species. But it would be far better if they could spot potential problems right at the beginning and build road networks that were intrinsically environmentally friendly.
Often that simply hasn’t happened, says Harvard landscape ecologist Richard Forman – largely because ecologists and road planners approach the problem differently. Ecologists study how organisms live and move across entire ecosystems and speak in terms of populations and dispersal patterns, while road planners and engineers take a much narrower view, typically focusing on what’s happening at the roadside itself. For instance, roads can alter streams hundreds of metres away, but road planners can easily overlook the problem if they look only at the bits of land immediately beside the roads. If only ecologists and engineers could find a shared language for talking about roads and their effect on the environment.
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Forman thinks he may have found common ground. The key is a concept he developed which he terms the “road-effect zone” – the total area over which a road exerts its ecological influence. The idea that a road’s influence extends beyond the pavement is scarcely new. But by drawing the REZ on a map, Forman has given road engineers an easy way to visualise a road’s ecological footprint. He hopes engineers will now find it easier to design road networks that tread more gently on the land. “Here’s a tangible, concrete zone where statistically significant ecological effects extend,” says Forman. “In planning for upgrades and new roads, even maybe maintenance, this is the zone that we should be looking at.”
The REZ concept is so new that most road designers haven’t even heard of it yet, but the few who have are enthusiastic. “Forman has translated the problem into terms that we can understand, and that’s a big step,” says Ian MacGillivray, a road engineer at the Iowa Department of Transportation. REZ maps provide information in a format engineers can easily use with their other planning maps, he says, so that ecological considerations can take an equal place at the table during the design process. And once engineers recognise and understand a problem, they can find solutions.
Mending fences
Wayne Kober, a transportation and environmental management consultant in Dillsburg, Pennsylvania, agrees. “This would promote building environmental quality in from the onset of the design process, rather than responding to controversy caused by an insensitive design,” he says. Kober believes REZ mapping could even help mend fences between environmentalists and road planners. “One of the side benefits of this approach is that it engages the whole community in recognising and working together to protect important ecological resources,” he says.
It’s not that road engineers and ecologists have never tried to solve environmental problems together before. But most of their work has focused on an isolated concern, rather than a road’s overall impact on the surrounding ecosystem. In Alberta’s Banff National Park, for instance, the federal government has installed fences, underpasses and overpasses along a 45-kilometre stretch of the Trans-Canada Highway, slashing road kill involving deer, elk and other hoofed mammals by 96 per cent. In the north-western US, salmon and sea-run trout must swim upstream to spawn. Road culverts can make this journey even more hazardous, so the states of Washington and Oregon are replacing old culverts with more fish-friendly ones.
Such programmes may solve isolated problems but they are not far-reaching enough, says Forman. “You can look at individual road kills or you can look at noise or you could look at stream degradation and various other road effects, and in each case, you would come to a different solution for the road if you look at it piecemeal like this,” he says. Engineers could do a better job if they looked at the big picture, he believes – one that took into account patterns of movement across landscapes, including everything from water to wildlife. The REZ framework may make this possible.
To demonstrate his new approach, Forman and his colleague Robert Deblinger, an ecologist with the Massachusetts Division of Fisheries and Wildlife, mapped the road-effects zone for a busy 25-kilometre stretch of four-lane highway west of Boston. The stretch of Route 2 they chose (see Diagram), which begins 10 kilometres west of the city of Cambridge, winds through forests, wetlands, open fields and residential neighbourhoods and carries 50,000 vehicles per day at speeds of up to 90 kilometres per hour.
The researchers ended up with an irregularly shaped REZ in which topography, landscape, land use and waterways all dictated how far the borders of the zone reached. In some areas, watershed disturbances such as draining wetlands and rerouting streams set the outer bound for the REZ. Elsewhere exotic plant species, which had spread from the roadside at least 1200 metres into the forest, determined the extent of the REZ. In other places road noise defined the boundary – Dutch studies have shown that it causes bird populations to decline (Conservation Biology, vol 14, p 36).
Sometimes, though, a road’s main effect might be harder to see, like the Iowa highway that blocked the migration of salamanders and disturbed both forest and wetland. Such habitat fragmentation can sometimes take a heavy toll, says Forman. A recent German study, for example, showed that bank vole populations living on opposite sides of four-lane highways in the forests of southern Germany and Switzerland interbred so little that they had become genetically different (Conservation Biology, vol 14, p 1066).
The far-reaching consequences of habitat fragmentation can carry the REZ far beyond road planners’ typically neat roadside corridors, says Forman. In this case, it’s especially tricky but not impossible to draw the boundary around the REZ, Forman notes. For example, over the past decade or two ecologists have gathered quite a bit of information on the minimum size a habitat patch can be to support particular sets of species. If a road bisects a woodland, for example, and neither of the resulting fragments is big enough, then species will gradually bleed away from both. The road effect zone should encompass the entire woodland.
By mapping the REZ along roads in many different ecosystems, Forman hopes researchers can learn to predict how future roads will influence their environments. Map the REZ along enough highways in high desert ecosystems, for instance, and you’ll have a blueprint for predicting where the REZ will lie along other highways in similar ecosystems. Armed with blueprints for a variety of ecosystems, researchers can decide where to build roads through such landscapes and pinpoint areas along the road that need special responses such as underpasses and noise barriers.
Networking
One of the strengths of the REZ framework, says Forman, is that planners can use it to grapple with the effect of entire road networks, not just individual roads. With a single map showing the REZs of all the roads in a region you can see at a glance which areas remain relatively pristine and which are already so degraded that further construction will make little difference. Likewise, while a single road may only limit animal migration in one direction, several roads can create islands of habitat that are cut off on every side. Such an overview should help planners reduce the overall impact of roads in large natural areas so they don’t become fragmented. For example, Forman estimates that nearly 20 per cent of the total land area of the US now lies within at least one REZ. With careful engineering of road upgrades to shrink REZs right down and minimise habitat fragmentation, Forman thinks that the fraction could be cut to 10 per cent or less.
Much of the detailed information needed to accomplish the reduction, in the US and elsewhere, may emerge from an organisation called the Infra Eco Network Europe (IENE). Brainchild of the Dutch Ministry of Transport, IENE is a network of 19 European countries sharing information and ideas about road ecology. The IENE project aims to reduce the environmental impact of roads and conserve biodiversity through research. A related programme, COST 341 (cooperation in the field of scientific and technical research), is creating a handbook for road planners describing how they can build roads with a minimum of habitat fragmentation. Success will depend on looking at the road network as a whole and planning road systems to suit the ecosystem, says Hans de Vries, an engineer at the Dutch Ministry of Transportation, Public Works, and Water Management.
Large-scale planning is essential, de Vries says, because sometimes there’s no effective solution. As an example, he cites the problems his group encountered with the tree frog. “The lesson we’ve learned here is that prevention is better than trying to mitigate the problem afterwards,” he says. Since they couldn’t fix the frog’s habitat loss, scientists tried to compensate for it instead. “We needed to create new stepping stones between the habitats,” says de Vries. There, too, an REZ-like overview of the road network was crucial because it helped identify relatively undisturbed areas suitable for new stepping-stone habitats as well as giving some idea how far apart to space them.
But no matter how successful REZs turn out to be, the approach has its limitations. “We will never be able to fully mitigate all the negative effects roads have,” says Marcel Huijser, a road researcher with the Dutch transportation ministry. And it’s difficult to know whether schemes will work in the long run. “It’s easy to find that a species uses a tunnel, but it’s hard to measure whether that’s enough to keep the population viable,” he says.
There’s also no getting around the fact that roads let people into areas they might otherwise leave alone, says Brian Horejsi, a Canadian wildlife scientist who studies grizzly bears. “The real issue is human behaviour. You can’t divorce the activities that come with roads from the roads themselves,” he says. “There is no track record of success here. We can’t just limit roads, we must define thresholds of human activity that we’re going to allow in sensitive ecosystems.” Horejsi says roads and the people they bring are one of the largest threats to grizzly bears, and road-less areas are essential for the bears’ survival.
No-go zones
“You can reduce the impacts of a road, and that’s good. But we need to realise that these technologies still don’t make a road invisible ecologically,” says Bethanie Walder, a forest ecologist and executive director of the Wildlands Center for Preventing Roads in Missoula, Montana. “We’re not saying we have to get rid of all roads. We’re just saying that there are some places where it’s not appropriate to build roads.” Crucial bear habitats and steep hillsides prone to erosion, for example.
Most people agree there needs to be more emphasis on road planning. “The debate about roads needs to shift away from the question of, ‘are roads good or bad?’ to the real question – how many roads do we need and where should we put them?” says David Haskell, an ecologist at the University of the South in Sewanee, Tennessee. Answering that question as a team is the key to creating environmentally friendly roads. “The only way we’re going to get solutions is if the transportation community gets engaged in working with the ecological community,” says Forman.
In Europe, the IENE is paving the way for this type of cooperation. In the US, they’re making progress even if it is less organised. “There’s a new awareness emerging now. We’ve hired a biologist to work with us, and we never used to do that,” says MacGillivray. “We used to think of things just in terms of the immediate road area. Today we look a lot further.”
Careful planning might also help reduce the ecological cost as less developed countries push roads into new regions such as the Amazon rainforest (91av, 27 January, p 10). “Roads are an absolute disaster for ecosystems, and the reason is that they let people in,” says Terry Erwin, an entomologist at the Smithsonian Institution in Washington DC. But REZs could help conservationists identify and fight for large enough blocks of relatively pristine nature.
“I’m optimistic,” says Forman. “I believe we’ve turned a corner. We can help society accomplish its goals of safe and efficient mobility while providing for natural processes and biodiversity. I see the two being perfectly meshable. This is a new dawn.”
