Wednesday, November 9, 2011

Road Construction and Kerinci Seblat National Park Part 2: The Edge Effect and other Ecological Impacts

Cartoon from here.
Few forces have been more influential in modifying the earth than transportation -- EL Ullman in The Role of Transportation and the Basis for Interaction, 1956.

Over the past couple of posts I've been discussing the "road problem" at Kerinci Seblat National Park.  Currently there are at least 32 proposals that would penetrate the interior of the park.  In the first post I talked about why people want roads.  In the second post I described how the lack of roads impacts residents of Kerinci valley while simultaneously suggesting that additional roads might not be the best solution to the problem.  Today I'm going to look at the ecological impacts of roads.  Though as an impartial and neutral researcher I have no dog in the fight, I do think it's important that policymakers have as thorough an understanding of what's at stake before making far-reaching and irreversible decisions.  Most people are not aware of the multitudinous ways that roads alter the environment.  From the actual surface itself to the "road corridor" (the road surface plus maintained roadsides and any parallel vegetated strips) to the interior several hundreds of meters away from the road itself, the road trace shapes the surrounding environment in obvious and not-so-obvious ways.  

Deforestation associated with new roads in Amazon basin.  From Mongabay.
The well-publicized arguments against roads through Kerinci Seblat National Park (TNKS) focus mainly on very visible secondary effects; anti-road folks are worried about the baggage that comes along with a road.  They argue that roads provide access for illegal loggers and poachers.  They also argue that the roads create avenues for frontiersman-farmers to open up new fields to cultivation.  All of these arguments are based on experiences common across Sumatra; in the loose regulatory environment it has proven very difficult to stop forest destruction stemming from roads.  In addition, dwellings and shops tend to be established along the road.  In addition roadkill is an ever-present concern when constructing new roads.  In the US, for example, motorists now kill more vertebrates than hunters, and it's estimated that one million vertebrates are killed each day on roads there.  But besides these commonly-cited concerns there are a whole host of more subtle direct and indirect effects from roads.  Though geographers have traditionally been very interested in roads, networks, and transportation, the relatively new field of transportation ecology has made the most significant contributions to our understanding of these complex impacts on the environment.

Lesser-known Impacts of Roads on Animals and Plants...

The most conspicuous direct impact of roads is on certain large wildlife species.  Specifically at TNKS conservationists worry about how the roads will affect one of the last remaining populations of Sumatran tigers, which are very endangered (1).  Tigers are what ecologists refer to as an "interior species"; this means that they avoid the edge areas of forests.  They also need large "patches" of forest to thrive.  The construction of roads fragments the habitat and thus divides the tiger population into smaller populations.  The tigers won't cross the road to find mates, and in the long run these smaller populations are much more vulnerable to extinction than one large, contiguous population.  Avoidance behaviors have many different causes; traffic noise, visual disturbance, pollutants, and predators moving along the road have all shown to contribute.  This is known as the barrier effect.  Roads also affect tiger livelihoods; Kerley et al (2002) showed that Amur tigers living in roadless areas stayed longer at kill sites, ate more meat, and thus lived healthier and survived longer than tigers living near roads. 

Roads have species-specific effects which also vary depending on location, traffic, and road conditions.  For example, one study showed that moderately traveled tropical roads (as opposed to heavily-used roads) have higher incidents of roadkill for amphibians and reptiles.  Possibly related to this is the fact that blacktopped roads absorb a great deal of solar radiation, re-radiating it at longer wavelengths as thermal energy.  In other words, the road gets hot, which attracts animals like reptiles that use the external environment to regulate their body temperature.

The Edge Effect...

Diagram from Proust Bushland Services
Roads create "ecotones", which are abrupt transitions between different ecosystems.  There are natural ecotones, like the transition between a meadow and a forest, but these are a part of nature and there is usually a systemic equilibrium surrounding them.  Roads emerge out of nowhere and are more or less permanent.  Ecologists use the term "edge effects" to refer to the specific characteristics of these places.  Mortality for certain species (many species of birds fall into this category) is greater at the edge because they are more visible to predators.  Researchers have demonstrated many other subtle effects of roads as edges; for example, vibrations associated with traffic may affect the emergence of earthworms from the soil thus negatively impacted crow populations!  Edges also affect plants; these are areas of increased light which allows opportunistic species to grow rapidly.  There are certain species of plants that are very good at colonizing edge areas, and roads open up new spaces for expansion.  In addition, seeds can be carried and deposited along roads by passing vehicles, which opens avenues for invasive species.  Constructing the road can help invasive species to spread as well.  One very well-known example of this is the spread of Imperata cylindrica, a type of grass commonly found in the US and known in Indonesia as ilalang.  This grass was originally introduced in Florida in the 1940s and 1950s to control erosion.  Though this grass doesn't spread very far on the wind, it moved quickly through the state by hitching a ride on road construction equipment.  In addition, the rhizomes (like roots that help the plant reproduce itself) got mixed up in road fill material which was spread at construction sites.  The rhizomes quickly took root, allowing the grass to spread.  Byt the 1980s the grass was considered a major problem throughout the state (2).  Air turbulence caused by vehicles also might help seeds to disperse. 

Other Ecological Effects

From Chicago Wilderness Magazine
In addition to direct and indirect effects on plants and animals, roads alter the environment in other ways as well.  The impacts of roads on hydrology and streams has been extensively studies.  Roads in hilly areas do a really good job of concentrating water flows.  This has some pretty esoteric impacts on watersheds and catchment areas which you can read more about in the references cited below.  More basically, though, concentrated water flows means that the flow has more energy and generally moves faster than in areas with no roads.  This means that more water enters rivers faster, increasing the occurrence of flooding and the rate of erosion.  Increased erosion creates a cascade effect; when more sediment is suspended in the stream the river gets turbid and it makes it harder on certain aquatic species.  At the same time, all of that sediment that has been eroded upstream has to be deposited somewhere, and so streams silt up and get shallower.  Shallower streams combined with the increased turbidity and less vegetated banks increases water temperature of rivers, which can stress certain species.  In addition, silt gathers behind dams, decreasing their useful life.  Though one road through the park is unlikely to have huge effects on drainage and streams, a lot depends on the way the road is constructed.  And though one road might not have as much of an impact, 32 roads definitely would.  In addition to these, roads serve as conduits for pollutants to enter streams.  

As you can see, there is a lot more to the road than meets the eye.  Researchers have learned much about the ecological impacts of roads, but there is still a tremendous amount of research to be done.  Much of the lessons of roads have been learned via experience.  Australia, the US, and the Netherlands are all countries where the impacts of roads have been studied after the fact.  Hopefully other countries, including Indonesia, can utilize this experience for better policy making. 


(1)  Estimates as to the number of Sumatran tigers remaining range from around 300 to 1000.  According to recent research, Kerinci Seblat is the biggest remaining habitat for the tigers and has the highest numbers.

(2)  Roads also helped the spread of fire ants, which were introduced to the US in the 1930s in Mobile Alabama.  Though the ants can thrive in any habitat, they are most often found within 150 meters of roads. 

References and For Further Reading....

Coffin, Alisa.  2007.  From Roadkill to Road Ecology: A Review of the Effects of Roads.  Journal of Transport Geography 15, pp396-406

Forman, Richard.  2003.  Road Ecology: Science and Solutions.   Washington, D.C.: Island Press.

Forman, Richard and Lauren Alexander.  1998.  Roads and Their Major Ecological Effects.  Annual Review of Ecological Systems 29, pp207-31

Kerley, Linda, John Goodrich, Dale Miquelle, Evgeny Smirnov, Howard Quigley, and Maurice Hornocker.  2002.  Effects of Roads and Human Disturbance on Amur Tigers.  Conservation Biology 16:1 pp97-108.

1 comment:

  1. I didn't know that roads could also be so destructive in more subtle ways than just clearing the path to create them! It seems that building a road always entails some sort of negative consequence. Although you did talk about the negative impacts that the roads have had on the environment of Indonesia, what about the effects that they have had on human populations, particularly in the cities? Are most roads built near areas of lower-income dwellings?