Smithsonian Scientists Say Roads Are the Number-One Threat to World’s Tropical Rainforests

October 14, 2009
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Infrastructure such as roads, canals and power and gas lines are pervasive features of human activity and are rapidly expanding in the tropics. Smithsonian scientists say that these linear clearings could be the biggest threat to the world’s tropical rainforests.         

“Clearing wide paths in any forest has a strong effect on the ecosystem, but these impacts are particularly acute in tropical rainforests,” said William Laurance, a biologist at the Smithsonian’s Tropical Research Institute in Panama. Laurance recently coauthored a paper in the journal Trends in Ecology and Evolution with Susan Laurance and Miriam Goosem. Susan Laurance is also a biologist at STRI, and all three authors are researchers at James Cook University in Australia.

The team used dozens of existing studies done in the Amazon, Australasia and Central Africa to emphasize that roads are the number-one threat to the world's tropical rainforests. The scientists urged that maintaining large areas of intact forests without roads should be the highest priority of conservationists worldwide.

Biologically, rainforests are characterized by a complex architecture and a uniquely humid, dark, stable climate. They sustain many species that are incredibly specialized for the forest’s interior and understory conditions. Other tropical species are susceptible to hunting, increased predation, invasive species and being killed by vehicles.

From a socioeconomic perspective, tropical rainforests are mainly in developing nations, many of which experience continued population growth, rapid economic development and intense natural resource exploitation. In many of these areas, industrial logging, oil and gas development and large-scale agriculture and mining provide an economic impetus for the expansion of road and infrastructure developments. “The roads and paved highways that this creates play a key role in opening forested regions to exploitation from hunters and miners—exacerbated by often weak enforcement of environmental laws in remote frontier areas,” said William Laurance. New logging roads make forests greatly more accessible to exploitation by hunters, miners and settlers. Disease and invasive species generally follow the influx of humans.

Roads and linear clearings also act as barriers that greatly affect water drainage, erosion and fire-maintained tropical woodlands, as well as animals. A striking feature of tropical forests is the high proportion of species that tend to avoid even narrow clearings or forest edges. Many species (such as those that are completely arboreal, adapted to flying in dense forests or depend on specialized food resources) are halted by linear clearings. 

Some species, however, do not avoid roads or other such clearings, resulting in what the scientists call “road-related mortality.” The most obvious form is often vehicle road kill. Anteaters with poor eyesight, slow-moving sloths and the Australian echidna—whose reaction is to freeze in the path of oncoming vehicles—are some of the many rainforest species that are particularly vulnerable to being struck by a vehicle.

The scientists also found levels of chemical and nutrient pollution to be elevated in areas where roads have been built. Effects of chemical pollution and nutrient runoff are especially serious for streams and wetlands near roads.

Limiting the width of roads, reducing vehicle speeds and maintaining a continuous forest canopy above roads are ways to reduce the impact on tropical rainforests. However, not building roads in the first place is the best option, the scientists suggest. “Actively limiting frontier roads is by far the most realistic, cost-effective approach to promote the conservation of tropical nature and its crucial ecosystem services,” said Laurance. “As Pandora quickly learned, it is far harder to thrust the evils of the world back into the box than to simply keep it closed in the first place.”

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