We live in an era of unprecedented road and highway expansion — an era in which many of the world’s last tropical wildernesses, from the Amazon to Borneo to the Congo Basin, have been penetrated by roads. This surge in road building is being driven not only by national plans for infrastructure expansion, but by industrial timber, oil, gas, and mineral projects in the tropics.

Few areas are unaffected. Brazil is currently building 7,500 kilometers of new paved highways that crisscross the Amazon basin. Three major new highways are cutting across the towering Andes mountains, providing a direct link for timber and agricultural exports from the Amazon to resource-hungry Pacific Rim nations, such as China. And in the Congo basin, a recent satellite study found a burgeoning network of more than 50,000 kilometers of new logging roads. These are but a small sample of the vast number of new tropical roads, which inevitably open up previously intact tropical forests to a host of extractive and economic activities.

“Roads,” said the eminent ecologist Thomas Lovejoy, “are the seeds of tropical forest destruction.”

Despite their environmental costs, the economic incentives to drive roads into tropical wilderness are strong. Governments view roads as a cost-effective means to promote economic development and access natural resources. Local communities in remote areas often demand new roads to improve access to markets and medical services. And geopolitically, new roads can be used to help secure resource-rich frontier regions. India, for instance, is currently constructing and upgrading roads to tighten its hold on Arunachal Pradesh state, over which it and China formerly fought a war.

Of course, roads are not just an environmental worry in the tropics. In forested areas of western North America, one of the best predictors of wildfire frequency is the density of roads. In Siberia, road expansion is promoting a sharp increase in logging and forest fires. And new roads in the Arctic could potentially alter epic mammal migrations.

But no other region can match the tropics for the sheer scale and pace of road expansion and the degree of environmental change roads bring. Road building has a range of direct impacts on rainforest ecology. In wet tropical environments, the cut-and-fill operations associated with road construction can impede streams, increase forest flooding, and drastically increase soil erosion. Roads also discharge chemical and nutrient pollutants into local waterways and provide avenues of invasion for many disturbance-loving exotic species.

Roads that cut through rainforests can also create barriers for sensitive wildlife, many of which are ecological specialists. Studies have shown that even narrow (30 meter-wide), unpaved roads drastically reduce or halt local movements for scores of forest bird species. Many of these species prefer deep, dark forest interiors; they have large, light-sensitive eyes and avoid the vicinity of road verges, where conditions are much brighter, hotter, and drier. A variety of other tropical species — including certain insects, amphibians, reptiles, bats, and small and large mammals — have been shown to be similarly leery of roads and other clearings.

In frontier regions, new roads often open up a Pandora’s box of unplanned environmental maladies

And by bringing naïve rainforest wildlife into close proximity with fast-moving vehicles, roads can also promote heavy animal mortality. For some creatures, especially those with low reproductive rates, roads could potentially become death zones that help propel the species toward local extinction.

Although the direct effects of roads are serious, they pale in comparison to the indirect impacts. In tropical frontier regions, new roads often open up a Pandora’s box of unplanned environmental maladies, including illegal land colonization, fires, hunting, gold mining, and forest clearing. “The best thing you could do for the Amazon,” said the respected Brazilian scientist Eneas Salati, “is to bomb all the roads.”

In Brazilian Amazonia, my colleagues and I have done studies showing that around 95 percent of all deforestation occurs within 50 kilometers of highways or roads. Human-lit fires increase dramatically near Amazonian roads, even within many protected areas. In Suriname, most illegal gold mining occurs near roads, whereas in tropical Africa we have found hunting to be so intense near roads that it strongly affects the abundance and behavior of forest elephants, buffalo, duikers, primates, and other exploited species. Roads can sharply increase trade in bushmeat and wildlife products; one study found that eight killed mammals were transported per hour along a single road in Sulawesi, Indonesia.

Paved highways are especially dangerous to forests. They provide year-round access to forest resources and reduce transportation costs, causing larger-scale impacts on forests and wildlife than do unpaved roads, which tend to become impassable in the wet season. The proposed routes of new highways often attract swarms of land speculators who rush in to buy up cheap forest land, which they then sell to the highest bidder.

Perhaps the most damaging aspect of paved highways is that they spawn networks of secondary roads, which spread further environmental destruction. For instance, the 2,000-kilometer-long Belem-Brasília highway, completed in the early 1970s, has today evolved into a spider web of secondary roads and a 400-kilometer-wide swath of forest destruction across the eastern Brazilian Amazon. As my colleagues and I showed in a 2001 study published in Science, large expanses of the Amazonian forest could be fragmented by the advance of new highways and roads in Brazil. According to our models, by the year 2020, rates of forest destruction would rise by up to 500,000 hectares per year, and the area of forest that remained in large, unfragmented tracts — exceeding 100,000 square kilometers — would decline by 36 percent.