A new study looks at the worldwide effects of habitat fragmentation. Photograph by Georg Gerster / Panos

The first paved highway across the Brazilian Amazon began, in the nineteen-seventies, as a narrow, hard-won cut through dense rainforest. The road, which connects the northern port city of Belém with the country’s capital, Brasília, twelve hundred miles away, was hailed as a huge step in the region’s development, and so it was: it quickly spawned a network of smaller roads and new towns, drawing industry to the Brazilian interior. But the ecological price was high. Today, much of the Belém-Brasília highway is flanked by cattle pastures—a swath of deforestation some two hundred and fifty miles wide, stretching from horizon to horizon. Across the planet, road construction has similarly destroyed or splintered natural habitats. In equatorial Africa’s Congo Basin, logging roads have attracted a new wave of elephant poachers; in Siberia, road expansion has caused an outbreak of wildfires; in Suriname, roads invite illegal gold mining; and in Finland, so many reindeer are killed by cars that herders have considered marking the animals with reflective paint.

“Roads scare the hell out of ecologists,” William Laurance, a professor at James Cook University, in Australia, said. “You can’t be in my line of business and not be struck by their transformative power.” Laurance has spent most of his career studying that power. Beginning in 1979, not long after the Belém-Brasília highway took shape, one of Laurance’s colleagues, Thomas Lovejoy, helped direct the selective clearing and burning of nearly four hundred square miles of intact forest in northwestern Brazil, near the city of Manaus—a deliberate act of habitat fragmentation that would become the world’s largest and longest-running experiment in tropical ecology. (Laurance joined the project in 1996.) Today, a study in Science Advances synthesizes results from Manaus with those from similar experiments worldwide, confirming what scientists have long suspected: no matter the ecosystem—forest, prairie, patch of moss—the effects of habitat fragmentation are ruinous.

The new study, led by Nick Haddad, a professor at North Carolina State University, and co-authored by Laurance and others, found that fragmented habitats lose an average of half of their plant and animal species within twenty years, and that some continue to lose species for thirty years or more. In all of the cases examined, the worst losses occurred in the smallest habitat patches and closest to a habitat edge. The study also demonstrates, using a high-resolution map of global tree cover, that more than seventy per cent of the world’s forest now lies within one kilometre of such an edge. “There are really only two big patches of intact forest left on Earth—the Amazon and the Congo—and they shine out like eyes from the center of the map,” Haddad said.

The trouble is that, for humans, if not for plants and animals, roads are useful. At last November’s G20 summit, in Australia, world leaders pledged to invest some sixty trillion dollars into new infrastructure by 2030. The International Energy Agency estimates that, between now and the middle of the century, more than fifteen million miles of new paved roads will be built worldwide. And with nine billion of us expected on the planet by 2050, the pressure to create more edges will only increase. That leaves Laurance and other ecologists with the unenviable task of deciding which roadless areas matter most. Late last year, in the journal Nature, he and a group of colleagues published a global map that pinpointed places where new roads could increase food production with relatively little environmental harm—and where the opposite was true. Laurance now hopes to carry out more detailed analyses in sub-Saharan Africa, the Amazon, Asia, and northern Australia.

So far, Laurance’s critiques of new road-building schemes have been well received, but he expects that to change. In the early two-thousands, he and a group of American and Brazilian scientists criticized Avança Brasil, a forty-billion-dollar infrastructure initiative of then President Fernando Henrique Cardoso. Laurance and his colleagues were pilloried in the Brazilian press as foreign meddlers; only when the administration changed were some of the more ecologically problematic projects modified. “You only really get kicked when you start talking about specific projects, specific roads,” Laurance said. “But that’s O.K. We’re used to it.”