Africa, still covered in large swathes of pristine wilderness, is likely to lose much of its biological wealth if dozens of new massive development projects—from highways to railroads to pipelines—get the green light, according to a new study. Most of the projects are designed to increase agricultural production and ease the transport of minerals such as iron and coal. Yet if all are built, they’ll create a spider web of some 53,000 kilometers of corridors through deserts, forests, and savannas—and a host of environmental disasters, scientists say. Even worse, they contend, most won’t help the continent feed its people, even though this is the primary justification behind many of the projects.

“Africa is undergoing the most dramatic era of development it’s ever experienced,” says William Laurance, an ecologist at James Cook University, Cairns, in Australia, and the study’s lead author. “No one disputes its need for food and economic development. But these corridors need to be built without creating environmental crises.”

The scientists’ study is a follow-up to a previous one they published last year in Nature warning about the unprecedented number of road and transportation projects being planned globally. In Africa, these range from a 4441-kilometer highway and railway corridor stretching from South Africa to the Congo to a planned West African corridor that will make a 4349-km-long arc between Dakar and Port Harcourt, Nigeria.

In their new study, the researchers assessed the potential of 33 such projects to improve agricultural production, and then quantified the environmental costs. Ten of the corridors are already being built, whereas 23 are still in the planning stage. Of these, nine are proposed upgrades to existing infrastructure.

To ensure they assessed each project using similar measurements, the scientists overlaid each corridor map with a 50-km-wide band centered on the road or railway. Within that band they estimated the human population and the area’s “environmental values,” which include the number of endangered and endemic animals, plant diversity, critical wildlife habitats, and the capacity of vegetation to store carbon and help regulate Earth’s climate. They estimated agricultural production for each corridor by looking at the surrounding habitat. Those cutting through savanna woodlands were deemed to have the most potential, followed by forests and desert shrublands.

To generate the most agricultural benefits from the corridors while limiting environmental costs, the scientists argue that they should be built in areas with low environmental values and high agricultural potential.

But their analysis shows that only five of the 33 projects would meet these dual goals, the team reports online today in Current Biology. Twenty-two of the projects are marginal, meaning that they’re important either for the environment or for agriculture, but not both. And six don’t meet either goal because they would cut through areas with high environmental values without delivering high agricultural returns. Simply put: They are bad for the environment and worthless for growing crops.

These six corridors, which include projects in East and West Africa, are “bad, bad, bad, especially for the equatorial regions, and should be canceled,” Laurance says. It’s not just the direct effect of the corridor itself, he points out. “It’s the indirect effects—a panoply of evils, from hunting to illegal mining—that come in the wake of a completed project. That’s what we worry about.” He points out that the first paved highway through the Brazilian Amazon, completed in the 1970s, is now a 400-km-wide slash of deforestation.

Because 10 of the marginal corridors are not yet underway, the scientists say that the projects’ sponsors still have time to rethink their plans. “If they improve or build the corridors in the right places, they can get a lot of economic return without destroying important habitats,” Laurance stresses.

But continuing with the 33 projects as now planned will utterly alter sub-Saharan Africa, Laurance and his colleagues say. If completed, the corridors will bisect 408 protected areas and disrupt the habitats of at least another 1800 reserves. “That suggests that a number of the continent’s famous areas for large mammals could be in jeopardy within our lifetime,” says Tim Caro, a wildlife and conservation biologist at the University of California, Davis, who was not involved with the study.

The work “is important because it shows just how intense development pressure is in sub-Saharan Africa,” adds William Ripple, an ecologist at Oregon State University, Corvallis, who was also not involved with the study.

But whether the countries heed the study’s recommendations and warnings is “another matter,” says Caro, who does research in East Africa. He worries that the people most likely to make the decisions are locals who seldom take into account the type of assessment that Laurance and his team have done. “What’s needed are country-by-country reports,” that can be sent to the right government officials and to the international agencies—and state-sponsored enterprises—footing the bills for these projects.

Laurance agrees, and hopes his team’s work will lead to these types of detailed studies. “But this is an enormous task. We need local partners to help us develop careful and stringent land use plans.” And the sooner the better, he adds. “These projects are happening now, even as we speak.”