As countries build more hydropower projects, new research warns that massive dams pose an extinction threat to mammals, birds and tortoises—at least in the Amazon.

Brazil’s Balbina Dam has turned what was once undisturbed forest into an artificial archipelago of 3,546 islands where many vertebrates have disappeared, according to a study published Wednesday by England’s University of East Anglia.

“We’re watching extinction unfold right in front of us,” says co-author Carlos Peres, a Brazilian professor at the university’s School of Environmental Sciences. “We uncovered astounding local extinction rates,” he says, even in areas that belong to a biological reserve and are protected from hunting.

The two-year study comes the same week that Brazil, in a joint U.S. announcement, pledged to restore 12 million hectares, or 46,332 square miles, of its forests—nearly the size of England—by 2030. It also promised to dramatically increase its use of solar, wind and geothermal energy. In fact, it’s planning to add floating solar panels to the Balbina Dam, located on the Uatuma River in the country’s northwestern rainforests.

Brazil currently gets most of its electricity from hydropower and like other developing countries, it plans to build hundreds of new dams to meet rising energy demand. Hydropower, often touted as “green” because of its water reuse, produces more electricity worldwide than all other renewables combined.

“Hydropower is an effective way, in many landscapes, to generate power,” says Peres. Yet its efficacy, he adds, depends on topography. In Brazil’s lowlands, a hydroelectric plant requires a large dam to raise the water level enough to create a cascade. In Colorado’s mountains, the rivers are on steeper slopes, so smaller reservoirs are needed.

As a result, Peres says, a hydropower plant in flat areas produces a lot less electricity per flooded acre than one in the mountains. It also loses a lot more carbon-storing trees and other vegetation, so its environmental costs are higher.

Prior studies have looked at these varying costs, including cuts in fishery revenue and the displacement of indigenous people. Peres says his research is different, because it looks at a wider area of land and range of vertebrates.

“We’ve looked at every single species greater than a pound,” he says, noting his team targeted three dozen species. The 250-megawatt capacity Balbina Dam, which began operating in 1989, inundated 3,129 square kilometers (1,208 square miles) of primary forests and created more than 3,000 islands.

Only a small number of the islands still have a diverse mix of wildlife. The researchers said the extinction rate on the three dozen islands they surveyed, which were mostly the larger ones, is 42 percent. They estimated that rate jumps to 70 percent for the entire reservoir area.

“There’s nothing surprising” about the results, says Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley. Still, he applauds the study as a “really nice detailed look” at the actual biodiversity impacts of large dams.

Kammen has done his own research. In a paper to be published this month, he estimates that the large hydro dams being built in eastern Malaysia will threaten most of Borneo’s birds and mammals. He co-authored another study, published in June, that says small-scale hydro projects and biogas generators would be cheaper energy alternatives.

“It’s all about attracting international investment,” Kammen says of the continuing lure of mega hydro deals. He says developing countries find it’s easier to secure financing for big rather than small projects.

Peres agrees. Mega dams “serve the interests of large engineering companies, not local communities,” he says, noting they often require the inefficient carrying of electricity across long distances via transmission lines.

Some communities have fought large dam projects—and won. Last year, Chile’s government canceled a controversial plan for five dams on two of Patagonia’s wildest rivers after facing strong public opposition.