Continued dam-building across Amazonia could threaten dozens of species with extinction, says a new paper published this month in the journal Biodiversity and Conservation. With 191 dams already dotting the Amazon basin and nearly 250 more planned, scientists say the region’s unique freshwater ecosystems are at serious risk. Major changes will be required from the Amazonian nations — most of all, increasingly unstable Brazil — to prevent further damage in the most biologically diverse area of the planet.

Hydropower is the favored energy source in the Amazon, where it is largely considered among the most reliable and price-competitive options. In Brazil, hydroelectric plants already account for about 80 percent of the electricity generated in the country, and the country is in the process of completing the 11,233-megawatt Belo Monte dam on the Xingu river, which will end up being the largest dam in the region. One reason is the mining industry: Dams provide a convenient way to power remote mining operations, and the construction of dams and subsequent redirecting of water supplies can also help facilitate the exposure of new mining sites.

But conservationists are increasingly concerned about the dams’ effects on both the natural environment and the indigenous communities who call it home. It’s an issue that, according to some, is only starting to garner mainstream attention, as most conservation efforts and media coverage until this point have focused on deforestation and the Amazon’s terrestrial resources — not its freshwater systems, which are equally important to the health of the rainforest.

“Something that’s galled me is the fact that there’s been very little on the cumulative impact of dams on Amazonian biodiversity,” said the new report’s lead author Alexander Lees, a postdoctoral research fellow at the Cornell Lab of Ornithology. “There are lots of people going out and doing consulting work or work with individual dams, but there was no overarching review on the effects.”

So Lees and a group of other researchers set out to conduct a literature review synthesizing the dozens of existing papers looking at the impacts of dams in the Amazon.

“The idea of this paper was to say there’s all these individual papers saying we’ve got this huge problem,” Lees said. “We’ve really done a coordinated effort to try to understand what’s happening.”

The review suggests that dozens of species in the Amazon, including fish, birds and even mammals, are put at risk by dam-building. And there are a variety of direct effects caused by dams that are responsible for the threats.

“Amazonia is not like a big homogeneous place,” Lees pointed out. “It’s…basically an archipelago of different islands separated by major rivers.”

Here are three ways dams are causing harm:

1. Upending unique aquatic ecosystems. Because the Amazon landscape is divided up by rivers and tributaries, many of the different “islands” Lees referred to are characterized by organisms that are only found there and nowhere else. Because these species are so specific to their habitats, any disturbance that significantly alters their environment can put them at risk of dying off.

To maximize their efficiency, dams are usually placed at a river’s headwaters, as close to the river’s source as possible. This practice can alter all the habitats further downstream by slowing or halting water flow, forming still reservoirs and allowing for silt and other sediment build-up, altering all the habitats farther downstream and halting the flow of important nutrients before they can make it to any other parts of the water system.

Naturally, fish and other aquatic-only organisms — even aquatic mammals, such as giant otters — are among the animals that are most affected by these kinds of changes. Preventing fish from migrating upstream means that many species can’t make it to their natural spawning grounds. Reduced flow can lead to deteriorating water quality, meaning some animals essentially suffocate from lack of oxygen or starve because their food sources are dying off. Additionally, many species are highly adapted to live in certain types of conditions — for example, certain levels of turbidity, or the amount of particles in the water — and these delicate conditions can be easily disrupted by dams.

The magnitude of these possible extinctions remains poorly understood because there are still so many species in the Amazon — fish in particular — that are poorly studied or not scientifically described at all. And the ways that all of these organisms coexist and depend on one another is still being explored.

2. Harming flying animals like birds and bats. But it’s not just aquatic animals that are believed to suffer from dam-building. Birds and bats also rely on certain habitats that depend on the flow of the water systems.

As Lees explained, undisturbed rivers in southern Amazonia are subject to seasonal rising and falling with the wet and dry seasons. At certain times of the year, water levels will fall and expose a series of rocky islands, which provide important habitat to many animals. Free-tailed bats and black-collared swallows, for example, use these areas as breeding grounds. Damming, however, can disrupt the seasonal cycle and flood out these important habitats.

Other birds — white-collared swifts, for instance — nest on waterfalls. But dams can eliminate these waterfall habitats entirely by stopping the flow of water, leaving their inhabitants exposed and vulnerable to predators.

3. How dams harm people and communities. All of these effects are worrying enough — but according to Lees, the indirect effects of damming are perhaps even more concerning. The thing about dam-building is that it tends to change human communities in the Amazon and alter the way people interact with the environment. Lees pointed to the Brazilian town of Altamira, which is located in the state of Pará close to the Belo Monte dam construction, as an example.

“It’s gone from this small Amazonian town to this megalopolis with…all the social ills you can imagine with unconstrained growth and no plans,” Lees said, citing an increase in crime and rising prices as examples.

Dams frequently bring floods of people into previously remote areas — and in addition to the problems this can cause for indigenous communities, it can also lead to an uptick in illegal deforestation. And the expansion of hydropower also aids in the expansion of mining and smelting, which draw even more human activity into the rainforest, are highly energy-intensive processes and can also introduce heavy metals and other pollutants into the environment.

So the environmental problems associated with damming are becoming increasingly clear. The solutions, on the other hand, remain murky. If hydroelectric power continues to be the energy source of choice in the Amazon basin, a great deal of work and some significant policy changes will likely be required to prevent catastrophic future impacts on the environment.

Most important is the need for a complete, basin-level analysis — beyond a review of the existing literature, which is still incomplete in many ways — of the impact of dams throughout the Amazon, said David McGrath, deputy director and senior scientist at the Earth Innovation Institute and a professor at the Federal University of Pará in Brazil. But there are currently many hurdles to doing so that need to be overcome with time and the adequate allotment of resources.

“We don’t have any sort of institutional capacity to really take a look at all of those, and I think that’s a real priority,” McGrath said.

The value of such an analysis lies largely in the need to understand the cumulative impacts of dams throughout the Amazon and the way their effects feed off of one another, said Leandro Castello, an assistant professor of fisheries at Virginia Tech. Castello recently published a similar paper in Global Change Biology examining the many threats to freshwater ecosystems in the Amazon.

“All rivers flow to the sea, which means the impacts of headwater dams propagate downstream and add to those of downstream dams,” he told The Washington Post by email. Merely evaluating the isolated effects of individual dams does not give a complete enough pictures of what kinds of changes the basin as a whole is undergoing.

For future environmental impact assessments to be meaningful, Amazonian governments will naturally need to abide by their recommendations, halting construction on projects that are predicted to cause significant ecological damage or lead to the extinction of species. Additionally, Lees suggested that policymakers should consider updating and modernizing existing dams, which may not always be functioning at maximum capacity, rather than continually adding new ones.

And in order to protect the region’s biodiversity, more stringent enforcement of protected areas will likely be necessary, Lees pointed out. Protected areas in the Amazon are sometimes still subject to illegal logging. And as another recent paper Lees co-authored points out, many of them also overlap with areas affected by mining interests or the influence of hydroelectric dams, and are sometimes diminished or dismantled in favor of development interests.

All of these recommendations will necessarily hinge on international cooperation among the Amazonian nations, McGrath said.

“They need a pan-Amazonian effort to look at this construction and do it before it’s too late, before we’re locked into a huge number of dams which are going to have progressively more severe impacts on the integrity…of the aquatic system,” he said.