The Balbina Dam is a large dam in the Amazon Basin. Our new research shows that the disconnected patches of forest are no longer able to support thriving environments.
One of the largest reservoirs in South America can be found in the north through the rainforest.
As the reservoir filled up, more than 3,500 islands formed as a result. The forest patches that used to be hilltop ridges are now forest patches.
For rainforest ecologists like us, the new landscape was a way to test theories of what happens when a forest and its many animals are restricted to smaller and smaller patches.
One of the main drivers of the crisis is the loss of habitat. We know that hydroelectric dams are one of the main ways humans are disturbing these habitats and that many developing countries are due to build many more dams.
Smaller islands that can't sustain viable populations are expected to disappear faster in the new landscape created after a dam. We expect other factors to play a role, such as whether a species is resistant to change.
At least the theory is true. We were able to see it in practice.
Over the past decade or so, scientists from many different institutions have made huge efforts to investigate which species are vanishing and which are still present in the Balbina reservoirs. We were able to put together those efforts.
We looked at 22 forest islands that were different in size. Three sites that were connected to the main forest and weren't islands were looked at as a baseline reflecting the scenario before damming.
Medium and large mammals, small mammals, diurnal lizards, understory birds, frog, dung beetles, orchid bees, and trees were recorded.
Science Advances publishes our results. Most of the diversity was held by a few large islands.
Smaller islands were hit hard. There were only a few species that were able to survive for more than 30 years. Medium-sized animals can be found in a relatively small area. Smaller mammals such as tapirs and jaguars need more space and may have disappeared from mid-size islands.
Many species went extinct on individual islands. Larger species were more likely to go extinct than smaller ones. The largest species of orchids Eulaema bombiformis or Eulaema meriana were widely distributed across the landscape.
This was also the case for large understory bird species, with the smallest ones being more widely spread.
The majority of the islands created by Balbina Dam are relatively small. 95 percent are small. The islands proved to be able to sustain low levels of flora and fauna.
When we visited the large islands of the Amazon, we were struck by the fact that they were dominated by species of animals and plants that were not native to the area.
In the wake of river damming, we expect this process to happen more and more. Future hydroelectric projects should not flood large expanses of forest and create lots of small islands.
The loss of biodiversity would be minimized by this. Future assessments of whether damming the world's mega-diverse tropical rivers is worth it should take into account the impact of biodiversity loss.
The TROPIBIO Project, the University of Porto, and Carlos Peres are all associated with the University of East Anglia.
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