Two new studies, published in the July issue of the journal Biological Conservation, have looked at the islands created after hydroelectric dams blocked rivers and flooded the areas behind them to create huge reservoirs. Dam developers consider these islands—previously the tops of tall hills—to be potential wildlife refuges where species can thrive despite the reduction in their original habitat.
“We can now show that this is not the case,” said Isabel L. Jones, a doctoral student at the University of Stirling in Scotland and the lead author of the first study.
Jones and her colleagues analyzed literature about reservoir islands around the world and found that these islands contain, on average, 35 percent fewer species than were present on the mainland. Declines were seen across every type of species, including mammals, plants, invertebrates, birds, and reptiles. The losses began within a year of flooding and got consistently worse each year, a process that continued for decades.
Jones said the extent of the biodiversity loss came as a shock. “We knew there was already a lot of evidence of local extinctions from thoroughly examining the existing published literature,” she said. “The surprise came when we put the evidence from those snapshot studies together to find that species loss continues year on year after dams are constructed and reservoir islands created, and that this pattern was consistent across all groups of plants and animals across every habitat investigated.”
Jones has visited islands in the 443,000-hectare reservoir created by Brazil’s Balbina Dam. Larger islands might still feel like “the real jungle,” she said, but others went to the opposite extreme. “If you’re on a very small island of a few hectares, for example, then there might be a few trees left or none at all,” said Jones, adding that the worst islands looked as if they were just grassy lawns that had been fully grazed by a capybara, a gigantic relative of the guinea pig that is native to the region.
The Balbina reservoir was the subject of the second study, which was led by Carlos A. Peres, a professor at the University of East Anglia in Norwich, England. Peres’ previous research found that the dam’s waters were inhospitable for both fish and the giant river otters that fed on them. This time he and his fellow researchers found that 90 percent of reservoir islands had a negative impact on birds. Only the largest islands, they discovered, supported large bird populations, and even those were described as being “on borrowed time” owing to the potential for destructive winds, fires, or other threats that could make the islands inhospitable.
These studies, along with another published in January describing how new dams could threaten one-third of the world’s freshwater fish species, make the point that the world’s 50,000 existing big dams, as well as the hundreds of dams planned or being constructed, could have a much greater impact on biodiversity than developers have estimated.
Author: John R. Platt