A bird that suffers badly when dams interfere with its food supply has been found to recover rapidly when the dams are removed. The finding is being taken as evidence that it is possible for damaged ecosystems to recover when past mistakes are righted.
Washington state draws most of its electricity from hydropower dams, but has paid a severe ecological price. Salmon that once used the rivers to breed are blocked by the dams. In some cases, solutions have been found (including the marvelous salmon cannon), but for many rivers, the higher reaches are salmon-free.
Dams not only affect salmon reproduction, they also have devastating consequences for species that feed on the spawning fish, with ripples through the food chain.
Some dams are judged to be not worth the price. As part of the process known as rewilding – most famously involving the reintroduction of keystone species – hundreds of dams in Washington and Oregon have been removed so that rivers once again flow freely to the sea.
Dr. Christopher Tonra of Ohio State University has published two papers almost simultaneously on the way dams affect the American dipper (Cinclus mexicanus).
The dipper is distinctive with its transparent second eyelid, allowing them to see underwater while protecting their eyes. Rather than diving for fish and coming swiftly to the surface, they walk underwater searching the riverbed for food. They eat juvenile salmon, but rely more heavily on aquatic insects that thrive on the nutrients that salmon bring.
In Ecography, Tonra writes that salmon “deposit rich marine-derived nutrients (MDN) that benefit the base of freshwater food webs.” Tonra confirmed his prediction that the MDN available to the dippers below dams would see them in better condition and breeding more successfully. These and other measures of dipper health proved that dams represent a major threat to upriver dipper survival.
Most dramatically, dippers with access to salmon were 20 times more likely to attempt to raise multiple broods in a year, and 13 times more likely to stay in the same area year-round than those whose food supply was interrupted by the dam.
Tonra confirmed that the difference lies in the salmon diet by showing that dippers nesting below dams have increased levels of the isotopes carbon-13 and nitrogen-15, both richer in marine food webs than on land.
It does not follow that because dams damage the ecosystem, removing them will solve the problems they cause. Many forms of environmental damage are almost impossible to reverse. However, Tonra has documented in Biological Conservation an impressive revival in dipper populations in the first year after the Elwha and Glines Canyon dams were removed.
His four-year study revealed more rapid restoration of salmon populations – and the species dependent on them – than was anticipated following the largest dam removal project in U.S. history.
"It's exciting to be able to show a real positive outcome in conservation. We don't always get that," Tomra said in a statement. "That these rivers can come back within our own generation is a really exciting thing."
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