A songbird species that flourishes on the salmon-rich side
of dams in the western United States struggles when it tries to nest on the
side closed off from the fish and the nutrients they leave behind.
But the songbird and the rest of the divided ecosystem
rebounds, faster than some experts expected, when dams come down and rivers are
allowed to resume their natural flow.
Two new studies led by Christopher Tonra, assistant
professor of avian wildlife ecology at The Ohio State University, illustrate
the stress dams impose on species that rely on salmon and the impact of dam
removal on the well-being of that wildlife.
The areas previously depleted of salmon are on a fast track
to recovery in a shorter time than he ever expected after the dam removal,
"It's exciting to be able to show a real positive
outcome in conservation. We don't always get that," he said. "That
these rivers can come back within our own generation is a really exciting
During his time conducting the studies in Washington, Tonra
watched reservoir beds that looked like moonscapes return to vibrant, rich
habitat and cascades emerge where none had been, at least for the last century.
"Watching that happen was just incredible," he
Tonra and his colleagues studied the American dipper, a bird
set apart by its unusual feeding style. Dippers, which are equipped with a
transparent second eyelid (think water goggles for birds), dive below the
river's surface and walk the riverbed scouring the rocky floor for meals,
mostly aquatic insects in their larval stage. They also eat some small fish,
including juvenile salmon when they're available.
The studies are the first to examine the effects of dams,
and dam removal, on the dipper, considered an indicator species and the only
bird of its type found in North America. Dippers that are faring well point to
a strong ecosystem in and around the river.
"These birds are right where aquatic and terrestrial
ecosystems meet," Tonra said.
Tonra and his colleagues spent four years in Washington's
Olympic National Park and surrounding tribal, federal and private lands. The
Elwha River winds through the park and is the site of the largest dam removal
in history. Crews started tearing down the Elwha and Glines Canyon dams in 2011
and concluded in 2014, freeing the path for migratory fish for the first time
in a century.
Salmon, which do most of their growing in the ocean, carry
marine-derived nitrogen and carbon back into freshwater systems when they
return to spawn and die. They benefit animals and plants, whether through
direct consumption or because nutrients find their way into plants and other
food, including larval mayflies and other insects for which the dipper dives.
"They're truly fertilizing the river and so that makes
its way all the way up through the food chain," Tonra said.
In one study, the researchers documented that American
dippers with access to salmon were in better physical condition and more likely
to attempt multiple broods of offspring in a season. They also produced larger
female offspring and were more likely to stay in breeding territories
year-round. The research, published early online, will appear in an upcoming
issue of the journal Ecography http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1600-0587
Tonra and his colleagues worked along four streams, three of
which were blocked to salmon either by waterfalls or dams. They banded the
birds, weighed them and collected blood samples. They looked at carbon and
nitrogen in the birds' blood to determine their level of marine-derived
The research team watched for multiple attempts to breed and
an inclination to stay in the nesting area year-round, and tracked what type of
food was delivered to nestlings.
The birds with salmon access had more marine-derived
nutrients and were 20 times more likely to attempt multiple broods. They were
13 times more likely to stay year-round and had an annual adult survival rate
that was 11 percent higher than their salmon-deprived peers.
The female birds with access to salmon had larger body mass,
suggesting they were healthier. Fledgling females raised in areas with salmon
also were larger.
The birds without access to salmon and food enriched by
their presence "weren't in very good condition and it looked like they
weren't attempting to breed as much," Tonra said.
And they took off after they fledged a single brood, presumably
for salmon-rich waters.
"Within the same river you basically have two different
populations," Tonra said.
There's good news in the team's second dipper study,
published in the December 2015 issue of the journal Biological Conservation:
Within a year of the Elwha Dam removal, Tonra and his colleagues were able to
document an increase in salmon-derived nutrients in American dippers.
Tonra was surprised, and delighted, by how quickly the
"It was pretty much as soon as the first dam came out
and fish were beating up against the second, wanting to go." Tonra,
previously with the Smithsonian Migratory Bird Center, worked with Kimberly
Sager-Fradkin of the Lower Elwha Klallam Tribe and Peter Marra of the
Smithsonian on both studies. Sara Morley of the Northwest Fisheries Science
Center and Jeffrey Duda of the Western Fisheries Research Center contributed to
the study published in Biological Conservation.
Tonra said he'd like to return to the Pacific Northwest soon
to measure changes in the birds' patterns and health since the dam removal.
He's hopeful that other birds and bats that feast on insects in the air and on
the trees near the river will become stronger as well.
The research was supported by the U.S. Fish and Wildlife
Service, the Lower Elwha Klallam Tribe, the National Oceanic and Atmospheric
Administration, the U.S. Geological Survey, the Smithsonian Institution and the