Oct. 3, 2008 -- A steep decline in the sea otter population in Alaska's Aleutian Islands has forced bald eagles nearby to change their diet, according to a new study.
Study leader Robert Anthony of the U.S. Geological Survey in Corvallis, Oregon, examined the prey remains in bald eagle nests on the islands to evaluate the eagles' diet between 1993 and 1994 and again between 2000 and 2003.
In the first time-frame, sea otters were abundant in the area, after recovering from near extirpation in the 1930s. By the late 1990s, numbers were down more than ten-fold and have not increased since. The second set of measurements corresponds to a low otter population.
Anthony found that between the two sampling times, the eagles' diets shifted from including otter pups, a kelp-dwelling fish called the rock greenling and a fish called the smooth lumpsucker to diets including greater numbers of seabirds and Atka mackerel.
The otters feed on sea urchins in the kelp forest. As long as sea otters are abundant, the urchin population is kept in check, which allows the kelp forest to thrive, said James Estes, of the University of California, Santa Cruz, who was also part of the study, published in the journal Ecology.
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Without the otters, the kelp forest declined, which appears to account for the eagle's switch to more seabird lunches.
The seabirds appear to have been good for the eagles, to the researcher's surprise.
"I would have guessed that we would have seen declines in eagles, but we didn't," said Estes. "The numbers remained constant. If anything, the reproductive success went up a little bit."
They speculate that the higher nutritional content of the seabirds may account for the improvement in the eagles' breeding success.
"I think the most interesting aspect of the results is the fact that these indirect effects of sea otters on near-shore communities can transcend a number of different species in the food chain and have an effect on another apex predator, the bald eagle," Anthony told Discovery News.
"You're jumping realms [from sea to land], which is very interesting," agreed Robert Steneck of the University of Maine's Darling Marine Center in Walpole, who was not involved in the study.
Estes has studied the decline of the otters in previous work. The cause of their rapid die-off appears to be attacks by killer whales.
"It was a huge surprise to us when the otter numbers collapsed in the 1990s," he said. "It was a huge surprise to see killer whales start to attack otters."
Although Estes observed only a dozen or so otter attacks by killer whales once the decline started, he had never observed an attack in previous years. His team's analysis suggested that extrapolating the number of observed attacks to the entire area accounted almost perfectly for the estimated otter decline.
"Then we did an analysis to say, 'How many killer whales would it take to kill 50,000 otters?', and it turned out it was, like, three," Estes said. "I think they just came in, ate the otters, and left. We happened to be there to see it."
Estes has proposed that the reason the killer whales turned to sea otter prey is the decline of larger whales in the region because of international whaling after World War II. "Big whales were an important prey resource for killer whales," he said.
"One of the interesting conservation implications is what we really need to do is recover the large whales," Estes said. Given international disputes over whaling, "it's a pretty controversial argument," he added.
"I'm very convinced by their argument," Steneck said. "There's no alternative that works as well as their case."
"We're now just starting to understand how connected various species are in ecosystems," Anthony said. "These indirect effects certainly are not ones that are easy to identify and study, but it tells us that if we are going to try to conserve sensitive species, we can't just look at the species in and of itself, we have to focus on the entire system."
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