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Alaska's Sea-Otter Decline Affects Health of Kelp Forests and Diet of Eagles

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Bald Eagle
Above: Bald Eagle. Photograph copyright © Norman Smith; used with permission. [larger version]

Sea otter
Above: Sea otter. Photograph courtesy of the U.S. Fish and Wildlife Service, Alaska Image Library. [larger version]
Who would have guessed that as the number of sea otters in Alaska's Aleutian Islands plummeted over the past 15 years, bald eagles would be forced to change their diet?

That's right. Two top-level predators, one in the ocean and one on the land, are linked through a marine food web that includes sea urchins, kelp, and fish—an effect never documented before.

"It's like an ecological chain reaction, affecting many different species and many different levels of the food web," said Jim Estes, retired marine-mammal specialist from the U.S. Geological Survey (USGS) and one of the authors of the new study published in the journal Ecology. Estes suggests that the sea-otter decline is due to an increase in predation by killer whales.

Researchers conducted bald-eagle surveys with colleagues during the sea-otter breeding seasons in 1993-1994, when sea-otter populations were high, and then again in 2000-2002, when they were extremely low. Without the sea otter's presence to control sea-urchin populations, severe kelp deforestation occurred.

The sea otter is a keystone species in near-shore coastal environments because of the pivotal role it plays in maintaining a healthy balance in kelp forests. By eating sea urchins—the biggest threat to kelp forests—sea otters controlled sea-urchin numbers and kept the ecosystem in balance. This ensured that these underwater plants could thrive and reach their maximum height of 250 ft.

Kelp forests provide habitat, shelter, and a buffer from waves and currents for numerous aquatic species. This marine environment also provided the bald eagle with a diet of marine fishes and sea-otter pups.

When sea-otter numbers crashed during the 1990s, the kelp-forest ecosystem began to collapse as well. Eventually, a deforested underwater landscape and lack of sea-otter pups for food forced the bald eagles to change their diets. 

"Bald eagles proved their extreme flexibility as a predator by simply shifting their diet to other available fish and seabirds when the sea otters were no longer present," said Robert Anthony, a USGS researcher with the Oregon Cooperative Fish and Wildlife Research Unit and lead author of the study. "Bald eagles are opportunistic feeders. They defend foraging territories against other bald eagles along these coastlines, and the terrestrial environment provides very little prey for them. So they forage over open water for most of their prey."

Although the diet of these bald eagles changed with the decline of sea otters, the scientists discovered that the birds actually produced more eggs and young, a fact that Anthony suggests may be a result of high caloric content in the eagles' increasingly seabird-dominated diet.

Anthony cautioned that although bald eagles quickly adapt to necessary dietary changes, other animals with more specialized diets might not be so adaptable. Therefore, said the authors, resource managers need to focus on entire ecosystems, not single species, when they develop management strategies.

Complete findings were published in the journal Ecology on October 10, 2008 (v. 89, no. 10), in an article titled "Bald Eagles and Sea Otters in the Aleutian Archipelago: Indirect Effects of Trophic Cascades" (p. 2725-2735). The authors are Robert G. Anthony (U.S. Geological Survey and Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oreg.), James A. Estes (Department of Ecology and Evolutionary Biology, University of California, Santa Cruz), Mark A. Ricca (U.S. Geological Survey, Western Ecological Research Center, University of California, Davis), A. Keith Miles (U.S. Geological Survey, Western Ecological Research Center, University of California, Davis), and Eric D. Forsman (U.S. Forest Service, Pacific Northwest Research Station, Corvallis, Oreg.). The article's abstract can be viewed online at URL http://dx.doi.org/10.1890/07-1818.1.

Related Sound Waves Stories
California Sea Otters: Population Recovery Continues at Slower Rate
September 2008
Collapsing Populations of Marine Mammals—the North Pacific's Whaling Legacy?
October 2003

Related Web Sites
Bald Eagles and Sea Otters in the Aleutian Archipelago: Indirect Effects of Trophic Cascades

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cover story:
Scientists Discover New Pacific Iguana

Tracking Sea Otters

Research Sea-Otter Decline Affects Kelp Forests and Eagles

Outreach International Coastal Cleanup

Awards Norling Receives Diversity Award

Publications November 2008 Publications List

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