for National Geographic News
The new diet is a surprising ripple effect of a changing food chain that includes sea otters, sea urchins, and underwater kelp forests and the fish that depend on them, researchers say.
When sea otters all but disappeared from the Aleutian Islands, it was a boon for spiky sea urchins, which otters eat.
The expanding urchin population, in turn, began gobbling up the area's underwater kelp forests. The forests declined dramatically, making coastal waters inhospitable to the kelp-dependent fish that were the primary food sources of local bald eagles.
The bald eagles adapted by switching to a diet of mostly seabirds, according to the study, published today in the journal Ecology.
"Sea otters have quite an effect on near-shore marine communities," said lead author Robert Anthony. "It ripples through the system and has indirect effects on a number of species, including bald eagles—another predator that feeds at the top of the food chain."
Even the researchers were surprised that the otter decline had effects that rippled through five species all the way to bald eagles, said Anthony, an ecologist with the U.S. Geological Survey and Oregon State University.
From Fish to Fowl
Anthony and colleagues had gathered detailed information on the Aleutian Islands' bald eagles in the early 1990s, when sea otter numbers were relatively high.
Researchers returned in the early 2000s, when there were about 90 percent fewer sea otters.
The researchers counted the number of breeding pairs of bald eagles and studied what the birds had been eating.
"Dinner" remains found in nests revealed a general switch from fish and mammals—including otter pups—to seabirds. The eagles were still eating some fish, mostly non-kelp-dependent species.
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