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Eagles "Cannibalizing" Other Birds as Otters Disappear

Rebecca Carroll
for National Geographic News
October 3, 2008
 
Some bald eagles in Alaska have switched to eating mainly other bird species, a new study says.

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.

The eagles had more young during the second visit, possibly because of the higher calorie counts of birds versus fish. The total eagle population, though, remained about the same size, Anthony said.

"The changes that were caused by the decline in sea otters appeared to be neutral—or maybe even positive—for bald eagles," he said. "They are a very adaptable species and are opportunistic predators.

As for what caused the otters to disappear in the first place, Anthony said: "We don't know for sure, but there is at least one plausible theory."

Whale of the Tale

Increased otter hunting by killer whales is the main direct cause of the 1990s Aleutian sea otter collapse, study co-author Jim Estes believes. He points out, though, that the current study is about the effects of the otter collapse, not its causes.

The whale issue is touchy for various reasons, including the national and international political intrigue that surrounds the great whales.

But "the story is way bigger than [otters, urchins, kelp, fish, and eagles] in my mind," said Estes, of the University of California, Santa Cruz.

Most killer whales, or orcas, mainly hunt fish. But some groups hunt other marine mammals, and whale experts can easily identify these mammal-eaters by their markings.

Mammal-eating killer whales traditionally fed on large whales such as sperm whales, humpbacks, and fin whales, Estes believes. But in the mid-20th century, industrial whaling nearly eradicated these food sources.

The decline of large whales led the mammal-hunting killer whales to smaller and smaller prey—first harbor seals, then sea lions, and now otters—Estes said.

(Read more from Estes on the Aleutian wildlife in a 2003 National Geographic magazine article.)

"Amazing" Numbers of Links

Bernie Tershy, also of the University of California, Santa Cruz, described the report as "interesting and plenty accurate."

It's "well accepted" from previous research that killer whales are the direct cause of the decline in Alaskan otters, Tershy, who was not involved in the study, said via email.

"The number of links impacted by the shift in killer whale diet is amazing," said Tershy, a former grantee of the National Geographic Conservation Trust. (The National Geographic Society owns National Geographic News.)

Holly Jones, a doctoral candidate at Yale University, published research earlier this year about the threat that invasive island rats pose to seabirds.

Jones expanded on the possible effects of the eagles' new hunting behavior.

"If eagles impact seabird populations dramatically … these effects could continue on to intertidal ecosystems and entire island communities," Jones, who also was not involved in the new study, said via email.

The otter-eagle study "demonstrates how important ecosystem linkages are and how fragile the links can be."
 

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