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.
"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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