CORVALLIS, Ore. – A newly published study has found that the decline of sea otters along Alaska's Aleutian Islands has forced a change in the diet of a terrestrial predator – the bald eagle. The study demonstrates the extraordinary complexity of marine ecosystems and how far-ranging the impacts can be when there is a population shift in a keystone species like the sea otter.
The research was published in the October issue of Ecology, the journal of the Ecological Society of America.
When abundant, sea otters prey on, among other things, sea urchins that attach themselves to the "holdfasts" of huge strands of kelp. The tiny urchins eat the tissue of the kelp, killing it, and when their population becomes too large, they can destroy entire kelp beds. These kelp beds are host to a variety of fish that historically have comprised a part of the bald eagles' diet.
The loss of kelp forest with the decline of sea otters has forced the bald eagles to adapt and target new prey, primarily seabirds, according to Robert Anthony, an ecologist at Oregon State University and leader of the Oregon Cooperative Fish and Wildlife Research Unit – a partnership between OSU, U.S. Geological Survey and Oregon Department of Fish and Wildlife.
"Bald eagles have evolved to become opportunistic foragers," Anthony said, "and they can survive on a wide, diverse diet. This kind of adaptive behavior may not work as well with other species, but bald eagles appear to have suffered no ill effects from the otter decline and, in fact, they have had more young, on average, when sea otter populations are at their lowest numbers.
"We think it may be because birds and mammals have a higher caloric content per unit weight than fish and the extra calories may have given a boost to their overall reproductive success," he hypothesized.
Although bald eagles have a reputation of being scavengers, that characterization is only partially true, Anthony said. Adults don't reach breeding age until five years old and the food they deliver to their young is almost always freshly killed – whether it is fish, mammals, birds or other sources of protein.
"During the five-year period before breeding age, they do scavenge some as they hone their hunting and foraging skills," Anthony said. "And they have adapted those skills to prey on a variety of birds, mammals and fish. In the Aleutian study, they shifted their diets toward seabirds instead of fish and sea otter pups after the decline in sea otters."
Co-author Jim Estes, of the U.S.G.S. and the University of California at Santa Cruz, theorizes that the decline of sea otters in Alaska may be tied to increased predation by killer whales. Bald eagles themselves preyed upon sea pups to supplement their fish diet and can fly off with a pup that weighs as much as 4-5 pounds.
"I once examined a bald eagle nest in the Aleutians that had eight skins from sea otter pups in it," Anthony said. "That's a lot of pups for one pair of eagles."
Sea otter pups are most vulnerable when they are very young and the adults dive down to forage on clams and sea urchins and leave their offspring floating on the surface.
Bald eagles nest in rocky cliffs in the treeless Aleutian archipelago, and defend small parcels of territory consisting of a mile or two of shoreline. They typically fly over the near-shore and can scoop up fish with their talons, dipping down no more than a couple of feet into the water. Unlike osprey, they don't dive into the water.
When fish are not abundant, they switch to other prey – in this case, seabirds. Anthony said seabirds are plentiful in the region and there is no evidence that the eagles' new diet has had an adverse effect on their population. However, he cautioned, the ecological relationships can extend far and wide.
"That is the lesson this study provides," he said. "The link between sea otters and bald eagles transcends five trophic levels, pointing out how complex and far-reaching the effects of sea otters can be on near-shore marine communities."
Journal
Ecology