Decline in Alaskan sea otters affects bald ea...( Sea otters are known as a keystone s...)

Sea otters are known as a keystone species, filling such an important niche in ocean communities that without them, entire ecosystems can collapse. Scientists are finding, however, that sea otters can have even farther-reaching effects that extend to terrestrial communities and alter the behavior of another top predator: the bald eagle.

In nearshore marine communities, towering kelp can reach heights of 250 feet and function much like trees in a forest, providing food, homes and protection for fish and invertebrates. The most important enemies of these giant algae are tiny sea urchins, only inches in diameter, which live on the kelp's holdfasts and eat its tissue. When urchin populations become too large, they can defoliate entire kelp forests, leaving only barren remains.

Enter the sea otter. Otters can eat the spiky urchins whole, making them the major urchin predator. The otters' presence keeps urchin populations in check and maintains the balance of the ecosystem.

Scientists have known about these kelp forest community interactions since the 1970s. But in the October issue of the journal Ecology, Robert Anthony and colleagues report that the presence or absence of otters can also affect the diet of bald eagles, a neighboring terrestrial predator. Anthony is an ecologist with the Oregon Cooperative Fish & Wildlife Research Unit of the U.S. Geological Survey and Oregon State University.

Bald eagles live in high densities along the Aleutian archipelago off the coast of Alaska and place their nests on islets, coastal cliffs and shoreline sea stacks. Historically, more than 90 percent of the eagles' food comes from the ocean. Sea otters once also occupied a large range of coastal marine environments near these islands, but in recent years, otter populations have declined in response to their own main predator.

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