Otters and Eagles

It turns out that changes in the Alaskan sea otter population are affecting the bald eagles that nest along the coasts. Otters, like eagles, are top predators and help maintain the ecosystems in which they reside.

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.

So what happens when the otters disappear?

"All of the available data point to increased numbers of killer whales as the direct cause of the sea otter decline in southwest Alaska," says coauthor Jim Estes of the U.S.G.S. and the University of California at Santa Cruz. "The otter decline has caused a phase shift in the coastal ecosystem from a kelp dominated phase state to a deforested phase state."

This shift means many fewer kelp forest fish for the eagles to eat. In response, the eagles have adjusted their foraging tactics. Anthony and his colleagues surveyed remains of bald eagle prey in their nests during 1993 and 1994, when otters were abundant and the kelp forests were healthy, and in 2000, 2001 and 2002, when otters were scarce and the kelp forests had collapsed. They found that when otters were abundant, eagle prey consisted of predominantly kelp-forest fish and sea otter pups. When the otters were rare, however, the proportion of marine birds in the eagles' diet was much higher.

It would be interesting to know how much this change affects marine birds, all of which have their own sets of ecosystem interactions.