When rare species eat endangered ones

This year completes the relocation of the largest nesting colony of Caspian terns in North America from one island to another, reports Daniel Roby of U.S. Geological Survey in Corvallis, Ore.

Dredges dumping spoils created Rice Island in the Columbia River, and in the mid 1980s, observers noted Caspian terns breeding there. By 2000, 8,000 pairs of the terns were nesting in the estuary, 10 percent of the species.

“Worldwide populations are generally not faring well,” Roby says.

Life looked good for the terns until studies in the late 1990s revealed that salmon smolts made up 74 percent of the birds’ diet. The birds caught 10.2 million of these young salmon, representing 11 percent of those swimming down the Columbia. The exact impact wasn’t clear, but the numbers sparked alarm. “The situation of salmonids is really quite desperate,” says Roby.

In an attempt to lower the toll, biologists tried to persuade the colony to move to East Sand Island, a dredge-spoil mound closer to the mouth of the Columbia. The new location offered more species of fish as alternative food.

To suggest that terns were already congregating on East Sand Island, biologists dotted it with models of the birds and patio speakers broadcasting recorded calls. Attracted to what seemed to be the vanguard of the breeding colony, real birds settled among them. Nesting success on the

new island surpassed that of the old colony.

In 1999, about 15 percent of Rice Island birds shifted. Now the whole colony has moved, and the birds consume 6.4 percent of the salmon smolts swimming down the Columbia. That’s an improvement, Roby says, but hardly an end to the debate over what do about one vulnerable species eating another one.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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