Many geophysicists suspected that the plumes come up from as far down as the
core-mantle boundary, which lies about 2900 kilometres under the surface (see
“Deep secrets”, żěè¶ĚĘÓƵ, 17 October, p 38). Now Thorne Lay of the
University of California at Santa Cruz and his colleagues have shown that this
seems to be true, at least for the Hawaiian hot spot.

Lay and his colleagues analysed seismic waves generated by 54 earthquakes in
Tonga and Fiji. As the waves travelled through the Earth to seismographs in the
US, they grazed the top of the Earth’s core southeast of Hawaii. The seismograms
show that warm rock there flows horizontally toward the base of the Hawaiian hot
spot, then rises vertically into the plume (Nature, vol 396, p
255).

This result could be a milestone in the debate about where such plumes
originate, the researchers say. “This will certainly be a provocative paper,”
says Lay. “If it turns out to be right, it will be a landmark.”

In another paper in Nature (vol 396, p 251), seismologists from the
California Institute of Technology report similar evidence for a hot blob at the
core-mantle boundary under the Iceland hot spot.

Orcas usually ignore sea otters, preferring seals and sea lions, which are much larger and provide lots of calories with each blubbery bite. To a killer whale, otters are like “hairy popcorn”, says Paul Dayton, a marine ecologist at the Scripps Institution of Oceanography in La Jolla, California. “The whales must be hurting to eat them.”

Nevertheless, orcas are now eating otters, claims James Estes, a marine ecologist at the University of California, Santa Cruz. Estes and his colleagues first saw an orca attack an otter in 1991. Since then, a dozen more attacks have been recorded.

Estes has also documented a sharp fall in otter numbers in recent years. About 90 per cent of them have vanished from a 1000-kilometre stretch of the central Aleutians. “The magnitude and spatial scale of this decline probably is unprecedented for any carnivore,” says Estes, who reports his team’s work in Science (vol 282, p 483).

Estes ruled out disease, toxic pollutants and starvation as causes for the otters’ plight. He then noticed that otters were thriving in a protected bay inaccessible to orcas. “At first I didn’t think it was possible, but we gradually realised that at least some killer whales had switched to preying on sea otters.”

Estes estimates that there are about 150 orcas in the central Aleutians, enough to account for the otters’ decline. “But it’s conceivable, and not unlikely, that it’s one small group of animals,” he says. As few as four orcas could have wreaked this havoc by each gulping down about 2000 otters per year.

The orcas’ new menu has set off a startling cascade. Populations of sea urchins, the favoured diet of otters, have exploded. As a result kelp beds, on which urchins dine, are disappearing. Kelp is the base of the coastal food web and provides habitat for countless fishes, says Estes. Its loss may affect seabirds, bald eagles, and other nearshore species.

“This is the kind of thing a lot of us have worried about,” says Mark Hay, a marine ecologist at the University of North Carolina in Chapel Hill. “Just a few top predators can alter their habits and have astounding effects on an entire ecosystem.”

It is not clear why killer whales have switched prey, but Estes points to a collapse in the populations of northern sea lions and harbour seals in the region during the past two decades. Biologists are still not sure of the reason for this collapse, but a report published in 1996 by the US National Research Council blames overfishing, warming of the North Pacific and whaling as the most likely causes.

Other experts say Estes’s conclusions about the orcas’ dietary shift are highly plausible. “I wouldn’t put it past them,” says John Ford, director of marine mammal research at the Vancouver Public Aquarium in British Columbia. “They are very adaptable, stealthy, and innovative predators.”

Orcas have even attacked moose in shallow waters, Ford says, and in Argentina have been known to beach themselves to grab sea lion pups.