Top-Down Lowdown: Predators shape coastal ecosystem

May 24, 2006 at 10:16 am

The health of southern California kelp forests may depend more on the ecosystem’s predator population than on the forest’s access to nutrients, researchers report. The finding suggests that fishing practices have a profound impact on these ecosystems.

IN CHARGE. The populations of predators, such as Kellet’s whelk (left inset) and kelp rockfish (right inset) in the seaweed forests of the Channel Islands seem to have a stronger impact on the health of the ecosystem than nutrient availability does. Halpern; (left inset) S. Lonhart; (right inset) Corbis

Kelp forests grow worldwide in shallow coastal areas with mild climates. The brown seaweed called kelp reaches from the ocean floor to the water’s surface, usually spanning 10 to 20 meters, says Benjamin S. Halpern of the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. Along western U.S. coasts, these ecosystems support up to 1,000 species of fish, plants, and invertebrates, he says.

Ecologists have long debated whether the number of predators—such as fish that feed on smaller creatures—at the top of the ecosystem’s food web or the availability of nutrients at the bottom of the web more strongly influences the condition of ecosystems.

Halpern and his colleagues studied kelp forests that surround the Channel Islands, about 25 miles off the coast of Santa Barbara. The group analyzed surveys of species’ abundance from 16 sites around the Channel Islands National Park. They also examined satellite data from 1999 to 2002 on chlorophyll concentrations—an indirect indication of nutrient levels—in the ocean waters surrounding the islands.

The “top-down” control accounts for 11 to 20 percent of the ecosystem’s pattern of species abundance, the team reports in the May 26 Science. The predator populations have 7 to 10 times as much influence over the ecosystem as the availability of nutrients does.

“No one has tested these two factors at the same time,” says Halpern. “How healthy a kelp-forest community is depends primarily on which predators and how many of them you have in the community.” Overfishing that depletes these predator populations could affect the ecosystem’s stability.

“I think this is a very powerful paper in terms of suggesting the strength of top-down influences,” says Robert S. Steneck of the University of Maine in Orono. “As we basically fish down global food webs in all these different ecosystems, we will in essence be restructuring communities.”

James Estes of the University of California, Santa Cruz agrees that the work is important to fisheries management. “It provides further evidence for the notion that overfishing has a strong effect on the ecosystem. It’s not just the [fish] stocks being taken out.”

But Michael H. Graham of Moss Landing (Calif.) Marine Laboratories notes that the new study may have underestimated the bottom-up effect. He points out that the 1999–2002 satellite data cover a period without an El Niño or La Niña event, two weather phenomena that can have large impacts on nutrient prevalence in kelp forests. Furthermore, the satellite measures chlorophyll concentrations near, but not in, the kelp forests.