Magazine article Science News

Top-Down Lowdown: Predators Shape Coastal Ecosystem

Magazine article Science News

Top-Down Lowdown: Predators Shape Coastal Ecosystem

Article excerpt

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.

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. …

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