Species Shift Distorts Marine Food Web

The ocean's interlinked food web of photosynthesizers, grazers, predators and detritus recyclers is being reconfigured. Invasive species have replaced extirpated native species and reshaped the food web.

Along the coast of California, northwestern Europe and Australia, the pattern is the same.

Extinction targets predators at the top of the food hierarchy. But new arrivals tend to be secondary consumers, mainly organisms that eat plankton.

Carnivores comprise 70% of the 133 marine species that have become extinct at a global and regional level. These include animals such as dolphins, several species of sharks, along with seagulls and sea otters.

For the coastal marine areas examined on three continents, 70% of the newly-established species feed further down the food chain. The newcomers are mainly herbivores that graze on plants and algae, on plankton or on decaying organic matter.

Among the 166 new species documented in San Francisco Bay, for instance, three-quarters feed at the second trophic level. Of these, 40% are macroplanktivores, small organisms that eat phytoplankton and zooplankton. Meanwhile, less than 4% of the Bay's invasive species are top predators. The number of primary producers, those that convert sunlight to energy through photosynthesis, remains about the same.

The result is a skewed food web. The new trophic regime is heavier in the middle and lighter on top than what existed before the human-caused extinctions and invasions.

Researchers warn that the consequences of distorting the marine ecosystem to this extent could be far-reaching. Disrupting the marine species mix can trigger further declines in native organisms, as has been the experience with kelp and fish in the Gulf of Maine.