Secret of California Invaders Revealed

Vanquished invader. Adding lots of native seeds allowed native grasses to nudge aside exotic plants such as this black mustard (Brassica nigra).

TUCSON, ARIZONA--Exotic plants from other parts of the world are the bane of the U.S. West, crowding out native species, increasing fire risks, and replacing better cattle fodder. A study presented here last week at the Ecological Society of America meeting has uncovered the secret to grassland invaders' success in California--seed supply. The study suggests it may be possible to restore original grasslands simply by sowing fields with lots of native seeds.

Since settlers began arriving in the 1500s, Mediterranean grasses and forbs have swept over the 9.2 million hectares of California grasslands--23% of the state's total area. These invaders are mostly annuals, while the native grasses they displaced are perennials. To find out if the exotic invasions can be reversed, Eric Seabloom of the National Center for Ecological Analysis and Synthesis and collaborators turned to ecological theory. They explored three possible reasons for the success of invaders: They dominate resources such as sunlight and water; they establish an impenetrable beachhead; or they produce more seeds than the perennials.

In a 4-year series of experiments in former crop fields at a reserve in Southern California, Seabloom's team ruled out the first two possibilities. Exotic annuals used less water, nitrogen, and sunlight than the natives, they found. And the natives could invade patches of exotic grasses if the researchers planted more native seeds. "Seed addition is crucial," Seabloom says.

The study shows that "there may be a very simple solution to restoration of grasslands in California," says ecologist Rebecca Shaw of the San Francisco office of The Nature Conservancy, a nonprofit that owns many private nature preserves. But Carla D'Antonio, an ecologist at the University of California, Berkeley, cautions that dumping more seeds to restore native grasses may not work in places where exotics have been growing for years and have built up a huge bank of seeds in the soil.

Related sites
Some background on California grasslands
Seabloom's site
National Center for Ecological Analysis and Synthesis

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