Superstud grass menaces San Francisco Bay.

Watch out, San Francisco. A salt-marsh grass from the East Coast is showing a new twist on conquering the world. The intruders may still be relatively rare, but they're overwhelming a widespread native plant through the power of superior pollen.

The invader, Spartina alterniflora, or smooth cordgrass, produces 21 times as much pollen as the native Spartina foliosa, report Donald R. Strong of the University of California, Davis and his colleagues. The intruders fertilize native species and produce aggressive hybrids. With its prodigious powers for siring seeds, S. alterniflora could rapidly swamp the natives, the researchers warn in the November American Journal of Botany.

Strong reminisces about the early 1980s, when biologists still cherished the. notion that most species had built-in genetic protections against hybridizing with other species. Suggest otherwise in those days, and "you'd have been laughed out of a meeting," he says.

Now, researchers recognize that an alien species--plant or animal--often can move into a town, mate with the natives, and beget fertile hybrid offspring until the local species disappears.

Previous research on biological intruders focused on rare natives succumbing to common invaders, Strong observes. He sees the cordgrass study as the first specific analysis of a reproductive edge that lets an uncommon invader make fast progress against a widespread plant.

Study coauthor Curtis C. Daehler of the University of Hawaii at Manoa in Honolulu explains that the invasion got an ironic start. In San Francisco in the 1970s, people trying to restore salt marshes planted commercially available species, which happened to be from the East. Strong calls it "a pushy New York cordgrass" that's moving in fast on the native "laid-back California kind of species."

After analyzing 50 plants of each species, the researchers found that the flower clusters of the invader create much more pollen than the native clusters. Also, in a greenhouse test with four plants of each species, the team placed pollen on native plants. Invader pollen germinated at almost twice the rate of the native species'. Finally, in another test, the invader-native crosses produced almost eight times as many seeds as native-to-native pollination.

It's no wonder, laments Strong, that these super sires "are spreading like wildfire." He worries about the "genetic pollution" of the native species' genome and predicts disastrous impacts on the other creatures of the bay. The hybrids spread farther down the mud flats than the natives do, so less mud will be open for shorebirds to forage and for harbor seals to park their pups. Both the invaders and the hybrids are also choking flood-control channels and bedeviling navigation.

Controlling the invader biologically is out, Strong says, because anything that eats the invader will probably also eat the closely related native. Importing insect pests from the East Coast would be "a horrible idea," he asserts.

A chronicler of alien invasions, Daniel Simberloff of the University of Tennessee in Knoxville says that the cordgrass tale underscores the need to stop invasions promptly. "We need early detection and rapid response."