SAN FRANCISCO - An exotic marsh grass is spreading throughout San Francisco Bay more rapidly than anticipated, threatening native plants and birds and prompting an urgent search for ways to eradicate it.

Spartina alterniflora, also known as Atlantic cordgrass, has expanded from 470 acres in 2000 to an estimated 2,000 acres in 2003, according to the California Coastal Conservancy's San Francisco Estuary Invasive Spartina Project. Those 2,000 acres are distributed over 69,000 acres of tidal marsh and mudflats throughout the bay.

"We're at the beginning of an explosive expansion of this plant," said Peggy Olofson, the project's director.

Spartina experts from around the world are meeting in San Francisco this week to discuss ways to eradicate the foreign invader and its hybrid offspring.

Atlantic cordgrass is a fast-growing weed that can grow up to 7 feet tall and cover mudflats in dense meadows that can have a profound impact on an ecosystem. Some countries import the weed to convert marsh into solid land.

Atlantic cordgrass was introduced in San Francisco Bay three decades ago to combat erosion and has since become the most dangerous nonnative species to take root there. It grows faster, denser and taller than its native counterpart, Spartina foliosa, which forms critical habitat for an assortment of shorebirds, fish and other wildlife.

Even more distressing, the East Coast cordgrass has mixed with native species and spawned hybrids that grow faster and stronger than their parents.

If left alone, the invader could destroy thousands of acres of bird and wildlife habitat, extinguish native plant species and clog tidal creeks and flood control channels. Researchers are worried about the impact on migratory birds because San Francisco Bay is considered the West Coast's most important estuary for such birds.

The advancing cordgrass invasion also threatens efforts to restore thousands of acres of wetlands around San Francisco Bay. The intruder has infested all 33 marshes where restoration work is currently under way, Olofson said.

"At the rate it's growing, it will soon become an insurmountable problem," said Debra Ayres, a researcher at UC Davis. "I am optimistic that it can be contained, but I am pessimistic about the bay ecosystem if it isn't contained."

In all the places where Atlantic cordgrass takes root, the transplant eventually dominates the ecosystem. In Willapa Bay in southern Washington state, the weed has destroyed thousands of acres of migratory bird habitat as well as oyster-growing areas.

Wildlife managers are removing small patches of spartina manually -- cutting, digging, mowing and smothering them. For larger areas, they are spraying a herbicide called Rodeo that is relatively effective at killing the weed. But so far, the invader is spreading faster than it can be eradicated.

Among the areas hardest hit by the spartina invasion is the East Bay shoreline near Hayward, where wildlife officials are struggling to contain more than 500 acres of the weed. The invader has already destroyed the habitat of two endangered species: the California clapper rail and the salt marsh harvest mouse, according to Peter Alexander, a spartina control manager at the East Bay Regional Park District.

"It's a huge headache, and it's still in its relative infancy," Alexander said. "It's absolutely a battle. It's a battle against time because it's doubling every year."