California Sea Grant College Program

Four species of nonindigenous Spartina cordgrass have been introduced to San Francisco Bay. One of them, Spartina alterniflora, from the East Coast, has hybridized with the native S. foliosa and become highly invasive. These hybrids and their backcrosses are problematic to conservation objectives as they colonize nearly every ecological niche of a marsh – high and low marsh elevations, and across a range of salinities and sediment types. Where established, the hybrids inevitably pollen swap with native Spartina, creating yet more hybrids. With time, rare wetlands can be converted into uniform expanses of grass. Efforts to eradicate invasive Spartina have been based largely on visually identifying the most threatening, biggest, tallest and thickest, red-stemmed, big-flowered cordgrass plants and spraying these with herbicide. The strategy has been highly success- ful in reducing the invasion’s size – from about 800 net acres in 2005 to about 50 net acres in 2011. But, it also appears to have selected for “cryptic” hybrids – cordgrass plants that resemble native Spartina but contain exotic genes.