Potential Impacts of Non-Native Spartina Spread on Shorebird Populations in South San Francisco Bay

San Francisco Bay holds 70% of California’s mudflats and provides habitat to more wintering and migratory shorebirds than any other wetland along the Pacific coast of the contiguous U.S. The bay’s mudflats are currently threatened by the spread of a non-native cordgrass, Spartina alterniflora, and associated hybrids, which grow at lower elevations than the native S. foliosa and can render large mudflat areas effectively unavailable to shorebirds for foraging. Using shorebird and benthic invertebrate survey data, tidal benchmark data, and GIS-based habitat data, we analyzed the potential effect of S. alterniflora on shorebird habitat in the South Bay by creating grid-based spatial models of shorebird habitat value and potential S. alterniflora spread. We developed 12 potential scenarios of habitat value loss for shorebirds based on assumptions about invertebrate density, inundation tolerance of S. alterniflora, and temporal availability of mudflat resources. Predictions of habitat value loss ranged from 9% to 80%. We identified the upper mudflats, due to their greater exposure time, and the east and south shore mudflats, due to the high numbers of birds detected there, as the areas of highest value to shorebirds in the South Bay. These areas also coincide with the areas of greatest Spartina invasion potential.

This project was made possible by funding from the California Coastal Conservancy, the State Resources Agency, and the CALFED Program. The analyses were requested by the Coastal Conservancy’s Invasive Spartina Project (ISP)—a coordinated regional effort among local, state and federal organizations dedicated to preserving California's extraordinary coastal biological resources through the elimination of introduced species of Spartina (cordgrass).

Report to Conservancy available in PDF format