Effects of a Non-Native Crab on the Restoration of Cordgrass in San Francisco Bay

Invasive species are known to alter the progress of ecological restoration, but effects of invasive consumers, and particularly those that can act through both trophic and physical disturbance mechanisms, remain poorly understood. Due to its impacts on vegetation and ability to alter marsh food webs, we predicted that a non-native crab, Carcinus maenas (European green crab), would negatively affect restoration plantings of a native cordgrass, Spartina foliosa1 (California cordgrass), in San Francisco (SF) Bay, in contrast with neutral or positive impacts predicted for a native crab, Hemigrapsus oregonensis (yellow shore crab). We implemented an experiment in SF Bay using caged plots of planted S. foliosa, to which we added or did not add crabs. We compared responses among these treatments, cageless, and partially-caged controls. Carcinus maenas treatments had the greatest reduction in S. foliosa stem density from June through August, and the lowest stem density in August relative to all other treatments. Hemigrapsus oregonensis effects were similar to controls. Both crabs reduced small invertebrate abundance by similar to 50% and led to shifts in community composition relative to controls. Changes to the invertebrate community and subtle shifts in soil conditions may contribute to the apparent negative impact of C. maenas. Crab trapping revealed more C. maenas in areas with Spartina in SF Bay, suggesting it uses Spartina as a refuge and that these effects are especially relevant to restoration activities. These results add to our developing understanding that invasive consumers can affect the path and pace of restoration and thus warrant increased attention in restoration planning and management.