Thesis

Distribution of an invasive crab and implications for cordgrass restoration

There is an atypical distribution of the non-native crab Carcinus maenas, rarely found at high elevations in the intertidal zone elsewhere but mainly located in high elevation habitats in San Francisco Bay. Perceived threat from subtidal predators at low tidal elevations may cause C. maenas to seek refuge in higher tidal elevations, potentially in recently restored cordgrass (Spartina foliosa) habitat where their foraging activity may cause damage. Distribution of and predation on C. maenas were quantified using trapping and tethering experiments across tidal elevations, and also among high elevation habitat types. I found that use of S. foliosa by C. maenas may be context dependent; influenced by types of available refuge and whether competitors are present. I evaluated the effects of crab activity within cordgrass patches using a field enclosure experiment. Results show deleterious effects on cordgrass caged with C. maenas. Additionally, there was a trend in positive effects of native crab Hemigrapsus oregonensis on S. foliosa health and survivorship; potentially due to higher levels of nitrogen in the sediment. These results directly inform S. foliosa restoration efforts in the Bay, and give us new insight in to invasive species impacts on restoration efforts.

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