Predators structure ecosystems by interacting with other trophic levels directly and indirectly. Indirect interactions can have large effects, yet are understudied in the context of restoration. We explored how predators and other environmental factors may impact crab distributions in the intertidal zone, and potential consequences of the resulting distributional shifts on restoration efforts of the native cordgrass (Spartina foliosa) in San Francisco Bay. Perceived threat from or direct consumption by subtidal predators at low tidal elevations may cause the non-native European green crab, Carcinus maenas, to seek refuge in high tidal elevations. These high elevations may include restored cordgrass habitat where crab foraging activity may cause damage. Crab distribution and predation on crabs were quantified using trapping and tethering experiments across tidal elevations and among different habitat types. We then evaluated the effects of crab activity within S. foliosa patches using a field enclosure experiment, caging newly planted cordgrass with and without crabs to assess crab impacts on cordgrass growth and survival.
We found that C. maenas, which is rarely found at high tidal elevations elsewhere, has an atypical distribution in the Bay in which it is more common at high elevations. Caged treatments with green crabs had significantly fewer S. foliosa shoots at the end of a three-month period than plots without cages or crabs, suggesting that crab presence may hinder successful S. foliosa establishment. This work has important implications for management as interactions between multiple trophic levels can confound restoration efforts but are rarely considered in restoration settings. Illuminating the intricacies of species interactions will make future restoration attempts more efficacious and informed.