The roles of crabs in forming tidal creeks in sinking salt marshes

Salt marshes provide many ecosystem services, but are threatened by sea-level rise (SLR). Many studies examining the impacts of SLR neglect the role of the marsh biota. In some areas, sea-level rise is leading to rapid headward erosion of marsh creeks, which are characterized by dense crab populations. Crab burrowing and herbivory might affect creek erosion, but little is known about how these processes vary among crab species. We conducted field and mesocosm experiments to examine the burrowing and herbivory rates of four common marsh crabs (Sesarma reticulatum, Eurytium limosum, Panopeus herbstii, Uca pugnax). We also conducted a long term plant removal experiment to mimic crab herbivory at 6 pairs of creek heads to determine the effects of vegetation loss on creek growth. Sesarma excavated the most soil (~146g/week/crab) and reduced both below and above-ground Spartina biomass. The level of bioturbation varied across the marsh but was highest at the creek heads. Creek heads with vegetation removed grew at a significantly faster rate (2.3 m/yr.) than control creeks (1.5 m/yr.). In sum, crabs may mediate creek growth in response to SLR by excavating sediments, by indirectly damaging plant roots and weakening their ability to bind marsh sediments, and by directly consuming marsh plants. Because different crab species differ in these effects, the net impact of the crab community on marsh responses to SLR is a function of the relative abundance of different crab species.