Salt marshes provide many ecosystem services to humanity, but are threatened by sea-level rise. Many studies examining the impacts of sea-level rise 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.
From 2007-2010, 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). Various densities were used to determine how each crab population affects Spartina productivity and the amount of soil excavation. Spartina productivity was measured by examining above and belowground biomass at the end of the experiment. Excavation rate was determined by daily collection of loose sediments that were deposited on the soil surface from crab burrowing activities.
Field densities of crabs were determined in various microhabitats (platform, live, live/dead, and dead) on the marsh. Also, a long term plant removal experiment at six bifurcated creeks was conducted to examine the effects of plant removal (mimicking the effects of crab herbivory) on creek growth. We used the mesocosm and field density data to determine the potential yearly excavation rate of each crab species.
The four crab species differed in their impacts. Sesarma excavated the most soil and strongly reduced both below and above-ground Spartina biomass. The other three species did not significantly impact Spartina productivity. Eurytium and Panopeus were rare and occurred at low densities while Uca was common throughout the marsh surface. Sesarma was rare on the marsh platform but was abundant at the creek heads. The level of bioturbation varied across the marsh but was highest at the creek heads. The crab community can turn-over the marsh surface multiple times per year. Creek heads with vegetation removed grew at a significantly faster rate than control creeks. In sum, crabs may mediate creek growth in response to sea level rise 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 sea level rise is a function of the relative abundance of different crab species.