Predatory crab size diversity and bivalve consumption in oyster reefs

Body size is widely recognized as an important functional trait of predators due to its influence on prey consumption rates and diet breadth. Yet it remains unclear how the diversity of this trait within predator populations affects prey communities. To test the effects of intraspecific predator size diversity, we manipulated the number of size classes (i.e. size diversity) in the xanthid crab Panopeus herbstii and measured their consumption of the bivalve community in intertidal oyster (Crassostrea virginica) reefs. In the experiment, the presence of large crabs, but not size diversity, significantly affected total prey biomass consumption. The largest size class of crabs effectively consumed all bivalve prey types whereas smaller crabs were restricted in diet breadth. As such, any treatment containing large crabs had significantly greater total prey consumption and more uniform consumption across the prey community than those without. We also investigated the potential for oyster harvest to alter crab population size structure at the study site (North Inlet, South Carolina, USA). Anthropogenic oyster harvest acts to compress the surficial shell layer in reefs and could reduce crab body size by reducing the availability of refuges for large crabs. Therefore, we tested for a relationship between the height of the shell layer and average crab body size in the field. In the field survey, average crab body size decreased with decreasing height of the shell layer. Thus, our data suggests that oyster harvesting practices have the potential to skew crab size structure towards a preponderance of small individuals, thereby compromising the trophic transfer that occurs in unperturbed reefs.