Extreme climate events have been implicated in ecosystem phase shifts in terrestrial and marine systems. As these events become more frequent, it is necessary to understand the ecosystem changes that may occur due to extreme events. Tropical storm Agnes in 1972 was a 100-year storm that reduced salinity and increased sedimentation throughout Chesapeake Bay. This storm has been blamed for the loss of seagrass in Chesapeake Bay, high mortality rates and recruitment failure in oysters Crassostrea virginica, and declines in abundance of the soft-shell clam Mya arenaria. We present a case study of M. arenaria, which we hypothesize has been maintained at low abundance due to predation by the blue crab Callinectes sapidus. Evidence for this hypothesis will be presented from changepoint analysis of time series, predator-prey ordinary differential equation models, and a field caging experiment. Changepoint analysis of time series was conducted on M. arenaria landings and C. sapidus abundance in the Chesapeake Bay from 1958-1992. Lotka-Volterra models were modified to include a sigmoid density-dependent functional response and analyzed for steady states. In a field caging study conducted in the York River, Virginia, 12 juvenile clams were placed in each of nine 0.25-m2 plots and collected after 5-8 d. Predator exclusion treatments were used to determine the relative contribution of crabs to clam mortality.
Results/Conclusions
There was an abrupt shift in clam abundance in 1972. Before the storm, crab abundance was positively correlated with clam abundance with a 1-y lag (r = 0.67, p = 0.01), indicating bottom-up control. After the storm, clam abundance was negatively correlated with crab abundance with a 2-y lag (r = -0.58, p = 0.01), indicating top-down control. We confirmed the presence of a coexistence steady state at low densities of M. arenaria, providing the theoretical proof-of-concept that clams can exist in a low-density stable state despite blue crab predation. In the field, clams exposed to predators suffered an increase in mortality of 76.6% as compared to caged individuals (p << 0.001). Blue crabs were likely responsible for most of the mortality of juvenile clams. The observations, theory, and mechanistic basis suggest that M. arenaria was subjected to a storm-driven phase shift to low abundance, which has been maintained by blue crab predation. As extreme weather events become more common with climate change, it is important to examine the potential for such perturbations to produce phase shifts that may permanently change basin-scale trophic dynamics.