The perfect storm: Extreme weather and predators drive phase shift in dominant Chesapeake Bay bivalve
The soft-shell clam Mya arenaria, once a biomass dominant in the Chesapeake Bay, has been in decline since 1972 and now exists in the Chesapeake Bay at record low levels. We hypothesize that tropical storm Agnes in 1972 resulted in a phase shift for M. arenaria, which was 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, parameterized using values from previous experiments and the literature, and analyzed for steady states. In a field caging study conducted in the York River, Virginia, juvenile M. arenaria were placed in 9 replicate 0.25 m2 plots at densities of 48 individuals m-2 and collected after 5-8 d. Predator exclusion treatments (uncaged, stockade, and full cage) were used to determine the relative contribution of blue crabs to clam mortality, as compared to larger predators such as the cownose ray Rhinoptera bonasus.
Changepoint analysis identified an abrupt shift in clam abundance in 1972. Before the storm, crab abundance was positively correlated with clam abundance with a lag of 1 y (r = 0.67, p = 0.01), indicating bottom-up control. After the storm, clam abundance was negatively correlated with crab abundance with a lag of 2 y (r = -0.58, p = 0.01), indicating top-down control. Predator-prey models confirmed the presence of a coexistence steady state at low densities of M. arenaria, providing the theoretical proof-of-concept that M. arenaria can exist in a low-density stable state in the face of blue crab predation. In the field, clams exposed to predators suffered an increase in mortality of 67% as compared to caged individuals (p << 0.001). Predator exclusion treatments confirmed that blue crabs were likely responsible for most of the mortality of juvenile M. arenaria. 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.