Reef height drives threshold dynamics of restored oyster reefs

Nonlinear threshold responses to biotic or abiotic forcing may produce multiple population trajectories dependent upon initial conditions, which can reinforce population recovery or drive local ex tinction, yet experimental tests of this phenomenon are lacking in marine ecosystems. In field experiments at 4 sites in 2 tributaries of lower Chesapeake Bay, we examined demographic responses (density and survival) of eastern oyster Crassostrea virginica populations to reef height and associated gradients in sediment deposition and habitat complexity. After 2 yr, oyster reefs exhibited diverging trajectories to ward either degradation or persistence, dependent upon initial reef height. Reefs higher than 0.3 m supported greater oyster density, survival, and reef complexity, whereas sediment deposition was reduced. Reefs lower than 0.3 m experienced heavy sediment deposition and were eventually buried. These observations (1) provide experimental evidence for threshold dynamics in marine species, (2) suggest that the collapse of oyster populations was largely due to anthropo genic habitat degradation that eliminated positive feed backs and which may have created an alternative reef trajectory towards local extinction, and (3) indicate an avenue by which oyster restoration is achievable.