  Native oyster populations in Chesapeake Bay have been the focus of three
decades of restoration attempts, which have generally failed to rebuild the
populations and oyster reef structure. Recent restoration successes and field
experiments suggest that high-relief reefs offset heavy sedimentation and
promote oyster survival, disease resistance and growth, in contrast to
low-relief reefs which degrade in just a few years. These findings suggest the
existence of alternative stable states in oyster reef populations. We developed
a mathematical model consisting of three differential equations that represent
volumes of live oysters, dead oyster shells (= accreting reef), and sediment.
Bifurcation analysis and numerical simulations demonstrated that multiple
nonnegative equilibria can exist for live oyster, accreting reef and sediment
volume at an ecologically reasonable range of parameter values; the initial
height of oyster reefs determined which equilibrium was reached. This
investigation thus provides a conceptual framework for alternative stable
states in native oyster populations, and can be used as a tool to improve the
likelihood of success in restoration efforts.
