Estuarine ecosystems and subsystems can and do exhibit alternate or multiple states of existence. The ability of an ecosystem to absorb disturbance and resist a change in state is termed ecological resilience, as opposed to engineering resilience, which is the time it takes a system to return to its original state. In the last decades of the twentieth century, ecologists observed that ecosystems were not static entities, but appeared to change in response to external and internal forces. In the Chesapeake Bay estuary, for example, some of the factors causing a state change were over-fishing, increased suspended sediment load, eutrophication, species invasion, and disease. The bay's responses to these forces were slow at first, but with the steady increase in the human population in the bay watershed and with its adherent development, the signs of a state change were dramatically evident. The oyster reefs, a major benthic subsystem or habitat that had dominated the bay for centuries, began to decline rapidly or crash. The benthic-dominated food web was replaced by a planktonic food web. Management efforts to restore the initial oyster-dominated system did not work, probably because they had a single species focus and because ecosystems are strongly nonlinear which means the path to restoration is different from that leading to the initial change of state and many more components of the ecosystem are involved in addition to the oysters.
See also: Mangrove Wetlands; Salt Marshes.
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