Ecosystem Resilience

Ecosystems like Chesapeake Bay are dynamic, changing from season-to-season and year-to-year. But the overall state of the ecosystem, whether it has clear water and abundant submerged aquatic vegetation or turbid water and copious algae, can be very resistant to change.

Ecosystem resilience describes how much stress a system can withstand without a fundamental change in state. But ecosystems in both healthy and unhealthy states may be resilient and resistant to change –– so called perverse resilience. Returning an unhealthy ecosystem to a healthier state can prove especially difficult since changes that occur in the transition from one state to another may be hard to reverse, like a rubber band stretched too far. This loss of function is called hysteresis.

Despite deforestation and diminished wetlands, until the 1970s the Chesapeake Bay existed in a stable state with abundant fish, shellfish, and underwater grasses. Over time, however, the accumulated loss of oysters, forests, and other buffers weakened its resilience and left the Bay vulnerable to the torrential runoff brought by Tropical Storm Agnes in 1972. Since then, many ecologists feel that the Bay has found a new steady state, but one with far fewer grasses and many more algal blooms. Getting to a state that more closely resembles the past will require a difficult push up a hill made steeper by the loss of the Bay's buffers.

Along the way we will likely cross key thresholds that cause nonlinear -- perhaps rapid -- changes in the ecosystem, such as the sudden resurgence of underwater grasses once waters become clearer.

To move the Bay towards a future state that more closely resembles the ecological state of the past calls for adaptive management, a process that matches input with response and re-evaluation to measure whether actions taken have engendered the desired effects.

The concept of resilience has helped Maryland Sea Grant frame its understanding about pursuing a realistic restoration trajectory for the Bay.

Useful Links

On the Road to Restoration
Chesapeake Quarterly, Volume 3, Number 3, 2004

The Resilience Alliance