Nature Notes – From the Backyard to the Biosphere. Succession

Succession: Evolving Understandings of Change in Natural Systems

If there is one certain thing in a world characterized by uncertainty, it is that change is inevitable. Understandings of why and how change happens in the natural world (succession, as it is called in ecological circles) have themselves morphed over time.

Once emphasizing predictability and stability, theories of succession have shifted dramatically in recent decades to the recognition of dynamism and uncertainty in processes of change in natural systems.

Succession was initially understood to be an orderly process of change following a predictable pattern:

1. a ?pioneer? ecosystem stage characterized by fast-growing, light-loving species capable of growing on poor-quality sites;

2. a predictable set of ?seral? stages, for example the transition from the pioneer stage to an ?old field? ecosystem characterized by increasingly dense shrub cover, shading out of herbaceous (non-woody) species, and eventual establishment of shade-tolerant species; and

3. the final equilibrium climax ecosystem (e.g., a mature mixed maple-beech forest with thick, rich soils and shade-tolerant climax tree species).

Such a rigid view of ecosystem change had a profound impact on environmental management understandings and strategies. The climax stage was considered the ultimate end point to which each ecosystem was progressing; as such, management was directed toward maintaining existing climax communities in a stable state, and directing ecosystems considered to be in earlier seral stages toward their final climax stage.

This idea of stability, of the static nature of natural systems, and the perceived need for ecosystem managers to maintain climax conditions led to management actions such as near continent-wide fire suppression.

These management actions were seen as a means of protecting ecosystems from disturbance; disturbances were viewed as reversing the process of succession and setting back ecosystems otherwise on their way toward the ideal stable climax state. The doctrine of managing for stability dominated protected-areas management throughout much of the 20th century and has led, in the eyes of many ecologists, to the severe decline in ecological integrity seen in many parks and other managed areas.

Recognition among ecologists of the failure of past stability-oriented management regimes resulted in a shift in environmental management understandings. Scientists, theorists, and practitioners began moving from a focus on stability to a recognition of the dynamic, unpredictable nature of systems as complex as interconnected ecological webs, and of the need for incorporating disturbance and change into management regimes.

This new perspective on ecosystem change has begun to alter the face of ecosystem management in significant ways. Disturbance factors such as fire and flooding, which used to be seen as having only negative impacts on ecosystems, are now seen to be critical to maintaining ecosystem integrity and productivity.

Fire, for example, is now known to replenish soil nutrients, release the seeds of certain species from dormancy and enhance germination rates, control disease and pest outbreaks, and provide the landscape-level heterogeneity now considered so essential to broad-scale environmental integrity. Incorporating disturbances into management plans is becoming increasingly commonplace.

So change really does seem to be the only constant–even in the way we perceive dynamism in natural systems and the actions we take based on our perceptions of how and why change happens. Good thing is, the evolution of theories of succession seems to be leading us down a smarter environmental management path for the future.

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