State-Transition Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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S tate -transition refers to a model of vegetation community dynamics that examines the interaction between processes and the multiple persistent communities that can arise from these processes. These models have been developed largely within the field or range management as a reaction to the shortcomings of the Clementian range succession (or range condition model), in which rangelands were viewed as a single climax community that was removed from that climax equilibrium state by stocking rate, grazing pressure, and precipitation, and was constantly in succession back toward that climax community. State-transition models differ from this approach by asserting that multiple persistent vegetation communities (states) can exist and that climatic factors, grazing, and stocking practices and vegetation autecology can cause these communities to shift between one state and other persistent alternative states (transitions). Transitions between states can be either reversible or nonreversible. Researchers have proposed the idea of transition thresholds to account for the reversibility of transitions. A given community can tolerate a certain amount of disturbance and still be able to transition back to its original state, but if the transition crosses a threshold, then the community will irreversibly transition to an alternative state.

From a theoretical point of view, state-transition models are considered to be nonequilibrium approaches to ecology. The classic range succession model is an equilibrium approach, because it proposes a single stable climax community that the grassland ecosystem would produce in the absence of grazing and drought; reducing grazing pressure thus causes succession back to this climax community under this model. That is, it views vegetation change along a single axis of change, where any disturbance initiates a linear path of succession back to climax. Succession, as a process of change, is essentially assumed to occur under negative feedback returning the system back to its homeostatic equilibrium state, without actually analyzing the interactions between the biota and environment that take place during this successional change.

Management under this perspective focuses strictly upon adjusting stocking rates to precipitation to keep the grassland as closely removed from the climax as possible. Nevertheless the range succession model was not able to explain or predict observed changes in community composition and other models were sought after. The state-transition approach, by contrast, is concerned with both process and pattern, and seeks to establish the causes of the transitions between persistent states. In identifying the causes, researchers must examine the intersection between disturbance, plant demography (reproductive and dispersal habits) as well as landscape heterogeneity (whether surrounding patches exert propagule pressure on a patch), in determining how a community shifts its species composition from one persistent state to another (if any). As such, it is best categorized as a dynamic nonequilibrium model. Furthermore, the model examines vegetation change along several axes of transition.

The state-transition model is considered to be an improvement on the range succession approach, but is better understood as a heuristic model and management tool rather than a comprehensive theory. The model is based on managerial criteria rather than ecological ones, and does not employ any historical criteria to define the desirable alternative states. Although it seeks to reconcile pattern to process, its method of inquiry does not extend beyond identifying alternate states and the causes of transition, without attempting to produce a formalized, theory of community change and stability.

State-transition approaches are utilized both as management tools and in research. Although rangeland management has been the field where statetransition approaches are predominantly applied, these models are becoming more common within restoration ecology as well. The general approach in management is first to identify the various alternative ecosystem states that are present in any given area of management, then to identify the causes of transition. Having identified the various alternative states, an assessment is then made for which states are acceptable for the given management strategy and which are not.

Having identified the causes for transitions between desirable and undesirable states, managers can then not only strategize rehabilitation of undesirable states, but can avoid the processes that cause desirable states to transition to undesirable alternative states. That is, state-transition models allow managers to be proactive in their management, rather than reactive, since, under the range succession model, the undesirable alternative states irrupt onto the landscape as unpredicted departures from the linear succession to climax. From a research perspective, the state-transition model is best understood as a heuristic device and a means of generating testable hypotheses. Neither the actual transitional processes nor thresholds are well examined empirically under this model and provide the focus for future research.

Some recent research that attempts to explain these transitions and thresholds seeks to reconcile alternative ecosystem state concepts with homeostatic ecosystem models. The approach proposes that each of the alternative states represent different stable homeostatic equilibria, arising through heterogeneity in the spatial distribution of various environmental factors. Transitions and thresholds are understood in terms of feedbacks, with positive feedback driving transitions across thresholds into alternative ecosystem states. Another similar approach employs chaos theory to describe these alternative states and the transitions between them. The alternative states are depicted as attractors, with the transitions between attractors arising as consequences of values the descriptive equations take. Complexity theory has also been applied to State-transition models, with the crossing of a transition threshold between alternate states analogous to a phase shift.

Bibliography:

D. Briske, S.D. Fuhlendorf, and F.E. Smiens, “State-transition Models, Thresholds, and Rangeland Health: A Synthesis of Ecological Concepts and Perspectives,” Rangeland Ecology and Management (v.58, 2005);
Randall D. Jackson, James W. Bartolome, and Barbara Alan Diaz, “States and Transition Models: Response to an ESA Symposium,” Bulletin of the Ecological Society of America (v.83, 2002);
Katherine N. Suding, Katherine L. Gross, and Gregory R. Houseman, “Alternative States and Positive Feedbacks in Restoration Ecology,” Trends in Ecology and Evolution (v.19, 2004);
Mark Westboy, Brian Walker, and Imanuel NoyMeir, “Opportunistic Management for Rangelands Not at Equilibrium,” Journal of Range Management (v.42, 1989);
R. Wilkinson, M.A. Naeth, and F.K.A. Schmiegelow, “Tropical Forest Restoration within Galapagos National Park: Application of a State-transition Model,” Ecology and Society (v.10, 2005).