Continued Development of Non-Equilibrium Ecology for Rangeland Applications

Updated 06/09/2006

Paul Meiman, Department of Rangeland Resources
Oregon State University, Corvallis, Oregon, and USDA-Agricultural Research Service, Northwest Watershed Research Center, Boise, Idaho

Dr. W. C. Krueger and Dr. Tamzen Stringham, Department of Rangeland Resources
Oregon State University, Corvallis, Oregon

Pat Shaver, USDA-Natural Resources Conservation Service
Grazing Lands Technology Institute, Corvallis, Oregon

Concepts of non-equilibrium ecology have been apparent in the literature for a long period of time. During much of the past century however linear models, focused on systems at or near equilibrium, have driven succession theory. Some of these models have dealt with multiple pathways while others have depicted only one pathway as a linear continuum. The quantitative climax approach developed in the late 1940’s is one such model and has been the workhorse of rangeland ecology and management since its development. This model was developed in the Great Plains and has been applied to rangelands across the U.S. In the Great Plains, mountain grasslands, and various other systems, the quantitative climax model often works very well. However, many instances have been identified where the quantitative climax approach has not adequately described observed vegetation dynamics. As a result, many ecologists have looked to non-equilibrium ecological concepts and developed state and transition models in an effort to more adequately describe vegetation dynamics.

Although the terms state, transition and threshold are commonly used in rangeland ecology, they have been defined by different individuals at various points in time. As a result, a universal set of definitions linking all of the terms together into a framework for state and transition models is lacking in the literature. A review of the literature reveals at least two different applications of the term ‘state’. One is a very broad interpretation where states encompass multiple expressions of plant communities, or seral stages of plant communities, among which reversible relationships are maintained. An alternative interpretation is of much more specific states that approximate seral stages or condition classes. Thresholds have been interpreted at least two ways, depending primarily on how authors have interpreted states. With specific states, thresholds are often placed between some, but not all states. In many of the broad applications of state, a threshold must be crossed in order for a state change to occur.

A variety of state and transition model applications present in the literature were analyzed paying particular attention to the advantages each approach offered to the management of rangeland systems. These advantages were then considered and used to aid in the formulation of draft definitions and a proposed conceptual model linking the concepts of states, transitions and thresholds together. States are defined in terms of abiotic and biotic factors that form the foundation and framework of rangeland systems. Transitions and thresholds are defined relative to states and the proposed conceptual model provides visual representation of how the various components of non-equilibrium ecology may interact. It is recognized and appreciated that the quantitative climax model is appropriate in many systems and therefore, the proposed conceptual model was designed to build upon, rather than to replace it. At the present time, the authors of this effort view it as the next step in the continued development of non-equilibrium ecological models for rangeland applications, and not as a finished product.

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Continued Development of Non-Equilibrium Ecology for Rangeland Application

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