Abstract: The introduction of Clementsian succession theory in early twentieth century represented a substantial advance in rangeland ecology. Succession theory as a central concept of equilibrium ecology had profound influence on development of rangeland science and natural resource management models. However, equilibrium ecology and the steady-state management model received severe criticism in 1970s and 1980s. On one hand, nonlinear vegetation dynamics was recognized in rangeland ecosystem research, which is inconsistent with the interpretation of the traditional succession and steady-state management. On the other hand, research in other parts of the world indicated that the rangeland concepts and range models developed in United States did not well apply to the ecosystems in Africa and Asia where the prevailing environments are characterized by frequent disturbances and alternative conceptual frameworks are called for to provide more realistic, accurate interpretations of range vegetation dynamics and scientific support to natural resource management. In this context, equilibrium ecology emerged in response to the dissatisfaction of the prevailing ecological theory. In particular, an ecosystem-level research in east Africa presents a view of pastoral ecosystems that are nonequilibrial but persistent (persistent nonequilibrium model), and further research resulted the recognition of the importance of relative proportion of key resource in the dry/cold season as the determinant of animal numbers and their potential impacts on vegetation. The state and transition models which emerged in 1980s represented an important alternative conceptual framework and were applied in 1990s to rangeland assessment as a replacement of range condition in USA. Resilience is currently replacing nonequilibrium as the dominant ecological concept because ecosystems that were previously considered nonequilibrium is now more appropriately interpreted as multiple equilibria in many cases. The advances in rangeland ecological theory have been accompanied by parallel progression of rangeland management model, notably a transformation of the role of human from resource users to resource stewards. The rangeland models in twentieth century were generalized as steady-state management with the overall goal of maximizing sustainable production of forage and livestock. The application of resilience as the most recent and important ecological advance has been extended to coupled social-ecological systems, leading to the development of the concept and procedures of adaptive management. Currently the above described models are in application to a greater or less extent and synthesis of different models for rangeland assessment and management will help develop improved framework for complex, adaptive social- ecological systems.

Key words: rangeland, ecosystem, nonequilibrium, resilience, state-and-transition models, natural resource management