The appropriate ecological model underpinning pastoral systems shifted from equilibrial to nonequilibrial in the 1980’s. The concept of balanced plant – herbivore interactions driving predictable successional change was deemed ill-suited to rangeland systems characterized by extreme climatic and spatial variability and event-driven nonlinearities in community composition. Nonequilibrium models were introduced in the late 1980’s to overcome the deficiencies that had been identified in traditional equilibrium models. The absence of equilibria between plant production and herbivore populations was attributed to high inter-annual rainfall variability in arid and semiarid rangelands, and the ability of plant production to recover more rapidly than herbivore numbers in years of favorable rainfall following multi-year drought. Rapid acceptance of nonequilibrium models posited that rangeland dynamics were primarily driven by environmental stochasticity, rather than the self-organizing capacity resulting from biotic feedbacks within rangeland systems. It is currently assumed that equilibrium and nonequilibrium dynamics coexist at unique scales, but these two distinct models have yet to be integrated. Climatic trends of warming, drying and more extreme events are projected to impact a majority of global rangelands further pressing the need to develop an effective, integrated model underpinning their function.
Results/Conclusions
Functional resource heterogeneity and ecological resilience may provide both mechanistic and conceptual justification for integration of equilibrial and nonequilibrial models. Functional heterogeneity enables herbivores to establish biotic feedbacks with a subset of key forage resources that are accessible in the dormant season, but they are largely uncoupled from abundant forage in the growing season. Nonequilibrial vegetation dynamics can be reinterpreted as the existence of multiple stable states organized around unique equilibria, rather than an absence of equilibrium created by environmental stochasticity. Reinterpreting the rangeland debate within the resilience framework avoids the perceived dichotomy between equilibrium and nonequilibrium dynamics, further clarifies distinctions between within and among state dynamics, and emphasizes mechanisms of system self-organization. However, accelerating change, both internal and external to pastoral systems, has identified the narrow scope of the rangeland ecology debate. This provides a strong justification for development of a comprehensive rangeland framework that envisions rangelands as adaptive social-ecological systems.