From balance of nature to hierarchical patch dynamics: A paradigm shift in ecology

A common assumption historically in ecology is evident in the term "balance of nature." The phrase usually implies that undisturbed nature is ordered and harmonious, and that ecological systems return to a previous equilibrium after disturbances. The more recent concepts of point equilibrium and static stability, which characterize the classical equilibrium paradigm in ecology, are traceable to the assumptions implicit in "balance of nature." The classical equilibrium view, however, has failed not only because equilibrium conditions are rare in nature, but also because of our past inability to incorporate heterogeneity and scale multiplicity into our quantitative expressions for stability. The theories and models built around these equilibrium and stability principles have misrepresented the foundations of resource management, nature conservation, and environmental protection. In this paper, we synthesize recent developments that advance our understandings of equilibrium vs. nonequilibrium, homogeneity vs. heterogeneity, determinism vs. stochasticity, and single-scale phenomenon vs. hierarchical linkages in ecological systems. The integration of patch dynamics with hierarchy theory has led to new perspectives in spatial and temporal dynamics, with explicit linkage between scale and heterogeneity. The major elements of the hierarchical patch dynamics paradigm include the idea of nested hierarchies of patch mosaics, ecosystem dynamics as a composite of patch changes in time and space, the pattern-process-scale perspective, the nonequilibrium perspective, and the concepts of incorporation and metastability. Both environmental stochasticities and biotic feedback interactions can cause instability and contribute to the dynamics observed at various scales. Stabilizing mechanisms that dampen these destabilizing forces include spatial incorporation, environmental disturbances, biological compensatory mechanisms, and heterogeneity absorption. Hierarchical patch dynamics incorporates certain "emergent properties" of ecological systems, such as metasiability or persistence at the meta-scale, as opposed to the transient dynamics that usually characterize local phenomena. In contrast to the stability that derives from an assumed self-regulation in a closed system, the concepts of incorporation and metastability deal explicitly with multiple-scale processes and the consequences of heterogeneity. The most important contribution of hierarchical patch dynamics lies in the framework provided for explicitly incorporating heterogeneity and scale, and for integrating equilibrium, multiple equilibrium, and nonequilibrium perspectives. "The balance of nature" does not exist, and perhaps never has existed. The numbers of wild animals are constantly varying to a greater or less extent, and the variations are usually irregular in period and always irregular in amplitude. Each variation in the numbers of one species causes direct and indirect repercussions on the numbers of the others, and since many of the latter are themselves independently varying in numbers, the resultant confusion is remarkable. - Charles Elton(1930).