Nonequilibrium Ecology and Ecosystem Resiliency
The model for understanding ecosystems has long been based in equilibrium theory and linear succession. In recent decades, it has been recognized that climatic succession may be a concept well suited to some ecosystems that remain largely undisturbed, such as the nonlogged mesic forests of the Pacific Northwest. Most ecosystems throughout the world do not remain undisturbed, however, and therefore have perturbations (i.e., disturbances) that shift the ecosystem to alternate states. Nonequilibrium theory emerged as a way to think about ecosystems under the influence of disturbance and increased competition (Folke 2006). Nonequilibrium theory is now coupled with resiliency theory and resiliency thinking to recognize that ecosystems are not only under the influence of disturbance shifting; they are also subject to the influence of humans in and on ecosystems.
Section 1: Nonequilibrium Ecology
Nonequilibrium ecology asserts that ecosystems under disturbance pressure have a limited capacity for autogenic balancing and reach thresholds at which the system shifts to an altered state. An altered state likely includes a change in plant species composition and dominance and concomitant shifts in animal composition and ecosystem processes.
Equilibrium Systems vs. Nonequilibrium Systems
To better understand nonequilibrium ecology, let’s compare an equilibrium system to a nonequilibrium system. Briske et al. (2017) in Rangeland Systems: Processes, Management and Challenges provide us with a concise table comparison.
| Type | Equilibrium systems | Nonequilibrium systems |
|---|---|---|
| Abiotic patterns | Relatively constant | Stochastic/variable |
| Plant-herbivore interactions | Tight coupling | Weak coupling |
| Biotic regulation | Abiotic drivers | |
| Population Patterns | Density dependence | Density Independence |
| Populations track carrying capacity | Dynamic carrying capacity limits population tracking | |
| Community/ecosystem characteristics | Competitive structuring of communities | Competition not expressed |
| Internal regulation | External drivers |
If we were to apply these characteristics to a real-life scenario such as an overgrazed and cheatgrass-invaded (disturbance) sagebrush perennial bunchgrass system, shifts in soil stability and hydrology (abiotic patterns) would be evident, limiting resources for the native community and opening up niches to which cheatgrass is well adapted. The vegetation community composition, through the external drivers of overgrazing and invasive species establishment and facilitated through feedback loops, crosses a threshold to shift from a native perennial bunchgrass community to a new state dominated by cheatgrass. This scenario is fairly well-known. What may not be as well-known, however, is that the process of transition from sagebrush perennial bunchgrass system to a cheatgrass-dominated system is nonequilibrium, but the altered state of cheatgrass dominanc