How Ecosystems Respond to Stress

Nearly all ecosystems are subject to periodic disturbances by natural events, such as flood, fire, drought, and insect infestation (Vogl 1980). When such perturbations are extreme, ecosystems of immense complexity undergo rapid transformation to systems of remarkable simplicity that are characterized by a scarcity of life forms and few or no symbiotic interactions. However, this transformation sets the stage for recovery, which allows the ecosystem to adapt to changing environments (Holling 1986). In healthy systems, therefore, these perturbations are seldom more than a temporary setback, and recovery is generally rapid (Odum 1969). By contrast to natural disturbances, anthropogenic stress is not a revitalizing agent, but a debilitating one. Stressed ecosystems do not recover; rather, further degradation may follow. Indeed, Odum et al. (1979) defined stress as a debilitating agent and perturbation (subsidy) as potentially beneficial. Anthropogenic stresses are of many specific types, but they can be classified into four main groups: physical restructuring (e.g., changes resulting from land use); the introduction of exotic species; discharge of toxic substances to air, land, and water; and overharvesting. Ecosystems lack the capacity to adapt to these stresses and maintain their normal functions and structure. Thus, stress results in a process of degradation, which is commonly marked by such signs as less biodiversity, reduced primary and secondary production, and lowered resilience (i.e., the capacity of an ecosystem to recover to its original state) to natural perturbations (Barrett and Rosenberg 1981, Odum 1985, Mageau et al. 1995).