Ecosystem stress and health: an expansion of the conceptual basis

The assessment of the ecosystem health and departures from it requires clarity of what the system, its structure, dynamics, and healthy conditions are. Available definitions provide inadequate tools to acquire this clarity and may lead to arbitrary diagnoses of ecosystem health but such diagnoses can be overturned on a variety of scientific, philosophical, or political grounds.Nested hierarchy of ecosystem structure compounds the difficulty in the assessment of stress and health because both states may occur simultaneously at different hierarchical levels: with stress at one level being a necessary condition of health at another.An approach based on a formal definition of system change is advanced. First, a conceptual model identifies a self-maintaining minimum interactive structure (MIS) at each level of ecosystem organization. Components of MIS are complementary, coordinated, and exchanging information — they are integrated. Function is defined as a contribution of a component to the maintenance of the whole. In this context health is viewed as persistence of the system at a given temporal and spatial scale. Impairment of the function is stress and is contrasted with change of system structure (loss, addition, or replacement of components of MIS) which is disturbance. Stress can be measured directly by changes of function or indirectly by changes in integration. Even though undesirable from the human point of view, a changed system may again be considered healthy.