Disturbance: A process or event that results in changes in the physical and biological characteristics of watersheds. Stress: Adverse effect on organisms or ecosystems, or the source of these adverse effects ('stressor').

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The Concept of Change

Change is an integral component of the watershed. The wide variations in river and stream flow regularly change shorelines, stream channels and stream corridors. Upland areas of watersheds also undergo changes due to ecological succession, disease, competition, human activity and other factors. Even when virtually undisturbed by man, the physical and biological characteristics of ecosystems do not remain constant over time, as plant and animal communities are continually altered in response to changes in each other and in the watershed's physiographic and climatic conditions. Natural changes do cause adverse effects on some components of the ecosystem, while benefiting others with new opportunities.

The processes and events that cause these changes are generally called disturbance or sometimes stress. The former term implies any change to a watershed's physical or biological characteristics, and the latter term is a more value-laden word that implies the adverse effects that change usually has on some part of the ecosystem. Terms like dynamic equilibrium are used to describe the state of existence in which ecological communities exist through and are modified by change. In many cases, periodic disturbance is required to foster ecological processes (e.g., flooding promotes nutrient cycling in riparian soils), or to complete the life cycles of various organisms (e.g., many coniferous trees rely on fire for seed release and dispersal). A certain amount of change is therefore unavoidable, essential and desirable in watershed ecosystems. For this reason, a key element in any concept of ecosystem health is an ecosystem's ability to evolve over time and to self-regulate following disturbance (Steedman and Regier 1990).

Take a moment: Think about the main types of change occurring in your watershed. What changes mostly involve natural components of the watershed ecosystem? What changes in the human community are occurring in your watershed? Are these changes or their magnitude new or unprecedented? What do you think is the outlook for your watershed's human and natural changes, in terms of dynamic equilibrium? You may wish to revisit these questions after completing this module.

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Section 2 of 31