9.2.1.2 Input-process-output model

An input-process-output model provides the elemental structure for a framework of cumulative environmental change. The three elements of input, process, and output are inherent in the notion of environmental systems, and parallel the basic parts of a stress response model. Each component is briefly elaborated on below.

Input refers to a stimulus which acts as the causative agent of change. Inputs may be differentiated by type, magnitude, and frequency. Key considerations for cumulative environmental change include whether inputs are single or multiple, similar or different in kind, continuous or discrete, short or long term, and concentrated (i.e., point source) or dispersed (i.e., non-point source).

Process alludes to the pathway or mechanism followed to transfer a unit of input into a unit of environmental change. It determines a system's ability to resist, absorb, or adapt to perturbation. Processes of accumulation may be additive or interactive. The latter implies feedback mechanisms, a concept to be included in a framework of cumulative environmental change.

Output or response represents a change in system structure (e.g., hierarchy, spatial) or system function (e.g., primary production, nutrient cycling) after perturbation. A typology of cumulative effects should distinguish changes in structure and function.