(introduction...)

Several concepts and principles contribute to the development of a conceptual framework of cumulative environmental change.

9.2.1.1 Model of causality

A cause and effect relationship exists between the perturbation and the response of the system. This causal model is fundamental to a framework of cumulative environmental change. The nature of the cause and effect relationship is complex because of multiple causation, feedback mechanisms, and variable system response.

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.

9.2.1.3 Temporal and spatial accumulation

Time and space are generic to each of the components of the input-process-output model of cumulative environmental change. A temporal perspective recognizes that in an environmental system exposed to continuous or repeated inputs, the interval between each input may be insufficient for system recovery before the next input occurs, resulting in temporal accumulation. Processes with lengthy feedback loops may contribute to time delays. The output or system response may differ over the short and long term, as the response frequently requires critical thresholds to be reached before cumulative effects are apparent.

A spatial dimension is also evident. In environmental systems subjected to multiple inputs, such as those from non-point sources, the spatial proximity between inputs may be too small to disperse each input, resulting in spatial accumulation. The additive and interactive mechanisms of environmental processes at local scales may increase and contribute to regional environmental change. System responses may also involve cross-boundary movement or alter the spatial pattern of a landscape.

9.2.1.4 Control factors

Several factors control the components of input, process, and output. They are not mutually exclusive and may act dependently or independently of each other. The factors listed below are described briefly in terms of their influence on system response to perturbation.

Boundaries. Spatial and temporal dimensions define the perimeter of a system and so distinguish it from the external environment. Boundaries determine whether inputs are foreign or internal to the system, and also establish margins to identify cross-boundary flows between systems. Cumulative effects assessments are generally characterized by broad temporal and spatial boundaries to incorporate the accumulation of environmental changes over long time frames (i.e., decades, centuries) and among spatial scales (i.e., local, regional, global).

Hierarchy. Each level of organization (e.g., individual, population, community, ecosystem) within a system operates with a degree of autonomy by functioning at time and space scales which differ from other levels in the hierarchy. Different levels of organization may be associated with varying types of system response. For example, a perturbation such as cultural eutrophication may eliminate or replace a single fish species (e.g., trout), but the aquatic system as a whole may remain intact. Thus, cumulative effects assessment investigates environmental changes within and among various levels of organization.

Organizational complexity. The degree of organizational complexity also influences the capacity of a system to respond to varying amounts of stress. Mature and complex organizations tend to resist cumulative effects characterized by small and short-term stress, but are more likely to succumb to severe and prolonged stress. Immature stages of organization are more likely to absorb or adapt to extreme events by rapid rebuilding of system structure and function. The response to cumulative effects differs between mature and rudimentary systems because as organizational complexity increases, the degree of specialization and connectivity among elements usually increases, and dynamic variability of system processes generally decreases.

Assimilative capacity. All environmental systems possess an assimilative capacity that regulates the amount of input a system can receive without degradation to a system component or process. A stress of high intensity and short duration may quickly exhaust the assimilative capacity of a system, resulting in sudden system response. A stress of low magnitude and frequent repetition may deplete assimilative capacity at a gradual rate. Low levels of repeated stress may result in increments of environmental change which accumulate over time and delay the system's response (i.e., time delay).

Thresholds. Accumulation of environmental change can result in a critical threshold. This is the point at which the intensity or duration of an input is sufficient to result in system response. Thresholds control the response function by defining the level at which a system can no longer resist or absorb inputs. Perturbations that exceed the critical threshold result in adaptation or breakdown. Analogous to assimilative capacity, critical thresholds may be reached quickly under high level of stress or incrementally under low stress levels.

Dynamic variability. Perturbation may force an environmental system or a system variable to function outside its normal operating range. Dynamic variability is a measure of the amplitude or degree of fluctuation beyond this range. It determines the capacity of a system to adapt to stress, whether extreme events or the accumulation of incremental environmental changes. Systems with high dynamic variability generally have a greater capacity to adapt to severe stress than systems with low variability.

Stability and resilience. Closely related to dynamic variability, the factors of stability and resilience also influence the manner in which systems respond to perturbation. Stability is characterized by a low degree of fluctuation around an equilibrium state and a rapid return to this state following stress. Resilience is distinguished by high variability and the ability of a system to maintain its structure and function. A system with low stability and high resilience is more likely to persist in the face of extreme stress than a system with high stability and low resilience. The latter can absorb incremental cumulative effects, but is vulnerable to change when thresholds are reached.

The influence of the control factors varies among the three components of input, process, and output so that some govern inputs (e.g., boundaries, assimilative capacity), others regulate processes (e.g., thresholds, dynamic variability), and still others determine the type of output response (e.g., hierarchy, organizational complexity, stability, and resilience).