9.2.2.3 Cumulative effects

Cumulative effects have been categorized in various ways. Lane et al. characterized four types of cumulative effects by their primary driving force (cause) and their basic spatial pattern (effect).

1 Type A effects are proponent-driven, large, single projects that induce environmental change over a large region (e.g., NATO low-level aircraft flights over Labrador).

2 Type B effects are proponent-driven, multiple projects (related or unrelated) that interact, resulting in spatially diffuse and complex environmental change (e.g., shoreline development along the Great Lakes).

3 Type C effects are ecosystem-driven (no identifiable proponent), catastrophic, or sudden events (natural or anthropogenic origin) with abrupt environmental changes (e.g., eruption of Mount Pinatubo in the Philippines, pollution from oil well fires in Kuwait during Operation Desert Storm).

4 Type D effects are incremental and widespread ecosystem-driven (no identifiable proponent) environmental changes attributed to diverse temporal and spatial processes (e.g., increase in the atmospheric concentration of carbon dioxide).

Table 9.3 A typology of cumulative effects

The emphasis here is on the spatial pattern and minimizes the recognition of temporal attributes. Also, effects are distinguished according to the source of the cumulative environmental change rather than characteristics inherent to different types of cumulative effects. Nevertheless, it makes a useful contribution by discriminating cumulative environmental change according to their cause-effect relationships.

A typology of cumulative effects which incorporates temporal and spatial attributes more explicitly is shown in Table 9.3. These effects can be broadly grouped into two categories. Functional effects refer primarily to the accumulation of time-dependent cumulative environmental changes. Temporal accumulation occurs when the interval between perturbations is less than the time required for an environmental system to recover after each perturbation. An example is harvesting renewable resources, such as forests and fish, at rates that exceed those of replacement. Time lags are exemplified in the continuous exposure to toxins in a food chain which may contribute to intergenerational genetic abnormalities.

Structural effects are primarily spatially oriented (e.g., space crowding, cross-boundary movement, fragmentation). Spatial accumulation is where the spatial proximity between each perturbation is smaller than the distance required to remove or disperse each perturbation. Space crowding is evident in the systematic collection of pesticide residues by farm drainage systems and its movement at a higher concentration into an aquatic system. This movement also demonstrates cross-boundary flow from one environmental system to another (i.e., agroecosystem to aquatic system). Spatial fragmentation is manifested in changes in size, shape, and contiguity of forests and wetlands in intensely farmed rural landscapes.

Other types of cumulative effects such as compounding, indirect effects, and triggers and thresholds are indicative of the manner of accumulation (Table 9.4). These types generally contribute to or manifest themselves as functional or structural effects, or both.

An attempt has been made to relate the types of cumulative effects to specific pathways of accumulation. Although all cumulative effects are potentially associated with each pathway to a certain degree, some effects are potentially more related than others. For example, time crowding, space crowding, and fragmentation are usually dominated by additive pathways. Time lags and cross-boundary movements are likely to also involve interactive processes. Compounding by definition is interactive.

Linking types of cumulative effects to pathways of accumulation enhances the understanding of system response to perturbation in two ways. First, it provides an indicator of potential cumulative effects in the future when unwanted changes in pathways occur. For example, incremental increases (additive processes) in the area and density of agricultural land drainage within a region might signify future cumulative effects such as fragmentation of wetlands. Similarly, drainage can result in space crowding by the systematic gathering of leached residue from widely dispersed farm inputs (e.g., fertilizers, pesticides), and cross-boundary movements by transporting contaminants downstream. Thus, linkage provides the basis for a predictive tool of analysis. Second, when a cumulative effect is observed and the cause is largely unknown (e.g., change in waterfowl migration, loss of ecosystem functions in a regional landscape), the association among effects and pathways can be used to trace and identify sources of cumulative environmental change. In this case, the association provides the basis for a form of hindsight analysis.