Scale ConceptsEnvironmental change is caused by a wide range of processes occurring through space and over time. As a basic conceptual framework, think of a watershed as a system incorporating a series of subsystems at smaller spatial scales (e.g., watershed > river corridor > tributary > reach > pool/riffle habitat > microhabitat spaces among cobblestones), each of which are affected by disturbances occurring at time intervals and spatial scales that are (as a very general rule) proportionally related to the magnitude of the subsystem. Scale is essential to evaluating change because the significance of a given change is very different from scale to scale. For example, channel form and streamside vegetation may seem constant when viewed over a time frame of months or a few years, but may be highly variable when viewed over a time scale of decades or longer. Similarly, a forest fire may be considered a rare and catastrophic event within the lifetime of a stand of trees, but the same fire may be a natural part of a larger cycle of recurrent fires when considered over a geological time scale.
Evolutionary Events vs. Developmental Processes: There is a distinction to be made between evolutionary events, which are extrinsic forces that create, significantly modify, and destroy systems at a given scale, and developmental processes, which are intrinsic, progressive changes following a system or subsystem's genesis or change as a result of an evolutionary event (Frissell et al. 1986). For example, global climate change may result in a shift in the distribution of a various tree species. In this case, climate change represents a large-scale evolutionary event resulting in a predictable set of developmental processes (i.e., shifts in forest community composition) at smaller scales. Similarly, a 100-year flood may be seen as an evolutionary event that scours away and destroys floodplain vegetation communities, giving rise to developmental processes such as forest succession in affected habitats.