Transitioning among scales: The cumulative effects of roads

“Cumulative effects” are the combination of influences affecting a particular entity of concern. The scales and influences relevant for understanding the combination of environmental effects on an entity—whether it is a species, a resource, or a societal value—vary according to the distributions and environmental needs of those entities. Because of their ubiquity and design, roads and associated road use are often among the influences that combine to affect entities of concern. Roads influence a wide variety of watershed and ecosystem processes, and the nature of these influences and of the interactions among influences varies from the site scale to that of the landscape as a whole. Analysis of the influences of roads on an entity of concern is most readily accomplished by first evaluating the distribution of the entity and identifying the kinds of changes directly affecting that entity. These steps define the scale at which overall influences need to be integrated to understand a change and identify the kinds of environmental changes that need to be evaluated. For each kind of change, mechanisms capable of generating the change can then be identified. A basic understanding of the direct and indirect effects of roads on hydrology, sediment, vegetation, and habitat use can then be applied to evaluate the extent to which the distribution of roads present can influence each of the relevant mechanisms.