FIGURE 1-2 Conceptual model (top) and actual response (bottom) of a biological system’s response to stress. The “Urban Gradient of Stressors” might be a single metric of urbanization, such as percent watershed impervious or road density; the “Biological Indicator” may be single-metric or multi-metric measures of the level of disturbance in an aquatic community. The right-declining line traces the limits of a “factor-ceiling distribution” (Thomson et al., 1986), wherein individual sites (i.e., data points) have a wide range of potential values for a given position along the urban gradient but are not observed above a maximum possible limit of the biological index. The bottom graph illustrates actual biological responses, using a biotic index developed to show responses to urban impacts plotted against a standardized urban gradient comprising urban land use, road density, and population. SOURCE: Top figure reprinted, with permission, from Davies and Jackson (2006). Copyright by the Ecological Society of America. Bottom figure reprinted, with permission, from Barbour et al. (2006). Copyright by the Water Environment Research Foundation.