Geographic Targeting of Increases in Nutrient Export Due to Future Urbanization

Urbanization replaces the extant natural resource base (e.g., forests, wetlands) with an infrastructure that is capable of supporting humans. One ecological consequence of urbanization is higher concentrations of nitrogen (N) and phosphorous (P) in streams, lakes, and estuaries. When received in excess, N and P are considered pollutants. Continuing urbanization will change the relative distribution of extant natural resources. Characteristics of the landscape can shape its response to disturbances such as urbanization. Changes in landscape characteristics across a region create a geographic pattern of vulnerability to increased N and P as a result of urbanization. We linked nutrient-export risk and urbanization models in order to identify areas most vulnerable to increases in nutrientexport risk due to projected urbanization. A risk-based model of N and P export exceeding specified thresholds was developed based on the extant distribution of forest, agriculture, and urban land cover. An empirical model of urbanization was used to increase urban land cover at the expense of forest and agriculture. The modeled (future) land cover was input into the N and P export risk model, and the ‘‘before’’ and ‘‘after’’ estimates of N and P export were compared to identify the areas most vulnerable to change. Increase in N and P export had to be equal to or greater than the accumulated uncertainties in the nutrient-export risk and urbanization models for an area to be considered vulnerable. The areas most vulnerable to increased N and P export were not spatially coincident with the areas of greatest urbanization. Vulnerability also depended on the geographic distribution of forest and agriculture. In general, the areas most vulnerable to increased N exports were where the modeled urbanization rate was at least 20% and the amount of forest was about 6 times greater than the amount of agriculture. For P, the most vulnerable areas were where the modeled urbanization rate was at least 20% and the amount of forest was about 2 times greater than the amount of agriculture. Vulnerability to increased N and P export was the result of two interacting spatial patterns, urbanization and the extant distribution of land cover. It could not be predicted from either alone.