A number of challenges remain for improving the utility of valuation exercises for decision-makers concerned with wetlands in urban areas. There are large gaps in our understanding of wetland ecosystems and how perturbations of such systems will result in changes in the production of various valued ecosystem services, despite advances in classification systems and the use of reference wetlands for assessment ( Brinson 1993, Brinson and Rheinhart 1996). For example, wetland ecosystems may have thresholds. Stress may be tolerated with little damage until a threshold is reached, at which point system function may change radically, giving rise to a large change in ecosystem services. Linking human actions in the surrounding landscape to changes in wetland ecosystem function is not well-understood but is crucial to evaluating which among a set of alternative choices are preferable.