Ecosystems vary in time and space. As ecologists extend their analyses of ecosystem structure and function to include potential goods and services, the uncertainty affecting assessments increases across both time and space. The interaction of ecological and social systems makes extrapolation of observations and prediction of future conditions exceptionally complex (Berkes et al., 2003; Gunderson and Holling, 2002; Gunderson and Pritchard, 2002). The challenges arise from the heterogeneity of ecosystems and values across space which complicates aggregation for assessment at larger scales, and from nonlinear system behavior that confounds forecasting. Recognition of the thresholds of change in both space and time is one of the principal challenges in ecological research.

It may be argued that almost all ecosystem functions can be performed by aquatic ecosystems at any scale. Indeed, Limburg et al. (2002) found that scaling rules describing production and delivery of ecosystem services are yet to be formulated and quantified (as noted in the preceding sections). However, there are clearly thresholds in the level of their relative importance. For example, individual wetlands in a watershed may each have the capacity to slow the flow of waters moving through them, but this function becomes important only when there are a sufficient number of wetlands in a watershed to significantly alter the flow of floodwaters downstream.

The complication in assessment of ecosystem goods and services arises because the scale at which functions become important is not always the same.

Front Matter (R1-R12)

Executive Summary (1-16)

1 Introduction (17-32)

2 The Meaning of Value and Use of Economic Valuation in the Environmental Policy Decision-Making Process (33-58)

3 Aquatic and Related Terrestrial Ecosystems (59-94)

4 Methods of Nonmarket Valuation (95-152)

5 Translating Ecosystem Functions to the Value of Ecosystem Services: Case Studies (153-208)

6 Judgement, Uncertainty, and Valuation (209-238)

7 Ecosystem Valuation: Synthesis and Future Directions (239-260)

Appendix A: Summary of Related NRC Reports (261-265)

Appendix B: Household Production Function Models (266-269)

Appendix C: Production Function Models (270-273)

Appendix D: Committee and Staff Biographical Information (274-278)