Passive use value captures the idea that people might enjoy satisfaction from "just knowing" (that is, enjoying the assurance) that a particular habitat is being maintained in good condition. There is no general expectation that passive-use value involves overt activities or is reflected in behavior. However, contributions to voluntary organizations that provide habitat preservation and political support for pro-habitat policies are consistent with passive-use value.

Together, use value, option value, and quasi-option value make up total economic value. It also includes bequest value, in that bequest motives assume that one's heirs will enjoy use or passive use. Total economic value includes all the kinds of economic value. There is no claim that economic value, however, constitutes the totality of value. As chapter 4 has made clear, there are many ways of valuing , but, total economic value then represents a comprehensive application of the economic way of valuing.

Methods of Valuation

Valuation relies on detailed information from the natural sciences . We might value an environment as an asset, in which case its value would be the net present value of the services that it provides and will provide. Alternatively, we might evaluate some proposed action (a project or policy); value would then be the net present value of the change in services that the environment will provide minus the cost of implementing the proposed action. Either way, valuation requires detailed knowledge of the service flows of the environment, of the costs incurred in preparing these services for human enjoyment, and of the responsiveness of service flows and costs to human interventions (Randall 1987 and NRC 1997 provide conceptual models of the valuation process). Much of that information must originate with experts whose specialties are far from economics, for example, ecologists and hydrologists. Economic valuation depends heavily on information that is fundamentally noneconomic.

Valuation also requires evidence of WTP and WTA. Evidence of WTP and WTA varies along two dimensions of quality: consistency with the conceptual framework of welfare-change measurement and reliability of the data themselves. For example, data generated by market transactions are convincing in at least one respect—paying money is the sincerest expression of WTP and accepting money and relinquishing an amenity constitute the sincerest expression of WTA. But the data might, for a variety of reasons, fail to measure the correct value concept. Price typically indicates marginal value (literally, the value of the next unit more or less than the status quo quantity—a small change); but a proposal might involve nonmarginal (big) changes. In addition, market distortions of various kinds might distort prices, markets might be incomplete or otherwise imperfect, and the environmental service involved might be nonmarketed. Data generated by contingent valuation or contingent policy referendums often can (because a researcher controls the valuation context) be addressed to the right value mea-

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