However, the marginal value (market price) does not correspond well to the total value of the good。 In order to estimate the total value of the good, it is necessary to examine the demand curve, since the total value equals the total WTP (i。e。 the total area under  the  demand curve  to  the  left  of  the  allocation  in  question)。   The

Fig。 1。 Market price paid vs。 total   value。

difference between market price paid and people’s WTP is known as Consumer Surplus (CS)。 Given that many of the environmental services are provided at no  cost (i。e。 have no price) but hold a positive value, it is necessary to obtain their demand curve in order  to reveal this value (which, in the case of zero price, will consist entirely of consumer surplus)。 Hence, it is evident that in the majority of cases, ‘‘non-demand curve’’ approaches are unable to capture the true value of the ecosystem。 Therefore, ‘‘demand curve’’ techniques should be employed, although they are more complex and costly to  apply。

3。2。2。Demand curve approaches

Demand curve approaches are pided to Direct and Indirect approaches。 Direct approaches, also known as state preference methods, include techniques that at- tempt to elicit inpiduals’ preferences directly by means of social surveys。 The most important of these techniques is the Contingent Valuation Method,  which  is  by far the most popular, since it is able to measure TEV of environmental goods and services [20,35,38e40]。

Indirect approaches, also referred to as ‘‘revealed preference methods’’, elicit preferences from the actual behaviour of inpiduals, based on market information。 Generally, two types of procedures have been applied to this type of  valuation:

(a)Household   production   function   methods  based on the demand for complements  and  substitutes (e。g。 Travel Cost Method in order to measure the demand for recreational  activities)。

(b)Hedonic price analysis of decomposing prices for market goods to extract embedded values for related environmental attributes (e。g。 studying the    housing

market to elicit people’s WTP to avoid environmen- tal impacts)。

3。3。Stated  preference methods

Contingent Valuation (CV) is a widely accepted and well-established method, since it offers certain advan- tages [38,41e44]。 The CVM relies on a direct question- naire approach, asking a sample of inpiduals to state their hypothetical maximum WTP for preserving an environmental asset or their minimum WTA  for suffering the loss of that asset。 Illustrative environmental applications of CV include: air and water quality, outdoor recreation, reduced risks from drinking water and groundwater contaminants, protection of wetlands, wilderness areas, endangered species, and cultural heritage sites。

In order to derive estimates for values associated with environmental assets, it is necessary to determine the population of interest。 Next, the usual practice is to: (a) define a representative sample, (b) describe a scenario of the survey in which the respondents have to answer, (c) take answers from the sample, (d) analyze the data, (e) infer the mean (or median) WTP of the population from the analysis, and (f) calculate the sum value multiplying the mean (or median) WTP by the total units of the population。

CVM has been in use for over 35 years and there are more than 2000 studies dealing with the topic [45]。 While the most known applications are those for natural resource damage assessments (e。g。 the Exxon Valdez oil spill), the vast majority of CV studies have been undertaken for assisting in decision-making procedures。 Thus far, CV studies have been conducted in over 50 countries by government agencies and international organizations [46] and are increasingly being used [38,39]。 However, due to the hypothetical character of the method and the fact that a social survey by means of questionnaire must take place, there is considerable controversy over whether it adequately measures peo- ple’s WTP for environmental quality。 The debate over the use of CV has two major points [46]。 The first one is whether or not non-use values should be included in an economic  analysis   [47],   since   it   is   necessary   for a consumer to physically use a good in order to get utility from it。 However, it is argued  that consumers may also get utility from a good without physically using it (e。g。 preserving the existence of a remote wilderness area)。  Consideration  of  passive-use  value in economic analysis is attributed to Krutilla’s [48] observation that many people value natural wonders simply for their existence。 The term ‘‘passive-use value’’ was popularised in the important 1989 U。S。 Appellate Court decision, Ohio vs。 The Department of Interior, which   mandated   that   such   values   be   included   in