to the owners of natural areas and their associated biodiversity. Taking present values of these fees would give a value for natural assets which could fit consistently within the SNA balance sheet account. However, this would not necessarily be a good measure of the economic value of the natural asset because park fees and other usage fees are generally not tapping the full willingness to pay of the people visiting and using the natural area. Additionally, it is important to note that the total willingness to pay to use the natural area is linked to the bundle of properties possessed by the area, including its biodiversity.

In countries where surveys of users have measured the willingness to pay (per person per day)<sup>7</sup> for the enjoyment of the natural area, it may be possible to reflect this in the balance sheet. While the SNA measures only the value of actual transactions, with few imputations, bringing values of natural areas into the balance sheet based on stated willingness to pay could be done if (i) due care is taken in the survey design, and (ii) there is evidence that willingness to pay actually varies as the properties of the natural area, including biodiversity, change. If implemented in national accounts, the result would be an imputed value of national wealth and an associated measure of national income which includes the imputed consumption of benefits over and above the actual user fee paid.

Ecotourism is another example of use value which provides economic benefits to natural tourism operators as well as park entry fees to government. If the natural areas visited by the ecotourists are privately owned then the capitalized rents generated by ecotourism already form part of the national balance sheet – although the land may retain its ‘natural’ characteristics, it is effectively a commercial natural resource. If the ecotourist visits a lodge in a national park, then the rents paid by the lodge owner to the government contribute to government income and therefore to the asset value of the national park.

Bioprospecting rents are also economic benefits accruing to the owner of a natural area, but Polasky _et al._ (2005) note that the expected values of bioprospecting rents are low, generally less than the financial costs of the conservation of natural areas. The value of the knowledge which can be gained from diverse species can in principle be measured and ascribed to the natural areas which are the source of the species.

As highlighted above, biodiversity is an integral part of the production of ecosystem services, and there is evidence that increasing biodiversity increases the productivity of natural areas. This

> 7 The stipulation that the surveys measure willingness to pay to use the natural area per person per day is important, since this is in effect a price, and the relevant quantity is the number of person-days demanded. Imputing the additional consumption of benefits from the natural area is therefore consistent with national accounting conventions which stipulate that final expenditure is the summing up of ‘p times q’ values, excluding consumer surplus. This suggests, however, that the survey should attempt to measure the demand curve for person-days of use of the natural area.

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