Though the existence of ecosystem sources is widely recognized, only the most preliminary attempts have been made at valuing most of them (e.g., Costanza et al. 1997; Daily 1997; Nordhaus and Kokkelenberg 1999; Daily et al. 2000).We are still far short of accurately quantifying and valuing many ecosystem services (Daily et al. 2000). Controversy over the attempt to do so by Costanza et al. (1997) highlights the lack of reliable and scientifically grounded estimates (e.g.,Ayres 1998; Bockstael et al. 2000;Toman 1998).