environmental economists, this translates the direct ecological yield as it would be on commodity markets: e.g. the value of water, timber, fish, or other commodities if immediately developed and sold at market price.Thus, an exchange value or 'price' is associated with the objects of value, regarded as natural capital, associated with ecosystems and this may be based on the ability of a system to produce yields each year that are exchangeable in operating markets and have existing exchange prices. Indirect use value attributed to indirect utilization of ecosystem services, through the positive externalities that ecosystems provide. The World Bank explains that these values are "derived from ecosystem services that provide benefits outside the ecosystem itself. Examples include natural water filtration which often benefits people far downstream, the storm protection function of mangrove forests which benefits coastal properties and infrastructure, and carbon sequestration which benefits the entire global community by abating climate change." Option value attributed to preserving the option to utilize ecosystem services in the future. Existence value attributed to the pure existence of an ecosystem. Altruistic value based on the welfare the ecosystem may give other people. Bequest value based on the welfare the ecosystem may give future generations. Given these types of potential ecological values, economists utilize a variety of methods to calculate those market values and measure non-market values. Standard environmental economic methods are used to place a monetary value on ecosystem services where there are no market prices. These include "stated preference" methods and "revealed preference" methods. Stated preference methods, such as the contingent valuation method ask people for their willingness to pay for a certain ecosystem (service). Revealed preference methods, such as hedonic pricing and the travel cost method, use a relation with a market good or service to estimate the willingness-to-pay for the service. Applying such preference based approaches has been criticized as a means of deriving the value of ecosystems and biodiversity and for avoiding deliberation, justification and judgment in making choices. The monetary value society attaches to ecosystem services depends on the income distribution. Valuation Results While the literature is still emerging, many important studies have resulting in striking valuations. One academic paper by de Groot et al., which synthesized more than 300 scholarly works collectively evaluating the 10 main biomes, "shows that the total value of ecosystem services is considerable and ranges between 490 int$/year for the total bundle of ecosystem