in monetary terms are: Avoided cost: Services allow society to avoid costs that would have been incurred in the absence of those services (e.g. waste treatment by wetland habitats avoids health costs) Replacement cost: Services could be replaced with man-made systems (e.g. restoration of the Catskill Watershed cost less than the construction of a water purification plant) Factor income: Services provide for the enhancement of incomes (e.g. improved water quality increases the commercial take of a fishery and improves the income of fishers) Travel cost: Service demand may require travel, whose costs can reflect the implied value of the service (e.g. value of ecotourism experience is at least what a visitor is willing to pay to get there) Hedonic pricing: Service demand may be reflected in the prices people will pay for associated goods (e.g. coastal housing prices exceed that of inland homes) Contingent valuation: Service demand may be elicited by posing hypothetical scenarios that involve some valuation of alternatives (e.g. visitors willing to pay for increased access to national parks) A peer-reviewed study published in 1997 estimated the value of the world's ecosystem services and natural capital to be between US$16–54 trillion per year, with an average of US$33 trillion per year. However, Salles (2011) indicated 'The total value of biodiversity is infinite, so having debate about what is the total value of nature is actually pointless because we can't live without it'. As of 2012, many companies were not fully aware of the extent of their dependence and impact on ecosystems and the possible ramifications. Likewise, environmental management systems and environmental due diligence tools are more suited to handle "traditional" issues of pollution and natural resource consumption. Most focus on environmental impacts, not dependence. Several tools and methodologies can help the private sector value and assess ecosystem services, including Our Ecosystem, the 2008 Corporate Ecosystem Services Review, the Artificial Intelligence for Environment & Sustainability (ARIES) project from 2007, the Natural Value Initiative (2012) and InVEST (Integrated Valuation of Ecosystem Services & Tradeoffs, 2012) Payments Management and policy Although monetary pricing continues with respect to the valuation of ecosystem services, the challenges in policy implementation and management are significant and considerable. The administration of common pool resources has been a subject of extensive academic pursuit. From defining the problems to finding solutions that can be applied in practical and sustainable ways, there is much to overcome. Considering options must