Ecosystem valuation is a widely used tool to find out the impact of human activities on an environmental system. It is done by giving an economic value to an ecosystem or its ecosystem services.

Value of ecosystem services
The simplest form of ecosystem valuation is that one looks at an ecosystem as if its ecological yield had the same value as it would be on commodity markets. That means one looks at the value of water, wood, fish or game, that is produced in that ecosystem.  Thus, a price can be put on the natural capital of an ecosystem based on the price of natural resources it yields each year.  

More complex arguments in ecosystem valuation regard nature's services and the assignment of values in a service economy to all that nature does "for humans".  Studies compiled by Robert Costanza in the 1990s argued strongly that even just considering the most basic seventeen of these services, the combined value of the ecosystems of the earth was worth more (US$33T) each year than the whole human exchange economy (US$25T) at that time (1995). Other studies have focused on the marginal value of ecosystem changes, which can be used in cost-benefit analysis of environmental policies.

In Natural Capitalism, 1999, Paul Hawken, Amory Lovins and Hunter Lovins advanced an argument to assign the value of Earth in current currency.  See value of Earth article for that and other examples of this extreme case of ecosystem valuation - biosphere valuation.

Four types of values can be assigned to ecosystems (Pearce and Turner, 1990):
 direct use value is the value attributed to direct use of ecosystem services;
 indirect use value is the value attributed to indirect use of ecosystem services, through the positive externalities that ecosystems provide;
 option value is the value attributed to preserving the option to use ecosystem services in the future;
 non-use value is the value attributed to the pure existence of an ecosystem and consists of three components: value based on the welfare the ecosystem may give other people; value based on the welfare the ecosystem may give future generations; and value based on knowing that the ecosystem exists.

Methods to estimate the value of ecosystem services which cannot be derived from market prices 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.

Is valuation economics, or ecology?
Such valuation, and that of the effectiveness of various environmental health measures that affect the value of life and quality of life, are usually thought to be part of economics.  Natural capital and individual capital are studied ecology as living systems, however, this does not extend to the economics of valuation by which they are related:

Considering "valuation" as an "economic not ecological issue" reflects the way these fields divide of the activities of humans versus non-humans in "making a living".  When humans go out to get food or homes, that is studied in "economics", but when non-humans do it, that is "ecology", though it is clear that there are motivations, methods and certainly bodily needs in common.

Since animals do not put explicit prices on ecosystems they use, but do behave as if they are valuable, e.g. by defending turf or access to water, it is mostly a matter of definition whether ecology should include valuation as an issue.  It may be anthropocentric to do so, since "valuation" more clearly refers to a human perception rather than being an "objective" attribute of the system perceived.  Ecology itself is also human perception, and such related concepts as a food chain are constructed by humans to help them understand ecosystems.  In many cases by those who hold that markets and pricing exist independently of any individual human observers and "users", and especially those who deem markets to be "out of control", ecosystem valuation is considered a (marginal, ignored) part of economics.  

Some versions of conflict theory focus on the role of resource scarcity in sparking or propagating human conflicts - in effect holding that the resources or ecosystems they fight over are being held so valuable that they are worth considerable risk of harm to control.  This is at least a relative notion of value and value at risk applied to ecosystem.

References
 Hanley, N. and C. L. Spash (1993). Cost-benefit analysis and the environment. Edward Elgar, Cheltenham.
 Pearce, D.W. and R.K. Turner, 1990. Economics of natural resources and the environment. BPCC Wheatsons Ltd., Exeter, UK. 378 pp.

Other websites
Environmental Valuation Reference Inventory
The EcoValue Project 
Ecosystem Valuation Toolkit 
 Ecosystem Valuation
Earth Economics' ESVs 

Economics
Green politics