ture of solar energy and subsequent production of food and fiber, maintenance of water quality, and maintenance of a genetic library that provides the raw materials for improved foods, materials, and drugs. While these services have long been taken for granted, society is beginning to realize that many functions of natural systems are not immutable and can be affected by human actions.
The definition of an ecosystem service is a matter of societal perception because it hinges on valuation. Of all the processes or functions carried on by ecosystems, only those contributing to the well-being of human society are considered services. On those rare occasions when the societal value of minimally managed functions of ecosystems is evaluated, different people reach different conclusions. The debate is even more heated when management actions to protect ecosystem services are proposed. What scientific evidence is necessary to facilitate the societal debate on value and help establish a reasonable level of management? Does the delivery of necessary ecosystem services depend on ecosystem health? Not only is additional, well-conceived research needed to clarify these relationships, but it is also crucial to be able to communicate the results of such research and its uncertainties to the wider society that is properly involved in the debate on values. These issues provide recurring themes in the following discussion of balance.
All ecosystem functions could possibly be viewed as ecosystem services and any distinction between the two as a reflection of the limits of human knowledge rather than an actual difference. In addition to the term ecosystem services, the term sustainable use is often used to describe human benefits from ecosystems. When an ecosystem service is being provided at a rate that meets society's demands without compromising future use, there is sustainable use of the ecosystem. However, as Costanza notes (in this volume): "The problem is that one knows one has a sustainable system only after the fact . Thus, what usually passes for definitions of sustainability are actually predictions of what set of conditions will actually lead to a sustainable system." The alternative is an unsustainable use that not only may fail to meet the needs of society but also may damage the ecosystem and impair the rate at which the desirable service is provided.
The least controversial examples of ecosystem services are those for which an economic value is easily derived. These economic values can then be incorporated into existing decision making tools. The rapidly developing field of ecological economics (e.g., Costanza, 1989, 1991, and in this volume) has identified several useful approaches. There are problems with these approaches, but in their absence ecosystem services are too often completely ignored as externalities.
Some cases of free market respect for ecosystem services rather than technological ones can be cited. Natural systems are replacing chemical technology for waste treatment (Hammer, 1989). Natural systems complement energy-using