"Sustainability" is often used in a qualitative sense. However, there is at present a great need to quantitatively measure (and monitor) its many qualitative aspects in real systems. Real systems are regarded as sustainable if they can maintain their current, desirable productivity and character without creating unfavorable condi tions elsewhere or in the future [1-4]. Sustainability therefore incorporates both concern for the future of the current system (temporal sustainability) and concern about the degree to which some areas and cultures of the planet are improved at the expense of other areas and cultures (spatial sustainability). That is, sustainability is to hold over both space and time. Sustainability encompasses many disciplines. For example, economic systems are not sustainable if they degrade their natural resource base and impoverish some sectors of the human population [5, 6]. Indices are needed that will measure sustainability through time, and over space, at several scales. These indices must also have the ability to aggregate the many disciplinary facets of sustainability, often incorporatedthrough a large number of environmental, social, and economic variables. Such a multidisciplinary dynamic system can be regarded as sustainable if it maintains a desirable steady state or regime', including fluctuations that are desirable (such as those that respond to natural disturbances [8]).
