which represent direct indicators of particular aspects of sustainability. Such issues were raised in the major report, World Conservation Strategy (IUCN, 1980, Section 4.1):
Sustainable utilization is somewhat analogous to spending the interest while keeping the capital. A society that insists that all utilization of living resources be sustainable ensures that it will benefit from those resources virtually indefinitely.
According to this view, sustainability requires that certain baseline conditions be guaranteed. Because of difficulties associated with monetization (among others), baseline conditions are best specified in biogeophysical terms that take into account both stocks and flows. For instance, biological terms appear the most informative when discussing issues such as the need of species for habitat, healthy food, clean water, adequate range, a minimum viable population, stress resistance, etc. If these and other baseline conditions cannot be provided, species are likely to go extinct.
Because of ambiguities and uncertainties about what constitutes sustainability and whether or not it is being achieved in particular cases, it is not surprising that discussions of sustainability frequently lead to debate. Questions concerning humankind's relationship to the environment, perception of growth, views on the helpfulness or harmfulness of technology, salience of equity issues (both intragenerational and intergenerational), population stress, species and cultural diversity, and the ability of our present socioeconomic-political systems to effectively address these issues are all relevant to this discussion. These issues help us to explore the preconditions for sustainability. Improved natural capital accounts can help us to explore our current state of affairs and hypothesize about how we got here and what the future might hold. Consensus on the range of diverse issues is not necessary for directing policies in a more sustainable direction.
The economist Herman Daly has provided a useful starting point by observing that:
[f]or the management of renewable resources there are two obvious principles of sustainable development. First that harvest rates should equal [or be lower than] regeneration rates (sustained yield). Second that waste emission rates should equal [or be lower than] the natural assimilative capacities of the ecosystems into which they are emitted. Regenerative and assimilative capacities must be treated as natural capital, and failure to maintain these capacities must be treated as capital consumption, and therefore not sustainable [our interjection] (1990:2).
These two "common sense" principles for the establishment of sustainable policies differ from traditional economic indicators in that they are biogeophysically based. To make such principles operationally useful one must have data on actual stocks and flows. We suggest that when the issue at hand is sustainability, economic valuation techniques are often too decoupled from the biogeophysical world to provide us with the necessary insights. Our approach eliminates the need for developing pseudo-market values for currently nonmarketed goods. The entire issue of assigning monetary values, with its uncertainty, problems of time value, cultural variability, added cost, etc., is effectively side-stepped. Such accounts could serve as biogeochemical satellites to complement the existing monetized national income accounts.