by humans. This is one of the few performance measures that not only attempts to quantify total human impacts, but does so in terms that are related directly to the environment's potential for support. Vitousek et al. (1986) estimate that humans appropriate 25 percent of global net primary production and almost 40 percent of terrestrial net primary production. By appropriation, they mean the sum of direct use of plants for food, fuel, fiber, and timber and the reduction in primary production that would otherwise occur through alteration of ecosystems by deforestation, decertification, paving, or other types of conversion to a less productive condition. They conclude that ". . . with current patterns of exploitation, distribution, and consumption, a substantially larger human population—half again its present size or more—could not be supported without co-opting well over half of terrestrial NPP [net primary production]" (p. 373). Regardless of whether this estimate is accurate, human use of resources and the impact of such usage on the life of the planet is clearly massive and growing.
Like performance measures, a diverse collection of methods is now used to assess the condition of environments or their components. These range from simple physical and chemical measurements to measures of the condition of individual organisms and methods for assessing the condition of entire ecological communities. Some conventional ecological measures, such as the biomass or size structure of a population, are useful as measures of ecosystem condition. Others, such as the Index of Biotic Integrity (IBI) (Karr, 1981), were developed for the express purpose of measuring ecosystem condition.
An extensive literature describes the relative merits of various measures for various applications. We do not attempt to review that literature here, but merely describe briefly some important methods. Introductions to the literature are provided by Schindler (1987, 1990) and Cairns et al. (1993).
Environmental conditions have traditionally been assessed with physical or chemical measurements, such as the pH of water or the temperature of air. These measures can be very informative and will surely remain important. However, it is often difficult to predict the ecosystem consequences of a change in physical or chemical conditions. In addition, conventional physical or chemical measures do not always detect important changes (Karr, 1991; Yoder and Rankin, this volume). As a result of these limitations, there is a trend toward reliance on a more balanced combination of physical and chemical measures plus direct biological criteria for assessing ecosystem condition.