Ecologists have long told us that all flesh is grass, which in turn is sunlight. Thanks in large part to the oil embargo in 1973, we appreciate that bread is not just sunlight, but also oil. And economists have shown that demand for a shirt produces a demand for steel. These are all examples of indirect effects. The techniques used to quantify them span systems ecology, engineering, and economics - a compelling example of cross-disciplinary fertilization. Understanding them yields insights in diverse applications, from bioaccumulation of pollutants in ecosystems to labor demand in economies.

In principle, one could discern all aspects of indirectness from the full diagram of flows between compartments in a system. In practice, we often desire, or accept, summary variables or indicators which are specific to a particular application and convey the concept more concisely, though often with a loss of details.

In this article, such indicators are discussed, often using explicit calculations applied to a simple, idealized two-compartment ecosystem. The indicators are energy and nutrient intensities, trophic position (TP), path length (PL), and residence time. Besides application to steady state, the concept is also extended to dynamic ecosystems such as those responding to perturbations. Finally, calculating energy intensity of goods and services in economic systems is discussed. The latter is a crucial step in determining the energy cost of living in a consumer society, and has specific application in analyzing consequences of an energy tax.

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