In a multicompartment system, interactions can be analyzed at three levels of aggregation/detail:
1. Single compartment (isolated). This addresses direct effects (e.g., an eagle eats mice but no grass). Traditional population biology often works at this level, which includes no indirectness at all.
2. Single compartment in-system. This addresses direct plus indirect effects (e.g., by eating mice which eat grass, the eagle is consuming embodied grass, which is embodied sunlight).
3. Whole-system. This addresses system-wide processes (e.g., to what extent is the entire system recycling phosphorus vs. leaking it immediately?).
This article concentrates on level 2. The indicators calculated are the property of a single compartment explicitly connected to other compartments in an ecosystem. Level 3 is beyond the scope of this article.
Level 2 analysis is the basis for most of the energy analysis started in the early 1970s. It led to then-surprising results such as these:
1. Only c. 60% of the energy to own and operate a car is the fuel in the tank. Around 15% is required to produce the car, and c. 25% is for parts, maintenance, insurance, registration, parking, etc.
2. Only c. 10% of the energy to make the car is consumed at the assembly plant. The remainder is consumed at the steel mill, glass works, iron mine, rubber plantation, etc.
3. Switching from throwaway to returnable beverage bottles saves energy and increases jobs.
A more recent example is that suburban living ('sprawl') is only c. 10% more energy-intensive than urban ('compact') living. A biological example is the trophic cascade, exemplified by consequences of recent wolf reintroduction into Yellowstone National Park. Adding wolves has suppressed elk activity, resulting in increased regeneration of browse vegetation.
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