Moving up one level from community ecology, ecosystems are all of the populations and communities in an area, along with their physical surroundings (non-living actors like sunlight energy, soil, water, etc.). It is important to remember that, in ecosystems, the flow of energy is one-way, while nutrients (the molecules and atoms that make up living things) may be cycled from one organism to another. Nutrients move from one organism to another when it is eaten (plants by herbivores, prey by predators, and anyone by parasites and decomposers). These who-eats-whom relationships are the basis for one of the central concepts of ecosystem ecology - trophic levels.

Trophic levels are defined by the energy source of the creatures in that level (Figure 37.2 and Table 37.1). The first trophic level (primary producers; plants) obtain their energy from sunlight. Organisms in the second trophic level (herbivores) eat the plants, obtaining energy and nutrients from them. The third trophic level eats the second, and the fourth eats the third. Note that the energy harvested by primary producers flows from one trophic level to the next, and is eventually lost to heat (the least-useful form of energy). This means that a continual supply of concentrated energy (sunlight) is necessary for ecosystems to continue to function. In contrast the actual atoms and molecules can be reused an unlimiede number of times - cycled from one organism to another. Recall from our discussions of metabolism that energy is required to arrange the nutrients into whatever form is needed. Rather than thinking of a set of trophic levels as a food chain, it is important to remember that even in simple ecosystems (Figure 37.3) many organisms eat at multiple trophic levels, in more complex food web arrangements.

Section 37.3 is an extensive analysis of the energy flow in a small ecosystem. Note that the plants in the ecosystem harvest about one percent of the available sunlight energy. This is echoed in succeeding trophoc levels - each transition results in the loss of most of the energy into less useful forms. The fact that there is less and less useful energy available at higher and higher trophic levels is the reason that biomass (total weight of all organisms in a trophic level) is skewed towards the producers and primary consumers). This fact also means that few organisms can eat above the fourth trophic level - there is simply not enough useful energy available.

The carbon cycle is a good example of nutrient cycling. Carbon atoms are pulled from the atmosphere by plants in the process of photosynthesis (energy being used to arrange matter for the purposes of the plant). Some of the energy in the sunlight is stored as chemical energy in the carbon containing compounds (sugars) synthesized by the plant. That energy can be used by the plant or by herbivores as they metabolize the sugars into carbon dioxide - the carbon dioxide is released to the atmosphere, while the energy is converted into less useful forms. Under natural conditions, these flows (and others; Figure 37.12 is important) are in equilibrium, and the levels of carbon dioxide in the atmosphere, ocean, sediments, etc. are more or less constant.

Human impact on the carbon cycle consists of decreasing the rate of photosynthesis as remaining forests are converted to agriculture uses. The carbon in the forests is returned to the atmosphere in bulk as the forests are burned (note that very little of the wood is used for anything). In addition, humans harvest energy from fossil fuels (carbon containing compounds with stored energy from the distant past). This burning results in the large-scale return of carbon dioxide to the atmosphere. This activity is a good introduction to our discussion of human impacts.