What is the basic component of an energy flow model? Figure 3-14 presents what might be termed a universal model—one that is applicable to any living component, whether it be plant, animal, microorganism, individual, population, or trophic group. Linked together, such graphic models can depict food chains (Fig. 3-15) or the bioenergetics of an entire ecosystem. In Figure 3-14, the shaded box represents the living, standing crop biomass of the component.
Although biomass is usually measured as some kind of weight (living or “wet” weight, dry weight, or ash-free weight), biomass should be expressed in calories, so that the relationships between the rates of energy flow and the instantaneous or average standing crop biomass can be established. The total energy input or intake is indicated by I in Figure 3-14. For strict autotrophs, this is light, and for strict heterotrophs, it is organic food.
The concept of trophic level is not primarily intended for categorizing species. Energy flows stepwise through the community according to the second law of thermodynamics, but a given population of a species may be (and very often is) involved in more than a single trophic level.
Therefore, the universal model of energy flow illustrated in Figure 3-14 can be used in two ways. The model can represent a species population, in which case the appropriate energy inputs and links with other species would be shown as a conventional, species-oriented food web diagram (see Fig. 3-15), or the model can represent a discrete energy level, in which case the biomass and energy channels represent all or part of many populations supported by the same energy source.
Foxes, for example, usually obtain part of their food by eating plants (such as fruit) and part by eating herbivorous small mammals (such as rabbits or field mice). A single box diagram could be used to represent the whole population of foxes if intrapopulation energetics were to be stressed. Figure 3-16 is an energy flow diagram for the red fox (Vulpes vulpes) fed a diet of rabbits under penned conditions. All values are expressed in kcal/kg body weight per day.
On the other hand, two or more boxes would be employed should the metabolism of the fox population be divided into two trophic levels according to the proportion of plant and animal food consumed. In this way, the fox population can be placed into the overall pattern of energy flow at the community or ecosystem level.
So much for the question of the source of the energy input. Not all of the input into the biomass of an organism, population, or trophic level is transformed; some of it may simply pass through the biological structure, such as when food is egested from the digestive tract without being metabolized or when light passes through vegetation without being fixed. This energy component is indicated by NU (not used) or NA (not assimilated; see Fig. 3-14).
That portion used or assimilated is indicated by A in the diagram. The ratio between A and I (that is, the efficiency of assimilation) varies widely. It may be very low, as in light fixation by plants or food assimilation by detritus-feeding animals, or very high, as when animals or bacteria consume high- energy food such as sugars or amino acids.
In autotrophs, the assimilated energy, A, is, of course, the gross primary production or gross photosynthesis. The analogous component (the A component) in heterotrophs represents food ingested minus food egested (feces). Therefore, the term gross primary production should be restricted to autotrophic production.
A key feature of the model is the separation of assimilated energy, A, into the P and R components. That part of the fixed energy, A, that is burned and lost as heat is designated respiration, R; that portion transformed to new or different organic matter is designated production, P. Thus, P represents net primary production in plants and secondary production in animals.
Secondary production (SP) in consumer individuals is composed of tissue growth and litters of new individuals. The P component is energy available to the next trophic level, whereas the NU or non-assimilated component (such as feces) enters the detritus food chain (that is, material such as feces becomes available to be broken down by bacteria and fungi).
The ratio between P and R and between the standing crop biomass, B, and R varies widely and is ecologically significant. In general, the proportion of energy going into respiration (maintenance energy) is large in populations functioning at higher trophic levels and in communities with a large standing crop biomass. R increases when a system is stressed. Conversely, the P component is relatively large in active populations of small organisms, such as bacteria or algae; in youthful, rapidly growing communities; and in systems benefiting from energy subsidies.