The relationship between the ‘’boxes’’ and the ‘’pipes’’ that is, between standing crops and the energy flows P, A, or I is of great interest and importance. As we have seen, the energy flow must always decrease with c=each successive trophic level. Likewise, in many situations, the standing crop also decreases. However, standing crop biomass is much influenced by the size of the individual organisms making up the trophic graphic group in question. In general, the smaller the organism the greater the rate of metabolism per gram of weight. This trend is often known as the inverse size – metabolic rate ‘’law’’. Consequently, if the producers of an ecosystem are composed largely of very small organisms and the consumers are large, the standing crop biomass of consumers may be greater than that of the producers even though, of course, the energy flow of the latter must average greater (assuming that food used by consumers is not being ‘’imported’’ from another ecosystem). Such a situation often exists in marine environments where the water is moderately deep: bottom – dwelling invertebrate consumers (clams crustaceans, echinoderms, and so on) and fish often outweigh the microscopic phytoplankton on which they depend. By harvesting at frequent intervals, man (as well as the clam) may obtain as much food (net production) from mass cultures of small algae as he obtains from a grain crop harvested after a long interval of time. However, the standing crop of algae at any one time would be much less than that of a mature grain crop.

To reiterate, standing crop is a measure of the amount of living material present at a particular time. Productivity is a rate to be expressed as energy flow per unit area per unit time. As indicated by the examples, these two quantities should not be confused; the relationship between the two depends on the kind of organisms involved. Standing crop can be used as an index of productivity only if production accumulates unused, as in a crop where harvest is deferred until the end of the season. If growth is used as fast as it is produced (as in a grazed pasture). Then standing crop cannot be used to estimate productivity.