The transfer of food energy from the source in plants through a series of organisms with repeated stages of eating and being eating and being eaten is known as the food chain. In complex natural communities, organisms whose food is obtained from plants by the same number of steps are said to belong to the same trophic level (troph =nourishment, the same root as in autotrophic and heterctrophic). Thus, green plants occupy the first trophic level (the producer level); plant eaters (herbivores, and so on) the second level (the primary consumer level); carnivores that eat the herbivores the third level (secondary consumers), and perhaps even a fourth level (tertiary consumers). It should be emphasized that this trophic classification is one of function, and not of species as such; a given spccupy one, or more than one, trophic level according to the source of energy actually assimilated. We have already called attention to certain algae that may depend in part on their own food and in part on food made by other algae; or the populations of men who utilize food from both plant and animal sources.

       Figure 3 -2 is an energy flow diagram of a food chain. This is a more detailed rendition of the last three modules in figure 3 -1. In this model we introduce the biological terminology used to describe come munity metabolism – and the quantities shown are on a daily, rather than annual, basis. The boxes represent the population mass or biomass, and the pipes depict the flow of energy between the living units. As already indicated, about half of the average sunlight impinging upon green plants (that is, producers) is absorbed by leaves and about 1 to 5 percent is converted into food energy by productive vegetation. The total assimilation rate of producers in an ecosystem is designated as primary production or primary productivity (pg or A in figure3 – 2). It is the total amount of organic matter fixed, including that used up by plant respiration during the measurement period. Net primary productivity (P n ) is the organic matter stored in plant tissues in excess of respiration during the period of measurement. Net production represents food potentially available to heterotrophy. When plants are growing rapidly under favorable light and temperature conditions, plant respiration may require as little as 10 percent of gross production so that net production may be 90 percent of gross. However, under most conditions in nature net production is a smaller percentage of gross, usually about 50 percent, as shown in the lower line of figure 3 – 2.

       As shown in figure 3 – 2 part of net primary production may be stored or exported, and part may beome an energy source for heterotrophy. NU and I, are apportioned within different kinds of ecosystems. Some portion of food ingested by consumers is usually not digestible or assimilable, so some of the energy is likely to be egested unused (NA in the diagram); this component may be stored, exported, or consumed by microorganisms or other heterotrophy. Consumers as well as producers must respire (R) a large part of the energy assimilated (A) so as to maintain structure and function (represented by the boxes in the diagram). Respiratory degradation of energy ‘’pumps out’’, that is, reduces entropy as is required to maintain a high level of organization. The R flows in figure 3 – 2 represent heat losses from biological components that were shown by the more generalized heat sink symbol in figure 3 – 1. Assimilated energy not respired is available for production (Pn P2 P3 …….) which can take the from of growth of new tissue and reproduction and growth of individuals (population growth). Production at heterotrophic levels is often known as secondary production to distinguish it from the primary production of plants. Any auxiliary energy or nutrients that reduces the cost of maintenance (respiration) enhances the rate of production. If the autotrophic level is subsidized, the increased production may be passed along the chain; if a consumer level is augmented, the ‘’downstream’’ links will be mostly affected, although secondary production can ‘’loop back’’ upstream so that even plants might benefit from organic matter not used by animals.

       Roughly speaking, the reduction of available energy with each link in the food chain (as required by the second law of thermodynamics ) is about two orders of magnitude at the first or primary trophic level, about one order of magnitude thereafter. By order of magnitude we mean by a factor of 10. If an average of 1500 kcal of light energy were absorbed by green plants per square meter per day, we might expect 15 to end up at net plant production, 1.5 to be reconstituted as primary consumers (herbivores), and 0.3 as secondary consumers (carnivores) – provided, of course, that there are adapted organisms present that can fully utilize these resources. Efficiencies in terms of percent energy transfer are thus on the order of 1 percent at the first level and 10 – 20 percent at the heterotrophic levels. Since meat is generally of higher nutritional quality, percent transfer tends to be higher at meat – eating levels. Since so much energy is lost at each transfer the amount of food remaining after two or three successive transfers is so small that few organisms could be supported if supported if they had to depend entirely on food available at the end of a long food chain. For all practical purposes, then, the food chain is limited to three or four ‘’links’’. The shorter the food chain, or the nearer the organism to the beginning of the food chain, the greater the available food energy. About ten times more people can be supported by 100 acres of corn of they function as primary rather than secondary consumers, that is, if they eat the corn directly rather than feed if to animals and then eat meat. If you prefer to eat meat then you must plan not to let the population become so dense as to preclude the meat – eating option. In debating man’s’’ world food problem’’ we must consider the quality as well as the amount that might be obtained from a given area of land or water.

        It should be emphasized that the scheme in figure 3 – 2 is a model useful for making comparisons with the real thing. Somewhat larger, and frequently quite a bit smaller, percentages may actually be involved under different conditions. Much remains to be learned, not only about the orders of magnitude in  different ecosystems but also about the upper limits. Since the efficiencies of transfer seem low in terms of man – made machines, man has often thought that he could improve on nature by increasing the percent of transfer of light to food, and food to consumer. However, when we consider the low quality of available energy, the fact that organisms unlike machines are self – maintaining, and the need for storage and diversity for future survival than it turns that nature’s efficiencies are just about optimum. (for further discussion of this important point, see odum and pinkerton 1955; H.T.odum, 1971.

       At this point it would be well to make clear a few points about units, ratios, and efficiencies. Too often our thinking is clouded by people who, unintentionally or intentionally, use units that are not comparable in forming ratios and percentages. Biomass varies in energy value, both quantitatively and qualitatively. Plant biomass runs about 4.5 kcal per ash – free dry gram, and animal close to 5.5 where energy is being stored, as in seeds or in bodies os migrating or hibernating animals; the values approach 7 pr 8 kcal. If plant matter is mostly cellulose and lignin (wood), its energy (4 kcal/g) is unavailable to most animals. The main point to remember about ratios and percentages when used as estimates of efficiency is that they should be dimensionless; that is, the same unit should be used for both the denominator and numerator of the ratio. This, it should be calories/ calories not calories/grams. Otherwise comparisons may not be valid. For example, poultry or catfish farmers may tell you they are getting one pound of meat for every two pounds of feed, implying a 50 percent efficiency of transfer. However, since the food Is highly concentrated, dry material worth 5 or more kcal/g, and the meat s ‘’wet weight,’’ worth only about 2 kcal/g, then the real energy out/ energy in efficiency is more on the order of 20 percent.