SECT. 4] COMMUNITIES OF ORGANISMS 429 



biological interaction (cf. Fager, 1957, for examples of the procedures in this 

 paragraph). 



5. Community Function 



After a community has been identified and its structure in terms of in- 

 dividuals and species has been described in quantitative terms, including 

 variations in time and space, the pathways and amounts of energy flowing 

 through the community can be related to the structural features. A good 

 description of the function of a community is provided by a paraphrase of a 

 statement due to Bertalanffy (1950): communities are not in being, they are 

 happening ; they are the expression of a perpetual stream of matter and energy 

 which passes through the community and at the same time constitutes it. 



Although others had previously investigated the food relations of organisms 

 in communities, it was Lotka (1925, reprint 1956) who first treated this in a 

 formal manner. He pointed out that "aggregates of living organisms are, in 

 their physical relations, energy transformers" and that what we have to con- 

 sider is "essentially the evolution of a system of energy transformers" and 

 "the progressive redistribution of the matter of the system among these 

 transformers". This brings up a point which, as Macfadyen (1948) has shown, 

 was not always clearly apprehended in earlier publications, i.e. that the matter 

 is recycled but the energy is transformed, eventually to heat, and finally lost 

 to the system. The transformers may work in parallel, as for example a number 

 of different herbivores feeding on one plant, or be coupled in a series, as for 

 example a food chain involving plant^herbivore-carnivore. Each of these 

 transformers in addition to using some of the available energy for the per- 

 formance of work acts as an accumulator, storing up energy which can be used 

 later by the individual or by the next step in the food chain. Lindeman (1942) 

 applied these ideas to a specific example, Cedar Bog Lake, using values from 

 the literature for estimation of the amounts of energy used in respiration, 

 growth, etc. within each trophic level. Some of his calculations were in error 

 and his choice of values from the literature does not always seem to have been 

 wise, but his paper is still of primary importance because it showed that thinking 

 about communities in these terms is possible and leads to interesting con- 

 clusions concerning the system and its parts. His definition of productivity, 

 following Hutchinson's (1953) suggestion, as the rate of contribution of energy 

 from the next lower trophic level is, however, confusing and not in accord with 

 the usual meaning of the word. It would seem better, as Odum (1956) suggests, 

 to consider net rate of increase in energy content, including losses due to 

 predation and death and decomposition, as productivity. Energy content is 

 preferred to biomass because of differences in composition. At the producer 

 level it is relevant to consider gross productivity as well as net, but only the 

 latter seems to have a clear meaning in higher levels. Whichever method is 

 used, the annual productivity decreases as one progresses to higher trophic 

 levels. This is another way of saying that larger animals spend a greater 



