430 FAGER [CHAP. 19 



percentage of their income on respiration than smaller ones do and that the 

 higher trophic levels are usually composed of larger and less numerous animals ; 

 the "Eltonian" pyramid. 



Lindeman's definition of efficiency is also rather special. It is the ratio of 

 the productivity (his definition) of the level being considered to that of the next 

 lower level. On this basis, efficiencies increase as you go to higher trophic levels. 

 A more reasonable definition would seem to be input minus respiration, divided 

 by input, (I — R)/I. This is nearly the same as the definition given by Patten 

 (1959). This would give a measure of the efficiency of a trophic level in trans- 

 forming input into accumulated energy. In this sense, efficiency decreases with 

 increase in trophic level for the same reason that productivity decreases. The 

 producer level is a special case. If one considers the energy available in sun- 

 light as the input, the efficiency is very low ; if one considers the gross photo- 

 synthetic production as the input, the efficiency is probably about equal to 

 that of the primary consumers — 50 to 75% (Ryther, Chapter 17). Harvey 

 (1950), using a definition like the preceding but with input equated to respira- 

 tion plus growth, calculates an efficiency of 70% for primary consumers (herbi- 

 vorous copepods) and about 5 to 10% for carnivores. A decision between the 

 two definitions would turn on the question of whether materials which were 

 ingested but which the organism had no chance of digesting, such as chitin, 

 should be included in the input and how losses due to predation or death and 

 decomposition should be handled. In either case, the amount of energy available 

 as food decreases as one goes to higher levels and, though this may be partly com- 

 pensated for by increased predatory ability, eventually becomes too small to be 

 exploited by larger animals. As a result, ascending food chains seldom have as 

 many as five links. Other than limiting the number of links in a food chain or 

 as an aid in deciding whether to harvest fish, zooplankton or phytoplankton, 

 there is some question of what efficienc}^ means in terms of the community as 

 a whole. Most communities are not rapidly filling up their habitats with stored 

 energy in the form of organic materials and, therefore, in terms of the above 

 definition, they must be quite inefficient. As Lotka (1925) says "the great world 

 engine or energy transformer composed of a multitude of subsidiary units . . . 

 seems, in a way, a singularly futile engine, which, with a seriousness strangely 

 out of keeping with the absurdity of the performance, carefully and thoroughly 

 churns up all the energy gathered from the source." 



In broad terms, it is now clear what proportion of the chemical energy 

 originally fixed by photosynthesis reaches each trophic level and how each level 

 portions it out among various activities. It would be more accurate to have 

 everything in terms of free energy instead of total energy content but this would 

 probably not materially change the picture. What is not available is a detailed 

 analysis of the mechanisms involved in a particular community, e.g. the 

 physiological and behavioral properties of species A which cause it to get only 

 10% of the energy available to a certain level while species B gets 60%, etc. 

 When one gets down to the problem of trying to set up an accurate balance 

 sheet for even a simple community, all sorts of troubles arise which are glossed 



