COMMUNITY ORGANIZATION: METABOLISM 



519 



level, and the sum of the several level 

 potentials equals the reproductive potential 

 of the whole web, that is, of the community. 



Such a calculation is of theoretical in- 

 terest, but is not of practical value, since 

 we lack sufficient autecologic data for most 

 species for calculating the reproductive 

 potential in any but most general terms. 



The data available refer to a relatively 

 few well-known species, levels, and com- 

 munities in which predation has played its 

 role. For example, certain parasites, vectors, 

 and commensals of man and his domesti- 

 cated allies, and his chief plant and animal 

 foods or sheltering materials ofiFer the best 

 sources of information. 



The annual "yield" or "crop" of bushels 

 of corn, or board feet of lumber, or pounds 



succeeding level of the web. Hence Xn is 

 the true productivity, or rate of yield of the 

 trophic level An. 



Following the slow accumulation of in- 

 formation by Birge and Juday concerning 

 Wisconsin lakes, Welch (1935), Juday 

 (1940), Hutchinson (1941), Riley (1941), 

 Clarke (1946), and Clarke et al. (1946), 

 to cite a few references, have discussed this 

 complex problem in terms of yields, annual 

 energy budgets and productivities. 



As noted by Lindemann (1942), this an- 

 nual yield of a trophic level, that is, the 

 total of organic material formed per year 

 (An), is in reality a value usually uncor- 

 rected for dissipation of energy by (1) res- 

 piration, (2) predation, and (3) postmor- 

 tem decomposition (see Table 45) . To these 



Table 43. Productivity Values for Cedar Bog Lake, Minnesota, in Gram-calories per Square 

 Centimeter per Year (After Lindeman, 1942) 



Recalculation suggests that respiration of photosynthetic plants should be 29.6, predation 

 16.4, and corrected productivity 119.2; that respiration of herbivores should be 6.0, and their 

 corrected productivity 16.4. ( Courtesy of Dr. L. C. Birch, University of Sidney. ) 



of beef or of fish per unit area or volume 

 represents a given amount of protoplasm 

 or of protoplasmic products. The annual 

 rate of production of this protoplasm is 

 known as "productivity." 



Hutchinson (cf. Lindeman, 1942) con- 

 siders productivity in terms of the transfer 

 of energy. Using the trophic level symbols 

 Al, A2, ... An discussed previously, he con- 

 siders any trophic level as receiving energy 

 and disbursing energy. Consequently, the 

 rate of change of energy content in a given 

 level may be considered as having a posi- 

 tive and a negative component: 



we must add energy lost by (4) incomplete 

 assimilation (feces), (5) catabolic wastes 

 other than respiration— e.g., nitrogenous ex- 

 cretory wastes— and (6) heat regulation for 

 homoiothermal animals. 



Consideration of energy transfer between 

 trophic levels leads naturally to the question 

 of the biological eSiciency of any trophic 

 level, or of the whole community. The 

 efficiency of trophic levels has been studied 

 by Hutchinson (cf. Lindeman, 1942). This 

 author considers the eflBciency of produc- 

 tivity of a level (An) relative to the 

 productivity of any previous level (a^) as: 



dAg 



dt 



= Xn + Xn' 



Xn 



100 



where Xn is the positive component and 

 represents the rate of contribution of energy 

 from An-i, the preceding trophic level; Xn' 

 is the negative component and represents 

 the sum of the rate of energy lost from An, 

 or the rate of energy given to A.14.1 or the 



We have here another concept: namely, 

 the efficiency of a part of a community with 

 reference to some other part. It will be 

 noted that productivity is a rate of pro- 

 duction, whereas efficiency is a ratio. Linde- 

 man (1942), using Hutchinson's efficiency 



