PREDATION AND EFFICIENCY IN LABORATORY POPULATIONS 235 



crop severely. The upper limit to ecological efficiency is partially set by the 

 necessity of maintaining some minimum standing crop of prey. 



(b) Gromh efficiency of individual Daphnia 



In defining individual growth efficiency the relevant time span will be taken 



as the entire pre-natal and post-natal life of the organism. Growth efficiency 



is defmed as : 



{calories in the body of an organism at age i) 



/calories consumed asfood\ /{calories consumed by its mother\ 

 \ from age to age i J \ in producing one young animal J' 



By this definition the energy utilized in producing young animals relates to 

 the growth efficiency of the young rather than to that of the mother. 



Growth efficiency has been computed for Daphnia pulex as a function of 

 age and of cell concentration in the medium. The computations were made 

 by Armstrong (i960), using data from Richman(i958) and Slobodkin(i959). 



In order to determine the caloric cost of egg production, Armstrong first 

 determined the cost per calorie of growth increment during the period 

 immediately prior to the beginning of reproduction. He then determined 

 the growth increment and food consumption during reproductive Hfe, 

 subtracted the appropriate cost of growth from the food consumption and 

 assumted that the residual food energy was utilized for egg production. The 

 cost per egg was then calculated by dividing the caloric content of the eggs 

 by the food energy available for reproduction and dividing this by the 

 number of eggs produced. 



Armstrong's resul"^ are summarized in Fig. 8. Growth efficiency up to any 

 age is a decreasing function of age. In excessively low concentrations of 

 algae, growth efficiency is low, since the effort expended in feeding is not 

 adequately compensated by the food acquired. At very high concentrations 

 of algae, growth efficiency is lowered by the inability of the gut to assimilate 

 all of the algae eaten. 



Growth efficiency is independent of method or intensity of predation 

 except to the degree that predation may alter the properties of the individual 

 animal and its local environment. 



It may reasonably be expected that Daphnia in nature are usually at or 

 below the optimal cell concentration for growth efficiency. The values of 

 growth efficiency determined for Daphnia seem to conform to comparable 

 determinations from other animals. 



(c) Population efficiency 



It is possible to defme a concept of efficiency which takes into account the 



