234 L- B. SLOBODKIN 



The degree of population size lability seems to vary from species to species. 



Population size in Daphnia ohtusa, D. pulex and Hydra oligactis is linearly 

 dependent on food consumption, with or without predation. 



Equation (i) or some biologically more meaningful equation of similar 

 form, may prove applicable to many species, since it seems to hold for an 

 herbivorous crustacean and a carnivorous coelenterate. 



THE EFFICIENCY OF DAPHNIA POPULATIONS 

 Richman (1958) determined the calories released by combustion o£ Daphnia 

 pulex and Chlamydomonas reinhardi. These analyses provide conversion 

 constants for translating the numerical data from D. pulex into energy units 

 thereby permitting an analysis of efficiency in D. pulex populations. 



The term 'efficiency' appUed to a natural population is ambiguous. Any 

 dimensionless number which is the quotient of two energies may be called 

 an efficiency. At least three different concepts of efficiency seem of ecological 

 interest. These will be discussed in order of simplicity. 



(a) Ecological efficiency 



The ratio of the steady state rate of production of yield to the steady state rate 



of food consumption by the population will be called ecological efficiency. 



The maximum observed ecological efficiency for the Daphnia populations 

 varies from 8- 5 to 12 • 5 per cent. The lower estimate is calculated as calories 

 of yield removed divided by calories of algal food provided. The higher 

 estimate results from correcting for food consumption by use of equation (i). 

 The maximum observed ecological efficiency of a Daphnia population from 

 which young animals are preferentially removed is no more than 4 per cent. 



Patten (1959) hsts twelve estimates of ecological efficiency derived from 

 field data. With the exception of one value of 75 per cent taken from Teal 

 (1957) and one of 21 per cent (Lindeman, 1942) all of these estimates fall in 

 the range 5-5-13 • 3 per cent. Wright (1958) found an efficiency of 10 • 5 per 

 cent for a zooplankton population. 



The maximum ecological efficiency values derived from the Daphnia 

 experiments are therefore in good agreement with observed ecological 

 efficiencies in aquatic environments in nature. 



Ecological efficiency is a function of the interaction between three trophic 

 levels; the predator, the prey and the food of the prey. More intense preda- 

 tion or more effective assimilation of food may increase ecological efficiency. 

 There is no direct relation between ecological efficiency and the standing 

 crop of the prey although it may be expected that exploiting a prey popula- 

 tion so as to maximize ecological efficiency will tend to reduce standing 



