PREDATION AND EFFICIENCY IN LABORATORY POPULATIONS 239 



This can be readily visualized by considering that the process of steady 

 state population maintenance consists of producing dead animals at a fixed 

 rate and with a fixed age and size distribution. The cost in calories of pro- 

 ducing a dead animal is given by its caloric content, ^S^., at the time of 

 death divided by its growth efficiency up to the age of death. 



In the absence of predation all of the energy consumed by a population is 

 used for maintenance so that: 



I=Pc=^^ (9) 



^x 



where D^ is the number of animals dying during the age interval x. 



Predation alters the distribution of deaths and may also change growth 

 efficiencies and even the S^, so that for a population subject to predation: 



I=P'{c+Ac) = i^^ (10) 



^x 



Solving for c, using equations (9) and (10), and substituting this solution 

 in equation (4), 



E ^i 



^ £'. P^ B. 



Ep, therefore, incorporates almost all the information that may be of 

 interest in exploiting a population. 



Growth efficiency of Daphnia decreases with age, while the population 

 efficiency associated with selective predation increases with the age of the 

 animals removed. This lack of correspondence is due to the fact that removal 

 of an animal that is about to die of other causes introduces minimal alteration 

 in the population death distribution. A scavenger utilizes a population with 

 infinite population efficiency. 



A predator is behaving with maximum prudence when his predatory 

 activities maximize the population efficiency of the prey. This can be done 

 by taking as yield slowly growing a^nimals with a minimal life expectancy 

 and low reproductive value. 



CONCLUSION 



Despite the dependence of population phenomena on precise experimental 

 conditions, a relatively simple equation predicts the functional dependence 

 of population size on an appropriately defined concept of predation in both 



