PREDATION AND EFFICIENCY IN LABORATORY POPULATIONS 233 



The simplest interpretation of these data is that the green hydrids use hght 

 energy in addition to animal food. Darkness alters the outcome of competi- 

 tion by ehminating this extra energy source. It seems possible, however, that 

 green hydrids do not feed as readily in the dark. In most of the populations 

 the green hydrids, when present, were individually smaller and more 

 numerous than the brown hydrids. 



A notable feature of the hydrid populations is the relatively high variance 

 between populations maintained under supposedly identical conditions. It 

 seems possible that part of this variance is due to the morphological flexibility 

 of individual hydrids. In hydrids, age and size are almost independent of each 

 other. An individual animal, deprived of food may reduce its size appreciably, 

 before it dies. 



(c) Immigration in Dugesia tigrina populations (Armstrong, i960) 

 The relation between size of individual and steady state population size is 

 most clearly demonstrated in Armstrong's experiments. He added new-bom 

 tails to populations of flatworms in proportion to the number of tails that 

 had been produced during the interval between census. 



Five populations were grown, two of which served as controls. Each 

 experimental population received either one, two or three new-bom tails 

 for every tail that was vegetatively produced by the population. All of the 

 populations reached a steady state in which numerical change was absent 

 and neither deaths nor births occurred. 



The number of animals in the populations, when they achieved this static 

 condition, was proportional to the rate of addition of new tails and inversely 

 related to the mass and length of the individuals composing the population. 

 Equilibrium could be attained with either a large number of small animals 

 or a smaller number of large ones. Animals m a population which achieved 

 numerical stability at forty-two animals were more than twice as heavy as 

 those in a population that was stable at 169 animals. The total dry mass of 

 the populations was shghtly greater for populations which were more 

 numerous. Possibly food was more effectively utilized when a large number 

 of small animals were feeding. 



CONCLUSIONS FROM THE NUMERICAL ANALYSIS 

 In all of the species examined, the surviving individuals in a population 

 subject to predation are different in reproductive rate, death rate, mean size 

 or some combination of these, from individuals in control populations. 



Predation lowers population size in these species. This cannot be expected 

 for all animals, however (cf Nicholson, 1957)- 



