PREDATION AND EFFICIENCY IN LABORATORY POPULATIONS 225 



Any extrapolation to the field must take into account the geometrically 

 closed nature of the laboratory microcosm. 



Three species of hydrid {Hydra oliqactis, H. littoralis, and Chlorohydra 

 viridissima), two species ofDaphnia {Daphnia obtusa, D. pulex) and one species 

 of flatworm (Dngesia tigrina) were used in these studies (Hyman, ipSQ'J* ^\ 

 Scourfield, 1942). Except where specific differences are concerned I will refer 

 to these simply as hydrids, Daphnia, and flatworms. 



Further details of the technique are available in the previously cited papers. 



POPULATION GROWTH 



The Daphnia populations came to a fluctuating steady state; the flatworm 

 achieved remarkable numerical stabihty; the hydrids were much less stable 

 and many of the populations died after an alteration in the laboratory 

 plumbing. Apparently heavy metals or other poisons persisted on the glass- 

 ware after the plumbing change. The hydrid experiments are being repeated 

 using disposable plastic Petri dishes, and much of the instability seems to 

 have disappeared in the new dishes. I will assume that steady state hydrid 

 populations would have been maintained, had the plumbing been satisfactory. 

 There is an element of doubt in this assumption, so that all of the hydrid 

 results must be considered contingent on repetition of the experiments. 



(a) Daphnia 



If a small number o£ Daphnia are placed in 50 cc of water and fed in the way 

 indicated above, there will be an initial period during which the number of 

 animals will increase and the individuals will all grow and reproduce rapidly. 

 This period of rapid growth may be sHghtly delayed if the original animals 

 were starved before the beginning of the experiment. In any case, no more 

 than six to ten days after the initial introduction the population will consist 

 of new-born animals and highly fertile adult females, producing fat laden 

 eggs at the rate of from six to fifty eggs per moult. Cladocera do not have 

 elaborate metamorphosis; young animals immediately begin to feed and 

 grow. 



After approximately twenty days population growth ceases and the 

 number of animals begins to decline. At this time the population contains 

 mainly small animals. From one-tenth to one-fifth of the animals are 

 reproductive adults. The amount of fat per egg, as indicated by visible fat 

 droplets, the number of eggs per brood and the colour of the eggs have all 

 changed during the early population history. At approximately the sixtieth 

 day no new eggs at all are apparent in the brood pouches of the adults. At 

 this time individual growth rates are low and mortality rates are high. 



