COMPETITION FOR COMMON FOOD IN PROTOZOA 



105 



In order to dimmish the concentration of the bacteria we made a 

 new smaller standard loop for preparing the "one-loop medium," and 

 also arranged experiments in which the one loop medium was diluted 

 twice ("half-loop medium") . The data obtained are given in Table 4 

 (Appendix) where every figure represents a mean value from the ob- 

 servations of two microcosms. This material is represented graphi- 

 cally in Figures 23, 24 and 25. 



Let us examine Figure 23. The curves of growth of pure popula- 

 tions of P. caudatum and P. aurelia with different concentrations of 

 the bacterial food show that the lack of food is actually a factor limit- 

 ing growth in these experiments. With the double concentration of 

 food the volumes of the populations of the separately growing species 

 also increase about twice (from 64 up to 137 in P. caudatum;^ X 2 = 

 128; from 105 up to 195 in P. aurelia; 105 X 2 = 210). Under these 



TABLE XI 



Parameters of the logistic curves for separate growth of Paramecium caudatum 



and Paramecium aurelia 



conditions the differences in the growth of populations of P. aurelia 

 and P. caudatum are quite distinctly pronounced : the growth of the 

 biomass of the former species proceeds with greater rapidity, and it 

 accumulates a greater biomass than P. caudatum at the expense of the 

 same level of food resources. 1 If we now express the curves of separate 

 growth of both species under a half-loop concentration of bacteria 

 with the aid of logistic equations we shall obtain the data presented 

 in Table XL This table shows clearly that P. aurelia has perfectly 

 definite advantages over P. caudatum in respect to the basic char- 

 acteristics of growth. 



(3) We will now pass on to the growth of a mixed population of 

 P. caudatum and P. aurelia. The general character of the curves on 



1 This is apparently connected with the resistance of P. aurelia to the waste 

 products of the pathogenic bacterium, Bacillus pyocyaneus (see Gause, Nas- 

 tukova and Alpatov, '35). 



