406 



POPULATIONS 



marizes the matter thus in terms of the 

 densities with wliich he worked: 



"In Tribolium, therefore, two agents . . . 

 cause the reduction in the number of eggs 

 found in the cultures as density increases, viz., 

 an actual fall in the rate of oviposition occur- 

 ring between 1.25 and 5 beetles per gram and 

 the eating of eggs which occurs at all densities 

 from 1.25 beetles per gram upwards and in- 

 creases in rate with increasing density." 



A further cause of the reduction in rate 

 of population increase in crowded cultures 

 is probably to be found in the disopera- 

 tions associated with self-conditioned flour 

 — disoperations that have been adequately 

 reviewed in a preceding chapter (p. 352). 



This particular instance of optimal popu- 

 lation density is of value, not only for the 

 thoroughness with which the phenomenon 

 has been established and the progress to- 

 ward an analysis of causal factors, but also 

 because the whole gamut of effects of im- 

 dercrowding, optimal density, and over- 

 crowding result from interactions within 

 a homotypic population and from the rela- 

 tions of the beetles with a relatively simple 

 environment. As Thomas Park has pointed 

 out (1941), we have here a simplified 

 analogue of the interactions on which 

 much of outdoor ecology depends (Clem- 

 ents and Shelf ord, 1939). We turn now 

 to a brief consideration of optimal popula- 

 tions when two species are present. 



Heterotypic Optima 



Certain problems related to heterotypic 

 populations have been discussed in a pre- 

 ceding chapter (p. 346). Optimal densities 

 exist in populations of mixed species. The 

 situation is complex and becomes rapidly 

 more so with each additional species in the 

 ecosystem. Certain relations can be illus- 

 trated by partially controlled laboratory 

 populations of the ciliate Oxytricha fallax 

 and of the bacterium, Pseudomonas fluores- 

 cens, on which it feeds (Johnson, 1933; 

 Cause, 1934). As in the original investiga- 

 tion, our presentation of the results is 

 simphfied by assaying the eflFects of popu- 

 lation densities of both constituent organ- 

 isms upon Oxytricha alone and disregard- 

 ing—in fact, experimentally avoiding— many 

 of the density eflFects on the bacteria. 



The concentration of the food organisms 

 modified the rate of population growth of 



the protozoans. Standard suspensions of the 

 bacteria in two drops of water that sup- 

 ported the highest rate of division of 

 isolated Oxytricha were called concentra- 

 tion X. In one series of tests, each isolated 

 oxytrichan yielded an average of more than 

 eleven ciliates in twenty-four hours (see 

 Fig. 144). Variations from this concentra- 



4X 



CONCENTRATION 

 2X X X/4 



X/IO 



I 



3.5 9.0 11.4 5.4 3.0 



Fig. 144. Optimal population density of bac- 

 teria for maximal production of Oxytricha. 

 (Data from Johnson.) 



tion of bacteria produced decided reduc- 

 tions in the numbers of protozoans. 



If too few oxytrichans are seeded into a 

 new culture, the phenomenon of under- 

 crowding is readily demonstrated in more 

 dense bacterial cultures. Certain of these 

 relations are outhned in Figure 145. Drop 

 suspensions with X concentration of bac- 

 teria yielded 10 -H Oxytricha per seeded in- 

 dividual in twenty-four hours, whether 

 started with one or with two individuals. 

 Consistent with this result, cultures of 2X 

 concentration also gave a 10 to 1 rate of 

 increase when inoculated with two individ- 

 uals, but yielded 8 for 1 in twenty-four 

 hours when seeded with only one oxy- 

 trichan. Cultures with 4X concentration of 

 bacteria showed still further reduction in 

 rate of increase, and some of the 5X con- 

 centrations approached, or even reached, 

 the limit of toleration for fission of the 

 Oxytricha tested. Earlier work by Chejfec 

 (1928) gave similar indications 



