398 



Relations between Species 



relation, the populations of the two species have certain reciprocal 

 relations. These may be illustrated by the well-known investigations 

 of Cause ( 1935 ) in which the interactions were demonstrated in 

 laboratory populations of protozoans (Fig. 10.14). Cause prepared 

 cultures in which the ciliate Paramecium caudatum, as prey, was 

 eaten by another ciliate Didinium iiasutiim, as predator. An ample 



__ Predator 



Predator 



III 



Predator 



Time 



Fig. 10.14. History of prey {Paramecium caudatum) and predator (Didinium 

 nastitum) populations in (I) homogeneous microcosm with initial seeding, (II) 

 heterogenous microcosm providing refuge for prey with initial seeding, and (III) 

 homogeneous microcosm with repeated seedings. ( From Allee et al., 1949, after 



Gause. ) 



bacterial population was present at all times as a source of food for 

 the paramecia. In the first type of experiment Paramecium and 

 Didinium were introduced into a medium equally available to both; 

 in the second type the microcosm was heterogeneous so that the 

 Paramecium could escape into one part of the medium inaccessible to 

 its aggressor. A homogeneous microcosm was again used in the third 

 type of experiment, but additional seedings of both species were in- 



