372 



POPCJLATIONS 



betAveen predator and prey predicted by 

 Volterra on theoretical grounds. 



Cause's experiments were of three de- 

 signs. In the first type a "homogeneous' 

 or clear medium was used into which bac- 

 teria were introduced along with one seed- 

 ing of Paramecium and one of Didinium. 

 In the second type a "heterogeneous" med- 

 ium was developed, consisting of a clear 



PREY 



PREDATOR 



H 



PREY 



PREDATOR 



PREY 



m 



PREDATOR 



TIME 

 Fig. 131. Predator-prey interactions between 

 two infusorian populations. The prey is Par- 

 amecium caudatum; the predator, Didinium 

 nasutum. Case I, "homogeneous microcosm 

 without immigrations "; Case II, "heterogeneous 

 microcosm without immigrations"; and Case 

 III, "homogeneous microcosm with immigra- 

 tions." (After Cause.) 



portion in which both ciliate populations 

 could Uve and a cloudy portion available 

 only to Paramecium. Into this heterogene- 

 ous medium bacteria, paramecia, and did- 

 inia were introduced, again as a single 

 seeding. The third set of experiments were 

 carried out in the homogeneous medium, 

 but, in addition to the initial seeding of 



prey and predators (the bacteria being 

 abundant, of course), Paramecium and 

 Didinium were added to the culture at reg- 

 ular intervals. This situation is designated 

 by Cause as a 'Tiomogeneous microcosm 

 with immigrations." 



The population interactions are diflFerent 

 in the three experiments. The results are 

 well summarized in Figure 131, from 

 which the following conclusions can be 

 drawn: In the homogeneous microcosm the 

 prey multiplies rapidly, thus providing a 

 dense culture for exploitation by the pred- 

 ators. The latter are efficient, find all the 

 prey, and bring about their extinction." 

 Then, with the food supply exhausted, 

 Didinium perishes. By making the micro- 

 cosm heterogeneous so that a refuge is 

 provided for the prey, an entirely difi^erent 

 end result is attained. The predator exploits 

 the prey available in the clear portion of 

 the medium and for a time multiplies 

 slightly. Some paramecia, however, re- 

 main or escape into the cloudy portion and 

 there cannot be eaten. The predator popu- 

 lation devours the prey it can get at and 

 then dies. The prey population, released 

 from the pressure of predation, now multi- 

 plies vigorously and establishes itself as a 

 successful culture of Paramecium, exploit- 

 ing, of course, the bacteria. 



The bottom chart of Figure 131 depicts 

 events when both species are introduced 

 into a clear medium with subsequent re- 

 introductions or "immigrations." Here, the 

 following things happen: 



" ( 1 ) At the first immigration into the 

 microcosm containing but few Paramecia the 

 predator did not find any prey and perished. 

 An intense growth of the prey began. (2) At 

 the time of the second immigration the concen- 

 tration of the prey is already rather high, and 

 a growth of the population of the predator 

 begins. (3) The third immigration took place 

 at the moment of an intense destruction of the 

 prey by the predators, and it did not cause any 

 essential changes. (4) Towards the time of the 

 fourth immigration the predator had already 

 devoured all the prey, had become reduced 

 in size and degenerated. The prey introduced 

 into the microcosm originates a new cycle of 

 growth of the prey population. Such periodic 



* This sort of result is probably largely 

 limited to experimental populations. It would 

 seem likely that in most natural populations 

 enough of the prey would escape by one 

 means or another so that complete extinction 

 would be avoided. 



