Competition 399 



troduced at regular intervals. In the first set of tests the prey popula- 

 tion at first grew rapidly, but, as the number of predators increased, 

 the prey gradually became extinct and thus caused the starvation of 

 the predators soon afterward (Fig. 10.14, Case I). In the second ex- 

 periments (Case II) since the prey could escape, their population con- 

 tinued to grow, but the predators died off. In the third set of tests 

 (Case III), in which the additional seedings simulated immigration, 

 oscillations of both populations were induced with the maxima and 

 minima of the predator population following those of the prey 

 population. 



Many illustrations of the interdependencies of prey and predator 

 shown in the foregoing tests are known in natural populations. The 

 removal of predators has allowed certain useful fish populations to in- 

 crease several fold (Huntsman, 1938; Foerster and Ricker, 1941). 

 The protection of deer from predation in the Kaibab forest resulted 

 first in their great increase and then in wholesale death from starva- 

 tion, as discussed in an earlier chapter (Leopold, 1943). In most 

 natural situations the predators are unable to find and kill all the 

 prey so that, after the reduction of the predator population, the prey 

 population can increase once more. Furthermore, since in nature 

 not all the prey are successful in finding refuge from their enemies, a 

 small part of the predator population usually manages to survive 

 periods of prey scarcity. 



The action of these reciprocating influences may cause oscillations 

 in natural populations of prey and predator even in the absence of 

 periodic immigration. For example, in the Hudson Bay watershed 

 the populations of lynx and of its prey, the varying hare, fluctuate 

 widely and with about the same period of approximately 914 years 

 (MacLulich, 1937). From the fact that the maxima and minima in 

 the lynx population follow closely those of the hare population, as 

 well as from other observations, the periodicity of lynx abundance 

 appears definitely to be controlled by the availability of its food, and 

 the oscillations of the hare population are influenced to some extent, 

 at least, by the extent of lynx predation. Host and parasite popula- 

 tions similarly affect one another in a generally reciprocal manner 

 (Debach and Smith, 1947, and Utida, 1950). The repercussions of 

 prey-predator and host-parasite fluctuations on the whole community 

 will be considered further in Chapter 12. 



To summarize, when two antagonistic species occur in the same 

 environment, these general types of result are possible: (1) If the 

 two species are ecological equivalents making common demand on 

 the environment, and if one inhibits the growth of the other more 



