POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



371 



cannibalistic pressure. Chapman spoke of 

 this self-hmitation by the population as 

 "environmental resistance" of which he 

 considered egg-eating to be an important 

 component. 



Thomas Park (1933) presented further 

 observations on Triholitim egg cannibalism. 

 He showed that the imago beetles move 

 through their flour medium essentially at 

 random and that, if the eggs are deposited 

 at random, as is presumably the case, the 

 rate of egg consumption thus becomes a 

 matter of the probability of a beetle find- 

 ing and eating an egg. This rate obviously 

 will vary with the number of adults (pred- 

 ators) and eggs (prey), and this pro- 

 vides an illustration of a coaction (canni- 

 balism) that is density-dependent (p. 

 405). Park also showed that Tribolium 

 imago males, virgin females, and fecun- 

 dated females eat eggs at statistically simi- 

 lar rates, although later observations by 

 Stanley (1942) and Boyce (1946) suggest 

 that females may be more cannibalistic 

 than males. 



Chapman and Baird (1934) recorded 

 egg-eating rates when male imagoes were 

 present in six diflFerent densities. They 

 found, according to expectations, that 

 "\\'hen a population was high in propor- 

 tion to the size of the environment, the 

 eating went on at a greater rate; and as the 

 population became lower the chance of a 

 beetle's eating an egg became less and 

 less." 



Crombie (1943) has recently carried out 

 a studv of cannibalism. He set up experi- 

 ments in which male Tribolium were pre- 

 sent in the following densities: 1.25, 2.5, 

 5. 10, 20, and 40 insects per gram 

 of medium. To each of these cultures 

 eggs were added at a rate of fifty-five each 

 twentv-four hour period, and the percent- 

 age of eggs eaten daily was ascertained. 

 These percentages for the six increasing 

 imago densities are as follows: 7.7, 17, 20, 

 39.7, 70.2, and 98.4. The differences be- 

 tween these figures (by #-test) are all sta- 

 tistically significant. Evidence is again pre- 

 sented showing that the percentage of eggs 

 eaten per unit of time is directly propor- 

 tional to predator density. 



This is an uncomplicated and straight- 

 forward illustration of predation. It occurs 

 within a single species; basically follows a 

 pattern that varies with prey and predator 

 concentrations; and the coaction involved. 



eating of an egg, is a relatively simple one. 



Interspecies predation is a problem of 

 greater complexitv than cannibalism and 

 also of much more general ecological signif- 

 icance. Such predation is certainly related 

 to natural selection (p. 48) and consti- 

 tutes in its own right an interesting aspect 

 of population study with obvious density- 

 dependent implications. It is the latter that 

 we touch on briefly here. 



It is clear that the predation of one spe- 

 cies population upon another ranges from 

 situations relatively simple to those exceed- 

 ingly complex. The coaction basis for the 

 simpler phenomenon depends upon some- 

 thing approaching "random searching" 

 (see Nicholson. 1933) by the predators, 

 with the prey, if not distributed at random, 

 at least not highly secluded or inaccessible. 

 When these requirements obtain, it would 

 be expected in a general wav and on a 

 priori grounds that (1) when the prey and 

 predator populations are both large, preda- 

 tion would be rather consistentlv intense: 

 (2) when the prey population is large and 

 the predator population small, the inten- 

 sity of predation per individual predator 

 would be high, but the total predation 

 light; (3) when the prey population is 

 small and the predator population large, 

 the total predation would be intermediate; 

 and (4) when the prey population and the 

 predator population are both small, the 

 total predation would be light. More com- 

 plex predation situations are established 

 when the prey population, upon meeting 

 the pressure of predation, compensates for 

 this in some manner. 



To illustrate certain points about preda- 

 tion by actual examples, we discuss briefly 

 an experimental study of predation in labo- 

 ratory populations of microorganisms, pre- 

 dation in fish populations, and predation in 

 higher vertebrate populations. 



An illustration of predation in laboratory 

 populations is seen in the investigations 

 of Cause (1934). Cause set up cultures 

 in which a bacterial population (Bacillus 

 vifoct/aneus) was at the base of the 

 food chain, a population exploited by 

 the ciliate Taramecium catidatum. The lat- 

 ter in turn was eaten by another ciliate, 

 D'diniiim nasutum. Thus, Didinium con- 

 stitutes the predator group and Tarame- 

 cium the prey group. Cause's interest lay 

 in seeing if he could reproduce the "classi- 

 cal oscillations" in growth form (p. 326) 



