370 



POPULATIONS 



species combinations with one form ini- 

 tially introduced at a numerical advantage 

 over the other. The experiments were run 

 for more than two years with examinations 

 each thirty days. Not all the conclusions 

 can be presented here, but these are per- 

 haps the more significant: (1) As single 

 species populations, Tribolium, Gnatho- 

 ceros, and Trogoderma each have a charac- 

 teristic growth form (see p. 320 and Fig. 

 113); (2) in mixed-species populations 

 Trogoderma and Gnathoceros are usually 

 driven out by Tribolium, although under- 

 standable exceptions occur; (3) in mixed- 

 species populations Trogoderma and Gna- 

 thoceros are more evenly matched, al- 

 though Gnathoceros is favored in most in- 

 stances; in populations consisting of all 

 three species, Trogoderma becomes extinct 

 first (at about 120 days), Gnathoceros 

 second (at about 510 days), while the 

 Tribolium populations gradually increase in 

 size, as the pressure from competition with 

 the other two forms is gradually reduced, 

 until they attain normal (i.e.. control) den- 

 sities. The particular factors involved in 

 this mixed-species study have not been 

 analyzed, but Hutchinson (1947) has dealt 

 with this case, among others, from a 

 mathematical viewpoint. 



A recent paper by Thomas Park (1948), 

 published too late for detailed inclusion 

 here, discusses competition between two 

 species of the same genus (Tribolium con- 

 fusum and T. castaneum) —a. competition 

 resulting invariably in the extinction of one 

 of the two forms. It is further shown that 

 the presence of a sporozoan disease alters 

 the extinction pattern markedly (p. 727).* 



INTRASPECIES AND INTERSPECIES PREDATION 



The ecologist is interested in predation 

 from several points of view: as a natural 

 historian studying morphological, physiolog- 



• Other mixed-species studies that deal 

 largely with laboratory populations of protistan 

 and arthropod species are the following: 

 Smaragdova (19S6), Vladimerova and Smimov 

 ( 1938), and the significant and analytical paper 

 of Crombie (1945) on population interactions 

 between Rhizopertha dominica (a beetle) and 

 Sitotroga cerealella (a moth) in renewed and 

 in conditioned media and on the relation of 

 these two species to a third competitor, Ort/zae- 

 philus surinamensis (a beetle). Crombie's 1944 

 paper dealing with larval interspecies competi- 

 tion has already been discussed. 



ical, and behavioristic adjustments be- 

 tween a prey and its predator; as a student 

 of evolution concerned both with the evo- 

 lution of such adjustments and with the re- 

 lation of predation to natural selection; as 

 a student of communities in which preda- 

 tion is an important component of the 

 food-chain nexus; and as a population biol- 

 ogist. The viewpoint of the last worker is 

 directed towards the statistical appraisal 

 of the effectiveness of predation as that 

 factor influences the growth form of both 

 predator and prey populations. Predation 

 thus emerges as a source of prey mortahty, 

 and this can have real quantitative conse- 

 quences for consumer and consumed ahke. 

 Errington, Hamerstrom, and Hamerstrom 

 (1940) contribute to this point when they 

 say, "One of the causes of the disputes 

 often elicited by the mere mention of pre- 

 dation is confusion of the fact that preda- 

 tors prey upon certain animals with eifect 

 that such predation may have on numbers 

 of the prey. The fact of predation may 

 usually be ascertained with relative ease 

 through field or laboratory studies; evalua- 

 tion of effect of predation upon popula- 

 tion is another matter and one just begin- 

 ning to receive a small measure of the at- 

 tention that is its due" (pp. 817-818). 



Leopold (1933, p. 231) hsts five varia- 

 bles that influence the annual direct mor- 

 tality from predation in a given species of 

 "game" on a given range. These are: 



1. The density of the game population 



2. The density of the predator population 



3. The predilection of the predator, that is, 

 its natural food "preferences" 



4. The physical condition of the game and 

 the escape facilities available to it 



5. The abundance of "buffers" or alternative 

 foods for the predators 



We shall not discuss these variables one 

 by one, but it is helpful to have them so 

 formalized. They will be partly illustrated 

 in the examples that follow. 



An experimental demonstration of canni- 

 balism in relation to densitv is afforded by 

 studies of the flour beetle Tribolium confu- 

 sum. Chapman (1928) pointed out that 

 adult beetles eat their own eggs— a coac- 

 tion of some importance in regulating the 

 upper limits of population growth of the 

 colony. It was his conception that when a 

 Tribolium culture reaches certain levels of 

 density, the excess eggs are removed by 



