POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 369 



jf^ollovving are some of the types of end 

 .esults that can occur when two species 

 compete as mixed-populations." 



1. One population may become extinct, 

 owing to the pressure exerted against it 

 by the other. This is the usual outcome of 

 competition when the two species are 

 ecological equivalents and, therefore, make 

 some common demand on their shared en- 

 vironment. (Crombie, 1947; T. Park, 

 1948.) 



2. The two populations may reach re- 

 spective equilibrium, or semiequilibrium, 

 particularly if they inhabit somewhat dif- 

 ferent niches. The nature of the equilib- 

 rium is specific for each species under the 

 conditions of experiment and tends to be 

 maintained so long as food is available and 

 the physical habitat is not appreciably al- 

 tered. Under such conditions the equilibria 

 are probably largely functions of biotic in- 

 teractions. These equilibria, of course, are 

 not fixed indefinitely; they shift with signif- 

 icant alterations of the environment. 



3. The populations may vary in synchro- 

 nous or semisynchronous fashion so that 

 either oscillations or fluctuations between 

 the two species are established. This 

 growth form pertains especially to pred- 

 ator-prey and parasite-host situations. As 

 Volterra pointed out (p. 271) in his "Law 

 of conservation of the averages," the mean 

 numerical density of both species can be 

 maintained over considerable periods of 

 time. 



These three statements are derived on 

 the basis of logic and experience; they are 

 in part a priori and in part based on em- 

 pirical observation. Crombie (1945) re- 

 examines various mathematical models such 

 as those proposed by Lotka (1932), Win- 

 using data derived from mixed-species studies: 

 see especially treatments of the coactions be- 

 tween Drosophila and yeast (p. 349); the rela- 

 tion of Didinium to Paramecium (p. 372); the 

 interactions affecting fecundity between four 

 genera of granary beetles grown under homo- 

 t}'pic and heterotypic conditions of culture 

 (p. 355); and the relation of conditioning 

 brought about by planarian worms to survival 

 of Procerodes (p. 360). The sections concerned 

 with "predation" (pp. 370-377) and with 

 "environmental conditioning" (pp. 352-361) 

 also contain general discussions directly perti- 

 nent for the mixed-species problem. 



• Elton (1946) has examined in a novel way 

 the effects of competition between populations 

 in relation to community organization. 



sor (1934), Cause (1935), and Kostitzin 

 (1937) and concludes that these corre- 

 spond to the following biological possibili- 

 ties: 



"(1) Each species inhibits its own potential 

 increase more than that of the other and both 

 continue to exist together; (2) the second 

 species inhibits the potential increase of the 

 first less than it inhibits its own, while the first 

 species inhibits the potential increase of the 

 second more than it inhibits its own, whatever 

 the initial number of the two species: the first 

 species drives out the second; (3) this is the 

 opposite to (2) and the second species drives 

 out the first; (4) each species inhibits the other 

 more than itself: one drives out the other de- 

 pending chiefly on the initial sizes of their 

 populations (cf. Park, Gregg and Lutherman, 

 1941)" (p. 364). 



An illustration of a mixed-species popu- 

 lation study is seen in the work of Park, 

 Gregg, and Lutherman (1941) that has 

 been already mentioned in other con- 

 nections elsewhere in this section. This 

 study analyzes in quantitative terms what 

 results when three genera of granary bee- 

 tles (Tribolittm confusum, Gnathoceros 

 cornutus, and Trogoderma versicolor) are 

 brought together as competitors. The ap- 

 proach consists in establishing control 

 populations of each of the three species 

 cultured singly and experimental popula- 

 tions cultured as planned mixed-species 

 groups. In an investigation of such design 

 it is possible to difiFerentiate those efiFects 

 upon growth form that are purely intraspe- 

 cies in origin from those that are interspe- 

 cies, i.e., those new relationships that 

 emerge when a taxonomically distinct and 

 competing population is added to the eco- 

 system. 



In these experiments a food medium was 

 developed that proved suitable for all three 

 species and could be sifted for census tak- 

 ing. The total habitat was kept as optimal 

 as possible by replacing the medium at 

 each examination period; by using the 

 same volume of flour in all populations; 

 and by maintaining temperature, humidity 

 and light at certain designated levels. A\ 

 regularized intervals counts of larval, 

 pvipal, and imaginal beetles were taken. 

 The populations were set up as follows: 

 controls, consisting of single species cul- 

 tures; experimentals, consisting of two spe- 

 cies introduced in initially equal numbers; 

 and experimentals, consisting of various 



