404 



POPULATIONS 



growth have been given in preceding 

 chapters together with other ecological im- 

 plications of such studies, and the present 

 discussion was appropriately foreshadowed 

 (p. 353). 



Data obtained by Chapman (1928) and 

 analyzed by Allee (1931) revealed a more 

 rapid early increase in population density 

 with an initial seeding of 0.125 beetles 

 per gram of flour than at lower (0.062 per 

 gram) or higher densities. The results are 

 summarized in Figure 141. As the graphs 



peated the experiment, using the converse 

 plan of holding the initial inocula constant 

 at one pair of beetles per bottle and in- 

 creasing the size of the effective environ- 

 ment in a regular series. Despite a reduc- 

 tion in the observed rate of reproduction, 

 the optimal density for early increase fell 

 at the same point, namely, at an initial 

 density of 1.125 beetles per gram of floui- 

 (Fig. 142). 



Another report of the same phenomenon, 

 based on still another strain of T. confusum 



Z A 8 16 32 64 



INITIAL POPULATION per 32gms. of flour 



Fig. 141. Optimal initial population of Tribolium with volume of medium held constant. (From 



Chapman and Thomas Park.) 



show, Chapman's populations started with 

 one pair of adult beetles in 32 gm. of flour 

 (0.062 per gram), and grew at a slower 

 rate per female per day during the initial 

 period of eleven days than did any other 

 density tested by him at that time. 



Thomas Park (1932) confirmed this part 

 of Chapman's experiment, using a different 

 strain of beetles of the same species and a 

 different type of wheat flour. The agree- 

 ment, even in rate of effective reproduction, 

 is as close as could be expected from inde- 

 pendent workers using different genetic 

 stocks at different locations and with a 

 few years intervening. Animals are not 

 physical automata and do not always give 

 identical responses even under approxi- 

 mately similar conditions. 



These two sets of tests were made by in- 

 troducing beetles in an ascending geomet- 

 ric series into 32 gm. of flour in each ex- 

 perimental microcosm. Park (1933) re- 



and a different set of initial relations, was 

 published by MacLagen (1932), who in- 

 troduced 8,8,8,4,8, and 16 pairs of beetles, 

 respectively, into 16,8,4,1,1, and 1 gm. of 

 flour. He maintained his cultures at a 

 sUghtly higher temperature and at a much 

 higher relative humidity than did Park. 

 Also, his stock cultures had been subjected 

 to intense crowding for six months and 

 had possibly been selected for ability to 

 withstand crowding. As shown in Figure 

 143, MacLagen, too, obtained a definite 

 optimum, but now at a density of two 

 beetles per gram of flour. 



So much for the demonstration of the 

 reality of an optimal population density 

 above the possible minimal one. Now for 

 the known factors. Park (1933), in initia- 

 ting this analysis, drew the inference from 

 certain experiments that the decrease in 

 rate of effective reproduction in oversparse 

 populations results from infrequent meet- 



