THE GROWTH FORM OF POPULATIONS 



329 



for our purposes since actual counts are 

 given in four day intervals as the cultures 

 contract. After the two forms had attained 

 equilibrium, they were placed as single 

 species into a medium of the two condi- 

 tioned types. 



These decUne and extinction curves are 

 instructive for their description of negative 

 growth as well as for the hght they throw 

 on conditioning. Cause, Nastukova, and 

 Alpatov oflFer these comments: 



"The analysis of the curves of decline shows 

 the existence of an essential difference between 

 P. caudatum and P. aurelia. The population of 



nected with the lower absolute values of 

 growth on the P. aurelia medium. It seems that 

 here the waste products of the species itself are 

 more toxic than those of the other species . . ." 

 (p. 230). 



Another example of decline and extinc- 

 tion of a laboratory population is reported 

 by Park, Gregg, and Lutherman (1941) 

 for mixed cultures of the granary beetles 

 Gnathoceros cornutus and Trogoderma ver- 

 sicolor (Fig. 123). Here, the essential fac- 

 tor was not environmental conditioning, for 

 this was eliminated by appropriate manip- 



1200 



ui 1000 



O 800 



600 



400 



200 



I I I L 



•— •- a 



__L 



m ' ' 



30 60 



120 



180 



480 



540 



600 660 720 



240 300 360 420 



POPULATION Af^E IN DAYS 

 Fig. 123. Population trends in competing cultiires of Gnathoceros cornutus (solid line) 

 and Trogoderma versicolor (broken line). The graph illustrates the decline and extinction of 

 the latter species. ( From Park, Gregg, and Lutherman. ) 



P. caudatum dies out rapidly and disappears 

 entirely on the eighteenth day. The rate of the 

 decline varies . . . under different conditions, 

 but these differences are small. Another state of 

 affairs is found in P. aurelia. When about 90 

 per cent of the population has already perished, 

 the remainder (the experiments were made 

 with pure cultures! ) adapt themselves to the 

 rather unfavourable conditions and continue to 

 live for a certain time. Later there appears a 

 second cycle of decline sharply separated from 

 the first. The second cycle of decline in P. 

 aurelia presents an extraordinary sensitiveness 

 to the homotypic and heterotypic conditioning 

 of the medium : ( 1 ) P. aurelia in pure culture 

 on a 'homotypic' medium possess a relatively 

 lower level of the second cycle than on a 

 "heterotypic" one (6 per cent as compared to 

 19 per cent . . .), and it is of shorter duration 

 (8 days instead of 14). The level is here taken 

 in a relative form and is apparently not con- 



ulation, but, rather, interspecies competi- 

 tion (p. 368). 



Special Cases 



In the introductory section of this chap- 

 ter we recognized a category of growth 

 form designated as "special cases." These 

 are accentuated, sudden changes that depart 

 radically from the normal pattern of equilib- 

 rium and refer particularly to population 

 spurts and crashes. This aspect has received 

 considerable attention, particularly from 

 workers studying the control of economi- 

 cally undesirable insects and mammals as 

 well as, of course, epidemics. These aspects 

 are not discussed in detail at this time 

 since the background is already laid 

 through our treatment of fluctuations and 



