THE GROWTH FORM OF POPULATIONS 



319 



a state of fluctuation. The data on fluctua- 

 tions of laboratory populations (especially 

 for protista and insects) are fairly numerous 

 and those on natural populations profuse. 

 In this section we select certain examples 

 that seem to provide a satisfactory view of 

 the range and character of fluctuation. 



Population fluctuations have been defined 

 as relatively asymmetrical de^^artures from 

 equilibrium. This does not necessarily imply 

 that they are characterized by their asym- 

 metry: in many cases they are rather regu- 

 lar over large periods of time. 



Laboratory Populations 



Figure 112 shows the fluctuations of 

 populations of the cihate Glaucoma pirifor- 

 mis cultured under three nutrient conditions 

 at constant temperatures (25° C). The pro- 

 tozoa of population IX were maintained in 

 a solution of caseinpeptone, KH2P04, and 

 distilled water. Population X had in addi- 



5 10 15 20 2 5 30 



DAYS 



Fig. 112. Population trends of the ciliate. 

 Glaucoma piriformis, in three different media. 

 (From Hall and Shottenfeld. ) 



tion a relatively small amount of thiamine 

 (vitamin Bj), while ten times the concen- 

 tration of this substance was added to XI. 

 The authors, of course, are interested in 

 showing that "the available concentration 

 of thiamine definitely affects the density of 

 population during the maximal stationary 

 phase and the secondary stationary phase 

 and also influences the time at which the 

 major phase of death begins." Thus our 

 interest lies in the three curves from about 

 day seven on. The trend is definitely down- 

 ward and is somewhat exaggerated by the 

 absence of thiamine. This is a favorable 

 illustration of population decline with min- 

 imal fluctuation. The final phase, that of 

 the actual extinction of the population, has 

 not been thoroughly studied, although the 



authors report that it is of long duration in 

 some cases, even exceeding eight months. 



An illustration of fluctuations in three ex- 

 perimental populations of granary beetles is 

 afforded by the study of Thomas Park, 

 Gregg, and Lutherman (1941) on Tribo- 

 lium confusum, Gnathoceros cornutus, and 

 Trogoderma versicolor. The first two are 

 tenebrionids, the last a dermestid. The pop- 

 ulations were maintained as single-species 

 cultures in the same kind and quantity of 

 medium and under controlled and identical 

 conditions of temperature, humidity, and 

 experimental manipulation. The trends, 

 therefore, reflect the pattern of growth of 

 these forms as species and can be compared 

 on that basis. Figure 113 shows the average 

 behavior of a number of the cultures. The 

 following points are to be noted: 



1. Tribolium confusiim builds up the 

 largest mean populations, followed by Tro- 

 goderma versicolor and Gnathoceros cornu- 

 his, in that order. 



2. Tribolium exhibits the minimum of 

 short, sharp fluctuations from one census 

 to another, but does display a single large 

 fluctuation (or oscillation) with a low 

 point around day 300. This is followed by 

 a rapid recovery to higher density. 



3. Gnathoceros does not display a single, 

 major fluctuation, but instead fluctuates 

 abruptly over a small range from count to 

 count, the tendency being for one count to 

 be high, the next to be low, and so on. 



4. Trogoderma grows most erratically of 

 the three. The individual population curves 

 for this species are quite different from each 

 other, a fact not true for Tribolium and 

 Gnathoceros. Thus, some Trogoderma cul- 

 tures start high and drop low and then par- 

 tially recover: others start low and build up 

 high; and still others start in between and 

 tend to stay there. 



5. The general conclusion so far as the 

 population fluctuations of these three forms 

 go is that, while fluctuation is characteristic 

 of all, the pattern of the fluctuation differs 

 between each and represents real species 

 differences at the group level. 



We return to this example in another con- 

 nection in a later chapter (p. 368) when 

 the problem of Interspecies Competition is 

 discussed. 



Natural Populations 



John Ford (1937a) reported on the popu- 

 lation fluctuations of CoUembola and mites 



