320 



POPULATIONS 



that inhabit tussocks of the grass, Bromus 

 erectus, during the winter months. This 

 author developed a sampling method which 

 enabled him to estimate the fauna in terms 

 of the number of organisms per square 

 meter. Figure 114 shows the extent of the 

 fluctuations during November, December, 

 January, February, and March. This figure 

 is concerned with the collembolan, Pseu- 

 dachorutes suhcrassus, and the mites, Asca 

 aphidioides and Hypochthonius pallidulits. 

 It will be noted that the former is about 



fauna and the drying up or wetting of dif- 

 ferent regions of the tussocks causes migra- 

 tions of certain species within them" (p. 

 111). 



Davidson (1944) notes that insect popu- 

 lations fluctuate in one of two general ways, 

 depending largely upon their reproductive 

 pattern. Certain species consist of individ- 

 uals present in all stages of development 

 and belonging to diflferent generations— 

 "complete overlapping generations." Other 

 species are dominated at particular pheno- 



4000- 



3000 



2000 



TRIBOLIUM CONFUSUM 



GNATHOCEROS CORNUTUS 



90 150 210 270 330 390 450 510 570 630 690 



POPULATION AGE IN DAYS 



Fig. 113. Population trends of three genera of granary beetles. (From Park, Gregg, and 



Lutherman. ) 



five times more numerous than the latter. 

 Ford draws a number of conclusions about 

 the fluctuation pattern, several of which are 

 as follows: "A fluctuation of the population, 

 with increases in November and December, 

 early February and late February, with in- 

 tervening minima, was shown to charac- 

 terize the Collembola and Acarina. . . . 

 The February minimum was shown to cor- 

 respond with a period of high evaporation 

 rate, during which contrary winds destroyed 

 the tussock structure. . . . Moisture is of 

 great importance for the existence of this 



logical intervals by individuals in essentially 

 the same stage of development— "incom- 

 plete overlapping generations." The latter 

 category, exempUfied by the collembolan 

 Smynthurus viridis in Davidson's study, ex- 

 hibits a distinctive upward and downward 

 trend in each generation. This results in a 

 growth form that rises high during "active" 

 seasons of the year and drops low during 

 "inactive" seasons when only eggs repre- 

 sent the species in nature. This must be a 

 relatively common type of fluctuation 

 among insects in temperate climates since 



