POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 335 



populations were more resistant to cold 

 than were the fishes. Some of the inverte- 

 brates appeared completely dead at the 

 time, but soon revived. 



Ward (1940) conducted an intensive 

 survey of the seasonal fluctuations of En- 

 tomostraca dwelling in ponds in the region 

 of Cincinnati, Ohio. She developed methods 

 for consistently sampling the population 

 turnover of Cladocera, Copepoda, Ostra- 

 coda, and Phyllopoda. These records of 

 abundance were correlated with a number 

 of physical, chemical, and biotic measure- 

 ments. Ward concluded that temperature 

 was the most important single factor con- 

 trolling entomostracan populations and that 

 much of its influence was direct upon the 

 organisms themselves. "Undoubtedly tem- 

 perature both directly and indirectly 

 played a great part in inducing seasonal 

 Huctuations in numbers and in controlling 

 the appearances and disappearances of var- 

 ious species as the yearly cycle progressed" 

 (p. 678). She confirms a much earlier opin- 

 ion of Birge (1898a) that temperature con- 

 trols reproductive rhythms in these forms 

 and that seasonal distribution is primarily 

 a description of the reactions of the Crus- 

 tacea to this factor as it affects produc- 

 tivity. 



A particular illustration is instructive. The 

 cladoceran Allonella karua, a southern 

 form, in all probability has recently ex- 

 tended its northern range to the Cincinnati 

 area and is present in the ponds studied by 

 Ward. "This species appeared abruptly and 

 in considerable numbers on July 2, 1938, 

 . . . when the temperature was 26° C, a 

 reading first recorded for that year in the 

 observations of the preceding week. It ap- 

 peared consistently at every collection 

 thereafter until September 24, when the 

 temperature fell below 20° C. for the first 

 time. With this drop to ]7° C, the species 

 disappeared completely from collections, 

 although it was several weeks before water 

 temperatures became much further re- 

 duced." 



Certain other probable direct effects of 

 temperature have been discussed in Section 

 II. On pages 103 and 104 the concept of 

 Heat Hardiness was developed; on pages 

 99 to 103, the concept of Cold Hardiness; 

 and on pages 207 to 211 some general re- 

 lations between heat and moisture, along 

 with an introduction to the climograph, 

 appear. 



HUMIDITY, AND HUMIDITY AND 

 TEMPERATURE 



The direct effect of humidity on popula- 

 tions, and the effect of humidity when as- 

 sociated with temperature, have been con- 

 siderably investigated. 



Experimental Populations 



Utida (1941) studied experimental popu- 

 lations of the Azuki bean weevil, Calloso- 

 bruchus chinensis, as affected by humidity 

 and population density, and his paper af- 

 fords an illustration of both density-inde- 

 pendent and density- dependent operations. 

 Figure 125 summarizes certain of the find- 

 ings when reproductivity, as indexed by 

 number of progeny, is assayed in the pres- 

 ence of a wide series of parent weevil 

 densities and under three conditions of rel- 

 ative humidity: 32 per cent, 52 per cent, 

 and 76 per cent. The graph clearly shows 

 that both density and humidity are potent 

 agents in terms of their effect on reproduc- 

 tion. Relative to the former it is to be 

 noted that the total number of progeny (at 

 76 and 52 per cent humidities) increases 

 with density until 192 parent weevils per 

 container is attained and then declines 

 sharply. The reproductive rate, when ex- 

 pressed as number of young per female, 

 was highest in the lowest density and de- 

 creased with each successive stage of par- 

 ental crowding. 



The effect of humidity is equally clear 

 and easily summarized: The order of pro- 

 ductivity in the three humidities is 76 per 

 cent, 52 per cent, 32 per cent. 



The component humidity effect that is 

 basically independent of density can be as- 

 certained and thus advanced as an illustra- 

 tion of a direct density-independent effect 

 induced by atmospheric moisture and in- 

 fluencing reproduction. Utida showed that 

 weevils maintained at essentially 100 per 

 cent relative humidity did not reproduce 

 effectively because a filamentous fungus got 

 established, which checked the normal 

 hatching of the weevils' eggs and made the 

 beans unsuitable as food for the larvae. 



Lund (1938) reports the effect of hu- 

 midity, among other things, on the produc- 

 tivity and longevity of adult Trichogramma 

 evanescens. This is a chalcid fly that, in 

 these experimental populations, parasitized 

 the eggs of the Angoumois grain moth, 

 Sitotroga cerealella. 



