POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



337 



This aflFords another illustration in which 

 an environmental factor, humidity, analyzed 

 for its eflFect on an organism, is found to 

 be primarily and, in part at least, density- 

 independent. As in most of these cases, the 

 possibility of density-dependent action rel- 

 ative to humidity is not excluded. For ex- 

 ample, it is quite conceivable that different 

 densities of chalcid flies would react dif- 

 ferentially to the saturation deficiencies just 

 discussed. But it seems clear that there is 



populations decreases. This is brought out 

 in Figure 126. The graph shows survivor- 

 ship definitely and consistently highest at 

 a saturation deficiency of 4 mm. (89 per 

 cent relative humidity), with a large de- 

 crease in percentage survivors against time 

 as the relative humidity drops 17 per cent 

 to 72 per cent, followed by still further 

 decreases at 55 and 27 per cent humidities. 

 Survivorship of the fleas when away from 

 the rats is then inversely proportional to 



TIME IN HOURS 



Fig. 126. The relation between saturation deficit and longevity of fleas. Curve I, relative hu- 

 midity 89 per cent, and saturation deficit 4 mm.; II, 72 per cent and 10 mm.; Ill, 55 per cent 

 and 16 mm.; and IV, 27 per cent and 26 mm. (After Uvarov.) 



an underlying direct effect exerted on the 

 individual members of the population. 



Natural Populations 



An interesting study combining research 

 in the field and laboratory and concerned 

 with both temperature and humidity is 

 reviewed briefly by Uvarov (1931). This 

 concerns the effect these factors exert on 

 adult flea populations and the consequence 

 of this for epidemics of bubonic plague. It 

 can be shown experimentally that as hu- 

 midity lowers, in the presence of a constant 

 air temperature of 32° C. and constant air 

 movement, the mean longevity of flea 



the saturation deficiency. Temperature also 

 is effective. If the saturation deficiency is 

 constant, a 10° C. rise in temperature 

 reduced Ufe duration by 50 to 66 per cent. 

 These experimentally determined facts 

 relate to the success or failure of epidemics 

 of plague. Uvarov comments on this point 

 as follows : "A variation of the saturation de- 

 ficiency from 5 mm. to 35 mm., such as 

 occurs in the plains of Northern India, 

 would, accordingly, shorten the average 

 duration of life of wandering rat fleas in 

 the proportion of fifteen to one. This ac- 

 counts for the fact that plague does not 

 maintain itself in an epidemic form when 



