POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 349 



sentially the primary cycle, density-depend- 

 ent phenomena may stem either from the 

 secondary cycle and, therefore, involve 

 coactions, or from those diflEerences in in- 

 tensity of the primary cycle induced bv 

 varying degrees of crowding, or from both. 



Population Pressure Resulting from Co- 

 actions 



One of the most thought-provoking illus- 

 trations of the coaction aspect of the popu- 

 lation problem is found in investigations 

 concerned with the relation of crowding to 

 fecundity in the fruit fly, Drosophila melan- 

 ogaster, despite the fact that the studies are 

 somewhat "artificial" from the ecological 

 viewpoint. This problem was formulated 

 and developed by Pearl (1932) and has 

 been extended by Bodenheimer (1938) and 

 especially by Robertson and Sang (1944), 

 We shall first review the findings of Pearl 

 and of Bodenheimer and then discuss these 

 briefly in the hght of Robertson and Sang's 

 more recent critical analysis (see also p. 

 396). 



In 1922 Pearl and Parker set up experi- 

 mental populations of Drosophila at initial 

 imago densities ranging from one to fifty 

 pairs per half-pint bottle. The bottles con- 

 tained a culture medium the surface of 

 which was inoculated with yeast. The num- 

 ber of progeny produced by the parents in 

 these densities was counted, and the result- 

 ing data showed that as population density 

 increased the number of offspring per bottle 

 decreased. In short, an inverse relation be- 

 tween productivity and crowding is estab- 

 lished under the conditions of this experi- 

 ment. 



In a later paper Pearl (1932) attempted 

 to answer these questions: (1) What phys- 

 iological process or processes that would 

 explain these results are actually influenced 

 by density, and (2) what factor or factors 

 dependent upon the density relations in- 

 fluence this process? In considering the first 

 point it was reasoned that the rate of re- 

 production must be a major factor varying 

 between the different densities. That is to 

 say, the flies in general would have to re- 

 produce faster at low than at high den- 

 sities if the observed result was to be real- 

 ized. Such a difference in reproduction 

 could involve an alteration of fecundity or 

 fertility, or both. Appropriate experimenta- 

 tion showed that fecundity was greatly 



affected by the number of imago flies in 

 the bottle; the oviposition rate, expressed 

 as a rate per female per day, fell rapidly 

 as the density increased. No important rela- 

 tions between crowding and fertility were 

 reported (p. 289). 



With this fecundity effect estabUshed, 

 Pearl attempted an analysis of the popula- 

 tion factors actually influencing oviposition. 

 Experiments were performed in which the 

 air volume in the bottles above the medium 

 was varied while the area of agar surface 

 was kept constant. The experiments showed 

 that, although the extent of the air space 

 had no marked relation to fecundity, the 

 extent of crowding of the flies on the agar 

 surface was highly important. Differently 

 put. Pearl's findings suggested that the chief 

 density coactions took place on, and per- 

 haps below, the surface medium and not 

 in the flying space above. 



Bodenheimer (1938) reports experiments 

 that substantiate some of Pearl's ideas. His 

 experiments were so designed that agar 

 volume and air volume could be varied 

 while agar surface was maintained con- 

 stant. It was found that the asymptotes at- 

 tained by the fly populations growing in 

 these various situations were the same re- 

 gardless of the volume manipulations. This 

 again indicates that the agar surface expo- 

 sure is important. 



To explain the decrease in fecundity with 

 progressive crowding. Pearl advanced the 

 following ideas based on experimentation 

 and observation: (1) Drosophila females 

 will not oviposit if they are in contact with, 

 or disturbed by, other flies; (2) crowded 

 flies stimulate each other so excessively 

 that energy is dissipated that might other- 

 wise be used in reproduction; and (3) 

 individual imagoes do not obtain their full 

 share of food (i.e., yeast growing on the 

 agar surface) under these crowded condi- 

 tions owing to the disturbance of their 

 feeding behavior brought about by their 

 neighbors. These three relations can be 

 thought of in ecological terms as density- 

 dependent competitive coactions. As cul- 

 tures become more crowded, the flies com- 

 pete with each other primarily for food and 

 perhaps for oviposition space, and this com- 

 petition results in lowered fecundity. Since 

 the coactions compound with density, egg 

 production drops as the flies get more 

 crowded. Finally this reaches a point, 

 above 100 flies per bottle, beyond which 



