Harmful Effects 



319 



ther growth of the colony is prevented. Organisms continuing to Hve 

 in the same medium tend to change or to "condition" it. Condition- 

 ing of flour by laboratory populations of the flour beetle Tribolium 

 confusiwi has been shown to cause reduced fecundity, extended dura- 

 tion of metamorphosis, and increased mortality (Park, 1941). The 

 same kinds of harmful changes are brought about by overcrowding 

 under natural conditions. In some situations aquatic animals may 

 aggregate to such an extent as to exhaust the oxygen supply; else- 

 where they may exhaust the food supply, produce harmful metabo- 

 lites, or displace one another mechanically. Of several hundred 

 oyster larvae originally setting on an old shell or other small object, 

 only a few will find sufficient space to develop into full-sized oysters; 

 all the others will eventually be killed by crowding. Where com- 

 petition for space is keen, oysters grow in a long slender form, un- 

 desirable for the market. Oystermen in cultivating their beds com- 



10 20 30 40 50 60 70 80 90 

 Mean flies per bottle 

 Fio. 9.4. Curve showing decrease in rate of reproduction in Drosophila as cul- 

 tures become more crowded. (Pearl and Parker, 1922.) 



