SECONDARY COMMUNITIES 17 



(24) into streams, ponds, and lakes. The effect of the industrial wastes 

 differs with their character. Sewage practically destroys all the life of a 

 stream or lake near the point of entrance, through the introduction of 

 many poisonous substances, through the increase of carbon dioxide 

 and ammonia and through the lowering of oxygen content. Nichols (25) 

 states that the oxygen above the entrance of the Paris sewer into the 

 Seine was 9 . 23 c.c. per liter, and immediately below 1 .05 c.c. per liter, a 

 reduction of almost 90 per cent. The typical swift-water fauna of Thorn 

 Creek at Thornton was reduced to practically nil by the opening of 

 the Chicago Heights sewage system. The common isopod (Asellus com- 

 munis) was the only animal able to withstand the conditions. At a 

 distance from a point of entrance of sewage the amount of plankton 

 is increased by its introduction because of the nitrogen and other food 

 for plants which it contains. Forbes (see 5a) reports that the amount 

 of plankton near Havana in the Illinois River has doubled since the 

 opening of the Drainage Canal. 



5. EQUILIBRIUM IN THE SECONDARY COMMUNITIES 



Equilibration means a restoration of balance in the numbers of 

 contending organisms of the community. For instance, as has already 

 been noted, the deer reached their maximum number with the correspond- 

 ing destruction of the carnivores by man. This indicated that the 

 primeval balance between the carnivores and the herbivores had been 

 disturbed. An entirely new balance has now been established through 

 the complete destruction of both the large hervibores and carnivores, 

 by man. Most of our knowledge of equilibration in communities has 

 resulted from the study of the secondary communities of parks and 

 agricultural lands. Concerning these Forbes (26, p. 15) has said: 



There is a general consent that primeval nature, as in the uninhabited forest 

 or the untilled plain, presents a settled harmony of interaction among organic 

 groups which is in strong contrast with the many serious maladjustments of 

 plants and animals found in countries occupied by man. [All our serious out- 

 breaks of insect pests are instances of these maladjustments.] 



To man, as to nature at large, the question of adjustment is of vast impor- 

 tance, since the eminently destructive species are the widely oscillating ones. 

 Those insects which are well adjusted to their environments, organic and inor- 

 ganic, are either harmless or inflict but moderate injury (our ordinary crickets 

 and grasshoppers are examples); while those that are imperfectly adjusted, 

 whose numbers are, therefore, subject to wide fluctuations, like the Colorado 

 grasshopper, the chinch bug, and the army worm, are the enemies which we 

 have reason to dread. Man should then especially address his efforts, first, 



