18 MAN AND ANIMALS 



to prevent any unnecessary disturbance of the settled order of the life of his 

 region which will convert relatively stationary species into widely oscillating 

 ones; second, to destroy or render stationary all the oscillating species injurious 

 to him; or, failing in this, to restrict their oscillations within the narrowest limits 

 possible. For example, remembering that every species oscillates to some 

 extent and is held to relatively constant numbers by the joint action of several 

 restraining forces, we see that the removal or weakening of any check or barrier 

 is sufficient to widen and intensify this dangerous oscillation, and may even 

 convert a perfectly harmless species into a frightful pest. 



Forbes mentions that cottony scale, a common pest in our parks, 

 was rare in natural conditions. The close setting of trees has favored 

 its increase. Close setting is nearly always a factor which has to be 

 considered. 



How do pests arise ? The recent rise of the wheat aphis may be 

 taken as an example. The spring of 1907 was very warm in the southern 

 part of the wheat belt, and the grain aphis, which is said to reproduce 

 freely at temperatures from ioo° F. to below freezing, was accordingly 

 able to reproduce without interruption from its parasites and enemies, 

 which do not become active at such low temperatures as occurred. 

 When the weather grew warmer and the enemies appeared the aphids 

 were so numerous that the work of the enemies was hardly appreciable. 

 But since they too, like the aphids, are rapid reproducers, w r ith such 

 favorable conditions they were able to increase rapidly. With their great 

 increase the aphids decreased and soon their numbers were far too great 

 for the available aphid food. The enemies therefore decreased because 

 of the absence of sufficient food, and this portion of the society was 

 accordingly restored to an approximate equilibrium. It is to be under- 

 stood that such an oscillation in the society is far-reaching in its effects. 

 It has been noted that such oscillations affect the whole community. 

 The birds and mammals find certain kinds of food abundant and accord- 

 ingly eat things different from what they do under different conditions. 

 Such fluctuations in the animal communities are constantly going on. 



The whole process may be summarized as follows : 



1. Weather conditions unfavorable to enemies and favorable to plant pest. 



2. Increase in pest. 



3. Increase in enemies. 



4. Decrease in pest. 



5. Decrease in enemies. 



6. Balance. 



