INTRODUCTION 31 



Balance in Nature 



The influence exerted by one animal or one group of animals on 

 another can hardly be estimated until one of them leaves the pic- 

 ture. In an established animal community which might be said to 

 be balanced, all groups are held in boimds by their enemies. Bal- 

 anced animal communities can be found the world over, and we are 

 only beginning to get a notion of the extensive ramifications of the 

 forces concerned in maintaining that balance. Quite clearly most 

 animals live in a state of repression because relatively few of them 

 become pests and overrun the country. About eighty-five years ago 

 someone who had admired the remarkable spirit of the English 

 sparrow in its native European home thought this hardy little bird 

 would be a cheerful addition on this side of the Atlantic, Conse- 

 quently, a few pairs were landed in Brooklyn. In the short years 

 that have elapsed, this sparrow has proved so hardy and free of 

 enemies here that it is now our dominant bird. Every city in the 

 United States, as well as many in Canada and Mexico, has a large 

 permanent population. Its nesting and perching habits in the heart 

 of great cities are a source of great annoyance and expense to 

 building owners. Also, they consume enormous quantities of the 

 farmers' grain. 



The story of the rabbit in Australia is likewise an interesting ex- 

 ample of the effect of balance or lack of it. Not many j-ears ago 

 Australia did not have a rabbit within its boundaries. It was hoped 

 and intended by English immigrants there, that a few imported pairs 

 of rabbits would increase sufficiently so that the old English sport 

 of riding to the hounds might be developed in Australia. To the 

 surprise and dismay of these people the rabbits flourished until now 

 they are jeopardizing the enterprises of man. Many men are kept 

 employed full time doing nothing but hunting rabbits. 



Again, we have an example of the effect of the natural agents 

 of repression. The Japanese beetle which was recently intro- 

 duced in the United States hy accident has ravaged the vegeta- 

 tion in several eastern states and threatens other areas. When our 

 investigators went to Japan to study the enemies of the beetle in 

 an effort to find a means of control, they had to search for weeks to 

 find a seriously infested area. So impressed are some biologists be- 

 coming with the potential danger of interfering with the natural 

 balance, that even when some irritating pest is under discussion, 



