290 PRINCIPLES OF ANIMAL BIOLOGY 



sparrow. A census of this species in the north central states over a five- 

 year period showed a minimum of 9 pairs and a maximum of 13 pairs 

 per 100 acres of land. In the northeastern states it was almost equally 

 steady at a lower level — 3 to 7 pairs per 100 acres. The number of eggs 

 laid by the English sparrow is such that, starting with the normal 

 number of pairs, about 260 individuals could have been produced in each 

 100 acres in one year. But in the long run the numbers did not increase 

 at all, and at no time mthin the five years were the sparrows excessively 

 abundant. Casual observation indicates that this stability is common 

 over longer periods. 



When some unusual event removes from the environment of a species 

 one of its chief limiting factors, the number of individuals may increase 

 enormously. Some of the best examples are found in the annals of eco- 

 nomic entomology. An insect plant pest imported into a new region 

 without the parasites which kept it in check at home may experience a 

 remarkable outbreak. The end of such "explosions" has, in economic 

 entomology, usually been brought about by introduction of the appro- 

 priate parasites. How they might end in the absence of help from 

 man is problematical. The pest might exterminate its only food plant, 

 resistant strains of the food plant might be developed through selection, 

 or some other parasite might find the newcomer a suitable host. 



Animal Communities. — Though the foregoing discussion deals mostly 

 with single species in relation to their environment, more ecological infor- 

 mation is often obtainable by a study of animal communities. A 

 community consists of all the species living in one general situation. 

 In a broad way, it is found that the species making up a community 

 tend to be the same in many localities of the same kind. As will be 

 seen later, similar ponds over a wide area have in part the same species 

 in them; lakes of like size and depth not too far from each other are apt 

 to contain many of the same species. These species are held together in 

 a community by their requirement of practically the same set of environ- 

 mental factors. Organisms requiring a given range of temperature, 

 moisture, oxygen, and light herd together where these features are to be 

 found. The constitution of communities is not rigid, for no two situations 

 are exactly alike. One lake may have slightly more oxygen or lower 

 temperature or clearer water than another. The difference may cause 

 the communities of the two lakes to differ in certain species, tliough they 

 are alike in most. Occasionally also two communities will differ in their 

 component species by the mere accident that one or two species have been 

 introduced into one but not into the other. 



Sometimes species are held together by some very specific relation 

 between them This relation may involve merely the nutrition of one 

 of the species. Many plant-eating insects favor, or are practically 



