CHAPTER LXVII 



ANIMAL ORGANISMS IN RELATION TO THEIR 



ENVIRONMENT 



ECOLOGY 



That field of zoology which deals with the relations of organisms to 

 their environment is called ecology. In other words, it is the study of the 

 animal in its home. The field is not new, for it is practically equivalent 

 to what has long been known as the natural history of animals, but the 

 name is new and the exact methods of modern ecology are of very recent 

 development. 



530. Facts of Ecology. — Observation reveals a varied distribution of 

 animal forms. In one locality with a certain character are found certain 

 animals, while in another of a different character there is quite a different 

 group. In fact there is no species of animal which is uniformly distrib- 

 uted. Not only is this true but it is a matter of common knowledge that 

 the animals which are active at night are not those which are active in 

 the daytime. The species of animals about us change from one season to 

 another ; birds come and go, and insects appear at their appropriate times. 

 One who observes life from year to year finds differences both in the 

 species to be found and in their relative numbers. The animals found 

 associated in a certain type of locality, however, are usually associated 

 together in any other locality of the same character, and so definite types 

 of animal communities can be recognized. The study of the causes of 

 these phenomena lies within the field of ecology. 



531. Relations of Animals to Plants. — Early in this text (Sec. 69) the 

 dependence of animals upon plants was emphasized. Directly or indi- 

 rectly plants are the basis of all animal food and the capacity of any 

 given region to support a large animal population depends upon the 

 amount of plant food available. Plants, however, serve not only as food 

 for animal organisms but also for shelter and concealment. For these 

 reasons the plant life has a very important influence— and in many cases 

 even a controlling one — over the animal life of a given area. It is dif- 

 ficult to study intelligently either plant or animal ecology without also 

 studying the other, and for this reason the term biota is commonly 

 employed in reference to all of the living organisms of a given area, 

 including both plants and animals. Owing to the fact that few plants 

 possess the power of locomotion, plant ecology presents fewer difficulties 

 than does animal ecology and has not only preceded animal ecology in 



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