SECTION V 

 ECOLOGY 



CHAPTER 33 



THE ECOLOGY OF ANIMALS 



Most of the previous discussion has been devoted to the individual 

 animal, its classification, its anatomy, its physiology, and finally its evo- 

 lutionary history. No matter how detailed such study may be, it does 

 not tell the full story of the living animal. No animal exists independently 

 of its environment. ^ Wherever it lives, it is part of a group or com- 

 munity of other animals and plants. Here it is affected not only by 

 these other organisms, but by the physical factors of the environment 

 itself. The study of how both the biotic and physical factors are inter- 

 related is known as ecology. More precisely ecology may be defined 

 as the study of plants and animals in relation to each other and to their 

 environment. 



In many ways ecology may appear to be a simple science, perhaps 

 merely an outgrowth of natural history. This, however, is deceptive, 

 for ecology must consider not only the animal's physiology and anatomy, 

 but also its relationships to other animals and to its environment. 



PHYSICAL AND BIOTIC FACTORS OF THE ENVIRONMENT 



In whatever type of habitat an animal lives, it is subject to a num- 

 ber of influences, both physical and biotic. Some of the physical fac- 

 tors included heat, wind, water currents, light, water, and presence or 

 absence of various salts. The biotic factors include such things as 

 population pressures, food supply, and relationships to other organisms. 



Heat. — All animals are affected to a greater or less extent by the 

 temperature of their environment. In general it can be said that those 



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