Part V 



ANIMALS AND 

 THEIR ENVIRONMENT 



CHAPTER 37 



Ecology 



The animals and plants living today are related not only by evolutionary 

 descent, as described in the preceding three chapters, but also by their 

 relations to each other and to the physical environment. One form may 

 provide food or shelter for another; it may produce some substance 

 beneficial or harmful to the second; or the two may compete for food 

 and shelter. The study of the interrelationships between living things— 

 both within species and between species— and their physical environ- 

 ment is known as ecology. Each organism, by the process of evolution, 

 has become adapted to survive in some particular kind of environment, 

 has developed a tolerance for a certain range of moisture, light, tem- 

 perature, wind and so on, and has developed certain relationships with 

 other living organisms in its immediate vicinity. Since the study of 

 ecology, and an appreciation of its prime importance in zoology, re- 

 quire a good background knowledge of the anatomy and physiology of 

 a wide variety of animals, the discussion of this topic has been reserved 

 for these concluding chapters. 



326. Ecosystems 



When any species of animal is carefully studied in the wild, it be- 

 comes clear that it is not independent of other living things, but is one 

 of a system of interacting and interdependent parts which form a 

 larger unit. Ecologists use the term ecosystem to indicate a natural unit 

 of living and nonliving parts that interact to form a stable system in 

 which the exchange of materials between living and nonliving parts 

 follows a circular path. Ecosystems may be as large as a lake or forest, 

 or one of the cycles of the elements (p. 755), or as small as an aquarium 

 jar containing tropical fish, green plants and snails. 



A small lake or pond is a classic example of an ecosystem small 

 enough to be investigated easily (Fig. 37.1). The nonliving parts of the 



753 



