CHAPTER 20 

 ANIMALS AND THEIR ENVIRONMENT 



Ecology deals with the relations of organisms to the environment. It 

 has to do primarily with those relations to environment which determine 

 the organism's characteristics, its success, its mode of life, and its distribu- 

 tion. Ecology is also concerned with the environment itself. Since the 

 things to which animals and plants are sensitive in the world about them 

 are not everywhere the same, any organization which the environment 

 may possess is of importance to living things. This organization is some- 

 times very intricate, and many an ecological study has been directed 

 toward an understanding of the environmental system, mthout imme- 

 diate reference to any particular organism. 



The environmental relations of organisms may be approached from 

 two different points of view: (1) that of the individual or single species, 

 in which case ecology comes very near to a limited physiology, and (2) 

 that of groups of species living in the same general situations and forming 

 what are called associations or communities. These two points of view 

 are successively adopted in this chapter. 



Temperature. — Each kind of animal is capable of carrying on its 

 metabolism only within a certain range of temperatures. At some point 

 within this range, usually above the middle but sometimes below, the 

 physiological processes work best. For most animals the lower limit is 

 slightly above freezing, while the upper limit is usually below 45°C. Fish 

 eggs develop best a few degrees above freezing, birds' eggs at about 40°C. 

 Some animals possess remarkable powers of adjusting themselves to tem- 

 peratures outside their usual range. Thus some of the pj-otozoa which 

 die when raised ^\dthin a short time to a temperature of 23°C. will endure 

 70°C. if the temperature is raised very gradually. 



Since temperature varies irregularly on the earth's surface from the 

 equator to the poles, with elevation above sea level, seasonally, and as 

 between day and night, it is obvious that animals must be so located that 

 their permissible temperatures are present and that their limits are not 

 overstepped. Ordinarily, species with a low optimum temperature must 

 live in temperate or cooler zones, those Avith a high optimum temperature 

 in tropical regions. The factor of dormancy also enters into the deter- 

 mination of geographic position. Most animals become torpid at suffi- 

 ciently low temperatures, and some endure actual freezing. Many of 



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