Are organisms slaves of their environment ? In the 

 sense that all living creatures require specific things 

 of their surroundings and cannot withstand others, 

 the answer is yes. Each of the physical and biological 

 conditions of the environment is an ecological factor 

 and has some influence upon life. 



Physical factors of the environment include general 

 climate, temperature, water, light, atmosphere, soil, 

 fire, and topography. General climate is the most in- 

 clusive factor; it is probably the most important in 

 determining the range of organisms. The other physi- 

 cal factors affect the local distribution of plants and 

 animals. Of these latter factors, temperature and 

 water are the most important for plants, because both 

 are so closely affected by other physical factors that 

 they often act as "measuring sticks'" of their environ- 

 ment. 



Biological (biotic) factors are the consequences of 

 life activities. The biotic environment, then, is the 

 influence of organisms on other living things. Natu- 

 rally some of these factors are direct and of intimate 

 nature; others are indirect. No matter how close or 

 remote the relationships among the organisms of an 

 area, each species is influenced by the other life 

 around it. 



In the present chapter many ecological factors, 

 physical and biological phenomena, are discussed in- 

 dividually. However, it must be realized that any 

 organism is subject to and responds to all environ- 



mental features simultaneously. Also, an environ- 

 ment is a synergistic phenomenon. A habitat is more 

 than the mere sum of its parts. The parts acting 

 together produce a dynamic total situation. Follow- 

 ing the treatment of ecological factors are brief state- 

 ments on ecological variation and ecological success. 



LIMITING FACTORS 



It is well known that humans like certain situations 

 and dislike others. Of the conditions man dislikes, 

 some are unpleasant, others are harmful, and many 

 are lethal. The reactions of other organisms are much 

 the same; however, each species responds in a par- 

 ticular manner to a given environment. These differ- 

 ential responses are indicated by the fact that some 

 creatures may live in many unlike places and under a 

 variety of conditions, whereas others have a limited 

 habitat. This is not due to chance, but to unlike re- 

 actions to present and/or past ecological factors. The 

 environmental factor or factors that cause such re- 

 striction of functions and partly determine the dis- 

 tribution of a species are called limiting jactors. 



TOLERANCE 



Limiting factors exist when the basic conditions 

 needed for growth and reproduction are absent. For 



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