5 



Tefuperature 



Temperature is perhaps the most commonly famiHar ecological fac- 

 tor. The great variation in heat conditions and their general influence 

 are evident to everyone. In contrast to many ecological factors tem- 

 perature may be measured with relative ease. Temperature is a uni- 

 versal influence and is frequently a limiting factor for the growth or 

 distribution of animals and plants. Even when an organism is dor- 

 mant, the chemical processes going on in its body are controlled by 

 the existing temperature. This factor is also important indirectly as 

 modifying the effects of other ecological agents. 



Temperature is the intensity aspect of heat energy. The capacity 

 aspect of this form of energy will also be considered in this chapter. 

 Differences in the heat capacity of the media, of the various types of 

 substrata, and of the bodies of organisms themselves are significant 

 in controlling susceptibility to temperature change. The amount of 

 heat in an ice-covered lake may change considerably without any 

 alteration of temperature, but in most natural situations increase or 

 decrease in heat also affects the temperature. Both aspects of heat 

 energy must be kept in mind, but temperature has the more predom- 

 inant direct influence on the lives of organisms. 



DISTRIBUTION OF TEMPERATURE 



Extremes of Temperature and of Tolerance 



In considering heat conditions in natural environments the extremes 

 of temperature may first be reviewed. In the open water of the 

 aquatic environment the temperature cannot drop below the freezing 

 point. This means that the temperature of the water in ponds is 

 never lower than 0°C and in the ocean never lower than about 



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