152 Temperature 



ance of temperature extremes played an important role in the evolu- 

 tionary development of the migration habit of birds and of other ani- 

 mals, even if this factor does not provide the trigger that sets off the 

 seasonal journeys each year. 



Action within Effective Range 



Although the action of temperature extremes may be drastic at 

 times, the plants and animals of any habitat spend most of their lives 

 at intermediate temperatures. We shall now consider the action of 

 the heat factor in the range between the minimum effective tempera- 

 ture and the maximum effective temperature. Just as with extremes 

 of heat and cold, the inffuence of a given temperature within the effec- 

 tive range depends upon the thermal conditions to which the species 

 of plant or animal is adapted. Many tropical, temperate, and polar 

 species have become attuned to the temperatures characterizing their 

 respective regions so that many of their life processes go forward at 

 approximately the same rate in spite of considerable temperature dif- 

 ferences. For example, the rate of oxygen consumption in certain 

 lamellibranchs is of the same order of magnitude for species living at 

 0°C in the Arctic, at 8°C in boreal seas, at 12°C in the Mediter- 

 ranean, and at 27°C in tropical waters (Thorson, 1950). Seasonal 

 adjustment to temperature is exhibited by the sand crab (Emerita 

 talpoida) in the Woods Hole area to the extent that its metabolism 

 at 3°C in winter is four times greater than in summer. As a result 

 the animal can be active and can grow during the winter when many 

 other species in the tidal zone become inactive (Edwards and Irving, 

 1943). The action of specific temperatures on life processes must 

 be considered for each species separately, and also for geographical 

 subspecies, but we can delineate certain generalities in regard to the 

 role of the heat factor within the effective range. 



Effect of Temperature on Biological Rates. The influence of tem- 

 perature on the rates of biological processes is most clearly seen in 

 poikilothermous animals and plants. Although mammals and birds 

 are definitely affected by the heat factor, the action is largely indirect 

 because these warm-blooded forms carry their own temperature 

 around with them. When acting directly, a rise in temperature of 

 10°C usually causes a doubling or a tripling of the rate of a biological 

 process, and the process is said to have a (^lo of 2 or 3 in accordance 

 with Van't Hoff's law. 



If you will take the trouble to listen to the cadence of the crickets on 

 several evenings, you will observe that the frequency of their chirping 



