156 Temperature 



Daphnia occurs at 23 °C, and the optimum from the point of view of 

 the largest number of young produced is found to be at 18°C (Mac- 

 Arthur and Bailhe, 1929). The ecological effect of these divergent 

 optimal values will depend upon whether they are applied to the 

 individual or to the population. The individual Daphnia lives longest 

 at one temperature, but the population as a whole grows the fastest 

 at another temperature. The optimum temperature for a species can 

 therefore be only a general concept. The optimum is best thought of 

 as a range of temperatures, and it is the range within which the or- 

 ganism as a whole functions best. 



TABLE 12 



Temperature Relations of the Cladoceran, Daphnia magna 



Other Effects of Temperature. In our discussion of temperature 

 thus far we have dealt chiefly with growth and survival. Many other 

 influences of this factor are of importance in the ecology of animals 

 and plants. A drop in temperature ordinarily means a decrease in 

 activity, especially among the cold-blooded animals and plants. As 

 winter comes on, insects and other invertebrates ordinarily become 

 dormant. In the spring on land the thermometer usually must climb 

 above 8°C before cold-blooded animals become abundantly active. 

 The snow flea, Collembola, forms an interesting exception to the fore- 

 going generality. These small black insects are often found swarming 

 on the surface of the snow among the trees in February or March. 

 Evidently the animals can absorb enough heat from the strengthening 

 spring sun in their protected microclimates at the base of trees to be 

 active. In the aquatic environment many animals are adapted to 

 carrying on a very active life at temperatures only a few degrees 

 above 0°C. No one who has hooked a salmon in a northern stream 

 will claim that a high degree of activity is impossible for cold-blooded 

 animals at these lower temperatures. 



Certain behavior patterns are influenced by the heat factor. Some 

 of the more primitive animals exhibit a thermotaxis, that is, an orienta- 

 tion toward a source of heat. Ticks are aided in locating their warm- 



