Mode of Temperature Limitation 159 



ceia, as described earlier, is able to withstand desiccation and freez- 

 ing but, since it is a sexual egg, it must be fertilized by a male before 

 it can develop (cf. Fig. 2.11). In Moina macrocopa, when the tem- 

 perature drops below about 14°C, or rises above 30"C, males appear in 

 the population and sexual eggs are produced. This control of egg type 

 and sex ratio is attuned to the fact that in temporary pools the oc- 

 currence of very high temperatures often precedes the drying up of 

 the pond in summer. At the other end of the year a drop in tempera- 

 ture will presage the possibility of freezing and the necessity that the 

 animal go into a life structure in which the severity of winter can be 

 endured. 



ACTION OF TEMPERATURE ON DISTRIBUTION 

 Mode of Temperature Limitation 



In many early attempts to describe the ecological action of tem- 

 perature an effort was made to correlate geographical distribution 

 with mean temperatures. Although we see many average tempera- 

 tures in print, we never encounter one in nature. A distinction must 

 be made between the use of a temperature value as an index and the 

 action of temperature as an agent controlling distribution. The limits 

 to the spread of a population are always set by ecological factors act- 

 ing on the individuals making up the population. In most situations 

 the individual is not subject to a temperature equal to the average 

 temperature of the region for more than a few hours of the day or 

 a few days of the year. 



In eastern Massachusetts the mean annual temperature is about 

 7°C but on many days in summer the thermometer stands near 24°C 

 and in the winter near — 10°C with extreme records far above and 

 below these values. As a consequence the fauna and flora of the 

 region are very different from what they would be if the temperature 

 remained in the neighborhood of 7°C all the year round. In the 

 latter event the local biota would be more like that of the Aleutian 

 Islands or of South Iceland where the mean monthly temperature 

 varies only from about 10 to — 1°C with an average of about 4°C. 

 We may conclude that the mean annual temperature is not the chief 

 aspect of this factor which controls distribution, but a more pene- 

 trating analysis of the action of temperature must be made. 



Careful consideration of the various thermal influences shows that 

 for the permanent life of a species in an area the following tempera- 

 ture requirements must be met: 



