Temperature and Moisture Together 181 



zones. The primary relationship is with the vegetation: these larks 

 are found wherever broad expanses of shortgrass occur regardless of 

 thermal conditions. Cone-feeding birds are similarly found wherever 

 suitable conifers are growing. The range of this type of bird may 

 include areas in one life zone where a northern conifer is growing and 

 also in another life zone where a southern conifer is growing. It is 

 obviously absurd to think of the range of this bird as primarily de- 

 termined by biothermal zonation. 



Temperature and Moisture Acting Together 



A discussion of temperature as an ecological factor should not be 

 concluded without calling attention again to the close interrelationship 

 between temperature and moisture in the terrestrial environment. 

 Emphasis has been placed on the fact that temperature affects relative 

 humidity and the rate of evaporation. The converse is equally true 

 that when evaporation and condensation occur, they tend to modify 

 the temperature. We are accordingly not surprised to find that tem- 

 perature and moisture often interact in such a way as to make it dif- 

 ficult or impossible to disentangle the individual effects of these two 

 factors. Does the salamander go into a sheltered place to keep cool, 

 to get warm, or to avoid excessive evaporation? Added to the com- 

 plication is the fact that for many organisms the indirect effects of 

 these factors may be more significant than the direct effects because 

 of their control of the vegetation. 



We have evidence that the influence of humidity on insects is often 

 greatly modified by the existing temperature. The dual effect of 

 these factors on the rate of development of the cotton boll weevil pro- 

 vides an admirable illustration (Fig. 5.21). This insect pest cannot 

 develop if the relative humidity is less than 40 per cent or more than 

 88 per cent, no matter how favorable the temperature may be. On 

 the other hand, the animal remains dormant regardless of humidity 

 if the temperature is lower than 17°C or higher than 39°C. Within 

 these ranges the speed of development depends upon the values of 

 both factors. At a temperature of 28°C, for example, the boll weevil 

 requires 21 days to develop under a relative humidity of 40 per cent, 

 but it develops in only 11 days if the humidity is between 60 and 65 

 per cent. 



In the foregoing example of the combined influence of temperature 

 and humidity on rate of development it is of interest to consider what 

 the limiting factor is. Picking the point on the diagram representing 

 a temperature of 24°C and a relative humidity of 55 per cent, we see 



