180 Temperature 



Although temperature relations are sometimes chiefly responsible 

 for the demarcation of certain life zones, it is obvious that moisture 

 is extremely influential in many instances, and other ecological factors 

 such as soil condition and length of day play contributory roles. The 

 positions of life zones cannot be universally correlated with any one 

 influence of the environment but are the result of a complex of in- 

 teracting factors. 



Nevertheless, an attempt was made by Merriam in 1894 to set up 

 zones based exclusively on temperature into which the entire fauna 

 and flora of North America could be divided. These were called 

 biothermal zones and, unfortunately, were accepted uncritically by 

 many ecologists. There are certain specific objections to the wide 

 application of Merriam's scheme besides the general warning that the 

 presence of an organism in a certain locality cannot be accounted for 

 by saying that it is a "member" of a certain biothermal zone ( Dauben- 

 mire, 1938). In the first place the heat requirement calculations for 

 Merriam's zones were based on 6°C as a threshold temperature for the 

 germination of wheat, and therefore obviously should not be directly 

 applied to other species of plants and particularly not to animals. 

 The assignment of birds and mammals to zones based on environ- 

 mental heat conditions is especially inappropriate since these warm- 

 blooded animals carry their own temperature around with them. 



Other objections to the use of Merriam's zones appear from a critical 

 understanding of the varied effects of temperature. For example, the 

 number of the degree days required for development is not constant 

 over a wide temperature range. Fifty days at 2°C above the thresh- 

 old have a very different influence from that of 5 days at 20°C above 

 the threshold, although the "thermal constant" would be the same in 

 both cases. Furthermore, as explained above, temperatures at seasons 

 of the year other than those used by Merriam may be critical in con- 

 trolling the distribution of many species. The fact that distribution 

 of a species may agree with certain isotherms of the region does not 

 necessarily prove a direct causal relation. The existence of many 

 minor species of plants is controlled by the presence of dominant or 

 accessory species. The geographical range of the dominant species 

 may in fact be controlled by temperature, or it may be dependent 

 primarily on the moisture factor, which in turn is influenced by tem- 

 perature. 



Animal populations also may exist in characteristic zones for reasons 

 that are only indirectly related, or unrelated, to temperature. Horned 

 larks, for example, are found all the way from Colombia to the Arctic 

 and occur in very divergent climates and hence in various biothermal 



