182 



Temperature 



that the speed of development will be increased if either the tempera- 

 ture or the humidity is raised. In this instance more than one limit- 

 ing influence is present. In certain situations, two (or more) in- 

 dependent factors may be unfavorable and botli of these must be im- 

 proved before the organism can continue its growth or extend its 

 range. In other situations, the limiting effect of one factor depends 

 upon the value of another, as in the development of the boll weevil. 

 The two factors acting together produce the limitation, and may be 

 referred to as a "limiting combination." The ecologist should always 

 be on the lookout for possible multiple adverse or modifying in- 

 fluences in the environment. Limiting combinations are probably in 

 operation under natural conditions more frequently than is now real- 

 ized ( Shelf ord, 1951). 



+ 10 



-10- 



1- Absolutely 



fatal 



Dormancy 



or death 



Dormancy 



No 

 development 



21 days 

 18 days 

 16 days 

 12 days 

 11 days 



Absolutely 

 fatal 



10 20 30 40 50 60 70 80 90 100 

 Mean relative humidity, % 

 Fig. 5.21. Generalized scheme indicating the interaction of temperature and 

 humidity in controlling dormancy and the number of days required for develop- 

 ment in the cotton boll weevil. (By permission from Animal Ecology by Chap- 

 man, 1931, McGraw-Hill Book Co.) 



An illustration of the combined action of temperature and moisture 

 on vegetation is the effect of these factors on the form of growth of 

 plants and on the life form of the whole plant. The climate exerts a 

 major control over the type of plant that can exist in each region, and 

 consequently the life form of the vegetation is to a certain extent an 



