Control by Need for Chilling 171 



ment will normally start again only after chilling, or not until the fol- 

 lowing spring. This timing not only fits the life cycle of the moth 

 but also is most fortunate for the survival of the vegetation in areas 

 where this pest is abundant. If reproduction continued throughout 

 the late summer and autumn months, the trees upon which the cater- 

 pillars depend for food would have no opportunity to recover from 

 the spring and early summer inroads of feeding. Even in species 

 without a recognized period of dormancy distribution toward warmer 

 regions may be limited by the fact that warm conditions inhibit cer- 

 tain necessary processes long before temperatures are reached that 

 would be immediately lethal. The southern ( equatorward ) bound- 

 ary for the barnacle, Baloniis halanoides, appears to be determined 

 by the isotherm for a minimum surface temperature of 7.2°C ( Hutch- 

 ins, 1947). Evidently this barnacle requires a certain period of low 

 temperature in order to complete its reproduction successfully; this 

 conclusion is supported by the fact that the species breeds during the 

 summer in the northern part of its range, but breeds only during the 

 winter in the southern areas of its distribution. 



The foregoing consideration of the limitation of distribution by 

 thermal requirements has called attention to the complexity of the re- 

 lationships involved. Temperatures that are harmful, or that are 

 necessary, differ widely with species, age, and physiological condi- 

 tion. Resistant stages of a species may be found far beyond the 

 boundaries of the area within which all parts of the life cycle may 

 be carried on. For marine bottom invertebrates, for example, Thor- 

 son (1950) has delineated three areas of distribution: (1) an exten- 

 sive area throughout which the adult animal may live at least vegeta- 

 tively; (2) a smaller area within the first in which the animal will 

 ripen its sexual products and spawn; and (3) a still smaller area 

 within the second in which the embryos and larvae will develop 

 successfully. Other types of animals and plants have diflFerent pat- 

 terns of temperature sensitivity at different life stages. As we have 

 seen, the effect of temperature may vary for different life processes 

 in the same organism or even for the same process in different parts 

 of the organism. Each species of animal or plant can live only in 

 those regions in which the temperature pattern is tolerable and ade- 

 quate for the needs of the species. The organism is restricted to 

 habitats in which the annual, seasonal, and diurnal fluctuations in 

 temperature occur at such times and in such magnitudes as to allow 

 all life processes to be completed but do not occur so as to cause 

 serious harm. The intriguing interdependencies outlined here cer- 

 tainly need and invite further investigation by ecologists. 



