164 Temperature 



the Gulf of Maine into the Bay of Fundy serve as an ilkistration of 

 this situation. Fish and Johnson ( 1937 ) report that adult copepods 

 brought into the Bay of Fundy find the temperature tolerable, breed 

 successfully, and produce eggs that are viable under the existing 

 thermal conditions. The temperature remains too low for the ensuing 

 larval stages, however, with the result that the new generation is 

 eventually killed off. The copepod population is consequently re- 

 duced until the next year when it is again renewed by currents from 

 the Gulf of Maine. Sometimes the harmful action of temperature is 

 quite indirect. Although the free-swimming life of the oyster, Cras- 

 sostrea virginica, is extended from 7 days at 25°C to 21 days below 

 20°C, the animal develops perfectly well at the latter temperature. 

 The principal significance of low temperature for the larvae is that it 

 prolongs pelagic life and hence increases the period during which 

 the larvae fall easy prey to enemies (Thorson, 1950). 



In some species the adult stage is more sensitive to extremes of 

 temperature than any of the younger stages. Excessive heat or cold 

 may reduce the vitality of the plant or animal as a whole, or it may 

 cause damage to specific tissues or functions. The organs of repro- 

 duction are frequently the most susceptible. For example, the north- 

 ward distribution of peach trees in the United States is limited by 

 damage to fruit buds by late frosts in the spring, although the other 

 parts of the plant remain quite uninjured. The foregoing discussion 

 will suffice to illustrate the fact that extremes of heat or cold may kill 

 off animals or plants at whatever season the most vulnerable stage of 

 development occurs. 



Control by Need for Minimum Amount of Heat. In order that a 

 species maintain itself in a given locality, the temperature at some 

 season must rise above the threshold, that is, above the minimum 

 at which the vital processes of the organism can go on, and it must 

 remain above that value for a period sufficient to allow growth and 

 reproduction to be completed. The great variation in the length of 

 the frostless season within the United States is indicated in Fig. 5.14. 

 Many organisms require temperatures considerably above freezing 

 at some season, but occasionally we find instances of plants or animals 

 that can get along with remarkably little heat. Lichens were found 

 growing on Antarctic nunatacs (rocky peaks protruding through the 

 ice) where extreme cold prevails for much of the year and the tem- 

 perature rises above 0°C on very few days. Similarly, a rich and 

 varied marine invertebrate fauna and associated flora are known to 

 grow and to reproduce along the coast of Greenland where the water 

 temperature never exceeds 0°C. 



