a species are curtailed, tlieii iiiliil)ited. In stiiiie organ- 

 isms, such as tisli, the u/^f^cr limit oj tolerance is 

 readied before activity is reduced to zero. At low 

 temperatures, the loiver limit of tolerance may he 

 readied wliile the animal is still cajjahle. potentially, 

 of considerable activity, and death is the result of 

 other factors. On the other hand, some organisms 

 may survive in an inactive or dormant state under 

 environmental conditions that do not permit activity, 

 only to become functional again when critical factors 

 rise above the threshold. Before the limits of tolera- 

 tion are reached there are zones of increasing physio- 

 logical stress. 



The species as a whole is limited in its activities 

 more by conditions that produce physiological dis- 

 comforts or stresses than it is hy the limits of toler- 

 ation themselves. Death verges on the limits of tol- 

 eration, and the existence of the species would be 

 seriously jeopardized if it were frequently e.xposed to 

 these extreme conditions. In retreat before condi- 

 tions of stress there is a margin of safety, and the 

 species adjusts its activities so that limits of tolera- 

 tion are avoided. There is variation in hardiness of 

 individuals within a species, so that some hardy indi- 

 viduals find existence possible under conditions that 

 disrupt other individuals. The population level of a 

 species becomes reduced therefore before the limits 

 of its range are actually reached. It is desirable to 

 test by acclimation and breeding experiments whether 

 these differences in physiological adaptiveness be- 

 tween individuals or populations are genetic or 

 phenotypic (Prosser 1955). 



Species vary in their limits of tolerance to tiie 

 same factor. The Atlantic salmon, for instance, 



THRESHgLD 



Limits of tolerance for some organisms 



TEMPERATURE 



High 



FIG. 2-2 Infsractlon betwean environment end cold-blooded 

 organisms: organism activity as a function of environmental 

 temperature (modified from Fry 1947). 



spends most of its adult life in the sea, but goes an- 

 nually into fresh-water streams to breed. Most other 

 marine fishes are killed quickly when placed in fresh- 

 water, as are fresh-water fish when placed in salt 

 water. The following terms are used to indicate the 

 relative extent to which organisms can tolerate vari- 

 ations in environmental factors. The prefix steno- 

 means that the species, population, or individual has 

 a narrow range of tolerance and the prefix eury- in- 

 dicates that it has a wide range : thus stcnohaline 



relation to distribution and population level — often a normal curve (modified from Shelford I" I )• 



^Lower limit of tolerance Upper limit of tolerance - 



High 



GRADIENT - 



•High 



The general nature of responses 



11 



