MULTIPLE-FACTOR AND SYNERGISTIC STRESSES 



729 



LOW 



HIGH 



ENVIRONMENTAL GRADIENT 



Fig. 2 The zones of lethality and compatibility. (From F.J. 

 Vemberg and W. B. Vernberg, The Animal and the Environment, 

 page 8, Holt, Rinehart, and Winston, Inc., New York, 1970.) 



For any one organism, a balance of external and internal 

 environments must be achieved which permits the functioning of the 

 diverse cellular components required for the intact organism to 

 survive and be successful. When extremes in an environmental factor 

 (or factors) are reached, an organism miay cease to function as a 

 viable, integrated entity. Between these extremes, however, it can 

 survive and function at various degrees of effectiveness. With this 

 concept, the zone of lethality (resistance adaptation, zone of 

 resistance) is that portion of the gradient representing the extremes 

 that cause the death of the organism; the zone of compatibility (the 

 biokinetic zone, zone of tolerance) is the central portion of the 

 gradient where life can be sustained (Fig. 2). 



Faced with this enormous complexity of the external and 

 internal environments, how can we with our limited knowledge 

 determine which factors are important and initially worthy of study? 

 Mason and Langenbein (1957) suggested the following criteria be 

 applied in assessing the ecological importance of an external factor: 



1. It must be operationally significant to an organism. 



2. It must be directly effective at some time during the life of the 

 organism. 



