BIOLOGY OF THE SEA-HOW THEY LIVE 31 



of the sperm whale with relation to temperature allows this species to live in 

 all oceans. 



Every species of animal has a valence for each of the many factors that de- 

 termine its distribution. There are valence values for food, temperature, salinity, 

 water clarity, light, pressure, enemies, breeding behavior, shelter, other animals 

 of the communitv, and all the other myriad factors that affect that animal at 

 any stage of its life. The distribution of any animal is usually not limited by 

 all of these factors but by only one or two. Figure 7 illustrates the law of the 

 miniviwn; an animal's distribution is limited by the factors to which the animal 

 shows the least tolerance. 



There are several clues to the discovery of limits to distribution. First, limiting 

 factors are often biotic, that is, have to do with the animals' relation to other 

 animals in the community, and these are sometimes very complex. Second, 

 limiting factors work on all stages of the life of an animal and animals are 

 limited at the stage where they show the greatest sensitivity to environment. 

 For instance, oysters grow in some brackish waters where they are seeded by 

 oyster farmers, but low salinity prevents their breeding there. Third, sometimes 

 the limiting factor is not deadly or even harmful in itself, but affects distribution 



I I 



J L 



^ LIGHT 

 OXYGEN 

 DEPTH 



I 



-|— I TEMPER/^TVBE 



H SALINITY 



j I WATETR CLARITY 



I I 

 -) 1 1 FOOD 



I I 



LOWER LIMIT I THE I UPPER LIMIT 



OF TOLERANCE I f^lNinUM' ^'^ TOLERANCS 



I I 



Fig. 7. Ecological valence and the law of the minimum. Each har represents a 

 valence value for one of the important factors in the life of an animal. Distribution 

 in this hypothetical case is limited hy low temperature tolerance and hy the high end 

 of salinity tolerance; this animal coidd not live in waters lower than a certain tem- 

 perature or of above a certain salinity. An animal's distribution is limited hy the 

 factors to which the animal shows the least tolerance. 



only indirectly. For instance, light in itself is not harmful to most nocturnal 

 animals, but these animals avoid light for protection against other animals 

 (Elton, 1935). Fourth, limits may often be discovered by noticing a decrease 

 in the numbers of animals near limiting conditions. Animals are most common 

 in optimum conditions or away from limiting conditions. Again, coral is a good 

 example. There are few corals in Bermuda where the lower limiting temperature 

 is reached and many in the Red Sea's optimum habitat. Fifth, rarely does any 

 one species of animal approach its physiological limits. Animals find suitable 

 habitats psychologically and travel the path of least resistance away from mar- 

 ginal conditions, unless they are forced there. 



