THE ENVIRONMENT 



333 



snow and ice 



tropical rain forest 



Figure 17.20 Vegetation zonation, implying both geograpiiic and altitudinal trends in North 

 America. 



munity. Finally, the highest and coldest, or .Mval 

 ^one, is found. This is the area of perpetual ice or 

 snow and is found only in the tallest mountains. Al- 

 though the zone occurs in the uppermost elevations 

 of the Sierras, such as Mount Lyell, the community 

 does not receive formal designation. Perhaps its lack 

 of a name stems from the paucity of plants and ani- 

 mals found there. The Sierran plants are mostly a 

 few species of algae and the animals are a few inverte- 

 brates. 



BIOTIC FACTORS 



In most instances, the physical factors of an en- 

 vironment are the governing forces within a com- 

 munity. However, certain activities of living organ- 

 isms have direct and to some extent governing effects 

 upon other species and the environment. These biotic 

 factors are created by relationships within and among 

 species. The strongest forces exist when different spe- 

 cies display close contact with one another, a phe- 

 nomenon called symbiosis. 



SYMBIOSIS 



Symbiosis is a much abused term. It has been 

 given many definitions, and many of them are still 

 used; however, its original meaning is literally "living 

 together" of unlike creatures. The original meaning 

 is followed here, so the word will denote any close 

 relationship among different species. 



There are many possible reactions when two spe- 

 cies are in intimate contact. The more important 

 interrelationships are (1) neutralism, in which neither 

 species is affected; (2) mutualism, in which both spe- 

 cies are benefited and both require the relationship; 

 (3) cooperation, in which both benefit but neither re- 

 quires the relationship; (4) competition, in which both 

 are harmed in the sense that each strives for the same 

 materials, especially food and shelter; (5) com- 

 mensalism, in which one is benefited and the other 

 unaffected; (6) amensalism, in which one is harmed 

 and the other unaffected; and (7) exploitation, in which 

 one is harmed and the other benefited. 



Further clarification of neutralism and exploitation 

 is necessary. Neutralism is probably a rare phe- 



