BIOLOGICAL FACTORS 



203 



population, the greater the modification of natural communities 

 by use and destruction. Whereas man was once essentially a de- 

 pendent in community structure, he is now more and more be- 

 coming the dominant organism everywhere. By cultivation, he has 

 eliminated natural vegetation from vast areas. Logging, even with- 



FlG. 96. Center of a burned swamp in iMaryland that once supported ma- 

 ture cypress-gum forest. Intense fire destroyed the forest and burned deep 

 into the peaty soil, which had accumulated through the years. Rebuilding 

 soil in the depressions, now filled with water, will require many years and 

 numerous generations of plants.— Photo by G. E Beaven. 



out subsequent cultivation, has changed the forests, and stands 

 equaling the original virgin forests will probably never occupy 

 most logged areas again. Cities, highways, airfields, and similar 

 products of man's living mean serious disturbance of natural vege- 

 tation. Drainage and irrigation projects, canals, road fills, and dams 

 result in soil moisture changes that promote the development of 

 quite different communities. Many similar disturbances can be 

 noted as a result of animal activities but always on a more local- 

 ized scale and consequently with less permanent effects. 



Fire is not peculiar to man's activities and, undoubtedly, oc- 

 curred here and there in North America before the white man 

 came. However, the conditions provided by lumbering operations, 

 and the constant use of fire, often with too little concern for its 



