210 THE STUDY OF PLANT COMMUNITIES * Chapter VIII 



others, overgrazing, and other destructive practices. Contour 

 plowing, strip cropping, terracing, and similar procedures check 

 runoff, hold water, and permit the rebuilding of rundown soils. 

 On wild lands and some submarginal cultivated lands, the re-estab- 

 lishment of natural vegetation is being encouraged where it should 

 never have been removed. Application of ecological principles in 

 such reclamation has generally paid good dividends. 



Not only has man disturbed or destroyed natural vegetation, 

 but he has also modified the environment, sometimes to his ad- 

 vantage. By irrigation or drainage, the soil moisture has been so 

 modified that great acreages have been brought under his control. 

 Enormous dams hold water in artificial lakes. When this water is 

 properly supplied to the surrounding soils, it transforms worthless 

 desert to highly productive agricultural land. Elsewhere drainage 

 systems put into lowlands have changed swampy, untillable soil to 

 some of the best truck and farming acreages. Not all drainage 

 projects have been profitable, however, especially those of muck 

 lands. Not all are equally productive, and cost of maintaining 

 drainage of some mucks is out of proportion to the crop yields. 

 Many such projects have been abandoned— to the joy of sports- 

 men and conservationists, who objected to the extensive destruc- 

 tion of homes and feeding grounds of all kinds of wildlife associ- 

 ated with these swamps. 



GENERAL. REFERENCES 



R. M. ANDERSON. Effect of the Introduction of Exotic Animal Forms. 



J. M. Coulter, C. R. Barnes and H. C. Cowles. A Textbook of Botany. 



(Vol. II : Ecology, pp.485-964.) 

 H. C. HANSON. Fire in Land Use and Management. 

 XV. A. McCUBBIN. Preventing Plant Disease Introduction. 

 S. A. WAKSMAN. Principles of Soil Microbiology. 



