Invading species may become particularly destructive if they find an 

 abundance of suitable food and no natural enemies. 



By destroying forests, man exterminated much of the wild life and brought 

 far-reaching changes in the soil and water relations of areas stretching across 

 states. 



Increasingly, our forest areas are being operated for continuous yield. 



By concentrating our forest growth in the best trees it is possible to in- 

 crease the yield of a given area. 



The four most serious dangers to the forests are ruthless cutting for profit, 

 fires, various insects, and various fungi. 



The extension of cities, the clearing of forests and the improvement of 

 farms, all result in exterminating various species of birds. 



Our attempts to utilize natural resources more thoroughly for our own 

 advantage often disturb balances and bring about epidemics. 



In general, the most effective way to fight an epidemic is with a counter- 

 epidemic, that is, a restoring of the biologic balance by encouraging the 

 natural enemies of the pest. 



EXPLORATIONS AND PROJECTS 



1 To see how man upsets the balance of nature, screen either or both of the 

 United States documentary films entitled The Plow That Brol^e the Plains and The 

 River. Relate the scenes shown in the films to conditions in the nearest region in 

 which man's activities are making rapid inroads on natural resources. 



2 Report on changes that have taken place in your own community or state, in 

 comparatively recent times, in the prevalence of {a) wild life, {b) forest land, {c) cul- 

 tivated crops, {d) weeds, or {e) domestic animals. Account for the reduction in 

 numbers of certain forms and the appearance and spread of new forms. Which 

 changes have produced results favorable to human beings? unfavorable? What 

 species should be further reduced, or what ones should be protected? Be sure you 

 have considered all factors in making your recommendations. 



3 To find out what can be done to conserve natural and human resources after 

 balance has been upset, investigate the work of the Tennessee Valley Authority in 

 its program of conservation, flood control and power development. 



4 To find out what methods of farming are least wasteful of our nation's soil, 

 investigate some of the better farming practices employed in scientific farming. 

 These methods include general farming, feeding of crops to livestock, careful tech- 

 niques in the handling of manure, rotation of crops, the use of cover crops, legume 

 crops, green-manure crops, commercial fertilizers, Ume, drainage, and methods of 

 tillage which lessen erosion. Report your findings. 



5 To investigate the succession of plant forms which take over an area after 

 the climax forest cover has been removed, visit areas that have been cut over re- 



598 



