30 PLANT SUCCESSION AND CROP PRODUCTION 



Among all the chlorophyll-bearing plants there is competition 

 for light as well as competition for moisture and nutrient mate- 

 rials in the soil. Thus, we see that the influence of the green plants 

 themselves upon the vegetation must be greater than the influence 

 of the non-chlorophyll-bearing plants, since there is active compe- 

 tition going on among the members of an association in addition to 

 the action of the micro-organisms- A plant association cannot be 

 regarded as completely closed until all the levels for abstracting 

 water, minerals, light, and air are filled with sets of species of 

 dissimilar habits and requirements. 



The animals are as important in the mixing of soil materials 

 as are plant roots. The importance of earthworms has been known 

 since Darwin's classic work on the subject. Snails, ants, beetles, 

 spiders, rodents, all contribute their quota toward mixing the min- 

 erals with vegetable mould and allowing air and water into the 

 substratum. Insects are important in cross pollinating plants and 

 in many species are essential to seed production. Insects may also 

 act injuriously in destroying plants and may even effect complete 

 destruction of a pure stand. The locust borer, to name only one ex- 

 ample, has almost eliminated the black locust from farm wood lots 

 and forests of the central states where it was long the favorite wood 

 for fence posts. Birds are agents in the dispersal of seeds and 

 spores. Rodents, rabbits, rats, and mice may destroy trees by prun- 

 ing the bark, and prairie dogs destroy grasses. Sheep and cattle 

 have been known to prevent the reproduction of some of the timber 

 trees in the grazing region of the National Forests of the Southwest 

 and West (Hill, '17), while the bison has been credited as being 

 one of the agents to delay the invasion of forest upon the plains. 

 Of all the animals, the destruction by man has been the greatest. 

 There are almost no virgin forests left in the United States, due 

 to man's activity. 



4. Summary of Climatic, Edaphic, and Biotic Factors 



It will doubtless have occurred to the reader that any classifi- 

 cation of factors of the environment will present certain difficul- 

 ties in the way of sharp distinctions between the subjects to be clas- 

 sified. The attempt made by the writer shows that the biotic fac- 

 tors cannot be escaped in talking of the other two sets, viz., in the 

 climatic factors the influences of forests upon climate are featured, 

 while the relations of soils and the micro-organisms are discussed 



