CHAPTER XI 

 BIOTIC FACTORS 



The mutual effects of plants upon each other, or the phytobiological 

 factors, have been discussed in the section on competition. A con- 

 sideration of the influences of man and animals is also necessary for the 

 complete understanding of vegetation. The present composition of 

 vegetation over vast stretches of the earth's surface is more completely 

 controlled by man and animals than is commonly supposed. 



1. FAUNA 



It will not be necessary here to return to the activity of soil organ- 

 isms (see p. 233). Little attention has been given to the sociological 

 importance of animals as agents of pollination and seed distribution. 

 Active seed-distributing animals may be the deciding factor in the 

 competition of plant communities pioneering on new land. The large 

 ants of the garigue (Messor harbarus) carry quantities of the seeds .of 

 Rosmarinus, Thymus vulgaris, and other species more than 50 m. and 

 thus hasten the spreading of these species over abandoned land. The 

 T. vulgaris facies of the Brachypodietum ramosi in southern France 

 may often be traced to seed distribution by ants. Similarly, birds and 

 mammals may contribute to the success of certain communities by the 

 distribution of their seeds. 



Even in prehistoric times large game animals probably contributed 

 to floristic equilibrium and to a modification of the vegetation of areas 

 frequented by them. This is the only explanation we have for the wide 

 jind saltatory distribution of the Lappula-Asperugo association, which, 

 characteristic of overhanging rocks in the Alps, is found in exactly 

 identical habitats in the south German foothills. 



Pasturing. — Farmers generally consider that moderate pasturing is 

 beneficial to plant growth. The soil is fertihzed; the distribution of 

 seeds, as well as their germination, is favored. Swiss farmers assert 

 that alpine pastures that are ungrazed for several years become rank 

 with weeds. 



Against this anthropocentric view it must be admitted that pastur- 

 ing always prevents the development of natural vegetation. It 

 retards, or prevents, the establishment and development of the climatic 



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