64 PLANT SOCIOLOGY 



often confined to a certain host or habitat and, much less frequently, to 

 one community exclusively. There are cases reported, however, 

 where fungus parasites are more strictly confined to a certain plant 

 community than their host plants. 



6. Competition is an important factor in developing fidelity. In 

 the severe struggle with better adapted species, the more imperfectly 

 adapted are confined to certain definite communities, although nothing 

 but competition prevents them from occupying other habitats, and 

 they actually do grow in other habitats when competition is removed, 

 as in garden culture. One recalls various characteristic species of 

 cultivated communities in vineyards, grain fields and gardens. Natu- 

 ral vegetation furnishes similar examples, e.g., the Mesobrometum of 

 central Switzerland with its characteristic annual species, such as 

 Arenaria leptodados, Cerastium brachypetalum, Saxifraga tridactylites, 

 Vicia tetrasperma and Myosotis collina. In the moist oceanic cHmate 

 of the alpine foothills the Mesobrometum, because of its open stand 

 (the perennial chamaephytes and hemicryptophytes often cover not 

 more than four-fifths of the ground), is the only grassland in which 

 these annuals can find space to grow. Similar conditions exist in the 

 Cyperus flavescens region of Central Europe, particularly in the 

 Eleocharetum where extreme habitat conditions obtain on account of 

 the short vegetative period after the annual sinking of the water table. 

 This causes a strict selection of a few species exclusive to the habitat 

 (W. Koch, 1926). 



Sociological Indicator Value of the Characteristic Species. — The 

 characteristic species furnish a connecting link between statistical- 

 structural studies and the ecological study of communities. Because of 

 their finer ecological adjustment the group of characteristic species and 

 to a lesser degree the differential species have very high value as 

 ecologic indicators. 



1: The characteristic species are primarily decisive for the floristic individuality 

 of a communityj 



2. They are collectively the best indicators of the ecological condition of the 

 community; 



3. They permit an estimate of the stage of development attained by a 

 community; 



4. They permit the drawing of conclusions as to the present and former dis- 

 tribution of certain communities. 



5. They are of special value in determining the natural affinities of plant 

 communities, thus makmg possible a classification of communities on a floristic 

 basis. 



Indicators of the Specific Ecology of Communities.— Since the 

 characteristic species are more sensitive to the determining ecologic 



