72 THE STUDY OF PLANT COMMUNITIES • Chapter IV 



tween presence and Constance. The latter has the advantage of 

 eliminating discrepancies resulting from sampling stands of un- 

 equal size. The lower classes of Constance are more uniform than 

 those of presence, for the larger the area examined the greater the 

 number of incidental species encountered. 



Constancy bears a relationship to the abstract community very 

 similar to that of frequency in the concrete community. The prob- 

 lems of minimal area are similar and can to some extent be reduced 

 by the use of species : area curves as used in frequency determina- 

 tions. Both concepts are concerned with homogeneity, the one 

 with that of the stand, the other with that of the abstract com- 

 munity. If Constance values are divided into five classes and these 

 are diagrammed as for frequency, the results are quite different. 

 Instead of two maxima as in frequency, only the classes represent- 

 ing irregular occurrences are high, and each succeeding higher 

 class is apt to include fewer species. 



Fidelity. — This character is indicative of the degree with which 

 a species is restricted to a particular kind of community. Species 

 may be grouped into five fidelity classes. 



Fid. 1 .—Strangers, appearing accidentally 



Fid. 2.— Indifferents, without pronounced affinity for any 

 community 



Fid. 3.— Preferents, present in several communities but pre- 

 dominantly in one of them 



Fid. 4.— Selectives, found especially in one community but 

 met with occasionally in others 



Fid. 5.— Exclusives, found completely, or almost so, in only 

 one community 



Species with fidelities 3-5 are termed characteristic species in a 

 community. Positive establishment of which species are character- 

 istic is possible only after all communities of a region have been 

 studied sociologically. Approximations can, of course, be made by 

 those of wide experience, but even then the assigned values must 

 be considered with skepticism. When fidelity values are accurately 

 determined, they contribute strongly to the recognition and classi- 

 fication of a community. However, studies of this sort have been 

 so few in the United States that it will be a long time before suf- 



