30 CONCEPT AND HISTORY. 



indicator values, the individual will play a necessary part. Its indications 

 are more minute and subject to greater error. While further quantitative 

 work will increase the accuracy and usefulness of individual indicators, at 

 present they are distinctly secondary. In fact this will probably always be 

 their relative position, inasmuch as they will serve to refine the major indica- 

 tions of species and communities. The question of species and community 

 values is much simpler than appears at first. It is not a matter of employing 

 one to the exclusior^ of the other, but of taking advantage of their complemen- 

 tary relation. There can be no doubt that the community is a more reliable 

 indicator than any single species of it. This is a necessary consequence of 

 the essential harmony of the important species as to physiological response 

 and factor control. The community not only affords a better norm for the 

 major indications, but it is likewise, so to speak, more finely graduated and 

 hence more sensitive, owing to the fact that no two of its dominants or sub- 

 dominants are exactly equivalent. It is also a better indicator of the whole 

 habitat, since it levels the variations from one point to another. 



The indicator value of a species depends primarily upon its r61e in the com- 

 munity. A secondary or subordinate species may be of little or no practical 

 value, in spite of the general rule. It merely accompanies the major species, 

 or as a subordinate accepts the conditions made by them, thus indicating minor 

 differences. It assumes practical value only in case of the destruction of the 

 dominants, as often happens in overgrazing and in deforestation. Even here 

 the real meaning of a secondary species is due to the fact of its association with 

 more important indicators. The significant species are the dominants and 

 subdominants which give character to definite communities. With these the 

 species and community values approach closely or merge completely. In fact 

 such species give their typical indication only where dominant. Their inci- 

 dental or scattered occurences may have meaning, but it is not the normal 

 one. In the present stage of our problem, then, attention should be focussed 

 upon the dominants and subdominants of the climaxes and their various seres. 

 When these have been correlated on the one hand with their efficient factors 

 and on the other with practical processes in agriculture, grazing, and forestry, 

 it will become evident whether an analysis of secondary species is profitable. 

 The dominant may well be regarded as the real basis of indicator study, so 

 conmianding is its r6le in the processes of vegetation (plate 5, a). 



Sequences. Every indicator owes its value to its position in a cause-and- 

 effect sequence. With this, however, must always be associated correspon- 

 dence with another cause-and-effect sequence. The value of the compass- 

 plant, Silphium ladniatum, as an indicator of corn production rests not merely 

 upon its preference for relatively moist rich soils, but also upon an experiential 

 knowledge at least of the production capacity of such soils. Up to the present, 

 our knowledge of indicators rests chiefly upon the basis of experience. In 

 emphasizing the point that this alone is usually inaccurate and insufficient, 

 there is no intention of failing to give it proper recognition. It is an essential 

 and often the critical part of indicator research, but its true value can be 

 obtained only by correlation with the other steps of the process. As a conse- 

 quence, it makes Uttle difference whether the approach has been through 

 experience or investigation. Both must be taken into account before the 



