III. KINDS OF INDICATORS. 



Basis of distinction. — Each plant or community serves as the immediate 

 indicator of a factor or group of factors. As a consequence, it may also be 

 employed to indicate the process or agency which causes or modifies the 

 particular factor, as well as that in which the factor or habitat is involved. 

 When the process is one set up or controlled by man, the plant likewise becomes 

 an indicator of practice, and gives direct service in land classification, agri- 

 culture, grazing, and forestry. The relations of the plant or community to 

 process and practice are direct corollaries of the basic principle that each is 

 the best possible measure of the conditions under which it grows. Such 

 measures merely require correlation with a particular process or practice to 

 be of immediate service. This is the inevitable sequence, whether indicator 

 values are the result of actual experience or the outcome of scientific investi- 

 gation. In the latter case, the correlation is merely more detailed or more 

 definite. Thus, while they all spring from the basic relation of plant or com- 

 munity to habitat, it appears desirable to distinguish indicators with respect 

 to the use made of them. On this basis, they may be recognized as factor 

 indicators, process indicators, or practice indicators. Furthermore, the 

 development of the field of paleo-ecology makes it desirable to extend the 

 application of indicator principles to the geological past. The sequence of 

 indications is essentially identical, but the results must be inferred from 

 present-day investigations, and hence it is desirable to speak of paleic indi- 

 cators in this connection. 



FACTOR INDICATORS. 



Basis and kinds. — Every habitat is a complex in which the factors are 

 almost inextricably interwoven. Each factor influences every other factor, 

 and is in turn affected by it. This relation should never be lost sight of, since 

 it is essential to the proper understanding of every factor indicator. Never- 

 theless, some factors are of such paramount importance in the habitat- 

 complex that it is desirable to relate the plants to them directly. This is 

 particularly true of the direct factors, water, light, temperature, solutes, and 

 soil-oxygen. The indirect factors, soil, slope, exposure, wind, and altitude, 

 can act only through these, but they too may be connected with plants as 

 indicators, whenever they exercise a compelling effect upon a direct factor. 



Each factor leaves a distinct impress upon a plant or community in propor- 

 tion to its intensity and the plant's habitual requirements. The plant becomes 

 an indicator of a particular factor to the more or less complete exclusion of 

 others only when the factor exercises the paramount limiting effect. This is 

 regularly the case when it is present in marked excess or deficiency, and hence 

 a factor indicator usually denotes one extreme or the other, or a tendency 

 toward it. Even in such cases, some at least of the other factors are con- 

 cerned in producing the particular intensity of the limiting factor or are 

 themselves affected by it. Consequently, each factor indicator not only 

 denotes the controlling or limiting factor, but also a sequence of factors related 

 to it either as causes or effects. A hydrophyte indicates deficient aeration as 

 well as excessive water-content, while a xerophyte as a rule marks high tem- 

 peratures and low humidity as well as low water-content. In some instances, 



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