FACTOR INDICATORS. 77 



two or more factors appear to be equally important, and the plant indicates 

 all of them. An excellent example of this is seen in alpine plants, where tem- 

 perature, water-content, and humidity are of almost equal importance, and 

 wind and pressure of much significance. The situation may be taken to 

 represent the factor-complex, and such plants may be said to indicate high 

 altitudes. 



Quantitative sequences. — It has already been pointed out that practically 

 every species has an optimum habitat, in which it exhibits its typical indicator 

 value. Outside the optimum or habitual habitat, it has a narrow range in the 

 direction of less favorable conditions for it, and a wider range in that of more 

 favorable conditions. The mere presence of a species or even of a community 

 can not be taken as evidence of its normal indicator value. Its actual value 

 can be determined only by reference to the normal habitat as well as to the 

 plants associated with it. It is this which makes dominance of the first 

 importance in arriving at indicator results. A plant is dominant only within 

 the range of essentially optimum conditions, and its control decreases in both 

 directions, but most rapidly toward less favorable ones. The behavior of the 

 individual plants is in close accord with these changes in abundance. The 

 species has its most typical form where it is dominant, and changes in size and 

 form usually furnish clear indications of departures from the optimum habitat 

 toward either extreme. Subdominance follows the same rules and has similar 

 values, though these are less striking than in the case of the dominants. In 

 the tall-grass prairies, the societies often approximate the value of dominants, 

 but in woodland and forest they are always strictly subordinate, and their 

 indications serve only for a minute analysis of the general conditions of the 

 forest. 



In the present condition of quantitative studies, serai and topographic 

 sequences must furnish the chief source of the indicator values of dominants 

 and subdominants. This will probably always be true to a large degree, but 

 the rapid growth of quantitative methods will afford a more detailed basis, 

 and one which can be understood in terms of factors as well as of plants. In 

 this connection, it must be recognized that a floristic census has slight value, 

 and that accurate results can be obtained only by the use of exact methods 

 which have dominance and sequence as their chief objectives. The floristic 

 outlook upon vegetation is a survival of the early days of distributional 

 plant-geography, and it must steadily decrease in importance as ecology 

 becomes truly quantitative in method and result. 



Climatic and edaphic indicators. — Every factor plays a part in the develop- 

 ment of a community as well as in the control of its final condition. In the 

 developmental habitats the local conditions, especially those of the soil, are 

 paramount, while in climax ones the general climatic factors are controlling. 

 The local or edaphic conditions find their expression in the serai dominants 

 and subdominants, and the communities which they constitute. The wide- 

 spread climatic conditions are reflected in the climax formation, associations, 

 and societies. As a consequence, it frequently becomes desirable to speak of 

 climatic and edaphic indicators. Certain factors, such as water and tem- 

 perature, will be represented by both climatic and edaphic indicators. Others, 

 such as light, solutes, soil oxygen, are primarily edaphic, while still others, 

 such as wind and pressure, may be either local or general. In the use of these 



