CONCEPT. 33 



The threefold basis of ecology is factor, function, and form (Clements, 1907: 

 1). As a consequence, every ecological fact has its indicator significance, 

 and it becomes possible to determine these just as rapidly as factor correla- 

 tions are made. The chief objective for the student of indicators is the cause- 

 and-effect relation, and his chief task to show how effects may be used as signs 

 of their causes. In a sense, the use of indicators reverses ecological procedure 

 inasmuch as it leads from effects to causes. Sooner or later it involves a more 

 or less complete system of reading all the evidence afforded by the responses 

 of plants and animals, whether as individuals or communities. 



With respect to its application, the scope of indicator work is far-reaching. 

 It not only furnishes a basic method in ecology, and especially in succession, 

 but it is also equally applicable in paleo-ecology. Because it gives us the 

 judgment of the plant upon the physical factors of the habitat, it is indispens- 

 able to studies of soil and climate in so far as they have to do with vegetation. 

 For the same reason, it is invaluable in land classification, and to the great 

 plant industries, agriculture, grazing, and forestry. While this is truest of 

 new regions, it holds to some degree for older agricultural communities as well. 

 It applies with especial force to the great unoccupied or poorly utilized inte- 

 riors of other continents, such as South America, Africa, Asia, and Australia, 

 and is not without meaning for large stretches in Europe. In short, wher- 

 ever plants grow, in field, forest, grassland, or desert, indicator results are 

 always of some, and usually of paramount, importance. 



In their relations to succession and to climatic cycles, plants exhibit some 

 of the most important indicator values. These involve quantitative relations 

 of abundance and growth which in conjunction with factor determinations 

 will give to ecology an accuracy and certainty more and more approaching 

 those of the physical sciences. As a consequence, it will become increasingly 

 possible to definitize ecological processes and principles, and to use them as a 

 basis for accurately forecasting the behavior of plants under changed condi- 

 tions. Such prophecy is possible at present in any region where an adequate 

 study of succession has been made. Its scope will be extended and its proba- 

 bility increased in just the proportion that instrumental, quadrat, and devel- 

 opmental studies of vegetation become the rule. 



Materials. — As has been suggested earlier, while every study of the actual 

 relation between habitat and plant is a possible source of indicator materials, 

 only those are of real value which are based upon instrumental, quadrat, or 

 successional investigations. The permanent foundation of indicator research 

 must be laid by those studies which employ all three methods. For these 

 reasons the published sources of indicator material are relatively few and 

 recent. They are largely American and are confined almost wholly to the 

 period since 1900. In fact, adequate ecological studies having indicator values 

 as their avowed objective are all subsequent to 1910, and are largely due to the 

 appearance of Shantz's paper on the indicator value of natural vegetation 

 in 1911. As a consequence, the present treatise is of necessity based primarily 

 upon the investigations of the author during the past 20 years and of Shantz 

 for the last decade or more. While these have had indicator plants as a 

 definite objective only since 1908, the preceding 10 years of instrument, quad- 

 rat, and succession work were an intrinsic part of the investigation. 



