II. BASES AND CRITERIA. 

 BASES AND METHODS OF DETERMINATION. 



Fundamental relations. — Plants serve as indicators by virtue of their 

 response to conditions about them. Every plant response has some signifi- 

 cance, the kind and degree of which must be subjects of exact determination in 

 each case. Some responses are obvious, others less evident, while still others 

 are invisible though demonstrable. All these, however, must be referred to the 

 habitat for the decision as to their meaning and their possible use as indicators. 

 It is clear that the causal relation of the habitat to the plant is the primary 

 basis of plant indicators. Each response is the effect of some factor or factor- 

 complex acting as a cause, and is consequently the indication of this factor. 

 The chief task of the investigator is the measurement of responses, and their 

 correlation with measured factors. 



In deciding upon possible bases for an indicator method, physiological 

 responses and physical causes must be given the place of first importance. 

 As further consequences of these must be considered the responses shown in 

 the development and structure of communities, i. e., the basic facts of associa- 

 tion and succession. The method of obtaining the facts in these four great 

 fields will continue to be both empirical and experimental. Experiment will 

 steadily increase in amount and value, but the result will be to refine and direct 

 observation and not wholly to displace it. In fact, the more completely 

 experiment is taken into the field, the more readily will observation reveal 

 the meaning of the innumerable natural experiments brought about by changes 

 of habitat and of climate. In this there is no intention of minimizing the 

 crucial value of experimentation, but rather to widen its scope so that all 

 experiments can be taken into account. This is especially important when 

 one recalls the slow advance in experimentation under natural conditions and 

 the insignificant area covered by it. The possibilities of this method have 

 been strikingly shown for many years at the Alpine Laboratory, where numer- 

 ous examples of natural transplanting in fragmented habitats verify and 

 extend the results of a relatively small number of artificial transplantings. 

 Similar results are to be obtained from natural experiments on a wider scale. 

 The value of Bouteloua gracilis as an indicator of climate was graphically 

 shown in the bad-land levels at Glendive, Montana, in 1917. The drying culms 

 of the current year were just half as tall as those of 1916 which still persisted 

 in the same mat. The rainfall for the two years was 26 and 12 inches, respec- 

 tively. Thus the inevitable adjustment of the short-grass cover to decreased 

 rainfall and water-content furnished results hardly to be surpassed by the 

 most carefully checked experiment. 



In indicator work, as in all adequate investigation, by far the best method 

 is that which uses all sources of information and does not emphasize one to 

 the neglect of others. While the very nature of indicators insures proper 

 consideration of habitat and plant, the study of each species must be accom- 

 panied by that of its associational and successional relations, and all four of 

 these objectives must be reached by the combined use of observation and 

 experiment, in which each must be utilized to the fullest capacity consistent 

 with accurate results. 



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