APPLIED ECOLOGY 351 



for agricultural purposes. With increasing knowledge, their selec- 

 tions became more effective as is indicated today by lands that 

 have been abandoned and that have remained so. In any agricul- 

 tural region, an experienced farmer knows the characteristics of 

 soils and habitats supporting local peculiarities of vegetation, or 

 often only a single indicator species. 



Such practices and beliefs are usually the result of trial and 

 error experiences, as well they must be, until the responses of a 

 crop plant are tested under the conditions indicated by native 

 vegetation. The knowledge has often been acquired after costly 

 experience. If the requirements of an introduced plant are known 

 and the characteristics of the habitats of native species are studied, 

 the guessing may be reduced. Selection of native species as indica- 

 tors of local conditions and fitting the ecological requirements of 

 appropriate cultivated plants to these conditions involves ecologi- 

 cal methods and thinking. Actually this is not easily accomplished, 

 because of our still limited knowledge of the ecology of both na- 

 tive and cultivated plants. It suggests the possibilities of the indica- 

 tor method, however, in an applied field. 



The scope of possible uses of indicators involves much of the 

 entire field of ecology, which necessarily limits the discussion 

 here. Clements' 57 exhaustive treatment explores most of the possi- 

 bilities of their application, and many of these we have considered 

 in other connections. Consequently, only certain practical aspects, 

 in which they have been successfully applied or might be further 

 expanded will be discussed. The available source material has been 

 brought together, and a review is available on the modern status 

 of the concept and its application. 212 



It may sometimes be difficult to recognize or select indicator 

 species. Those with restricted distributions and those tolerating 

 only narrow ranges of habitat conditions should be most useful. 

 Such plants should show responses to. minor habitat differences. 

 Thus it follows that similar local conditions in different climatic 

 areas would probably support different indicators. Also the same 

 species might not always be indicative of the same things through- 

 out its range. Differences in geological or cultural history might 

 make it necessary to interpret the significance of an indicator since 

 it need not always be the same. It is rather generally agreed that 



