BASES AND METHODS OF DETERMINATION. 47 



THE ASSOCIATIONAL BASIS. 



Nature of association — The association of two or more species in a community 

 is due to one or two of the following three reasons: (1) general similarity of 

 functional response to controlling factors; (2) dependence upon the reactions 

 of the dominants modifying these factors; (3) dependence upon the autophytes 

 as hosts or matrices. The last two reasons also explain as a rule the presence 

 of the animals of a community as well. Hence it is obvious why one species of 

 a community should indicate the actual or probable presence of the others 

 regularly associated with it, and likewise the corresponding factors. This 

 principle is susceptible of extended application, but it is nowhere more striking 

 than in the case of relict herbs of a former forest. Though axiomatic, it must 

 be used with some care, since no two species are exactly alike in response and 

 indication, and since successional factors often enter to cause confusion. 



The occurrence of a dominant indicates not only the presence or possibility 

 of its associated dominants, but also that of the related subdominants, second- 

 ary species, hysterophytes, and animals. This is as axiomatic as it is patent in 

 the case of an actual community in the field. This relation becomes of real 

 indicator significance where the community is partially or largely destroyed, 

 when it is rapidly changing, or is but incompletely known, especially in the case 

 of fossil vegetation. A subordinate species likewise indicates other subordi- 

 nate species as well as the controlling dominants, except in those plants which 

 occur in two or more associations or formations, as well as in different serai 

 stages. Even hysterophytes have a distinct indicator value when they are 

 restricted to particular hosts. Moreover, it is clear that the associational rela- 

 tion signifies that animals may often be indicators of plants, as well as plants 

 of animals. 



Dominants. — A dominant is the most important of all indicators. This is due 

 to several reasons. The first of these is that it receives the full impact of the 

 habitat, usually throughout the growing period. The second reason is that it 

 reacts upon the controlling factors, and thus modifies the response of its asso- 

 ciates. It also marks the progress of succession and consequently is bound up 

 in a sequence of dominants, with the result that it affords both developmental 

 as well as associational indications. In addition, it shows great abundance 

 over extensive areas and occupies a wide range. In fact, its very dominance 

 is the sign of its success under the conditions where it controls. However, it 

 is necessary to recognize that a dominant species is not always dominant, and 

 that its control may be local and developmental in parts of its range, while it is 

 extensive and climax in the main portion. Bouteloua gracilis is one of the most 

 exclusive of climax dominants in its typical area, the short-grass association of 

 the Great Plains, but it becomes a co-dominant or merely a successional one in 

 the related associations of the grassland formation, and on the edge of adjacent 

 climaxes, such as the chaparral and the sagebrush. In the Stipa-Koeleria 

 prairies it is subclimax on the ridges and drier slopes, while in the Aristida- 

 Bouteloua desert plains it is usually subclimax also, but in the valley plains and 

 swales it is truly climax. In all three associations it possesses indicator value 

 as a dominant, but this value is different in each one, both as to its associates 

 and the relative conditions. Near the edge of its range it loses its dominance 

 and becomes merely a subordinate member of the community with a greatly 

 modified or restricted significance. 



