264 



study and experience for solution as do those of taxonomic classi- 

 fication. The criteria that have been used in delimiting and classi- 

 fying associations have been almost as various as writers upon the 

 subject. 



Jaccard (1902:350) says, "Im allgemeinen ist der Bestand be- 

 stimmt durch die dominirende Art oder Arten". He was the first 

 to set up a mathematical criterion for distinguishing associations. 

 The association- or community-coefiicient (Gemeinschaftscoefficient) 

 is obtained by dividing the number of common species, in the two 

 areas under consideration, by the total number of species^ in them. 

 For example, area A has 100 species, area B has 120 species, 60 of 



which are common to the two areas. Then iqq -|- 120 — 60 ~ ^"^'^ 

 per cent, the community coefficient. For areas which are in the same 

 association and in the same locality this coefficient ought to be fairly 

 high. That even this method has its limitations Jaccard recognized 

 when he said, "Sie entsprechen zwar gewissen Differenzen in den 

 okologischen Bedingungen der verglichenen Territorien, aber es 

 besteht zwischen dem absoluten Werth dieser Differenzen und dem 

 der Gemeinschaftscoefficienten keine mathematische Proportionalitat." 

 The same method was independently arrived at by Professor S. A. 

 Forbes in a statistical study of Illinois Fishes.* 



Besides the floristic composition told by mathematical methods, 

 associations are usually appreciated by any or all of the following 

 characteristics: (i) the presence of one or more dominating spe- 

 cies, (2) the presence of tension lines at their boundaries, (3) the 

 presence of evidence of dynamic succession, usually shown at or near 

 the tension line, (4) the presence of a uniform environment, (5) the 

 inability of species of different associations to mix, and (6) the pres- 

 ence of similar vegetative forms and environmental adaptations. 



The association itself is composed of one or more principal or 

 dominating species, termed the dominant species, which give the fun- 

 damental character to the association. In some associations the donv 

 inant species may be the only species, but more usually the interstices 

 between the plants of the dominant species are occupied by what are 

 termed secondary species. Frequently secondary' species by their 

 showiness give the color tone to the association. Where this varies 

 from season to season, these different appearances are termed the 

 seasonal aspects. Succession occurs when, in a given area, one asso- 

 ciation displaces another. Successions trend toward a definite cli- 



*0n the Local Distribution of Certain Illinois Fishes: An Essay in Statistical 

 Ecology. Volume VII of this series, Article 8. 



