April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 169 



I would describe as developmental any succession which is marked by definite 

 progression toward a climax. 



Criteria of Classification. — Recognizing, first of all, that every individual 

 association is a member of a particular developmental series, the classification 

 of plant associations from the dynamic point of view has to do fundamentally 

 (1) with the relation of the individual association to the climax of its par- 

 ticular series, and (2) with the recognition and coordination of different 

 types of succession in their relation to cause and trend, origin and climax. 



B. Types of Succession in Relation to Cause 



Different successional series may be grouped, first of all, with reference 

 to the various agencies by which the succession is brought about — i.e., with 

 reference to the underlying causes of succession. From the causational point 

 of view three types of succession are commonly distinguished, vie, biotic, 

 physiographic (or topographic 52 ), and climatic. Biotic succession is brought 

 about wholly through the influence of plant and animal agencies. Physi- 

 ographic succession is instituted primarily by changes in physiography — by 

 changes either in topography or in the physical or chemical nature of the 

 substratum. Climatic succession is caused by changes in climate. 53 To these 

 three types may be added two others, viz., anthropeic and pyric. Anthropeic 

 succession is brought about through the influence of human agencies, pyric 

 succession through the influence of fire. 



In the series of far-reaching ecological transformations which the vegeta- 

 tion of the earth has undergone during the long course of geologic time 

 climatic change unquestionably has played a leading part, and second only 

 has been the part played by physiographic change. But while the influence 

 of climatic change in instituting marked transformations in vegetation may 

 sometimes be expressed within ecologically brief periods of time — periods 

 which are measured in centuries or even in years — on the whole, in its relation 

 to the more familiar successional phenomena of the present day, it seems to 

 be of relatively subordinate importance. The same may be said of the influ- 

 ence of physiographic change, except for those comparatively localized situ- 

 ations, as along streams and seacoasts, where active erosion or deposition is 

 taking place. The vast majority, at any rate, of the successional changes 

 with which we are immediately concerned are instituted primarily by biotic 

 agencies, secondarily by anthropeic and pyric agencies. In attempting, there- 

 fore, to solve the relatively contemporaneous problems of dynamic plant soci- 



53 The concept of "topographic" succession, originally proposed by Cowles ('01), 

 I have here modified to the extent of including (under the head of physiographic succes- 

 sion) successional phenomena which are due to changes in soil as well as those due 

 to changes in topography. 



03 For discussion of these three types of succession see especially Cowles ('01 and 

 '11). The various habitat factors concerned are outlined more fully in section III of the 

 present paper. 



