COMMUNITY SUCCESSION AND DEVELOPMENT 



563 



of diverse rank, extent, and complexity. 



For example, any assemblage of organ- 

 isms having a relatively constant habitus 

 and occuping a certain type of habitat or a 

 series of habitats, irrespective of area or 

 volume occupied, may be considered as a 

 "community." The forest with its associated 

 animals is a community, as is the coral reef. 

 Both have many features in common, but 

 they are communities of different rank. The 

 former is relatively independent of adjacent 

 communities, whereas the latter is depend- 

 ent upon the adjacent water for its food 

 supply. Forests in general are more closely 

 similar to the sea than to the coral reef, 

 and both sea and forest are major com- 

 munities in the sense defined previously. 



Consequently, any discussion of com- 

 munities presupposes the philosophic con- 

 templation of the content of the concept 

 and its applicability. We recognize that 

 nearly innumerable communities, and types 

 of communities, may be considered and 

 studied within the frame of the major com- 

 munity concept. In this book the term 

 "community" is used both in its loose, tra- 

 ditional sense, and in keeping with the 

 philosophically consistent concept of the 

 major community, as defined on page 436. 



It is obvious that lability is a char- 

 acteristic of life, whether at the cellular, 

 organismal, populational, or community 

 level. Since communities are composed of 

 organisms and their environments, it follows 

 that communities change. We have ex- 

 amined certain of these changes in terms 

 of organismal response (Chaps. 4-17), pop- 

 ulations (Chaps. 18-24), community me- 

 tabolism (Chap. 27), and community peri- 

 odism (Chap. 28). 



In addition, the community undergoes an 

 orderly series of broader changes. It comes 

 into being, grows, may reproduce itself by 

 a mass budding in some direction, may 

 shift its boundaries; it then matures, be- 

 comes senescent, and may perish. When a 

 community dies, or more usually as it is dy- 

 ing, its area is occupied by another com- 

 munity. This process is repeated, commu- 

 nity succeeding community, until a rela- 

 tively stable community occupies the area. 

 This is a bald statement of the process of 

 ecological succession, and stands in need 

 of examination, as there are numerous ex- 

 ceptions, variations, and controversial points 

 involved. 



In general, ecological succession is an 

 orderly, progressive sequence of replace- 

 ment of communities over a given point, 

 area, or locality. Research has demonstrated 

 that this sequence is directional, is capable 

 of description, that the rate of change can 

 be measured under certain conditions, and 

 that the end product may be recognized or 

 predicted in some cases. In succession, the 

 entire sequence of communities, from its 

 inception to the terminal product, is spoken 

 of, collectively, as the sere. Changes taking 

 place within the sere are spoken of as serai 

 changes or phenomena. The early stages of 

 a sere are termed pioneer communities, and 

 the relatively stable end product is known 

 as the sere climax. 



The causes responsible for succession are 

 numerous, complex, often interacting, and 

 not fully understood. In the first place, such 

 causal influences may be separated as 

 physical and biotic. 



Many physical factors are in continuous 

 operation. For example, there are the wide- 

 spread, slow geologic and geographic proc- 

 esses of erosion and deposition by wind, 

 precipitation, flowing water, and wave ac- 

 tion. Erosion and deposition affect the in- 

 animate and animate portions of a com- 

 munity, both directly and indirectly, and 

 produce physiographic succession. This 

 form of succession was clearly recognized 

 by Cowles (1899, 1901, 1901a): 



"Having related the vegetation largely to 

 topography, we must recognize that topography 

 changes, not in a haphazard manner, but 

 according to well-defined laws. The processes 

 of erosion ultimately cause the wearing down 

 of the hills and the filling up of the hollows. 

 These two processes, denudation and deposi- 

 tion, working in harmony produce planation; 

 the inequalities are brought down to a base 

 level. The chief agent in all of these activities 

 is water, and no fact is better established than 

 the gradual eating back of the rivers into the 

 land and the wearing away of coast lines; the 

 material thus £;athered fills up lakes, forms the 

 alluvium of flood plains, or is taken to the 

 sea. Vegetation plays a part in all these pro- 

 cesses, the peat deposits adding greatly to the 

 rapidity with which lakes and swamps are filled, 

 while the plant covering of the hills, on the 

 contrary, greatly retards the erosive processes. 

 Thus the hollows are filled more rapidly than 

 the hills are worn away. As a consequence of 

 all these changes, the slopes and soils must 

 change; so, too, the plant societies, which are 

 replaced in turn by others that are adapted to 

 tlip new conditions." 



