212 THE STUDY OF PLANT COMMUNITIES * Chapter IX 



States, Dr. Henry C. Cowles (1899) described the development of 

 vegetation on the sand dunes of Lake Michigan. Later (1901) he 

 described the vegetation of Chicago and vicinity, as it is related to 

 physiography, in so logical a fashion that a pattern for studies of 

 community dynamics was established. His papers also served to 

 stimulate similar investigations by others. Beginning at about the 

 same time, the publications of Dr. F. E. Clements, then working in 

 Nebraska, included much that served to shape our present con- 

 cepts of succession. The culmination of his ideas appeared in his 

 exhaustive treatment of the entire subject of plant succession, 56 

 which remains a basic source of reference today. 



THE CONCEPT 



Plant communities are never completely stable. They are char- 

 acterized by constant change, 73 sometimes radical and abrupt, 

 sometimes so slow as to be scarcely discernible over a period of 

 years. These changes are not haphazard, for within a climatic 

 area, they are predictable for a given community in a particular 

 habitat. This means, of course, that similar habitats within a cli- 

 matic area support a sequence of dominants that tend to succeed 

 each other in the same order. Contrasting habitats do not support 

 the same sequence of communities. As a result, any region with 

 several types of habitats will have an equal number of possible suc- 

 cessional trends. 



CAUSES 



A detailed consideration of the relationships of organisms to 

 their environment should make it clear that major changes in the 

 composition of a community can only follow changes in the en- 

 vironment. The specific, immediate cause of a particular change 

 of species may not always be obvious because of the interrelation- 

 ship of controlling factors. Two general types of habitat change 

 may cause differences in the community. Development of the 

 community causes parallel developmental changes of the environ- 

 ment, and physiographic changes can likewise modify the envi- 

 ronment materially. 



Developmental changes of the environment result from reac- 

 tions upon the habitat by the organisms living there. To illustrate : 

 Accumulation of litter affects runoff, soil temperature, and the 



