III. INITIAL CAUSES. 



Significance of bare areas. — Seres originate only in bare areas or in those in 

 which the original population is destroyed. They may be continued, with or 

 without change of direction, by less critical modification of the habitat or by 

 the invasion of alien species. It is a universal law that all bare places give 

 rise to new communities, except those which present the most extreme condi- 

 tions of water, temperature, Ught, or soil. Of such there are few. Even fields 

 of ice and snow show algal pioneers, rocks in the driest desert bear lichens, 

 caves contain fungi, and all but the saltiest soUs permit the entrance of halo- 

 phytes. From the standpoint of succession, water is the most important of 

 bare habitats, and it is almost never too extreme for plant life, as is shown by 

 the invasion of the hot springs of Yellowstone Park by various algse. 



Habitats are (1) originally bare or (2) bare by denudation. The former are 

 illustrated by water, land produced by rapid emergence, such as islands, con- 

 tinental borders, etc., lava flows and intrusions, deltas, ground moraines, etc., 

 dunes, loess, etc. Denuded habitats arise in the most various ways, and are 

 best exemplified by bad lands, flooded areas, burns, fallow fields, wastes, etc. 

 The essential difference between the two is that the new area is not alone 

 developmentally different in never having borne a plant commimity, but is 

 also physically different in lacking the reactions due to successive plant popu- 

 lations. The last consideration is of profound importance in the develop- 

 ment of the new vegetation, and serves as a primary basis for distinguishing 

 successions (plate 3, a, b). 



Modiflcations of development- — ^While a new sere can arise only after the 

 destruction of a community in whole or in part, striking changes in the course 

 or rate of succession may occur in existing coromunities. These are only 

 modifications of development, and are not to be mistaken for the beginnings 

 of new successions. A successional stage may persist beyond the usual period, 

 and become a temporary climax, or, more rarely, it may become the actual 

 climax. On the other hand, the rate of development may be accelerated, and 

 certain normal stages may be combined or omitted. New stages are some- 

 times interpolated, or the usual climax may be succeeded by a new climax. 

 The direction of development may itself be changed anywhere in its course, 

 and may then terminate in the usual climax, or rarely in a new one. These 

 are all changes within the succession, and are continuative. They must be 

 kept distinct from the destructive changes, which free the habitat for new 

 invasions and can alone initiate succession. Developmental modifications 

 are produced either by changes in the habitat factors or by changes in the usual 

 course of invasion. It is possible also that the two may act together. The 

 habitat may be modified in the direction of the successional reaction and corre- 

 spondingly hasten the rate of development, or contrary to the reaction and 

 thus reduce the rate, fix an earlier climax, or change the direction. In the 

 case of invasion it is obvious that the failure of the dominants of a particular 

 stage to reach the area would produce striking disturbances in development. 

 Likewise, the appearance of alien dominants or potential climax species would 

 profoundly affect the usual life-history. 



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