CAUSES OF SUCCESSION. 



CAUSES OF SUCCESSION. 



Eelation of causes. — Since succession is a series of complex processes, it 

 follows that there can be no single cause for a particular sere. One cause ini- 

 tiates succession by producing a bare area, another selects the population, a 

 third determines the sequence of stages, and a fourth terminates the develop- 

 ment. As already indicated, these four processes — ^initiating, selecting, con- 

 tinuing, and terminating — are essential to every example of succession. As 

 a consequence, it is difficult to regard any one as paramount. Furthermore, 

 it is hard to determine their relative importance, though their difference in 

 r61e is obvious. It is especially necessary to recognize that the most evident 

 or striking cause may not be the most important. In fact, while the cause or 

 process which produces a bare habitat is the outstanding one to the eye, in 

 any concrete case, it is rather less important if anything than the others. 

 While the two existing classifications of successions (Clements, 1904; Cowles, 

 1911) have both used the initiating cause as a basis, it seems clear that this 

 is less significant in the life-history of a climax formation than are the others. 

 This matter is discussed in detail in Chapter IX. It will suffice to point out 

 here that the same sere may result from several initial causes. 



Kinds of causes. — All of the causative processes of succession may best be 

 distinguished as initiating or initial, continuing or ecesic, and stabilizing or 

 climatic. At first thought, the latter seems not to be a cause at all but an 

 effect. As is shown later, however, the character of a successional development 

 depends more upon the nature of the climatic climax than upon anything else. 

 The latter determines the population from beginning to end, the direction of 

 development, the number and kind of stages, the reactions of the successive 

 stages, etc. Initial causes are those which produce a new or denuded soil 

 upon which invasion is possible. Such are the chief physiographic processes, 

 deposition and erosion, biotic factors such as man and animals, and climatic 

 forces in some degree (plate 2, a, b). 



Ecesic causes are those which produce the essential character of vegeta- 

 tional development, namely, the successive waves of invasion leading to a 

 final climax. They have to do with the interaction of population and habitat, 

 and are directive in the highest degree. The primary processes involved are 

 invasion and reaction. The former includes three closely related processes, 

 migration, competition, and ecesis. The last is final and critical, however, 

 and hence is used to designate the causes which continue the development. 



Proximate and remote causes. — In dealing with the causes of development, 

 and especially with initial causes, it must be borne in mind that forces in nature 

 are almost inextricably interwoven. In all cases the best scientific method in 

 analysis seems to be to deal with the immediate cause first, and then to trace 

 its origin just as far as it is possible or profitable. Throughout a climax for- 

 mation, physiography usually produces a large or the larger nmnber of devel- 

 opmental areas. The influence of physiography in this respect is controlled 

 or limited by the climate, which in its turn is determined by major physio- 

 graphic features such as mountain barriers or ocean currents. These are 

 subordinate as causes to the general terrestrial climates, which are the outcome 

 of the astronomical relations between the sun and the earth. As a conse- 

 quence, physiography may weU be considered the immediate initial cause of 



