IV. ECESIC CAUSES. 



Nature. — As has been indicated, succession owes its distinctive character 

 to the communities which succeed each other in the same area. This character 

 is given it by the responses or adjustments which the conmiunity makes to 

 its habitat, namely, migration, ecesis, competition, and reaction. These are 

 the real causes of development, for which a bare area does little more than fur- 

 nish a field of action. To them is due the rhythm of succession as expressed 

 in the rise and fall of successive populations. They may well be regarded as 

 the paramount causes of succession, since their action and interaction are 

 the development of vegetation. As every sere must begin with a denuded 

 area and end in a climax, it is clearer to treat them along with initial causes 

 and climax causes. 



AGGREGATION. 



Concept and r6Ie. — ^Aggregation is the process by which germules come to 

 be grouped together (Clements, 1905:203; 1907:237). It consists really of 

 two processes, simple aggregation and migration. These may act alone or 

 together, but the analysis is clearer if each is considered separately. By sim- 

 ple aggregation is understood the grouping of germules about the parent plant. 

 Even in the fall of seeds there is often some movement away from the parent 

 plant, but it can not properly be regarded as migration, unless the seed is 

 carried into a different family or into a different portion of the same colony 

 or clan. The distinction is by no means a sharp one, but it rests upon two 

 factors of much importance in vegetation. The first is that movement within 

 the parent area bears a different relation to ecesis from movement beyond the 

 parent area. The second fact is that simple aggregation increases the indi- 

 viduals of a species and tends to produce dominance, while migration has the 

 opposite effect (plate 14 a). 



Simple aggregation may operate by seeds and fruits, by propagules, or by 

 both. The method of aggregation plays an important part in determining 

 the germules in secondary areas, and in the initial stages of a sere. In this 

 respect it is essentially like migration, and will be considered in connection 

 with the discussion of the parts used as migrules. 



Eflfects of simple aggregation. — Aggregation usually modifies the composition 

 and structure of existing communities. This effect is seen most strikingly 

 where the vegetation is open, though it is readily disclosed by the quadrat in 

 closed communities. The increase of population in the case of the pioneers 

 of a bare area is mainly a matter of aggregation. Conspicuous examples of 

 this are found in areas with unstable soU, such as gravel-slides, blow-outs, 

 bad lands, etc. The influence of aggregation is especially important in com- 

 munities which are destroyed by fire, cultivation, etc. In many instances 

 the change in soil conditions is slight, and the course of succession is deter- 

 mined by the number of germules which survive. If the number is large, as 

 in certain forest areas, the resulting sere is very short, consisting only of the 

 stages that can develop while the trees are growing to the size which makes 

 them dominant. When the niunber of aggregated germules is small or none, 

 the selective action of migration comes into play, and the course of develop- 

 ment is correspondingly long. 



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