76 ECESIC CAUSES. 



and ecesis, and the production of new centers from which the species may be 

 ejttended over a wide area. Contrasted with continuous invasion is inter- 

 mittent or periodic movement into distant regions, but this is rarely con- 

 cerned in succession. When the movement of invaders into a community is 

 so great that the original occupants are driven out the invasion is complete. 

 This is characteristic of the major stages of succession, though there are neces- 

 sarily transitions between these, often of such character as to require recogni- 

 tion. Major stages, and especially subclimaxes and climaxes, often undergo 

 partial invasion without being essentially changed. While the permanence 

 of invasion varies greatly, the terms "permanent" and "temporary" are 

 purely relative. In each sere initial and medial stages are temporary in 

 comparison with the climax. The initial stages of a primary sere may last 

 for centuries, but they must finally pass in the course of development. Climax 

 stages are permanent, except in the case of destruction or an efficient change 

 of climate. In the geological sense, however, they are transient stages of 

 the geosere. 



Manner of invasion. — ^Bare areas present very different conditions for in- 

 vaders to those found in plant conamimities. This is due to the absence of 

 competition and often of reaction. Conditions for germination are regularly 

 more favorable in plant communities, but the fate of seedling and adult is 

 then largely determined by competition. Open communities are invaded 

 readily, closed ones only with difficulty, it at all. It is important to recognize 

 that a community is not necessarily open because part of the siuface is bare. 

 Secondary bare areas usually afford maximum opportunity for invasion. This 

 is due partly to the lack of competition, but -especially to the fact that condi- 

 tions are more or less optimum for the germination and growth of a wide 

 range of species. Primary areas, on the contrary, present only extremes of 

 water-content, and thus exclude all invaders except a few pioneers. 



Invasion into a bare area may be lateral, peripheral, or general. It is 

 lateral in all land areas bordered by deep water, siuce successful invaders can 

 reach it only from land commimities. It may be bilateral when the water is 

 shallow enough to contain amphibious species and the area sufficiently wide to 

 permit a gradual change of conditions. When the bare area lies between two 

 different terrestrial associations the movement is regularly from both direc- 

 tions, if conditions are not too extreme. If it is surrounded by an association 

 or consocies, particularly a climax one, the invasion takes place all along the 

 edge. When the area is large the invaders move forward into it by repeated 

 advances, often producing temporary zones. In small areas such a zonal 

 invasion is typical when species invade by propagules. In many secondary 

 areas, especially burns and abandoned fields, the migration is general, and the 

 area is more or less completely covered in the initial stage. 



In all invasions after the first or pioneer stage the relative level of occupants 

 and invaders is critical. A community may be invaded at its level, i. e., by 

 species of the same general height as those in occupation or below or above 

 this level. When invasion is at the same height the level has no effect and the 

 sequence is determined by other features. If it is above the level of the occu- 

 pants, the newcomers become dominant as they stretch above their neighbors 

 and soon give character to a new stage. This is typically the case with shrubs 

 and trees, in which the close dependence of the sequence of stages upon life- 



