CHAPTER XIV 

 INVASION AND SUCCESSION 



287. Invasion. The movement of one or more plants from 

 one area into another and their estabUshment in the latter con- 

 stitute invasion. Invasion may concern an individual or a num- 

 ber of individuals of one or more species. It may take place 

 between plant groups of any kind, though it is most conspicuous 

 between different formations. Invasion is a regular occurrence 

 between adjacent formations, but it also takes place into re- 

 mote ones, as a result of long carriage by wind, water, birds, rail- 

 roads, or vessels. 



iMigration and ecesis are both necessary factors in invasion. 

 The former carries the spore or seed into the area to be invaded. 

 In ecesis the spores or seeds germinate and grow and the new 

 plants become established after more or less adjustment. In 

 practically all terrestrial forms invasion is possible only when 

 migration is followed by ecesis. In the case of aquatic forms 

 distributed regularly by water, ecesis is of little or no importance, 

 and migration often becomes identical with invasion. 



288. The manner of invasion. Since ecesis is largely con- 

 trolled by competition, the manner and amount of invasion are 

 determined by the presence or absence of vegetation. Soils with- 

 out vegetation are either new or denuded, while areas covered 

 with vegetation are either open or closed. Each kind of area 

 presents different conditions to invaders. Naked habitats, i.e., 

 rocks, talus, dunes, are usually invaded with great difficulty, 

 because of their xerophytic character, and the slowness with which 

 invaders secure a foothold, in spite of the lack of competition. 

 Denuded habitats usually offer the best opportunities for invasion. 

 They ordinarily contain a large number of disseminules ready to 

 spring up wiien the original occupants are destroyed. The sur- 



270 



