INVASION. 75 



below the dominant one develop in the order of position, the lowermost first, 

 before the shrubs have developed their foliage. This effect is of com'se seen 

 most clearly in the aspects of deciduous forests, in which the lowest layer 

 consists chiefly or wholly of prevernal or vernal species. A similar and some- 

 times equally conspicuous sequence of layers occurs in prairies (plate 19, a, b). 



INVASION. 



Nature and role. — ^Invasion is the complete or complex process of which 

 migration, ecesis, and competition are the essential parts (Clements, 1904: 

 32; 1905:210; 1907:270). It embraces the whole movement of a plant or 

 group of plants from one area into another and their colonization in the latter. 

 From the very nature of migration, invasion is going on at all times and in all 

 directions. For our purpose it is necessary to distinguish between invasion 

 into a bare area and into an existing plant community. The former initiates 

 succession, the latter continues the sere by producing successive stages until 

 the climax is reached. Invasion does not cease at this point necessarily, 

 especially in the presence of artificial processes. As a rule, however, invasion 

 into a climax community is either ineffective or it results merely in the adop- 

 tion of the invader into the dominant population. From the standpoint of 

 succession only those invasions need be considered which people bare areas 

 or produce a new developmental stage. It is obvious that practically all 

 invasion in force is of this sort. 



Effective invasion is predominantly local. It operates in mass only between 

 bare areas and adjacent cormnimities which contain species capable of pioneer- 

 ing, or between contiguous communities which offer somewhat similar condi- 

 tions or contain species of wide range of adjustment. Invasion into a remote 

 region rarely has any successions! effect, as the invaders are too few to make 

 headway against the plants in possession or against those much nearer a new 

 area. An apparent exception is foimd in the case of ruderals introduced into 

 new countries by man, but these rarely come to be of importance in succession 

 until they have been domiciled for many years. The invasions resulting from 

 the advance and retreat of the ice during glacial times were essentially local. 

 They spread over large areas and moved long distances only as a consequence 

 of the advance or withdrawal of the ice: The actual invasion at any one time 

 was strictly local. Invasion into a new area or a plant community begins with 

 migration when this is followed by ecesis. In new areas, ecesis produces reaction 

 at once, and this is followed by aggregation and competition, with increasing 

 reaction. In an area already occupied by plants, ecesis and competition are 

 concomitant and quickly produce reactions. Throughout the development 

 migrants are entering and leaving, and the interactions of the various processes 

 come to be complex in the highest degree. 



Kinds of invasion. — ^Local invasion in force is essentially cmdimjums or 

 recurrent. Between contiguous communities it is mviual, imless they are too 

 dissimilar. The result is a transition area or ecotone which epitomizes the 

 next stage in development. By far the greater amount of invasion into exist- 

 ing vegetation is of this sort. The movement into a bare area is likewise 

 continuous, though it is necessarily not mutual, and hence there is no ecotone 

 during the earlier stages. The significant feature of continuous invasion is 

 that an outpost may be repeatedly reinforced, permitting rapid aggregation 



