SUCCESSION 239 



plants invade grassland, when the trees or shrubs become more or less uni- 

 formly scattered in an open woodland or open thicket. Here the grassland 

 takes on an altogether different appearance superficially, though it is usually 

 unchanged, except beneath and about the invaders, where either adaptation 

 or extermination results. Finally, it should be borne in mind that the inva- 

 sion of a particular formation, especially in the case of layered thickets and 

 forests, often takes place at two levels, at the height of the facies and below 

 the facies. 



INVESTIGATION OF INVASION 



287. The methods to be used in the study of invasion are those already 

 described elsewhere. The migration circle is of the first importance because 

 it makes it possible to secure an accurate record of actual movement. Quad- 

 rat and transect are valuable, but from their nature they are more service- 

 able for ecesis than for migration. All of these should be of the permanent 

 type, in order that the fate of invaders may be followed for several years at 

 least. Permanent areas furnish evidence of the changes wrought in the 

 actual vegetation, while denuded ones can serve only to show the potential 

 migration and ecesis of the constituent species. Transition zones and areas 

 are special seats of invasion ; they are best studied by means of the belt tran- 

 sect and the ecotone chart. The movement of a line of invaders or of scat- 

 tered outposts is traced by the use of labeled stakes at the points concerned. 

 It is clear that this method will yield conclusive data in regard to the great 

 invasions between regions, such as the movement of species guilds, the ad- 

 vance of the forest frontier, etc. When invasion is scattered, factor instru- 

 ments can not be used to advantage, but where the invading line is well 

 marked, or where extra-formational areas occur, a knowledge of the phys- 

 ical factors is a great aid. 



An invasion that has been completed can not be studied in the manner in- 

 dicated. A method of comparison must be used, in order to determine the 

 original home of the invaders. For this an exact knowledge of the contig- 

 uous formations and of the abundance of the species common to all is a pre- 

 requisite. With this as a basis, it is usually a simple task to refer all the 

 species of the formation cdncerned to their proper place in the groups, indig- 

 enous, derived, and adventitious. . 



SUCCESSION 



288. Concept. Succession is the phenomenon in which a series of inva- 

 sions occurs in the same spot. It is important, however, to distinguish clearly 

 between succession and invasion, for, while the one is the direct result of the 



