INVASION AND SUCCESSION 273 



vasion. Partial invasion takes place when the number of in- 

 vaders is sufficiently small that they may be adopted into the 

 formation without changing it materially. Partial invasion is 

 more freqvient though less conspicuous than complete invasion. 

 When an invader persists for a few years and then disappears, 

 invasion is temporary. This is usually the case in the earlier 

 stages of succession which replace each other more or less rapidly. 

 Permanent invasion occurs when a species becomes permanently 

 established in a more or less stable formation. It is characteristic 

 of the grassland and forest stages of succession. 



291. Indigenous and derived species. Species or forms that 

 have arisen within a formation, or have belonged to it since its 

 origin, are indigenous. Species that have invaded the formation 

 at a later date are derived. The latter are termed vicine when 

 they are fully established invaders from adjacent formations or 

 regions, and adventitious when they have come from distant for- 

 mations. Derived species which are unable to establish them- 

 selves permanently are adventive. 



Experiment 69. Invasion. Stake a quadrat in an area denuded by 

 flooding, or in consequence of cultivation or building. Trace the en- 

 trance of invaders, and determine their source in so far as possible. 

 Stake a second quadrat in the transition area between two formations, 

 and determine to which formation the various species belong. 



292. Succession. The process by which a series of invasions 

 occurs in the same spot is termed succession. This is always 

 the result of invasion, but not all invasion leads to it. The num- 

 ber of invaders must be large enough or their effect sufficiently 

 controlling to bring about the gradual disappearance of the orig- 

 inal occupants. Partial or temporary invasion rarely causes 

 succession. On the other hand, it is the regular consequence 

 of complete and permanent invasion. Succession depends pri- 

 marily upon invasion in such quantity and of such kind that the 

 reaction of the invaders upon the habitat prepares the way for 

 further invasion. The characteristic presence of stages, or for- 

 mations, in a succession is due to invasion and reaction. In a 

 denuded habitat, for example, migration from the adjacent for- 

 mations is constantly taking place, but a relatively small num- 

 ber of migrants become established. These reach a maxinuun 

 development in size or number, and in so doing react upon flic 



