BARRIERS 59' 



case of tumble-weeds and other anemochores, clinochores, etc. 

 A marked effect of vegetation in decreasing invasion arises 

 from the closed association typical of stable formations and of 

 social exclusive species. In these, the occupation is so thor- 

 ough and the struggle for existence so intense that the inva- 

 ders, though fitted to grow under the physical factors present, 

 are unable to compete with the species in possession for the 

 requisite amount of some necessary factor. Closed associa- 

 tions usually act as complete barriers, while open ones 

 restrict invasion in direct proportion to the degree of occu- 

 pation. To this fact may be traced a fundamental law of 

 succession, viz., the number of stages in a succession is deter- 

 mined largely by the increasing difficulty of invasion as the 

 habitat becomes stabilized. Man and animals affect migra- 

 tion directly, though not obviously, by the destruction of 

 disseminules. They operate as a pronounced barrier to 

 ecesis wherever they alter conditions in such a way as to 

 make them unfavorable to invading species, or when, by 

 direct action upon the latter, such as grazing, tramping, par- 

 asitism, etc., they turn the scale in the struggle for existence. 

 The absence of insects adapted to insure fertilisation is some- 

 times a serious barrier to the establishment of adventicious 

 or introduced plants. The presence of parasitic fungi, in so 

 far as they destroy the seeds of plants, acts as an obstacle 

 to migration, and restricts or prevents ecesis in so far as the 

 fungi destroy the invaders, or place them at a disadvantage 

 in the struggle for existence. 



Physical barriers are typically permanent in character, 

 while biological ones are either permanent or temporary, 

 depending upon the permanence of the formation and the 

 constancy of the physical factors which determine it. A 

 stable formation, such as a forest, or meadow, which acts as 

 a decided barrier to invasion from adjacent vegetation, may 

 disappear completely, as a result of a land-slide, flood or 

 burn, or through the activity of man, and may leave an area 

 into which invaders crowd from every point. Often, with- 

 out undergoing marked change, a formation which has pre- 

 sented conditions unfavorable to the ecesis of species of 

 mesophytic character, may by reason of a temporary change 



