INVASION. 



77 



form is most evident. When invasion is below the existing level it has no 

 direct influence upon the dominant species. Such invaders normally take a 

 subordinate place as secondary components of the community. In rare 

 instances they play an important or decisive part by virtue of some advan- 

 tageous competition form, such as the rosette or mat, or of some unique reac- 

 tion, as in Sphagnum (plate 20 A, b). 



Barriers. — ^A topographic feature or a physical or a biological agency that 

 restricts or prevents invasions is a barrier. Topographic features are usually 

 permanent and produce permanent barriers. Biological ones are often tem- 

 porary and exist for a few years or even a single season. Temporary barriers 

 are often recurrent, however. Barriers are complete or incomplete with 

 respect to the thoroughness of their action. They may affect invasion either 

 by limiting migration or by preventing ecesis. It has been generally assumed 

 that their chief effect is exerted upon migration, but it seems clear that this 

 is not the case. Even in the case of extensive barriers, such as the ocean, the 

 influence upon ecesis is decisive. 



Barriers are physical when due to some marked topographic feature, such as 

 an ocean, lake, river, mountain range, etc. All of these are effective by virtue 

 of their dominant physical factors. They prevent the ecesis of the species 

 conaing from very different habitats, though they may at the same time serve 

 as conductors for plants from similar habitats. This is especially true of 

 water-currents and mountain ranges. A body of water with its excessive 

 water-content is a barrier to mesophytes and xerophytes, but a conductor 

 for hydrophytes. Deserts set a limit to the invasion of mesophytic and hydro- 

 phytic species, while they favor that of xerophytes. By its reduction of tem- 

 perature, a high mountain range restricts the extension of plants of lowlands 

 and plains. It is also more of an obstacle to migration than most physical 

 barriers, because of the difficulty of movement up its slopes. Any bare area 

 with extreme conditions is a barrier to the invasion of communities beyond. 

 It is not to be regarded as a barrier to the development of succession upon it, 

 since the proper pioneers are always able to invade it. 



Biological barriers. — ^Biological barriers comprise plant communities, man 

 and animals, and parasitic plants. The limiting effect of a plant community 

 is exhibited in two ways. In the first place, an association acts as a barrier 

 to the ecesis of species invading it from associations of another type, on account 

 of the physical differences of the habitats. Whether such a barrier be com- 

 plete or partial will depend upon the relative unlikeness of the two areas. 

 Shade plants are imable to invade a prairie, though the species of open thickets 

 or woodland may do so to a certain degree. A forest formation, on account of 

 its diffused hght, is a barrier to poophytes, while a swamp, because of the 

 amount and kind of water-content, sets a limit to the species of both woodland 

 and grassland. Such formations as forests and thickets act also as direct 

 obstacles to migration in the case of tumbleweeds and other anemochores, 

 clitochores, etc. Closed communities likewise exert a marked influence in 

 decreasing invasion by reason of the intense and successful competition which 

 all invaders must meet. Closed associations usually act as complete barriers, 

 while more open ones restrict invasion in direct proportion to the degree of 

 occupation. To this fact may be traced the fundamental law of succession 

 that the number of stages is determined largely by the increasing difficulty 



