Ecesis 429 



mountain range is a barrier preventing the dispersal of plants that 

 require a warm, moist soil for their growth. 



Biological barriers are sometimes more obscure in their mode of 

 operation, but they may be equally effective in preventing the spread 

 of certain species. The action of biological barriers is perhaps most 

 easily seen when larger plants are involved. If an extensive area is 

 completely occupied by trees, for example, other species of trees with 

 closely similar ecological requirements cannot enter the area. If the 

 forested area is large, the dispersal of the other species will be stopped 

 unless their seeds can be carried over or around the barrier. In the 

 same way a grass sod acts as a biological barrier to the dispersal of 

 other herbs from adjacent areas. Undisturbed prairie grass regularly 

 prevents the invasion of trees even though in some situations the 

 climate can "support" trees once the prairie sod is broken. Com- 

 munities in which all available space for certain types of organisms 

 has been occupied are termed closed communities as far as these kinds 

 of organisms are concerned. Although these communities thus pre- 

 vent the invasion and the dispersal of certain species, they may allow 

 species of very different requirements to enter. 



Ecesis. In order for new species to invade an area they not only 

 must have some means of reaching the new locality but also must be 

 able to grow and reproduce under the conditions found there. Or- 

 dinarily the first invaders of a bare area are plants, and for them the 

 physical features of the soil and the climate are of primary importance 

 in determining whether or not ecesis, or successful establishment, can 

 take place. The pioneer assemblage— usually of plants— first estab- 

 lishing itself in a new area is sometimes referred to as a coloni/ but 

 care should be taken to avoid confusion with the use of the same 

 term for groups of social animals. After the ecesis of the pioneers 

 other species will arrive at the area and will gain a foothold if they 

 can. The presence of certain plants is usually a primary factor in 

 determining whether or not invading animal species will be able to 

 establish themselves. Accordingly, the first animals to arrive will be 

 primarily concerned with whether the vegetation which they find is 

 suitable. The general dependence of the distribution of animals 

 upon that of plants is not limited to the early stages of colonization, 

 but applies in more advanced communities, especially in the ter- 

 restrial environment. 



Later arrivals at the area being invaded must be able not only to 

 tolerate the physical conditions but also to compete successfully with 

 the species already present if they are to establish themselves. The 

 absence of a species in a given area does not necessarily mean that it 



