266 PLANT PHYSIOLOGY AND ECOLOGY 



invasion. They often prevent or decrease migration, but as a 

 rule their action is largely confined to ecesis. With respect to 

 their nature, barriers are usually distinguished as physical and 



biological. 



283. Physical and biological barriers. Physical barriers limit 

 invasion by virtue of some marked physiographic feature, such 

 as an ocean, lake, river, mountain range, or desert. Biological 

 barriers comprise vegetation, man and animals, and plant para- 

 sites. Physical barriers act through their dominant physical 

 factors by preventing the ecesis of species coming from very differ- 

 ent habitats. A body of water is a barrier to mesophytes and 

 xerophytes; deserts set a limit to the spread of mesophytic and 

 hydrophytic plants. A mountain range is usually an obstacle to 

 migration as well as to ecesis. 



Formations, such as forests and thickets, etc., sometimes act 

 as direct obstacles to the migration of tumbleweeds and other 

 wind-distributed plants. Their greatest influence in decreasing 

 invasion arises from their closed nature. When the competition 

 is intense, the invading species, though suited to the general con- 

 ditions of the habitat, are usually unable to secure a foothold. 

 Man and animals affect migration by the destruction of dissemi- 

 nules. They act as a barrier to ecesis whenever they make con- 

 ditions unfavorable to invaders, or when they turn the scale in 

 the struggle for existence, through grazing, tramping, parasitism, 

 etc. The absence of insects necessary for pollination sometimes 

 acts as a serious barrier to ecesis. Parasitic fungi affect migra- 

 tion in so far as they destroy seeds, or reduce the number pro- 

 duced. They may prevent or restrict ecesis by destroying the 

 invaders, or by placing them at a disadvantage in the competition. 



284. Influence of barriers. Physical barriers are permanent 

 as a rule, while biological ones are often temporary. A forest 

 or meadow, which often acts as a barrier to invaders from adja- 

 cent vegetation, may disappear as a result of a land-slide, flood, 

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

 which plants crowd from every direction. Such barriers as para- 

 sites and herbivorous animals may appear or disappear at any 

 time. When the physical factors of a habitat form a barrier, 

 these may be so modified by climatic changes that they no longer 

 act as such. A meadow ceases to be a barrier to prairie xero- 

 phytes during a period of unusually dry years. Similarly, dry 



