78 ECESIC CAUSES. 



of invasion as the area becomes stabilized. Man and animals affect invasion 

 by the destruction of germules. Both in bare areas and in serai stages the 

 action of rodents and birds is often decisive to the extent of altering the whole 

 course of development. Man and animals operate as marked barriers to 

 ecesis wherever they alter conditions unfavorably to invaders or where they 

 turn the scale in competition by cultivation, grazing, camping, parasitism, etc. 

 The absence of pollinating insects is sometimes a curious barrier to the com- 

 plete ecesis of species far out of their usual habitat or region. Parasitic fungi 

 decrease migration in so far as they affect seed production. They restrict or 

 prevent ecesis either by the destruction of invaders or by placing them at a 

 disadvantage with respect to the occupants. 



Changes in barriers. — ^A closed formation, such as a forest or meadow which 

 acts as a decided barrier to invasion, may disappear completely as the result 

 of a land-sKde, flood, or bum, and leave an area into which invaders crowd from 

 every point. A temporary swing of climate may disturb the balance of a 

 community so that it permits the entrance of mesophytes which are normally 

 barred, and one or more stages of succession may be omitted as a consequence. 

 On the other hand, a meadow or swamp, for example, ceases to be a barrier to 

 prairie xerophytes during a period of unusually dry years, such as regularly 

 occurs in semiarid regions. A pecuhar example of the modification of a barrier 

 is afforded by the complete defoliation of aspen forests in the Rocky Mountains. 

 As a result, they were invaded by poophytes, producing a change of develop- 

 ment identical with that found in the usual aspen clearing. Nearly all xero- 

 phytic stretches of sand and gravel, dunes, blow-outs, gravel-slides, etc., as 

 well as prairies and plains in some degree, exhibit a recurrent seasonal change 

 in the spring. As a result, the dry, hot surface becomes sufficiently moist to 

 permit the germination and growth of invaders, which are normally barred 

 out during the rest of the year. The influence of distance as a barrier has 

 already been indicated imder "Migration." 



