Dispersal ami Invasion 427 



causes changes in the physical nature of its biotope. Plants with- 

 draw material from the soil as they grow and return it when they die, 

 but the material returned to the soil is not in the same form. Humus 

 accumulates, pH changes, moistiu-e content is modified, and other 

 alterations of the environment discussed in previous chapters are 

 brought about. 



The changed conditions caused by the varied activities of the in- 

 habitants of the area may favor the growth of species other than those 

 that have been dominating the scene. When this occurs, different 

 species will soon get the upper hand. These may either be species 

 already present in a subordinate capacity, or invaders from the out- 

 side. As one or more species take over the dominant position, a new 

 community will be formed; its establishment constitutes a step in the 

 ecological succession of the area. 



Dispersal and Invasion 



The establishment of the pioneer community on a bare area and 

 the replacement of this community and of subsequent communities 

 as ecological succession goes forward are dependent in the first in- 

 stance upon the existence of means by which new species can reach 

 the area. The ways in which animals and plants can invade new 

 areas are extremely varied. Certain non-motile forms may be carried 

 by wind or water currents for great distances; other species ride as 

 hitchhikers on or in the bodies of various larger animals. The action 

 of the air and water media in providing dispersal and the many special 

 adaptations of eggs, seeds, and adults for transportation have been 

 described in earlier chapters. 



The organism's own locomotion is responsible for the dispersal of 

 many animals and of some motile plants. The success with which the 

 starling extended its permanent breeding range westward in the 

 United States from its point of introduction is indicated in Fig. 12.1. 

 The great distances traveled by ducks after the breeding season and 

 before undertaking the southward migration to winter grounds have 

 already been mentioned (Fig. 9.19). Many other species probably 

 wander radially from the breeding grounds in similar fashion before 

 going to their winter quarters. Animals with such ample powers 

 of dispersal can push rapidly into new areas suitable for their 

 permanent invasion. Although other animals cannot travel at such 

 speeds as those represented by the flight of birds, their incessant 

 pressing against their boundaries nevertheless accomplishes wide 



