it becomes drier and is also changed 

 chemically. In time certain smaller spe- 

 cies of trees invade the area, taking the 

 place of the shrubs, and eventually full- 

 sized forest trees will dominate the scene. 

 Various stages in such a succession are 

 shown in Fig. 12.3. 



The existence of zonation in a com- 

 munity does not necessarily mean that 

 succession is going on since distinct hori- 

 zontal subdivisions may occur in a static 

 community, as described in the previous 

 chapter. Furthermore, the change in 

 the vegetation is not always self-induced 

 but may be caused by outside influences. 

 Sometimes the conversion of a swamp to 

 dry land is brought about primarily by a 

 lowering of the water table caused by a 

 physiographic change. But in other in- 

 stances, as in the situation described in 

 the preceding paragraph, the vegetation 

 itself is chiefly responsible for building 

 up the land as ecological succession goes 

 forward. 



At the same time that the vegetation 

 is undergoing these profound changes in 

 the hydrarch succession, the animal life 

 of the community is correspondingly 

 altered. Fish, beavers, and muskrats 

 will gradually be excluded and terrestrial 

 vertebrates will enter. Less conspicuous 

 but just as significant will be the mani- 

 festations of succession among the in- 

 vertebrates and the microorganisms. 

 These trends in the animal members of 

 the community are indicated schemati- 

 cally in Fig. 12.4 for a hydrosere in Illi- 

 nois. The changes in bird species asso- 

 ciated with a hydrarch succession are 

 shown in Table 19. The changes in the 

 Fig. 12.2. Five methods by which vegetation invades deeper water from the 

 pond margin: (a) spike rush, (b) tussock sedge, (c) loosestrife, (d) cat-tail flag, 

 and (e) sphagnum and heath. ( Needham & Lloyd, 1937, Copyright, Cornell 



Univ. Press.) 

 433 



