432 Succession and Fluctuation 



hypothesis in which "vegetation is conceived as a pattern of popula- 

 tions, variously related to one another, corresponding to the pattern 

 of environmental gradients." 



The climax community remains in possession of the area until some 

 unusual change causes its displacement. The biotope and its biota 

 may be completely destroyed by a cataclysm such as a volcanic erup- 

 tion or extensive erosion; serious but incomplete destruction may 

 result from a forest fire, flood, or hurricane. On the other hand, the 

 climax community may be displaced less violently by removal of the 

 dominant species through lumbering or the attacks of parasites. 



Types of Succession 



Primary Succession. Ecological succession that begins on a bare 

 area where no life has existed, or where the previous fauna and flora 

 have been completely destroyed, is known as primary succession. 

 Habitats that become available for initial colonization include: new 

 islands, sand bars, deltas, or glacial moraines; recently formed ponds; 

 fresh alluvial, shore, or volcanic deposits; and various types of sub- 

 strata exposed by erosion. These diverse areas may be classified as 

 xeric, mesic, or hydric according to whether the initial moisture con- 

 ditions are dry, intermediate, or wet. Seres starting from these types 

 of situations represent xerarch, mesarch, and hydrarch succession, 

 respectively. 



A striking example of primary succession, and a classical one, is 

 the hydrarch succession in which a pond and its community is con- 

 verted to dry land with an entirely different community. The vege- 

 tation rooted along the margins of the pond is able to push out from 

 shallow water into deeper water in a variety of ways ( Fig. 12.2 ) . As 

 the vegetation invades the open water, the margin of the pond is 

 reduced. At the same time the growth of the plankton and of other 

 aquatic organisms adds organic matter, and much of this is deposited 

 on the bottom. Beavers, muskrats, and other animals carry material 

 into the pond, deciduous vegetation blows in from the shore, and silt 

 is carried in from surrounding land. Rafts of vegetation from the 

 pond margin drift offshore, strand, and take root, thus establishing 

 islets that grow in size until they meet and also join the shore. At 

 the same time the area available for completely aquatic plants, such 

 as the water lilies, becomes reduced. As the free water is changed 

 to swampy land, the water lilies and similar species give way to 

 sedges and rushes, and these are subsequently replaced by heaths and 

 shrubs. As succession continues, the soil is further built up, so that 



