440 Succession and Fluctuation 



phytoplankton and of zooplankton in natural waters is in part caused 

 by temporary modification of the medium by the organisms them- 

 selves and in part by seasonal changes in the physical environment, 

 as discussed in Chapters 8 and 13. The special circumstances of suc- 

 cession in arctic areas in which the substratum is repeatedly dis- 

 turbed by frost action are described by Hopkins and Sigafoos ( 1951 ) . 



Succession on a small scale, but of a complex nature, takes place 

 on and in the trunks of fallen trees. The sound wood, cambium, and 

 bark are first attacked by a group of boring insects, saprophytic fungi, 

 and various kinds of microorganisms. This pioneer assemblage is 

 followed by a series of more elaborate communities that cause and 

 accompany the further disintegration and decomposition of the tree 

 trunk. The presence of these organisms attracts predators and 

 scavengers until a very large number of species may be represented 

 in the dead-tree-trunk biotope. Mosses, lichens, ferns, and, later, 

 higher plants find the rotting log a favorable substratum for growth. 

 With the establishment of autotrophic plants a new cycle of con- 

 structive growth begins, in which the new pioneers take root literally 

 as well as figuratively from the remains of the previous cycle. 



The building up of coast lines is another activity in which the course 

 of succession and the physiographic changes are mutually interde- 

 pendent. The ecological steps involved in and following the forma- 

 tion of sand dunes along the southern shore of Lake Michigan as one 

 community succeeds another have been studied by a number of ecol- 

 ogists. In simplest terms the course of succession was found to in- 

 volve the capture of moving sand by grass, and the development of 

 communities dominated successively by cottonwoods, pines, and oaks, 

 leading to the beech-maple forest. Subsequent investigation has 

 shown that the succession is far more elaborate and also more flexible 

 than was at first supposed and that several interlocking channels for 

 advancement or recession may be followed by the various series of 

 communities involved, as indicated in Fig. 12.6. 



Along marine coast lines in tropical regions a well-known land- 

 building succession takes place (Fig. 12.7). Here red mangroves 

 ( Rhizophora mangle ) work out from the shore by dropping viviparous 

 seedlings that will root only in water more than about 25 cm deep. 

 By means of spreading, stilt-like roots this species of mangrove can 

 maintain itself in and slightly below the tidal zone in spite of wave 

 action. Mud that collects around the dense jumble of roots builds up 

 the bottom, causing the gradual elimination of the red mangrove, and 

 prepares the way for the establishment of the black mangrove 

 {Avicennia nitida), the seedlings of which will grow only in water 



