444 Succession and Fluctuation 



next, followed by mussels (Scheer, 1945), whereas in other situations 

 barnaeles, tube worms, or tunicates attach at an early stage (Woods 

 Hole Oceanographic Institution, 1952). The varied surfaces of the 

 attached organisms, and particularly the crevices between them, pro- 

 vide abodes for a host of mobile forms that join the community in 

 these later stages and that could not inhabit the biotope if it were not 

 for the presence of the earlier arrivals. Thus, in these small habitats, 

 as well as in the larger areas considered, the early inhabitants change 

 the conditions so as to cause their own displacement and the estab- 

 lishment of successive new communities. 



Modification of Succession 



The course of succession may be modified by unusual natural 

 circumstances, or frequently by the hand of man. Succession may 

 be changed, for example, by the browsing of an overly abundant 

 population of deer in a forest. Deer do not browse on all species of 

 the vegetation indiscriminately but have certain favorite items. If 

 the deer become numerous, the most appetizing plants will be grazed 

 down excessively and the unpalatable types will be given an ad- 

 vantage. In this way the species composition of the vegetation and 

 the course of succession may be considerably altered. If man permits 

 overgrazing by cattle on the range, the usual sequence of succession 

 may be reversed so that the vegetation is pushed back from perennial 

 grasses to annual weeds (Graham, 1944, Ch. 9). 



Repeated forest fires sometimes bring succession to a halt at a com- 

 munity stage that is different from the climax in neighboring areas. 

 In the sandy soils near Plymouth, Massachusetts, and on Cape Cod 

 the prevailing vegetation consists chiefly of pine and scrub oak, and 

 this appears to be a climax condition determined partly at least by 

 fire. On an undisturbed island in a large pond within this region a 

 mature stand of beech, maple, oak, hemlock, and yellow birch has 

 developed. This island has not been burned over during the memory 

 of present residents, and it may well be that because of the protection 

 of the water no fire has occurred on the island for generations. The 

 existence of this island community suggests that other neighboring 

 regions might progress beyond the pine and scrub oak stage if the 

 interference of forest fires was prevented for a sufficient time. The 

 pinelands in some parts of southeastern United States may similarly 

 represent a climax community controlled by fire rather than by climate 

 (Fig. 12.9). 



Modification of succession by special factors may also be observed 



