442 



Succession and Fluctuation 



shallower than about 25 cm. As the bottom is raised and the soil 

 becomes drier, buttonwoods replace the mangroves, and these are 

 subsequently succeeded by the climax palm community. 



In the situations described thus far the controlling organisms are 

 plants. On land, plants are usually the dominant members of the 

 community, and vegetation development is chiefly responsible for 

 causing succession, but in some communities animals are found to 

 exert control. For example, birds nesting in colonies are sometimes 

 so abundant that their activities and their droppings cause a change 

 in the vegetation. In this way a rookery of herring gulls on Kent 

 Island off the coast of New Brunswick caused the elimination of a 

 grove of spruce trees and its replacement by grass. 



.Climax 

 community 



MANGROVE SUCCESSION 

 Transition Salt-marsh Pioneer community 



Red 

 mangrove 



Viviparous 

 seedling 



Ui High tide 



'^'-"/♦Seedling 



f- — Low tide 



Soil line 



Underlying rock 

 Fig. 12.7. Diagram of succession along the margin of tropical shores as seen in 

 southern Florida. (Modified from Davis, 1940.) 



Control of the environment by animals is more commonly found 

 in aquatic habitats where sessile forms are prominent. The photo- 

 graphs in Fig. 12.8 show two stages in an all-animal warfare in the 

 tidal zone. Oysters became established on the sea bottom in this 

 biotope, but mussels soon began attaching to the oysters' shells. 

 Gradually the growth of the mussels smothered the oysters, and the 

 latter were replaced by an almost continuous carpet of mussels. Sub- 

 sequently, barnacles became attached to the shells of the mussels in 

 suflBcient numbers to kill them. After the death of the mussels their 

 shells broke loose from the bottom and the barnacle population was 

 swept away by wave action so that no enduring change was brought 

 about in this instance. 



Frequently both plants and animals are involved in well-defined 

 succession in the littoral zone as described by Dexter ( 1947 ) for mud- 



