Succession 51 



pioneers. These with the progressive physical and chemical changes 

 which occur in all habitats prepare the way for other plants and 

 animals. Then during a transition stage the number of species in- 

 creases, and an area often becomes thickly populated. Finally cer- 

 tain species which, because they happen to fit the conditions at the 

 particular time and place, become the dominant types and thus 

 constitute what ecologists call a climax formation. When a bit of 

 land sinks below the ocean it is populated in a more or less orderly 

 way by marine plants and animals. The same is true when a por- 

 tion of sea bottom is elevated into the air, when a landslide denudes 

 a part of a mountain side, or when any environmental change opens 

 a new area into which plants and animals may spread. 



Succession and spreading together continually give animals op- 

 portunities to explore and occupy new situations. A slight fluctua- 

 tion in environment may change the course of succession and lead 

 to a different climax formation. A particular species, if accidentally 

 introduced into an area early, becomes dominant; whereas if it 

 arrives later in the successional series and has to compete with or- 

 ganisms that are already established, it may never be able to do so 

 (Scott, 1910) . Succession results in part from the effects of envi- 

 ronmental changes, particularly those brought about by the pres- 

 ence of certain organisms which prepare the way for others, and in 

 part from competition between species. Competition and differences 

 in the physical-chemical qualities in environments bring about the 

 segregation of animals into specific habitats. 



McDougall (1943), Pyefinch (1943), and Weiss (1948) have 

 studied the repopulation of bare rocks and other hard surfaces in 

 the ocean. At first, such types as barnacles and oysters predominate, 

 but these usually are later smothered by hydroids, sponges, tuni- 

 cates, and bryozoans. All these sessile animals vary with the seasons. 

 They have no chance of attaining terrestrial life. 



