ECOLOGY 777 



Finally the biologic and physical data are correlated, the major and 

 minor communities of the region are identified, and the food chains and 

 other important ecologic relations of the communities and the particular 

 adaptations of the animals and plants for their role in the community 

 are studied. 



349. Community Succession 



Any given area tends to have an orderly sequence of communities 

 with time, which change together with the physical conditions and lead 

 eventually to a stable mature community or climax community. The 

 entire series of communities is known as a sere, and the individual 

 transition communities as serai stages or serai communities. These 

 series are so regular in many parts of the world that an ecologist, rec- 

 ognizing the particular serai community present in a given area, can 

 predict the sequence of future changes. The ultimate causes of these 

 successions are not clear. Climate and other physical factors play some 

 role, but the succession is directed in part by the nature of the com- 

 munity itself, for the action of each serai community is to make the area 

 less favorable for itself and more favorable for other species until the 

 stable, climax commimity is reached. 



One of the classic studies of ecologic succession was made on the 

 shores of Lake Michigan (Fig. 37.9). As the lake has become smaller 

 it has left successively younger sand dunes, and one can study the stages 

 in ecologic succession as one goes away from the lake. The youngest 

 dunes, nearest the lake, have only grasses and insects; the next older 

 ones have shrubs such as cottonwoods, then evergreens, and finally a 

 beech-maple climax community with a rich soil full of earthworms and 

 snails. As the lake retreated it also left a series of ponds. The youngest of 

 these contain little rooted vegetation and lots of bass and bluegills. 

 Later the ponds become choked with vegetation and smaller in size as 

 the basins fill. Finally the ponds become marshes and then dry ground, 

 invaded by shrubs and ending in the beech-maple climax forest. Man- 

 made ponds, such as those impounded by dams, similarly tend to be- 

 come filled up, becoming first marshes, then dry land. 



Ecologic succession can be demonstrated in the laboratory. If a 

 few pieces of dry hay are placed in some pond water, a population of 

 bacteria will appear in a few days. Next, flagellates appear and eat the 

 bacteria, then ciliated protozoa such as paramecia followed by predator 

 protozoa such as Didinium emerge. The protozoa, present as spores or 

 cysts in the pond water or attached to the hay, emerge in a definite suc- 

 cession of protozoan communities. 



Biotic communities typically show a marked vertical stratification, 

 determined in large part by vertical differences in physical factors such 

 as temperature, light and oxygen. The operation of such physical factors 

 in determining vertical stratification in lakes and the ocean is quite 

 evident In a forest there is a vertical stratification of plant life from 

 mosses and herbs on the ground, then shrubs, low trees and tall trees. 



