Fluctuation within the Community 451 



ties and by the mammals, birds, and other animals that move back 

 and forth between various communities of the biome. For example, 

 beavers are members of a pond community within a forest biome and 

 also are members of an aspen community that borders the pond. 



The biomes are the result of the equilibria established by living 

 inhabitants with all aspects of the climate of the region. Since the 

 biomes are characterized by major communities of distinctive life 

 form, they correspond to certain of the life zones that are based on 

 the same life forms. The chief biomes that occur in a series from the 

 equator toward the poles coincide with the principal continental life 

 zones described in Chapter 5. Some biomes tend to be continuous in 

 extent and form a more or less definite unit, but others are discon- 

 tinuous and the parts may be widely separated geographically. 

 Communities of the same life form as a biome but too small to con- 

 stitute a separate biome are referred to as biome types. 



The clearly defined major biomes of the land masses of the world 

 are shown in Fig. 12.11. The sea may be regarded as constituting 

 an additional biome, but the plants of the open ocean do not exert 

 the controlling influence on the biotope experienced on land. All 

 parts of the sea are interconnected, and many kinds of plankton and 

 nekton move readily from one region to another. The biogeographic 

 subdivisions of the sea are therefore based principally on physical 

 features of the environment, as has been described in previous chap- 

 ters and is further elaborated by Ekman ( 1953 ) . 



The principal biomes on land are: tundra, taiga, temperate decidu- 

 ous forest, temperate rain forest, grassland, desert, and tropical forest. 

 In certain regions, as in central North America, the dividing line be- 

 tween biomes tends to run longitudinally, but for the most part the 

 chief biomes of the world are arranged in a general latitudinal se- 

 quence. For a fuller description of the biomes of the world the 

 reader is referred to the general account of Cain (1944) and Allee 

 et al. (1949, Ch. 30) and to such special treatments as Richards 

 (1952) and Beard (1953). 



FLUCTUATION WITHIN THE COMMUNITY 



When the cHmax community has been established, can we then 

 take a long breath and say that at last the ecosystem will be entirely 

 constant? The answer is. No. Even within the relatively permanent 

 and stable climax community fluctuations occur— sometimes of con- 

 siderable magnitude. It is true that in some situations the community 

 remains relatively steady. Sometimes, the life cycles of the members 



