LAND AND SEA COMMUNITIES 247 



for subordinate communities, including seasonal ones (Clements, 

 1936) . 



In connection with the above terms, as well as those later discussed 

 under succession, it should be borne in mind that each has a concrete 

 or local application and, in addition, a general one. Thus, the pioneer 

 family of tiger beetles on a particular dune or sandhill may recur in 

 all identical habitats in the local area, or through an extensive region, 

 to constitute the pioneer family of a dune succession. The community 

 formed in the climax true or mixed prairie by Psoralea tenuijlora 

 exists today in thousands of separate fragments, as a consequence of 

 topographic diversity and especially of disturbance, but the local 

 examples are best regarded as parts of an extensive community. The 

 same is true of nearly all fresh-water communities. The validity of 

 this conclusion is all the clearer in the true prairie, for example, where 

 an originally continuous association of wide extent has been frag- 

 mented by tillage into many thousand pieces, often but a few acres 

 in extent. 



General Comparison of Land and Sea Communities. It is obvious 

 from the foregoing that size is an important criterion for the rank or 

 the significance of communities. The next paragraph indicates, in 

 very general terms, the comparative magnitude of climax and serai 

 communities of the land and sea bottom. It is based upon grassland 

 as one of the most extensive of North American biomes (Clements, 

 1920; Weaver and Clements, 1929; Shantz and Zon, 1924), and upon 

 the marine communities of the North Atlantic and North Pacific 

 (Petersen, 1914; Jensen, 1919; Davis, 1923; Shelford and Towler, 

 1925; Shelford, 1935). 



The following facts are brought out relative to the size of various 

 units. The biotic formation (biome), such as the North American 

 grassland, may cover as much as 1,000,000 square miles and be 

 divided into five or six associations covering 100,000-300,000 square 

 miles. Although the association is accepted as the most uniform unit, 

 there is still variation which the original workers such as Warming 

 had not discovered. A large association like the mixed prairie may 

 show faciations characterized by the dropping out or addition of 

 species which are as large as 9,000 square miles or even larger, and 

 lociations or local variation may be as large as 900 square miles. The 

 climax is more or less continuous but may be much fragmented at its 

 periphery. 



Serai communities are generally very much smaller and usually 

 much fragmented. A continuous serai area of 200,000 square miles 

 illustrated by the southeastern pine forest, or still larger areas of 



