28 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



same area. Moreover, the climate and climax themselves are never 

 entirely static, the relative stability being disturbed in varying degree 

 through cycles of different intensity and duration. The briefest of 

 these are exemplified by seasonal and annual cycles, expressed in 

 aspection and annuation, phenomena that are also exhibited by serai 

 communities, though to a lesser degree. This is likewise true of the 

 less regular effect of the sunspot cycle of 11 years and its double or 

 triple period. The longer solar cycles may comprise thousands of 

 years, as may also such physiographic ones as cycles of erosion, while 

 back of the major climatic and land-form cycles lie the still greater 

 ones of mountain lifting and continent building. To these necessarily 

 correspond the outstanding changes in biomes and the climates that 

 control them. When it is realized that cycles of different kind and 

 length are operating at the same time, with the phases now in con- 

 junction, now in disjunction, it is evident that habitat and climax are 

 in constant fluctuation. Apart from serai areas, however, most of this 

 has to do at any particular moment with the behavior, size, and 

 abundance of the constituent (not from habitat) species, which thus 

 serve as indicators of habitat variations. 



It is evident that the subdivisions of the climatic feature of the 

 habitat will correspond to the various divisions of the climax itself, 

 though there is no immediate need for a parallel series of terms. In 

 addition, there are not only the serai habitats or sereces, but likewise 

 one to match each distinct successional stage. There are also the 

 small habitats of individuals, ranging from a huge oak or banyan to 

 sedentary aphids and mites, or sessile algae and fungi. Furthermore, 

 the grouping of organisms in layers supplies a certain warrant for 

 dividing the habitat in the case of those limited to a particular layer 

 (Shelford, 1913, a; Yapp, 1922). However, it is obviously illogical to 

 carry such analysis to the extreme of speaking of the root or shoot 

 habitat of a particular individual or species, as has been suggested by 

 some workers. 



Finally, a large number of animals are concerned with two or 

 more habitats. This is a characteristic relation for insects with aquatic 

 larvae in the hydrosere, and also for migrating birds and fishes and 

 some mammals. These may occupy two or more climates each year 

 or during their life history, and the birds and mammals in particular 

 may live in both climax and serai habitats. It appears significant 

 that in the forest biomes of North America, at least, the larger mam- 

 mals are often best represented in subclimax areas, and the birds in 

 forest margins, where herbs and shrubs alternate in a transition belt 

 or an ecotone, or in serai stages. 



