ALTERNATIOx\ AND ZONATION 293 



In an extensive formation the same consocies alternates 

 between those areas that are similar. When the formation is 

 broken up and occurs here and there in separate examples, a 

 consocies often alternates from one to another of these. 



317. Normal alternation of species. The alternation of species 

 is the most typical feature of formations. The areas of a habitat 

 which show minor differences in physical factors or in compe- 

 tition are occupied by individuals of one or more species able 

 to adjust themselves to such differences. Individuals of svich 

 species tend to recur in all areas essentially similar, the inter- 

 vening areas being occupied by species more or less different. 

 Individuals that are scattered alternate as well as those that 

 grow in groups, but their alternation is necessarily much less con- 

 spicuous. In habitats not too heterogeneous a number of species 

 are sufficiently adjustable so that they occur throughout the 

 entire formation. In the prairie formation Festuca, Koelera, 

 Panicum, and Andropogon occur practically throughout, excei>t 

 in the moist ravines which are really meadows. Astragalus, Pso- 

 ralea, Erigeron, and Aster grow on slopes and crests, but they are 

 much more abundant in certain situations. Other plants, Loma- 

 tium, Meriolix, Anemone, Pentstemon, etc., recur in similar situ- 

 ations upon different hills. Lomatium alternates between sandy 

 or sandstone crests; Meriolix and Pentstemon occur together 

 upon dry upper slopes, while Anemone alternates between slopes 

 and crests. 



318. Numerical alternation. All species that alternate show 

 a variation in abundance from one area to another. Frequently 

 the difference is slight, and can be determined only by the use 

 of the quadrat. Sometimes the variation is so great that a facies 

 is reduced in number until it is less important than certain prin- 

 cipal or secondary species. A principal species often undergoes 

 a similar reduction in importance. This arises from the fact that 

 the similar areas are sufficiently different to affect the abundance 

 without entirely suppressing the species. This result is due 

 partly, and sometimes wholly, to competition. It is especially 

 noticeable in places where a species is ])laced at an increasing 

 disadvantage. Numerical alternation is often due to recent in- 

 vasion, in consequence of which the species has not yet had time 

 to reach its normal abundance. Astragalus crassicarjnis, for 

 example, grows on nearly all the slopes of the jjrairie formation. 



