RELATIONS AMONG SPECIES 



387 



Zones were the first units that allowed some level oi 

 local recognition of areas, and the so-called refined 

 schemes often provide no better understanding than 

 does Merriam's scheme. 



Present reduction in use of Life Zones came from 

 the following criticisms of the concept: (1) It is in- 

 flexible, not allowing the use of additional larger or 

 smaller categories. (2) It indicates that zonation is 

 across the continent, whereas geological history and 

 present organisms indicate a definite difference be- 

 tween the east and the west. (3) Basing the scheme 

 strictly upon temperatures during the period of 

 growth and reproduction allows for many possibilities 

 for error. For example, other parts of the year can 

 bejust as important, and daily and seasonal extremes 

 might be more important than the averages that were 

 used. (4) Often there is no close correspondence 

 between Life Zones as originally designated and the 

 actual distribution of flora and fauna. (5) Few or- 

 ganisms appear to be confined to the Hudsonian 

 Zone, which seems to be little more than a diluted 

 Canadian Zone. (6) Finally, perhaps because of mis- 

 interpretation, it seems that there were distinct errors 

 in temperature computations and hence in the final 

 Life Zone map. 



COMMUNITY TYPES 



The "association types" of G. E. Nichols were pres- 

 ent in 1923. At the present time reference is made to 

 "community" or "vegetation types" rather than to 

 association types because of possible confusion of 

 meaning of the term "association." A community 

 type is classified on the basis of the growth forma- 

 tion, or appearance, of a group of organisms. On this 

 basis, forests, woodlands, marshes, deserts, and like 

 habitats are recognized. 



Although this scheme has great value in referring to 

 a particular group of organisms, it is not used to 

 imply distribution. For one thing, the scheme pro- 

 vides little chance for drawing lines between com- 

 munities, because species are not considered. For ex- 

 ample, one community type can consist of many dif- 

 ferent and remotely related communities, and the 

 community type of a single community can be modi- 

 fied near the margin of its area. 



Community types contain units designated on the 

 basis of over-all adaptations (the growth formations). 

 Therefore, if this scheme is applied to show over-all 

 local responses between organisms and their environ- 



ment, it can be of value. However, except for a broad 

 interpretation of four Vegetation Types (Figure 

 19.25), most community types have received little use 

 in modern treatments of distribution. Moreover, 

 most of the community types probably will remain 

 limited to the role of convenient summaries of the 

 adaptations of organisms. 



forest 



iiiiii/iiiiimiiiniiJilniiHHlillll 



woodland 



savanna 



prairie 



^ ^ w \w ^ m 



steppe 



Ss^lM 



g:£P 



scrub 



Figure 19,25 The moior vegetation types: forest, woodland, gross- 

 lond, and desert. In forests the trees form a closed canopy above the 

 ground. As this canopy opens a woodland develops. The transition 

 between a woodland and grassland is recognized as a sovonno. Grass- 

 lands are of two types, prairies and really a grassland-desert transition 

 of bunch grasses called steppes. Deserts generally hove a more or less 

 open shrub vegetation called scrub. A particular scrub vegetation called 

 chaparral is found outside desert areas in western North America. 



BIOMES 



The biome consists of a single plant formation and 

 its animals. It normally is composed of a broad geo- 

 graphic area having a single type of climate and the 

 potential of a single climatic climax community (Fig- 

 ure 19.26). By implication it contains all serai stages 

 leading to the climatic climax, all nonclimatic cli- 

 maxes, and all serai stages leading to the nonclimatic 

 climaxes in the area of a particular climatic type. 



