of them caused by microrelief, are important in defining com- 

 munity boundaries which may be sharp or clear-cut, transitional 

 or diffuse, and mosaic-insular. ^^^ It appears that in some regions 

 the vegetation may be a complex continuum of populations 

 which are adjusted to the intricate pattern of environmental 

 gradients. ''^ However, because of the competition between species 

 differing in capacity to utilize the environmental resources, com- 

 munities are often more sharply delimited than the habitats. An 

 example of the resulting complexity in vegetation is seen in the 

 Cairngorm Mountains in Great Britain, well described by Met- 

 calfe^ ^^ as follows: "The result in toto is a vegetational patchwork 

 related to local physiognomy, the units of which repeat over the 

 whole area as the physiognomic features repeat, and in which the 

 units can be differentiated from each other, not only by the 

 presence or absence of the more sensitive or dependent species 

 but also by the relative abundance and cover of the more tolerant 

 and independent species." Even the smallest and poorest com- 

 munities, containing only the most common species, indicate the 

 ecologic conditions of their habitats by variation in quantitative 

 composition. Mosses and lichens, lacking root systems, are 

 especially sensitive indicators of moisture conditions.'^ 



In summary, several to many environmental factors are func- 

 tionally decisive in every area. Each environmental factor is 

 usually characterized by macro-, meso-, and microgradients, the 

 smaller ones being part of the larger. The gradients may be con- 

 tinuous or discontinuous, and they are generally variable rather 

 than uniform throughout their extent. In some cases a portion or 

 all of a certain gradient may change with time. Each factor 

 tends to be operative within a limited range of independence of 

 others. Intergradation of the gradients of all factors results in a 

 complex pattern of habitats in every region. A nongradient en- 

 vironment probably never exists, even if a peneplain should 

 develop by geomorphic processes. Populations and communities 

 are in dynamic adjustment to the intergrading steady-state 

 habitats, and also to changing habitats if they are present, so 

 that a complex pattern of climax and successional vegetation 

 types results. The communities tend to be more discrete than the 

 habitats. 



Habitat Patterxms, CHaMKges, aM%<l Climax • 141 



