fifth phase, resuking in degradation to the mineral soil and re- 

 newal of the cycle. 



The rate of change in space of one or more factors may be very 

 gradual and take place over an extensive area, or it may be 

 abrupt and very limited in extent. The rate of change usually 

 varies throughout the gradient, rather than being uniform, be- 

 cause of the variability in topography, soils, and other conditions. 

 The mixed prairie in western Nebraska and Kansas has been in- 

 terpreted as a broad continuum between the tallgrass prairie 

 and the shortgrass vegetation of the Great Plains, resulting from 

 an elongated gradual gradient. Where the change is abrupt, 

 either a discontinuity or a narrow transition belt is found, as, for 

 example, on steep mountain slopes where a series of sharp 

 changes occur (see Figure 5-1), or along seacoasts where salinity 

 and soil moisture may favor distinct zones. 



Relatively rapid changes in time of one or more environmental 

 factors may occur which are unidirectional rather than fluctua- 

 tional in nature. There may be a gradual decrease in the avail- 

 ability of soil moisture, as in the hydrosere, and this may occur 

 along the entire gradient or at various rates in different portions. 

 Where such a unidirectional change in gradient occurs there is 

 an associated directional change, or succession, in communities 

 on the same area; for example the cattail-reed stage is replaced 

 by sedge-marsh, which in turn may be succeeded by a shrub 

 stage — the first of these probably showing a more rapid decrease 

 in soil moisture than the others. In time unidirectional changes 

 slow down or terminate, and a more or less steady-state 

 environmental complex, with a complex of climax communities, 

 prevails. 



Vegetation is an indicator of considerable reliability of the gra- 

 dients of the environment. In the southern Appalachians it has 

 been shown that the populations of a number of species and 

 communities are related to the complex pattern of environmental 

 gradients. 203-204.206 £ach factor has a pattern of gradients which 

 is related to those of other factors, so that a great complex of in- 

 tergrading habitats results, accompanied by a very complex 

 arrangement of communities. 



The differences, therefore, in gradients in a landscape, many 



140 • Dyzmamics o£ Comxnvmnities 



