community in one or a combination of characteristics such as 

 change in kinds of species, dominance, phenology, and growth 

 rate. The seasonal variation in the stage of vegetation develop- 

 ment can sometimes be related to the fluctuation of a factor or a 

 complex of factors in the habitat. The date of blooming of 7 

 species in Utah had a range of 44 days, and this was related to 

 the date of snow disappearance which had a range of 19 days.^^ 



Many communities persist for long periods of time with only 

 minor fluctuation in composition. The fluctuations in a steady- 

 state community are rarely of such magnitude that any species is 

 completely eliminated. It is expected that this kind of change 

 would be less apparent in the dominant and characteristic 

 species. Composition change is probably greater in the early 

 stages of succession and less as the steady-state condition is ap- 

 proached. If there is actually a change in species composition in 

 the climax, then such a change is of a relatively temporary dura- 

 tion. Fluctuation may also be manifested in the abundance of a 

 species, and this is probably more common than fluctuation in 

 floristic composition, but even in abundance most species prob- 

 ably exhibit only slight to moderate changes. The fluctuation in 

 abundance of each species tends to be damped and varies 

 around an intermediate position, and as the steady-state is ap- 

 proached the amplitude decreases. 



If fluctuation changes are of sufficient amplitude to be signif- 

 icant and of sufficient prominence to be recognized, then it is im- 

 portant to differentiate such changes from other kinds of changes 

 (directional or successional change and cyclic change) so that 

 there will be no confusion as to the kind and significance of the 

 changes. 



Directional Change 



Directional change is concerned with a noncyclic, reasonably 

 orderly sequence of perceptibly different communities on a given 

 site. A directional change from a less to a more complex com- 

 munity may be considered as a progression, and a change from 

 a more to a less complex community as a retrogression. In some 

 cases the directional change may be deffected to a degree. When 



148 • Dyimanmics of CoxnsnvmMmities 



