58 TH E STUDY OF PLANT COMMUNITIES • Chapter IV 



counts, but plentifulness may rapidly be estimated according to 

 some scale of abundance similar to the following : 



1. very rare 



2. rare 



3. infrequent 



4. abundant 



5. very abundant 



Such estimates are particularly useful when several similar stands 

 of uniform composition are to be surveyed within a limited time. 

 Assuming the sampling is adequate, the determination of actual 

 numbers by counting is of greater value because it permits the ex- 

 pression of density, which is abundance on a unit-area basis. 



Density is the average number of individuals per area sampled. 

 Since it is an absolute expression, the significance of density in in- 

 terpretation may be overemphasized unless one remembers that it 

 is an average value. Not all species with equal densities are of equal 

 importance in a community, or need they be similarly distributed. 

 If ten individuals of a species are counted on a series of ten plots, 

 the density is "one" regardless of whether they are all found in 

 one plot or one in each of the ten plots. It becomes necessary, 

 therefore, to interpret density values or to specify other charac- 

 ters that, combined with density, serve to complete the picture. 

 One such value is frequency. 



Frequency — This value is an expression of the percentage of 

 sample plots in which a species occurs. In the example above, the 

 plants that were all found on a single plot would have a frequency 

 value of 10 percent, whereas, if they had occurred in every plot, 

 the value would be 100 percent. Thus frequency becomes a very 

 useful value, when used in combination with density, for then not 

 onlv the number of individuals is known but also how they are 

 distributed in the stand. These two characters are of prime impor- 

 tance in determining community structure and, taken together, 

 have a variety of uses far beyond those of other quantitative values. 

 The use of frequency as a single determination in analytic pro- 

 cedure has proven unsatisfactory, although numerous attempts 

 have been made to show its adequacy. 



It should be emphasized that frequency values cannot be com- 



