360 



Annals New York Academy of Sciences 



a similar segment of the community will have been studied regardless of the 

 dominance of any species that may be present. 



We have found in natural rivers which are relatively free from pollution the 

 communities are composed of many species with most of them having relatively 

 small populations. These findings support the theory set forth by Thiene- 

 mann (1939) that optimal environments support many species composed of 

 relatively small populations. Furthermore, the numbers of species do not 

 change greatly from season to season in the same area nor do they change very 

 much from area to area collected at the same time. For example, in table 2 



Table 1 



The Number of Species and the Percentage of the Specimens in Populations of 



Dominant Species Observed when Varying Numbers 



OF Specimens are Counted 



* The percentage of specimens counted composing the dominant species. A dominant 

 species is one that is represented by 1000 or more specimens when 5000 or more specimens are 

 counted. 



t These are the number of specimens which had to be counted to place the mode in the 

 second interval when a truncated normal curve is constructed from the data. 



are shown the data for these statements derived from studies of the Savannah 

 River. 



When the numbers of species found in different natural soft water rivers, for 

 example the Savannah River, the Red Clay Creek, and the Wateree River, 

 are compared, they do not vary greatly. The total number of species for the 

 Savannah River (South Carolina) was 188; Red Clay Creek (Delaware), 145; 

 Wateree River (South Carolina), 181. Considering only those species repre- 

 sented by more than 6 specimens when 7000 or more specimens are counted, 

 we find Savannah River, 85; Wateree River, 89; Red Clay Creek, 76. The 

 reason that 6 or more .specimens have been used for estimating that a species 

 is established in a given area is that if a truncated normal curve is constructed 

 those species represented by 4 to 8 specimens will have better than a 50 per 



