Patrick: Structure of Diatom Communities 365 



changes in the natural environment of a river by changes in the quantitative 

 abundance of specific kinds of species. 



In conclusion, our studies have shown that diatom communities can be best 

 characterized by consideration of the kinds of species, the numbers of species, 

 and the relative sizes of the populations of the species that comprise the com- 

 munity. An excellent way to consider the relative sizes of the populations of 

 all the species studied is by the construction of a truncated normal curve. The 

 presence of certain kinds of species may tell us much as to the quaUtative char- 

 acteristics of an environment. The best means for determining quantitative 

 shifts in the environment is by considering the shift in numbers of species and 

 the ratio of the number of species with small populations to those with large 

 populations. Perhaps the reasons that the numbers of species do not vary 

 greatly is that there are similar numbers of niches for species occupancy in 

 ecologically similar types of streams. Also, at any one time there are prob- 

 ably more species available to inhabit natural eutrophic or mesotrophic areas of 

 streams than there are niches available for species occupancy, thus, each niche 

 is filled with a different species. The reasons that the kinds of species vary con- 

 siderably in streams of these types are the continually varying yet favorable en- 

 vironment; the availability of species which have their best development in 

 different conditions of the environment; and the ability of diatom populations 

 to quickly expand or contract with changes in the environment. 



References 



Patrick, R. 1961. A study of the numbers and kinds of species found in rivers in eastern 



United States. Acad. Nat. Sci. Phila. 113(10): 215-258. 

 Patrick, R., M. H. Hohn & J. H. Wallace. 1954. A new method for determining the 



pattern of the diatom flora. Acad. Nat. Sci. Phila. No. 259. 

 Preston, F. W. 1948. The commonness, and rarity, of species. Ecology. 39: 254-283. 

 Thienemann, a. 1939. Grundzuge einer allgemeinen Okologie. Arch. Hydrobiol. 35; 



267-285. 



