364 Annals New York Academy of Sciences 



logical factors have operated to greatly limit the numbers of species which 

 can survive in these particular kinds of ecological conditions. 



A few studies which we have done indicate that in springs in which the en- 

 vironment is fairly constant the numbers of species composing a diatom com- 

 munity may be much less than in the very variable environment of an eutrophic 

 or mesotrophic, natural river. It seems that it is the highly variable, yet con- 

 tinuously favorable, environment of natural rivers of these types that is largely 

 responsible for the great diversity of species that make up these communities. 



The fact that the numbers of species remain fairly similar, although the kinds 

 of species vary considerably, suggests that there are a similar number of niches 

 for diatom species in ecologically similar natural areas and more species are 

 available than there are niches for them. Thus, each niche is occupied by a 

 different species. The lack of similarity in kinds of species present is probably 

 in part due to the highly variable environment in a natural river and the avail- 

 ability of species which have their best development in different variations of 

 the environment. Because diatoms have very rapid reproduction under favor- 

 able conditions the populations of certain species can quickly increase, whereas 

 populations of other species decrease beyond the limits of collectability or dis- 

 appear. 



Another important consideration in the study of diatom communities is the 

 kinds of species composing the communities. By careful consideration of the 

 kinds of species associated together, a qualitative evaluation of many of the 

 characteristics of the environment can be made. However, because of a lack 

 of data as to the complete physiological requirements of any species in nature, 

 it is very dangerous to say that the lack of any species indicates that the spe- 

 cific characteristic of the environment under consideration is not there, be- 

 cause the lack of any factor essential for the life of an organism may eliminate 

 it, although all other factors of the environment may be favorable to it. Also, 

 it is hazardous to use changes in the population sizes of specific species as a 

 basis for saying that the quantitative nature of a given environmental factor 

 has changed. For example, we studied two areas in the Guadalupe River 

 which were not over 500 yards apart. The structural environmental charac- 

 teristics of the two areas were very similar. Because no tributaries or pollu- 

 tion entered the river between these two areas during the time of this study, 

 the characteristics of the water were very similar. This was substantiated by 

 chemical analyses. When similar segments of the communities of diatoms 

 were studied the percentages of the community composed of specimens of dom- 

 inant species were very similar, 95 and 90 per cent, respectively (table 3). 

 However, the sizes of the populations of the dominant .species and the kinds 

 of species varied considerably. At Station 2, the population size of Gompho- 

 nema affinis var. insigue was twice that found at Station 1. The population 

 of Gomphonema parvulum was 38 per cent larger at Station 1 than at Station 2. 

 At Station 2 Navicula sp. had a population of 1900 specimens and Nitzschia 

 palea had a population of 2400 specimens yet neither of these species were 

 present at Station 1. Only one of the dominant species, .\avicula Iripunciala 

 var. schizonemoides had populations of similar size at the two stations. 



It is only as a result of thorough and continuous study of an environment 

 and the species living in it that one can venture to describe the quantitative 



