SECT. 4J COMMUNITIES OF ORGANISMS 433 



appropriate unless diatoms make up most of the phytoplankton crop. It also 

 requires identification of plankton patches and repeated sampling of the same 

 patches. Because of the mechanism of division in diatoms, there is a progressive 

 decrease in mean cell width in a diatom population which is growing and this 

 decrease can be related to the number of divisions which have occurred be- 

 tween samplings. Given the initial standing crop, the number of divisions and 

 the average volume of plant material per diatom, one can estimate the theoreti- 

 cal net production. Given the observed standing crop of diatoms at some time 

 after the initial measurement and an estimate of natural (non-grazing) mortality 

 (Cushing uses the number of intact, empty frustules to get this), a comparison 

 can be made with the theoretical net production and a grazing rate for herbi- 

 vores can be estimated from the difference if it is assumed that the change in 

 the diatom population with time is a function only of the division rate, the 

 grazing rate and the natural mortality rate. When applied to a major, and 

 perhaps rather simple, food chain, diatom-CaZamts-herring, off the northeast 

 coast of England, it gave reasonable results. This may be a special situation but 

 it seems that the method might be more widely applied. 



6. Community Theory 



In an earlier section the methods used for describing the structure of com- 

 munities were discussed. This structure is concerned with two concepts : order 

 and complexity (Bray, 1958). The order of a system may be considered as 

 directly related to its content of negative entropy or to its information content 

 or to the amount and kind of energy within and flowing through the system, 

 with particular emphasis on the availability of the energy within it. Complexity 

 is a measure of the number of parameters needed to define a system fully in 

 time and space. These two concepts are not necessarily related although in- 

 creasing complexity often goes with higher order. The difference is perhaps 

 most clearly shown by an example presented by Patten (1959): a single cell 

 of Bacterium pycnoticus has been estimated to contain about 10 13 "bits" of 

 information compared to about 2xl0 28 "bits" for a man; 2 x 10 15 cells of 

 B. pycnoticus, therefore, contain as much information as a man but certainly 

 are not as complex. 



As a community develops from a situation where there are no organisms 

 present in a volume of water or on an area of substratum to a condition of more 

 or less complete filling up of the space or area by organisms, the order of the 

 community increases. It may be regarded as acquiring negentropy from the 

 environment, storing it as coded information in the organisms and then con- 

 verting it to enthalpy and entropy as required. At first the negative change in 

 entropy made possible by import of complex organic substances far over- 

 balances the increase of entropy associated with irreversible processes such as 

 respiration. If plants are part of the community, import may become less and 

 less important as they take over the role of suppliers of negative entropy 

 through photosynthetic processes. Even after photosynthetic production has 



15 — s. ii. 



