SECT. 4] COMMUNITIES OF ORGANISMS 417 



micronutrient, etc., in plankton communities. Watt (1947) has discussed this 

 problem in connection with a number of plant communities and has concluded 

 that in many cases a community is best considered as a sequence of inter- 

 related processes which lead to a certain assemblage ; this then is broken down 

 to nearly the initial conditions and the sequence starts again. At any one point 

 in time, all phases are found at some point in the space occupied by the com- 

 munity. Something like this seems to happen in Mytilus beds on exposed 

 rocks ; e.g. a buildup from bare rock to a crop of Mytilus with its associated 

 organisms which is too heavy to withstand winter storms ; practically bare 

 rock exposed by the breaking off of sections of the bed and a sequence of stages 

 leading again to the Mytilus. Although the different phases might be separated 

 for convenience of study, it seems more reasonable to consider them all as parts 

 of the Mytilus community. 



A multi -species assemblage has certain properties which are not shared by 

 the individual species populations, in the same sense that the mortality rate 

 or birth rate or age structure of a population are properties not shared by 

 individual organisms in the population. There is a structure in terms of numbers 

 and kinds of organisms in each of the trophic levels represented ; there is an 

 overall pattern of flow of energy and matter through the assemblage ; there is 

 a multiplicity of possible pathways for the flow of energy and matter and this 

 multiplicity confers stability on the assemblage (MacArthur, 1955); there is 

 often an ordered succession of species abundances in either time or space or 

 both ; there are effects upon the environment which are the result of several 

 species working in sequence. From a theoretical point of view, there seems, 

 therefore, good reason to study assemblages or communities and the species 

 interactions occurring within them. 



From a practical point of view, the study of communities is exceedingly 

 complex and, with the present state of our knowledge, a complete study is 

 probably impossible. A major difficulty is taxonomy. No one person has much 

 hope of becoming a competent taxonomist in all the groups which will be 

 represented in even a moderate-sized community, certainly ranging from 

 bacteria, fungi and algae to small invertebrates and, perhaps, to higher plants 

 and large vertebrates. There are a number of approaches to the solution of this 

 difficulty. One is to cut off arbitrarily at some size level(s) and study just a part 

 of the community (this is what the exponents of the study of single species do 

 in an extreme way) ; another is to guess at the functions of the organisms 

 present and talk in terms of energy flow and the biomass of producers, herbi- 

 vores, carnivores and decomposers (this tends to obscure the biological nature 

 of the system in a cloud of thermodynamics) ; another is to take identifications 

 to large classificatory groupings and then assign numbers or letters to what 

 seem to be separate entities within these groupings (this is much like suggesting 

 that a modern organic chemist should be satisfied to report that he analyzed 

 the leaves of a certain plant and found three aromatic compounds and two 

 heterocyclic compounds, none of which he has identified but all of which he 

 thinks are important). The first approach can include a respectable part of 



