416 FAGER [chap. 19 



detail. However, anyone who has looked at the plants and animals living in the 

 intertidal region or at samples of plankton or benthic organisms is aware of 

 the existence of similar, although not identical, recurrent groups of species. 

 The dissimilarities tend to be emphasized when one looks at the rarer species ; 

 the similarities often appear more important when one considers the more 

 common species. It is true that no species will be represented in a sample 

 unless its ecological tolerance range covers the conditions in the environment 

 sampled, but among the factors determining these conditions are the presence 

 and activities of the other organisms. As Elton and Miller (1954) have pointed 

 out, organisms do not exist in nature as isolated single-species populations 

 which have to cope only with the physical environment but rather are in- 

 fluenced by and in turn influence all other organisms with which they come in 

 contact, directly or indirectly. Four of the biotic relationships which are con- 

 sidered important factors in the limitation of numbers of a species are between 

 different species : interspecific competition, herbivore-plant relations, predator- 

 prey relations, parasite-host relations ; and only one, intraspecific competition, 

 might be studied out of the community context. The relative importance of 

 the physical and biological aspects of the environment changes from place to 

 place, the biological generally becoming more important as the physical 

 become less variable, but neither is ever entirely ineffective. When studying 

 assemblages in nature it is necessary, then, to conceive of a complex network 

 of action-reaction involving both physical and biological factors, and defined 

 by a set of co-ordinates in time and space. It will seldom, if ever, happen that 

 such a network will have sharp and impassable limits but it will often be found 

 that there are broad areas of considerable internal similarity bordered by 

 relatively narrower regions of rapid change, these in turn grading into other 

 areas with internal consistency. No one of the internally similar areas will be 

 exactly like another but if each must be treated as an unique individual, 

 generalization will be difficult. One needs, therefore, to decide how often and 

 how precisely an assemblage must be repeated in space before it can be con- 

 sidered a general enough phenomenon to merit careful study in the hope of 

 discovering some widely applicable principles. This decision ultimately rests on 

 experience and intuition, but there are some objective ways of identifying, 

 describing and comparing assemblages which will help. These are discussed in 

 the section on community structure (page 424). 



Variation in time must also be considered. Should each seasonal phase be 

 considered as a separate community or should they be thought of as aspects of 

 one community? The plant ecologists take the latter view; e.g. a meadow 

 community is accepted although the spring flora may be quite different from 

 the summer flora and that from the autumn and winter floras. This seems a 

 reasonable point of view for there is continuous change throughout the year 

 and any subdivision would seem to be unnecessarily arbitrary. The same ques- 

 tion arises in connection with succession. In the normal course of events, 

 cleared or partially cleared patches will be formed in intertidal and benthic 

 communities and may be, by grazing, predation, exhaustion of a particular 



