424 FAGER [CHAP. 19 



try to remain aware of its limitations, particularly if the groups found agree 

 too well with his preconceptions. One fact which cannot be too strongly empha- 

 sized is that all groups based on samples, however formed, depend strongly on 

 the sampling techniques used, the sample size chosen, the areas over which 

 sampling was done, the season or time of day during which sampling was 

 carried out, the distribution patterns of the organisms being sampled, the 

 mobility and sensitivity of the organisms, etc. They are in this sense "artifacts" 

 of the sampling program. This does not deny the existence of recurrent groups 

 of species in nature, but does require that very careful consideration be given 

 to the relation between the sampling and the organisms before the groups 

 based on samples are equated with natural groups. No doubt the fact that 

 results may be relatively easily checked in the case of terrestrial plant com- 

 munities has been one of the main reasons leading to the advanced development 

 of community studies in plant ecology. 



4. Community Structure 



After a community has been identified, it can be described in terms of various 

 distributional statistics of the component species, in terms of the behavior and 

 physiology of the species or in terms of trophic levels and energy flow. These 

 are, of course, not really separable but they may be so considered as an aid to 

 understanding the community. The following discussion will assume, contrary 

 to fact, that, by the use of an appropriate grouping procedure, the near-surface 

 plankton communities of the North Pacific have been identified as to species 

 composition and their positions in the region have been determined. A start 

 has been made on this and it is hoped that the suggestions made here may 

 provide an impetus to work in this region and elsewhere. The first requirement 

 for further analysis of the communities would be a large number of samples 

 with the appropriate characteristics (size of net, depth and volume sampled, 

 etc.) taken throughout the year and, preferably, at randomly determined 

 positions within the area of each community. On the basis of these one could 

 start to describe in some detail the structure of the communities in terms of 

 the distribution patterns of each species. 



Perhaps the first statistics to be looked at would be the related ones of fre- 

 quency and abundance. The first gives some idea of the spread of a species 

 throughout a community. Species with high frequency and high fidelity (see 

 below) will be good indicators of the presence or absence of a particular com- 

 munity in an unknown area. Abundance may be thought of as either the mean 

 number of individuals per unit volume, the mean number of individuals per 

 unit volume in the samples in which the species was present or the number of 

 individuals relative to those of other species within each sample. The clearest 

 picture of overall abundance is probably given by a combination of frequency, 

 the second abundance measure suggested, and a statement of whether the 

 organisms are aggregated, randomly dispersed or evenly spaced. A convenient 

 index number expressing the extent of aggregation is the constant k of the 



