428 



[chap. 19 



Table I is an example of the sort of information which it is suggested should 

 be obtained for each species as a basis for understanding the structure of the 

 community. 



Table I 



Statistics Associated with Three Hypothetical Species of Small Planktonic 

 Crustacea Found in Samples Taken within the Area Occupied by a Particular 



Plankton Community 



Species A : An important and typical member of the community. 



Species B : Most probably a stray from another community, perhaps distant. 



Species C : A member of some importance in the community but not typical. 



Additional information can be obtained from the numbers of individuals of 

 different species within the samples characteristic of the assemblage. In order 

 to determine whether any particular sample or group of samples is better than 

 the others, as indicated by higher numbers of individuals of the species in the 

 community, the set of samples is ranked in terms of each species separately 

 and the ranks are summed for the different samples. These sums are then 

 tested for concordance (Kendall, 1955, pp. 94-102). Significant concordance 

 indicates that, despite species differences, the community in general agrees on 

 what is a good or bad habitat. One can, therefore, pick out the "best" and 

 "worst" samples and examine their characteristics in the hope of getting an 

 idea of conditions favorable and unfavorable to the community as a whole. 

 Pairs of species can also be examined for correlation between abundances, 

 always remembering that multiple comparisons within the same set of data 

 considerably change the significance levels (e.g. the probability of getting at 

 least one comparison significant at the 0.05 level when the ten possible pairs 

 of comparisons between five species are made is about 0.40 not 0.05). Despite 

 this limitation, a highly significant correlation, either positive or negative, 

 probably indicates something worth investigating further in terms of possible 



