Total (pet) 



12 14 16 18 20 22 24 26 28 



Replicates 



Figure 2. Effects of increasing number of replications on number and 

 percent of species caught from a population (Oliver and 

 Slattery, 1976) . 



(2) Total Number of Individuals and Number of Individuals Per 

 Species . Animals are usually recorded in two ways: total number of 

 individuals and number of individuals per taxonomic group or species. 

 In Table 3, the 19 species found by Oliver and Slattery (1976) have been 

 ranked and three main groups delineated on the basis of abundance. Means 

 per sample and their 95-percent confidence limits are listed for the 

 four sample sizes. The 19 species are distinguished by the following 

 characteristics: (a) They contain 87 percent of the total number of indi- 

 viduals of all species collected (by group--90 percent mollusks, 90 per- 

 cent crustaceans, and 87 percent worms); (b) variance-to-mean ratios 

 are greater than 1 in all cases and less than 7 with the exception of 

 Prionospio cirri f era and Mediomastus oatiforniensis; and (c) the mean 

 abundance per sample is greater than 1.5 (Oliver and Slattery, 1976). 

 The lower cutoff point in the ranking is somewhat arbitrary, since abun- 

 dance decreases gradually and there is no clear-cut distinction between 

 the last species included and the first one excluded. Dominance of a few 

 species can be seen by the fact that 90 percent of the individuals are 

 contained in 4 of the 19 species. For most of the species, the confi- 

 dence limits decrease sharply from 4 to 8 replicates and continue to 

 decrease to 28 replicates. Exceptions are some of the species with the 

 highest variance-to-mean ratios. 



Table 3 also shows changes in the estimate of the total number of 

 individuals per sample and individuals of major groups per sample. 



14 



