Individual species estimates . Variance associated with abundance 

 (counts) estimates for individual species is greater than that described 

 for total community density. Three species, Teredo navalis , Balanus 

 crenatus , and Mytilus edulis , collected at Giants Neck and Effluent in 

 1980 were selected to illustrate the range in the precision of the 

 estimates for single species (Fig. 12). The 95% confidence interval 

 about the mean of six replicates for these species ranged from 119% of 

 the mean for T. navalis at Giants Neck in November 1980 to 20% of the 

 mean for M. edulis at Giants Neck in August 1980. 



Four rather than three replicates appear most appropriate for 

 maintaining the precision of these estimates with a reduction in sampling 

 effort. After six replicates the confidence interval for T. navalis is 

 within 73-119% of the mean; that for J3. crenatus is within 35-93% of the 

 mean, and M. edulis is within 20-23% of the mean. In comparison, after 

 four replicates these percents are 105-200%, 49-101%, and 27-35%, respec- 

 tively. 



The differences in precision between total community estimates of 

 abundance and individual species estimates of abundances might be related 

 to panel location, low abundance, or species subtitution. Panel location 

 was previously shown to affect individual species abundances by increasing 

 inter-panel variance. For example, the distribution of T\ navalis at 

 Giants Neck in November 1980 was shown to have considerable bias due to 

 panel location, which resulted in the low precision of the six replicate 

 mean. However, the low precision of the abundance estimates for T. 

 navalis in November 1980 and 15. crenatus in May 1980 at Effluent was 

 primarily caused by their low abundances (or patchiness) on the panels. 

 Species substitution from one replicate to the next has not been identified 



25 



