On the measure of sampling effort used in species accumulation curves

We agree with Willott (2001) that number of individuals would be an appropriatemeasure of sampling effort to compare species accumulation curves among sites, but thereason is not sampling bias. It has been shown theoretically that number of individualsis an unbiased effort unit when density varies among sites, although our results with batsdo not prove this.2.We compared our results using nights and individuals as measures of effort in batspecies accumulation curves, but did not detect changes in the estimated richness usingthe two measures.3.We maintain our position about the utility of species accumulation curves as practicaltools for inventory assessment, even for very diverse groups. To compare curves fromdifferent sites, we agree that number of individuals may be an unbiased measure ofeffort. But to give practical recommendations for sampling, we suggest that effort alsobe expressed in standard sampling units.