On the Measure of Sampling Effort Used in Species Accumulation Curves

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