On the measure of sampling effort used in species accumulation curves.
This article is a reply to a paper by Willott [Journal of Applied Ecology (2001) 38, 484-486], which was written as a critique of an earlier paper by the authors [Journal of Applied Ecology (2000) 37, 149-158]. Moreno and Halffter agree with Willott that number of individuals would be an appropriate measure of sampling effort to compare species accumulation curves among sites, but argue that the reason is not sampling bias. They state that it has been shown theoretically that number of individuals is an unbiased effort unit when density varies among sites, although their results with bats (Phyllostomidae and Mormoopidae) do not prove this. They compared 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. The authors maintain their 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, they agree that number of individuals may be an unbiased measure of effort. But to give practical recommendations for sampling, it is suggested that effort also be expressed in standard sampling units.