Ricotta argues against the existence of a unique measure of biodiversity by pointing out that no known measure of α-diversity satisfies all the adequacy conditions that have traditionally been set for it. While that technical claim is correct, it is not relevant in the context of defining biodiversity which is most usefully measured by β-diversity. The concept of complementarity provides a closely related family of measures of biodiversity which can be used for systematic conservation planning. Moreover, these measures cannot be (...) replaced by summary statistics but must rely on inventories of biodiversity surrogates at candidate sites for conservation. (shrink)