The value of biodiversity is usually confused with the value of biological resources, both actual and potential. A sharp distinction between biological resources and biodiversity offers a clearer insight into the value of biodiversity itself and therefore the need to preserve it. Biodiversity can be defined abstractly as the differences among biological entities. Using this definition, biodiversity can be seen more appropriately as: (a) a necessary precondition for the long term maintenance of biological resources, and therefore, (b) an essential environmental (...) condition. Three values of biodiversity are identified and arranged in a hierarchy: (1) the self-augmenting phenomenon of biodiversity maintains (2) the conditions necessary for the adaptive evolution of species and higher taxa, which in turn is necessary for providing humans with (3) a range of biological resources in the long term. Two broad policy implications emerge: increments of biodiversity should not be traded off against biological resources as if they were the same, and the conservation of biodiversity should be a constraint on the public interest, not a goal in service of the public interest. (shrink)