What do we mean by diversity? The path towards quantification

Theconcept of biological diversity has evolved from a simple count ofspecies to more sophisticated measures that are sensitive to relativeabundances and even to evolutionary divergence times betweenspecies. In the course of this evolution, diversity measures haveoften been borrowed from other disciplines. Biological reasoning aboutdiversity often implicitly assumed that measures of diversity hadcertain mathematical properties, but most of biology’s traditionaldiversity measures did not actually possess these properties, asituation which often led to mathematically and biologically invalidinferences. Biologists now usually transform the traditional measuresto «effective number of species», whose mathematics does support mostof the rules of inference that biologists apply to them. Effectivenumber of species, then, seems to capture most (though not all) ofwhat biologists mean bydiversity.