For 10 points, identify this property that is generally described by the number of genera or species growing in a given area, which decreases with the extinctions of species.
This property is the subject of a set of three metrics proposed by R.H. Whittaker, the second of which expresses the difference in composition of two distinct sites within the area being studied. The generation of this phenomenon is the subject of the Janzen-Connell hypothesis, which explains it in terms of negative density dependence due to pathogen specificity. For a forest, this property is predicted to be highest when (*) disturbances are neither too rare nor too common. This property is measured by an index named for Shannon and Wiener, and the IUCN's Red List indicates threats to this property. For 10 points, identify this property that is generally described by the number of genera or species growing in a given area, which decreases with the extinctions of species.
This property is the subject of a set of three metrics proposed by R.H. Whittaker, the second of which expresses the difference in composition of two distinct sites within the area being studied.
biodiversity [accept answers like "the number of species" before the giveaway]