Introduces the Site Diversity Index (SDI) for agarics. This is an index of agaric biodiversity which can be used to compare different sites or which records exist.

The index is calculated by grouping the species into 7 frequency classes (based on the number of records in the BMS FRD). Each recorded species increases the SDI by the score of its frequency class (1 for "Common", through to 2300 for "Extremely Rare").

The SDI profile expresses the no of spp that have been found at the site as a %age for each frequency class. The steepest part of the slope moves towards the rarer classes as a site progresses from under-recorded to well-recorded so can be used to suggest where future recording effort should be targetted.