MISCELLANEOUS: RARITY - INFO
There are many plants which have several sub-species and sometimes it is only one particular sub-species which is actually scarce, othertimes several sub-species. But often one of the sub-species is also common, but occasionally not and they are all scarce to various degrees. Where it is only a select sub-species which is scarce, the sub-species in question is written out in full next to the Common Name (using 'ssp.' as the abbreviation for sub-species). For plants lacking sub-species, it is the species itself which is scarce.
Plants endangered or of low abundance by some measure are listed in the IUCN Red Data List (an XLS document) of 2005 with changes to 2010 (list courtesy of BSBI website), but this is extremely convoluted to work out and not easy to interpret afterwards either. It attempts to classify plant scarcity into several categories, the simplest interpretation of which is:
But those are not easy definitions with which to work. Clive Stace devised an alternative method using the E, R, RR, and RRR designations described here.
This method is a more practicable definition of Rarity; one that can more easily be determined and one that is based on the number of hectads (10km x 10km squares - of which there are a total of 3859 in the BI) which any one plant occupies in the British Isles since 1987. The rarity can then be determined directly from the BSBI distribution maps, the squares are even counted for the Reader when the appropriate year ranges are selected.
Current Examples (these may change as the years progress if the authorities move the starting year or revise the definition):
As the reader may themselves appreciate, the above latter definition of 'rarity' has several major flaws: