These plants are rather rare, some so rare as to be in danger of extinction in the United Kingdom.
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:
- Scarce overall, but locally common
- Scarce, local, but seldom common
- Local, and rare
But those are not easy definitions with which to work. Clive Stace devised an alternative method using the 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.
- R Uncommon. Found in not more than 250 hectads but more than 100 hectads.
- RR Scarce. Found in fewer than 101 hectads but more than 16 hectads.
- RRR Rare. Found in less than 16 hectads.
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:
But the value of Clive Staces' rarity value using his definition is that the 'rarity' can easily be ascertained from published distribution maps without leaving the house.
- Unless the Authors reading of the rules are incorrect (?), then it takes no account of any population decline (or increase) decade by decade. [If the rules were to say to take the whole decade with the least number of hectads since 1987, then it would indeed take into account the decline as years progressed, but the rules don't seem to say that - but your Author has assumed that in allocating rarity categories to plants (since he cannot find a definitive rarity list!)]. So, what was once uncommon in 1987 could have become R (rare) by 2013 but strict adherence to the definition would still classify it as R (uncommon).
- A plant belonging to the RRR (rare) category could, in theory, have more plants than one in the R (uncommon) category, etc.
- A plant belonging to the RRR (rare) category could actually occupy more land area than one in the R (uncommon) category, etc.