These plants are rather rare, some so rare as to be in danger of extinction in the United Kingdom.
There are three categories of Rarity, here listing most abundant first:
But those are not easy definitions with which to work.
- Scare overall, but locally common
- Scare, local, but seldom common
- Local, and rare
There is now 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 at least it is easily ascertainable from published distribution maps ~
- Unless the Authors reading of the rules are incorrect (?), then it takes no account of any population decline decade by decade. [Were the rules were to say to take the whole decade with 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.