The Essex Naturalist
71
National rarity status categories
Red Data Book and Nationally Scarce categories are given for each species.
Nationally Local and Common status has been obtained from the RECORDER
software. Regional county status, Frequency Ratios and the percentage of
recorded tetrads with records for each species are given according to criteria
defined later in this paper.
The National Red Data Book species (Rare or R species).
The Nationally Rare species are those falling within the Status categories
defined in the British Red Data Books (Shift, 1987) and provisionally updated by
Falk (1991). These are internationally recognised species listed in the various
Red Data Books published by, or under the auspices of, the International Union
for the Conservation of Nature (IUCN). Species included may not be informally
removed or transferred between categories.
RDB species are defined as those species occurring in up to 15 ten km squares
in a national context. There are four categories as follows:
RDB 1 Taxa in danger of extinction and whose survival is unlikely if the
causal factors continue operating. These include:
Species known from only a single locality since 1970;
species restricted to habitats which ate especially vulnerable;
species which have shown a rapid and continuous decline in
the last twenty years and are now estimated to exist in five or
fewer localities;
species believed extinct but which would need protection if
re-discovered.
RDB 2 "Vulnerable". Taxa believed likely to move into the Endangered
category in the near future if the causal factors continue operating.
These include:
Species declining throughout their range;
species in vulnerable habitats;
species whose populations are low.
RDB 3 "Rare". Taxa with small populations which are not at present
endangered or vulnerable but which are at risk. These include:
Species which are estimated to occur in fifteen or fewer
localities.
RDB K Taxa suspected to fall within the RDB categories but which are at
present insufficiently known to enable placement.