72
The Essex Naturalist
There are two Nationally Rare (RDB) ant species recorded in Essex one of which
(Solenopsis fugax) has not been recorded since Donisthorpe (1927). In the light
of recent knowledge on its national distribution Myrmica bessarabica (= specioides),
the second species, would have its Rare status revised to Nationally Scarce (Na).
The National Scarce species (Scarce Nb and Na species)
Nationally Scarce species are those falling within the Nationally Notable
categories introduced by Ball (1986). They are species which are estimated to
occur within the range of 16 to 100 ten-kilometre squares of the British
National Grid system since 1970. The specific categorisations of species have
been revised since their inception for a number of taxa including aculeate
Hymenoptera, revised by Falk (1991).
Notable species are subdivided as follows:
Na species estimated to occur within the range of 16 to 30 10-kilometre
squares of the National Grid System.
Nb species estimated to occur within the range 31 to 100 10-kilometre
squares of the National Grid System.
There are four Nationally Scarce ant species recorded in Essex, two Notable
A and two Notable B species.
The Nationally Local species
Nationally Local species are species which are either restricted to a particular
habitat type or to a particular geographical area, or to widespread species which
are intermediate in status between common and scarce species (Ball in Archer
1995). Without exception, nationally "Local" designations in this paper have
been obtained from the RECORDER software. Twelve species recorded in Essex
are in the Nationally Local category.
Regional status categories used in this paper
There are many problems associated with assigning rarity categories to species
and in the comparison of species distribution from county to county, not least in
the difficulty of ensuring adequate recording coverage and scientifically
comparable data. In order to make any kind of sense nationally, and to allow
comparative assessments of sites in nature conservation and planning terms it
seems essential that regional Red Data Book categories are comparable.
Unfortunately at present there is little evidence of a consensus on how to define
regional status categories and comparison between counties is difficult.
Plant (1993) provides a comprehensive example of the definition of regional
distribution and status categories for the LNHS London Area. Malton &
Pearson (1993) define Dorset Scarce Species as occurring in three or fewer sites
in the county and Archer (1998) defines Yorkshire Rare species as species that