74
The Essex Naturalist
Frequency Ratios as a ratio of post 1980 tetrad/10km square records are used
here to allow an assessment of the frequency of Essex ant species and the
isolation of their populations. Unfortunately this is still far from a satisfactory
method of assessing the isolation of populations: one tetrad record in one 10km
square will provide the same Tetrad Frequency as fifteen tetrad records in fifteen
10km squares and the fifteen 10km square records may be grouped together in
one part of the county, or separated and spread across the county.
Tetrad percentage and Recorded 1km square percentage
There are 350 tetrads and 484 one km squares with ant records in the county
out of a total of 990 and 3958 respectively. Whilst the number of recorded 1km
squares is only 12% of the total, it must be remembered that much of the county
consists of arable farmland with virtually no diversity of ants or other wildlife.
For each species the number of post 1980 tetrad records is given as a
percentage of the total number of tetrads within the county. On the assumption
that the tetrad records are evenly dispersed throughout the county this figure
will provide an approximate estimate of average population density. Of course
populations are rarely dispersed evenly. In Essex they may be confined to coastal
areas, to particular habitats which are unevenly represented in the county or the
populations may be clustered together for other reasons.
The number of post 1980 1km records is given as a percentage of the total
number of recorded 1km squares. This figure will enable comparison with other
counties allowing for differences both in county area and recording coverage.
Watsonian Essex Rarity categories
It is impracticable at present to address most of the problems associated with
varying coverage and comparability of fieldwork (e.g. Rich 1998) but in this
paper regional rarity categories based on distribution are defined by the number
of 1km records as a percentage of the total number of 1km squares in the county
with ant records. This provides some measure of comparability with other
counties for the factors of recording coverage and area.
These Watsonian Essex Rarity categories are defined as:
Essex Extinct species species not recorded in the county for more than 50
years.
Essex Rare species post-1980 records from 1% or less of the 1km squares
in the county covered for ant species (i.e. five or less
1km square records). This category is approximately
equivalent to the Dorset Scarce and Yorkshire Rare
Species used by Mahon 6k Pearman (1993 and Archer
(1998) respectively.
Essex Scarce species post-1980 records from more than 1% to 5% of the
1km squares covered.