129
Tetrad Percentage
There are 314 tetrads and 415 one km squares with harvestmen records in the county out of a
total of 990 and 3958 respectively. Whilst the number of recorded tetrads and 1km squares
(monads) is only 32% and 10.5% of the total respectively, it must be remembered that much of
the county consists of arabic farmland with virtually no diversity of harvestmen 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 rarely are 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.
Tetrad Frequency
The number of post 1980 letrad records is given as a percentage of the total number of recorded
tetrads. This figure will enable comparison with other counties allowing for differences both in
county area and recording coverage.
The provision of Tetrad Frequencies as well as Monad Frequencies for each species should
allow comparison with the status categories used in Morris (1998), as well as providing
comparability for counties where data is only held on a tetrad basis.
Monad Frequency
The number of post 1980 1km records (monads) 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 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 harvestmen records. This provides some measure of comparability with other
counties for the factors of recording coverage and area.
As noted above the definitions used are essentially the same as those in Harvey (1998) but with
the addition of a County Ubiquitous category to allow improved comparability with Morris
(1998). Analysis of over 52,000 records in Essex for spiders and aculeate Hymenoptera shows
that the average tetrad frequency for species is approx. 1.4 times the 1 km frequency. The
"County Ubiquitous" category here should therefore be comparable to that found in Morris. The
resultant 'Ubiquitous' species are very much in line with the author's experience in the field.
Essex Naturalist (New Series) 16 (1999)