The Essex Naturalist
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have been found in six or fewer 1km squares (solitary wasps and bees) or four or
fewer 10km squares since 1970 (social wasps), five or fewer 10km squares since
1961 (ants) and seven or fewer 10km squares since 1980 (social bees).
Some of the most important factors to be considered include not only the
rarity and frequency of a species but also threats to populations, sites and
habitats and any documented declines or increases in distribution or
frequency either nationally or regionally. Species associated with habitats
under threat regionally, species restricted to habitats or sites that have
become fragmented and isolated, species associated with habitats under threat
nationally but which are well represented in the county and species
associated with specific habitats where the county contains a large percentage
of the national habitat will all constitute taxa deserving regional
conservation status.
In this paper regional categories are provided for species both on the basis of
their distribution in Essex and on perceived threats to their county populations
or habitats. This means each species is given two regional status categories, a
Watsonian Essex Distribution category and a Watsonian Essex Threat Category.
Distributional data is also provided on Frequency Ratios, Tetrad and Covered
1km sq. percentages as defined below:
Frequency Ratios
Species may occur in a very restricted number of 10km squares yet be quite
common and widespread within these areas with high tetrad numbers. Species
occurring in a small number of 10km squares but with low tetrad numbers
indicate a scattered, possibly even widespread, distribution but now with isolated
sites and populations. These are the vulnerable species that require the greatest
nature conservation effort (Pearman, 1997). The huge losses of semi-natural
habitat in Essex make the isolation of populations a very real problem for many
species. Even the more widespread species which occur in many more 10km
squares but with very low tetrad numbers may be under much greater threat of
decline through loss or degradation of habitat than apparent from a 10km
distribution map.
Pearman calculates a Frequency Ratio of tetrads/10km square by comparing
the number of tetrad and 10km square records for a species. With every tetrad
thoroughly covered and a species found in every tetrad the maximum Frequency
Ratio is 25. This figure is unlikely to be approached except in some very
common and widespread species. A very low Frequency Ratio however may
indicate that a species should be of nature conservation concern even though
the 10km square distribution may suggest a widespread and common species.
Significantly Pearman demonstrates that many Scarce plants have very low
Frequency Ratios compared to some RDB species which are quite common and
widespread where they occur.