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may be under significant threat in the county. Distributional data is also provided on Frequency
Ratios, Tetrad and Monad Frequencies as defined below:
Frequency Ratios
Species may occur in a very restricted number of 10km squares vel 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 arc 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 1 Okm or tetrad distribution map.
There are the common species that are able to take advantage of a wide variety of habitats and
the pioneer species which can rapidly move into newly created ruderal habitats but which
gradually disappear as succession takes place. However many of the scarcer and more restricted
invertebrate populations do not seem to be able to move from one site to another easily. These
species evidently have limited powers of dispersal or are not able to enter established
ecosystems and compete successfully. As suitable habitats and populations become fragmented
re-colonisation of isolated patches becomes less likely. The smaller and more fragmented
wildlife habitat becomes the less likely it is that areas will provide the complex of conditions
required for each species, resulting in tire extinction of local populations.
Archer (in Archer and Burn, 1995 & Archer 1996) has shown that there is a clear relationship
between the number of aculeate Hymenoptera species recorded at a site and its area. Thus the
larger the area of the locality the more species are present. For sites where the species lists are
more or less complete this relationship is linear and represented by the regression line given by
the equation In S = C + z In A where S is the number of species. A is the area in hectares, C the
interception on the Y-axis and z the slope or regression coefficient of the relationship. Reduction
in the area of a site will reduce the species complement and there will be a resultant loss to
biodiversity. Despite the difficulty of defining a 'site' it is evident that the isolation of sites and
the reduction in habitat area drastically affects the ability of populations to survive.
Pearman calculates a Frequency Ratio of tetrads/lOkm 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.
Frequency Ratios as a ratio of post 1980 tetrad/l0km square records are used here to allow an
assessment of the frequency of Essex harvestmen 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; the fifteen 10km square records may be grouped
together in one part of the county, or separated and spread across the county.
Essex Naturalist (New Series) 16 (1999)