Additional identifications a brief summary




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CHIGWELL ROW WOOD

ADDITIONAL IDENTIFICATIONS

A BRIEF SUMMARY

January 2007


P.R.Harvey

32 Lodge Lane

Grays, Essex RM16 2YP

01375 371571


  1. SUMMARY





    1. A list of the species recently identified from previously unidentified material collected at Chigwell Row Wood by Barbara Schulten and John Ismay in 2002 is provided in Appendix 3, together with national and Essex statuses. This list and all the resultant records are also provided as an Excel file.




    1. A total of 121 species was identified from 1733 specimens in the taxonomic groups Arachnida (spiders, harvestmen and pseudoscorpions), aculeate Hymenoptera (bees, wasps and ants) , Dermaptera (earwigs), Isopoda (woodlice), Myriapoda (centipedes and millipedes) and Orthoptera (grasshoppers and crickets).




    1. These species include two Nationally Scarce Notable A taxa, five Nationally Scarce Notable B and nineteen Nationally Local species, as well as seven included in the Essex Red Data list. In addition one Nationally Scarce centipede is New to Essex.




    1. If all the data is examined, including that in the Ismay & Schulten report (2002) then a Species Quality Index (see appendix ) gives a figure suggesting an assemblage of national importance.




    1. Specialised survey for invertebrate groups not targeted by the Schulten and Ismay survey would undoubtedly find many more species of significance and interest.


  1. ANALYSIS ON THE BASIS OF NATIONAL STATUS





    1. In order to provide a broad indication of site quality, an Invertebrate Index can be calculated (Ball, 1986). Using this method, invertebrates recorded are scored according to their national status as follows:

RDB species 100 points

Notable - Na species 50 points

Notable - Nb species 40 points

Notable - N species 40 points

Local species 20 points



Common/Unknown species no score


    1. Adding together the scores in each category provides the overall Invertebrate Index.




    1. However, increased effort at a site will increase the number of species recorded and in time the Invertebrate Index will also increase, rendering use of the Invertebrate Index alone of limited value unless all the sites in a comparison are thoroughly and equally recorded. In order to take account of the overall number of species recorded (as a measure of recording effort) in assessing the rarity value of a species assemblages, a Species Quality Index (SQI) may be calculated through simple division of the Invertebrate Index gained from a site by the number of species recorded at that site.




    1. In this way, SQI is effectively the average number of points which each recorded species is worth. A site with ten recorded species, all of which were of Red Data Book status and so with a species value of 100 points each, would score 10 x 100 points to give an invertebrate index of 1000. Dividing this index by the ten species gives an SQI value of 100.




    1. In fact, sites always support a mix of common and scarcer species so that the SQI value is always well below 100. How far below, is a measure of the value of the site to invertebrate ecology on a national scarcity basis. As a general guide, a “good” invertebrate site in England would have an SQI value of at least 5.00 after moderate recording coverage. An “excellent” site might have a value of 7.50 and any site with an SQI value approaching 10.00 is almost certainly of national significance.




    1. If the data from the current identifications is combined with those provided in the Schulten & Ismay report (2002) then the following summary of species status is found:




Status

Score

Count

Index

RDB3

100

1

100

RDBK

100

2

200

Data Deficient, [formerly RDB1]

100

1

100

Scarce

40

8

320

Notable/N

40

9

360

Notable/Na

40

9

360

Notable/Nb

40

19

760

Local

20

120

2400

Unknown

0

58

0

Recently described

0

1

0

Common/Other

0

282

0













Total




510

4600

Species Quality Index







9.0




    1. The Species Quality Index value of 9.0 is a high value, indicating an important assemblage.



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