131
Essex Vulnerable
species known from only two extant populations within the county.
Essex Rare species which are restricted to habitats or sites known to
be under threat in the county e.g. from agricultural improvement,
development or inappropriate management.
species which only occur at sites within the county known to be
vulnerable.
species winch have shown a continuous decline over the last twenty
years and are now known to exist in less than 1% of the covered post
1980 one km squares in the county.
Essex Threatened scarce species with an Essex rarity status of Rare or Scarce and with
a Frequency Ratio of less than 1.5 and Tetrad percentage of less than
1.0 indicating widely isolated populations within the county.
Essex Scarce species which are restricted to habitats or sites known
to be under threat in the county e.g. from agricultural improvement,
development or inappropriate management.
species which have shown a continuous decline over the last twenty
years.
Regionally Important
species which are important in a national context because the county
contains a significance proportion* of the national populations.
species which are important in a national context because they have a
close association with a specific nationally threatened habitat
included in the Biodiversity Action Plan.
species which are important in a national context because they are
dominant in or peculiar to a specific habitat for which the county
contains a large percentage of the national habitat.
*thc author has used the following definitions to define a significant
proportion of the national population, using the definitions of RDB
and Notable categories in Shirt (1987). Ball (1986). revised by Falk
(1991):
RDB species
all species not already defined as Essex Endangered
or Essex Vulnerable are included;
Notable A - species with records from 3 or more ten km squares,
representing between 10-20% of the known national
distribution;
Notable B - species with records from 5 or more post-1980 ten
km squares, representing between 5-10% of the
known national distribution.
Essex Unknown species for which there is insufficient data available to determine a
reliable threat status but winch may be under significant habitat or
site threat.
Essex Naturalist (New Series) 16 (1999)