Essex Red Data threat statuses

The Essex threat statuses used for Arachnida and aculeate Hymenoptera are based on the definitions used for ants (Harvey 1998) and harvestmen (Harvey 1999). Essex Threat statuses have not yet been evaluated or assigned for most other taxonomic groups.

WATSONIAN ESSEX THREAT CATEGORIES

These Watsonian Essex threat categories are defined as:

• Essex Endangered
  • species which are known as a single extant population within a single post 1980 one km square or only from threatened or vulnerable sites in the county.
  • species believed extinct in the county but which if rediscovered would need protection.
• 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 which 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.

*the following definitions have been used 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 at least 10% of the known national distribution;

Notable B - species with records from 5 or more post-1980 ten km squares, representing at least 5% of the known national distribution.

Essex Unknown species for which there is insufficient data available to determine a reliable threat status but which may be under significant habitat or site threat. 

Species of Arachnida, aculeate Hymenoptera and Syrphidae (Diptera) not assigned an Essex Threat status are assumed to be under no direct threat of severe decline or extinction in Essex at the present time or the county is not known to contain a significant proportion of the national populations of the species. Other taxonomic groups have not been assigned a threat status.

Essex Red Data List