The Essex Naturalist 75
Essex Local species post-1980 records from more than 5% to 15% of the
1km squares covered. It is almost inevitable that these
species will have a restricted status confined either to
a particular habitat type or to a particular geographical
area within the county, or will be species with a
widespread distribution but whose populations are
isolated by habitat fragmentation.
Essex Common species post-1980 records from more than 15% of the 1km
squares covered. On the assumption that there is
reasonable county coverage, this category will consist
of the most widespread and numerous species.
Watsonian Essex Threat categories
Increasingly our wildlife is threatened by habitat degradation, destruction and
fragmentation. National Red Data Book categories should already provide
assessment of the threat at a national level. The following regional threat
categories assess threat at a regional level but also with a national
perspective.
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 from a
single threatened or vulnerable site in the county.
species with no records in the county for more than 50
years and believed extinct.
Essex Vulnerable species which are known as a single extant post 1980
population in the county but for which are further
populations are considered probable.
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
five or fewer 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.