Fen Management Handbook - Scottish Natural Heritage

Mollusca (snails, slugs and mussels)
The mollusc fauna of fens are at their richest in sites with high calcium levels, and so are better
represented in rich-fens. The fauna can be rich, and though the number of uncommon species is
small, they include some very rare and greatly declined species. The most important are the narrowmouthed
whorl snail Vertigo angustior, Desmouslin’s whorl snail V. moulinsiana and slender amber
snail Oxyloma sarsi. Some rare aquatic molluscs are associated with drainage ditches and pools in
fenland areas, though they are more strongly associated with grazing marshes: pea mussel Pisidium
pseudosphaerium, large-mouthed valva snail Valvata macrostoma, lesser whirlpool ram’s-horn Anisus
vorticulus, and shiny ram’s-horn Segmentina nitida.
Trichoptera (caddisflies)
Caddisflies are well-represented in fens. Sites containing a range of water bodies of varying
vegetation, size and permanence give the best overall assemblages. There will be additional species
if the site adjoins a river or lake, or if it is spring-fed. The number of uncommon species strongly
associated with fens is limited, however. The leptocerid Erotesis baltica requires shallow, dense,
submerged vegetation in clear water; three species without very recent records (Grammotaulius
nitidus, Limnephilus pati and L. tauricus) have so few records that their habitat requirements are
scarcely known.
Orthoptera
There are few British Orthoptera, and most of these are of restricted southern distribution. This is
therefore not a group of great relevance on most fens. Two species, the mole cricket Gryllotalpa
gryllotalpa and the large marsh grasshopper Stethophyma grossum, had, historically, a strong
association with fens and were locally common, but have since declined to possible extinction or to
virtual or complete loss from fenland habitats.
Further information on the groups of invertebrates associated with fens is available in Buglife (2006):
but not all groups are included, and there is implied variation in the definition of fen in the accounts of
different groups in this multi-authored work.
1.1 Identification of invertebrate interest features
Invertebrate species and assemblages which qualify as notified features are identified according to
the Guidelines for selection of biological SSSIs (NCC, 1989), or according to similar guidelines that
have been used to select ASSIs in Northern Ireland, or which appear on the Habitats and Species
Directive Annex II.
The two most prescriptive chapters (18 & 19) in the SSSI guidelines are those for butterflies and
dragonflies. The guidelines for dragonflies are now superseded by criteria for determining key
Odonata sites (French & Smallshire, 2008). The third chapter (17) dealing with all other invertebrates
does not list species at all but instead gives a series of principles to identify species worthy of SSSI
notification. Using the SSSI guidelines, and working guidance developed since, the invertebrate
specialists in each country agency have identified which species are currently known to be notified
features.
In simple terms, applying the guidelines results in a site being designated either because it has
one or more important invertebrate assemblage or, a strong population of one or more named rare
or threatened species. However, qualifying for notification on the criteria does not guarantee a
site being designated: other factors, notably threat, are also considered. The SSSI system can be
regarded as a selection of sites fulfilling the criteria.
The development of CSM Guidance for Terrestrial and Freshwater Invertebrates (JNCC, 2008)
provides further guidance on the identification of interest features, attributes, targets and methods of
assessment.
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