Guidelines for the Selection of Biological Heritage Sites
7. Section 2: Species Guidelines
7.8 INVERTEBRATES: MOLLUSCS (MOLLUSCA)
Application (all invertebrate guidelines)
Identification of sites on the basis of their invertebrate animal species should take account of the needs of many invertebrates for habitat and structural diversity, both at the 'macro' and 'micro' scales. Small-scale habitats of particular importance to some invertebrates, such as dead wood or small patches of bare ground, occur and indeed may shift their position over time within large scale habitats e.g. woodland or grassland, on which the invertebrates also depend. Moreover the larval and adult phases of the same species often need quite different habitats. Invertebrates generally have annual life cycles, and their survival on a site depends on the continued availability of the right mixture of habitats at the right time of year - every year. Such factors should be taken into account when determining site boundaries.
Species lists, especially for less well-recorded groups of invertebrates, should be regarded as tentative. They include species recorded in Lancashire since 1950, those not recorded since 1978 being shown with an asterisk, or other notation.
It should be noted that the category 'nationally scarce', referring to species believed to occur between 16 and 100 10km squares of the National Grid, is sometimes subdivided in relation to invertebrate groups into 'Notable A' and 'Notable B'. This subdivision has not been recognised in the following guidelines.
Justification
In Lancashire, as elsewhere, there are more species of invertebrate animals than of all plants and other animals combined. Many invertebrate groups and species have declined dramatically in recent decades, and their conservation is a matter of widespread concern in Europe. Until recently, it was thought that if sites were selected (and managed) on the basis of their botanical interest, then the invertebrates would automatically be catered for too. This is not so: it is important that due regard is paid to the contribution that invertebrates make to biodiversity, and of their habitat needs, in their own right, insofar as available information allows.
With such a large number of invertebrate groups, and the relative lack of knowledge about many of these, it is impossible to develop individual sets of guidelines for every species-group at the present time. Where there is an adequate database for a particular species-group relating to Lancashire, specific guidelines have been devised and appear below. For the remainder, there is a single, 'catch all', section which follows the standard approach adopted throughout the species section.
iv) MOLLUSCS (MOLLUSCA)
Application (all mollusc guidelines)
Acceptable evidence of the presence of mollusc species includes finds of living animals or empty shells. The following guidelines relate only to terrestrial and freshwater species.
Justification (all mollusc guidelines)
Molluscs are one of the few non-insect groups of invertebrates to be relatively well-recorded in recent years, although recording in Lancashire is patchy. They have poor powers of dispersal, and some terrestrial species are particularly good indicators of long continuity of habitat conditions.
Mo1 Any site which regularly supports a population of a species of mollusc listed in Schedule 5 of the Wildlife and Countryside Act 1981 (as amended) or in British Red Data Books: 3. Invertebrates other than insects.(1)
Application
This species to which this guideline applies include:
| Lymnaea glabra | Mud Pond Snail |
| Margaritifera margaritifera | Freshwater Pearl Mussel |
| Vertigo angustior | Narrow-mouthed Whorl Snail |
Justification
The species in the above category are either threatened or rare in Britain and there is a national responsibility for their conservation.
The whorl snail listed is now very rare in Britain, although, interestingly, it is much more widespread as a fossil from the Flandrian period.
Mo2 Any site which regularly supports a population of a 'nationally scarce' species of mollusc.
Application
The species to which this guideline applies include:
| Abida secale | Large Chrysalis Snail |
| Acicula fusca | Point Shell |
| Clausilia dubia | Craven Door Snail |
| Pisidium pulchellum | A pea mussel |
| Vertigo alpestris | Mountain Whorl Snail |
| Vertigo pusilla | Wall Whorl Snail |
| Vitraea subrimata | A glass snail |
Justification
Nationally scarce species are recorded only from 16-100 10km squares (inclusive) in Britain; their conservation is a matter of national concern.
Mo3 Any site which regularly supports a population of a species of mollusc which occurs at 3 or fewer localities in Lancashire.
Application
Any site for species in this category which are not covered by Guidelines Mo1 or Mo2 should be considered. The species to which this guideline applies includes:
Pomatias elegans Round-mouthed Snail
Justification
This species, although frequent in some parts of England, is rare in Lancashire. Subject to further localities being colonized or discovered, they may provisionally be regarded as 'endangered' in a County context.
Mo4 Any site which regularly supports a population of a species of mollusc which is recorded from more than 3 localities in Lancashire, but which could be at risk because of recent rapid decline, small populations, habitat loss or change, or is at the edge of its British range, where such populations contribute significantly to the distribution pattern or the total population size of that species in the County.
Application
Sites in the above categories (not included under Guidelines Mo1 or Mo2) may be considered for inclusion where they significantly extend the known geographical range of a species in Lancashire, or supports a significant proportion of the estimated total County population of that species. The species to which this guideline may apply include:
| Acanthinula aculeata | Prickly Snail |
| Aplexa hypnorum | A bladder snail |
| Ashfordia granulata | Silky Snail |
| Azeca goodalli | Three-toothed Snail |
| Balea perversa | Tree Snail |
| Bathyomphalus contortus | A ramshorn snail |
| Cecilioides acicula | Blind Snail |
| Cernuella virgata | Striped Snail |
| Cochlodina laminata | Plaited Door Snail |
| Columella aspera | A whorl snail |
| Helicigona lapicida | Lapidary Snail |
| Leiostyla anglica | English Chrysalis Snail |
| Leucophytia bidentata | A hollow-shelled snail |
| Limax cinereoniger | Ash-grey Slug |
| Milax gagates | Smooth Jet Slug |
| Perforatella subrufescens | Dusky Snail |
| Pisidium lilljeborgii | A pea mussel |
| Pupilla muscorum | Moss Snail |
| Pyramidula rupestris | Rock Snail |
| Spermodea lamellata | Plaited Snail |
| Sphaerium rivicola | A freshwater cockle |
| Unio pictorum | A freshwater mussel |
| Unio tumidus | A freshwater mussel |
| Vallonia pulchella s.s. | Beautiful Snail |
| Vertigo antivertigo | Marsh Whorl Snail |
| Vertigo substriata | A whorl snail |
| Viviparus contectus | Lister's River Snail |
Justification
Such species, whilst not qualifying under Guideline Mo3, are nevertheless either of very restricted distribution, or exist only as small scattered populations, in Lancashire.
(1) Bratton (1991)