Atlas of the land and freshwater molluscs of Britain and Ireland

The eagerly awaited second edition of "the atlas" has finally appeared, 23 years after its forerunner, to the great delight of those who have contributed to the mapping scheme over the years. So how does the new compare with the old? The new edition has a smaller format, a hard cover and reliable binding. The introductory text is far more extensive and reviews the 'Early history of recording'. The Mapping scheme' (its origin, field procedures and completeness), 'Factors influencing distributions' and The history of the British fauna'. The new format, identical to other atlases published by Harley Books, allows space for brief comments on habitat, history and status (including Red Data book criteria where appropriate) to accompany the distribution maps for each species. Drawings of each species appear as vignettes in the top left-hand corner of each map. These have been culled from various sources (some have been specially drawn) and are a welcome addition, although the quality of the images is somewhat variable. Following the 'Distribution maps and species accounts', which obviously form the main component of the book, are six further maps of environmental factors that influence many of the distributions (January and July mean temperatures. Annual rainfall, Calcareous rocks. Maximum altitude and Sulphur dioxide). Every locality mentioned in the text is then listed together with its 10 km grid reference. An exhaustive list of recorders follows, although it is worth noting that most of the new records that appear in this Atlas were submitted by a tiny subset, perhaps no more than about 40 dedicated enthusiasts. I mention this in case others are deterred from initiating a comparable scheme by what appears to be an impressively large work-force. The Atlas concludes with a select Bibliography of over 200 references and a useful Index that lists synonyms in italics alongside species names, still current.

As in the first edition, the distribution maps have been plotted on a 10 km square basis (using technology made available at the Biological Records Centre, ITE, Monks Wood). Three symbols appear on the maps: records made in or after 1965, records made prior to 1965 (the majority of these belong to the period 1880-1914) and fossil occurrences (Lateglacial and Postglacial). The maps therefore differ from those of the first edition in plotting as old records those earlier than 1965 rather than those earlier than 1950, and in showing Lateglacial, as well as Postglacial (=Flandrian) records for all species, rather than just a select few. This has resulted in the inclusion of maps for several species (e.g. Columella columella, Discus ruderatus, Nesovitrea petronella, Cochlicopa miens, Trochoidea geyeri, Helicopsis striata, Margaritifera auricularia and Pisidium vincentianum) that have never been found living in the British Isles in historical times. Although the Atlas focuses specifically on land and freshwater species, a few semi-marine snails of saltmarsh and brackish water are also mapped, including three (Heleobia stagnorum, Truncatella subcylin- drica and Paludinella littorina) that were omitted from the first edition. Greenhouse aliens and chance 'casuals' have been excluded, as have records of a few introduced species that flourished briefly before dying out (e.g. Cemuella neglecta at Luddesdown, Kent). The nomenclature has been updated in line with continental usage but such changes are conservative and have been kept to an absolute minimum.

At the outset of the mapping scheme in 1965, the intention was to record an average of 60 of the estimated species living in each 10 km square. The new coverage map is certainly impressive and suggests that this target has largely been achieved for most parts of the British Isles. Comparison of the coverage maps in the two editions shows substantial improvement, particularly for parts of Ireland and Scotland. Many of these new records have resulted directly from specially organized mapping expeditions by members of the Conchological Society. Some regions have been mapped intensively, especially those counties that have undertaken tetrad surveys (Devon, Cardigan, Bedfordshire, Suffolk and the Isle of Wight). Other areas remain poorly known (Nottinghamshire and Staffordshire). Despite these biases, the coverage is now suffi- ciently good to allow confidence in the reality of the maps.

The maps provide excellent data that should enable us to formulate hypotheses about controlling factors. Many maps can be interpreted in terms of geology or climate and are an elegant confirmation of the primacy of these factors in controlling distributions. Many calciphile species (e.g. Pomatias elegans) show a strong affinity for chalkland soils. Others, such as Pyramidula rupestris, demonstrate a liking for other sorts of limestone, particularly the Carboniferous Limestone, although the accompanying map of Calcareous rocks does not allow such discrimination. Not surprisingly, the calcifuge Zonitoides excavatus shows a general avoidance of any limestone substrate. One unex- pected finding is the aversion that some species (Trichia striolata and Lauria cylindracea) appear to show to the Triassic bedrock of the Midlands. Other distributions are obvi- ously determined by climate. The distribution of Perforatella subrufescens, for example, coincides with areas of highest annual rainfall. Other distributions are less easily explained, that of Ashfordia granulata presenting a particularly perplexing problem.

Additional species have been added to the British fauna. Since 1976, one aquatic species (Ferrissia wautieri), four species of land snail (Vertigo genesii. Vertigo modesta, Perforatella rubiginosa and Paralaoma caputspinulae) and one species of slug (Tandonia rustica) have been discovered living in the British Isles for the first time. The three records of Corbicula fluminea from East Anglia were discovered too late for inclusion.

A further outcome of the mapping scheme has been the stimulation of research into the taxonomy of various poorly known groups, especially certain families of slug. The systematics of arionid slugs, in particular, has undergone something of a revolution in the last twenty years and the maps for certain segregates in this group are consequently the least satisfactory in the entire Atlas. Tetrad surveys conducted during the last 20 years have separated these newly recognized taxa but this has not been done generally. Consequently, these segregate maps tend to reflect the location of these new surveys, rather than 'real' distributions. Devon is not therefore an arionid hotspot! No attempt has been made to map the distributions of the segregate species of Lymnaea. Molecular and other evidence suggests that both the palustris and peregra groups are comprised of a number of genuine species. It is therefore ironic that we still cannot identify what are likely to be our most widespread species of aquatic snail.

One of the important objectives of mapping is to illuminate change and this is why old and fossil records are included in the survey. Some alarming patterns emerge, which give rise to serious conservation concerns. For example, Myxas glutinosa and Segmentina nitida have contracted their ranges dramatically during the last century. It is imperative to understand the precise causes of these declines if we are to save the species as part of our fauna. Lymnaea glabra has also declined significantly but far less dramatically. Amongst the land snails, Leiostyla anglica, Spermodea lamellata and several species of Vertigo (pusilla, substriata, moulinsiana, genesii, geyeri, alpestris and angustior) show evidence of long-term decline, whereas other species (Balea perversa, Helicigona lapicida, Pupilla muscorum and Helicella itala) appear to have disappeared from many sites in eastern England during the last hundred years. Readers may be puzzled by the increased number of pre-1965 records on some of the new maps. This does not always reflect the discovery of new populations of dead shells but rather the more assiduous abstraction of records from the older literature and especially from unpublished note- books. Not all the maps present such a gloomy picture. A few introduced species appear to have expanded their ranges during the last 25 years. Hygromia limbata and H. cinctella have both spread beyond their South Devon strongholds. The latter, first noted in Paignton in 1950, has established colonies in Worcestershire and has extended eastwards as far as Kent. The vermiform slug, Boettgerilla pallens, has spread in even more spec- tacular fashion and is now distributed over most parts of the British Isles, since its first discovery here in 1972.

The Atlas is the culmination of nearly 40 years work by members of the Conchological Society. The success of the mapping scheme can be attributed to two main factors. First, the simplicity of its field procedure, which enabled the participation of a central core of enthusiatic amateurs. Second, the whole venture has been coordinated since its inception by Michael Kerney. The records submitted to him, as Non-marine Recorder, have all been carefully scrutinized and dubious records deleted, unless supported by voucher specimens. The role has been far from passive and has required both patience and tact. To ensure the accuracy of records, he has essentially provided an informal identification service and he has become famous for answering enquiries by return post. This has required enormous dedication and the success of the scheme stems largely from this commitment.

Most people involved with the mapping scheme will already have purchased their copy of the Atlas and this review is intended to entice those less directly involved or readers from abroad. The publication of this Atlas is an important milestone that I hope will have two direct consequences. First, I hope it will inspire people to initiate comparable schemes in those European countries where these do not exist. Second, if this happens, I hope that one day I will own an Atlas showing molluscan distributions for the whole of Europe, plotting maps on a 50 km square basis. Such atlases already exist for plants (Atlas Flora Europaea) and mammals (The Atlas of European mammals, Poyser Natural History). This Atlas of the land and freshwater molluscs the British Isles is a valuable contribution to this end.