Zoogeographical studies on the land Mollusca of the Province of Dalsland (SW. Sweden)

For many years the Museum of Natural History in Goteborg has undertaken faunistic studies in the Province of Dalsland in SW Sweden. This region is of special interest not only because of its ecological diversity but because it straddles the border between the Boreal and S. Scandinavian regions, a boundary reflected by the northern limit of oak. Moreover, calcareous rocks occur within the Precambrian Dal series in the central and northern parts of the Province and these contribute to the ecological diversity of the region.

This book provides detailed distribution maps for all 73 species of land molluscs known from the Province. Thirteen of these species are regarded as human introductions, the rest are believed to be indigenous. The distributions are shown as 'dot maps', rather than 'grid maps', and are based on data from 422 localities collected mostly by the author during the 1970s and 1980s. Older data obtained between 1938-1967 is also incorporated. The distribution patterns have been analysed and eight main groups recognized. These local distributions are then discussed in relation to the overall range of each species in Scandinavia. The Zoogeographical affinities of the fauna are discussed and full ecological data provided.

Eight species, including Spermodea lamellata, Ena obscura, and Balea perversa, apparently have their northern limit in this part of Sweden, whereas part of the southern range ofZoogenetes harpa also falls in this region. From a British perspective there were some surprises amongst the maps. The slug Arion ater, so abundant throughout most of the British Isles, was found in only 16-6 of the sites sampled. Moreover Deroceras reticulatum occurred in less than 10, whereas D. agreste was found in no less than 11-7 of sites. Arion circumscriptus (6) proved to be surprisingly scarce and far less comon than A. silvaticus (22.1). There were twice as many records for Aegopinella pura (21-3) than for A. nitidula (9-7), which in this region is said to be an inhabitant of'calciphilous woodland' (p. 19). Nesovitrea hammonis is not unexpectedly the most common and widespread species in Dalsland with Columella aspera, Punctum pygmaeum, Euconulus fulsus and Clausilia bidentata all occurring in over 50 of the sites.

The distributional limits for 39 species were accurately mapped and found to coincide with various topographical and geological boundaries, which are also shown on some accompanying maps. Furthermore, land molluscs from ten selected 5x5 km squares, which has previously been studies floristically, were analysed statistically and compared with the botanical data.

Dalsland, with an area of only 4136 sq km, is one of the smallest provinces ofSweden and this survey is therefore akin to some of the county tetrad (2x2 km) schemes presently underway in Britain. It will therefore be of considerable interest to those engaged on such projects and to everyone with an interest in the distribution and ecology of land snails. The author is to be congratulated for the sustained mapping over several years and for bringing the results to fruition in such an elegant manner.