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This is the first publication of the series summarising the current state of knowledge on mollusc biodiversity and distribution in Wielkopolska (W. Poland). Here, we give a historical background of the research conducted so far in the region; we describe the geography and environmental conditions which have determined the present shape of the region’s malacocoenoses. We also present a short summary of the results of the studies carried out in the region. Data from 89 publications and 73 unpublished papers (4,155 localities) show that 167 mollusc species were recorded from Wielkopolska during the last fifty years.

We present the list of molluscs recorded from Wielkopolska (W. Poland) during the last fifty years. It includes the 167 species (47 freshwater snails, 92 terrestrial gastropods, 28 bivalves) recorded from the region between July 1st 1958 and July 1st 2008. The number of localities recorded during the last 50 years is given for each species. The list is based on data from 4,155 localities described in 89 publications and 73 unpublished papers. Most of the information was published in Polish or not published at all, and thus inaccessible to a wider malacological audience.

We present the results of a terrestrial mollusc inventory in the Cybina River Valley (W. Poland). The species richness in 39 sites served as a biodiversity criterion for the whole area. The total number of 3,193 specimens collected represented 51 species (22 families), i.e. about 60% of terrestrial snail species of the whole Wielkopolska region. Species regarded as rare in the region and in the whole country constituted 22% of the whole material. Two of them are especially noteworthy: Vertigo angustior Jeffreys (Annex II, EU Habitats Directive), and Ruthenica filograna (Rossmassler) which is very rare in the region.

The paper presents results of two-year (2003–2005) monthly studies on terrestrial malacocoenoses in an ash-elm forest (Fraxino-Ulmetum) near Duszniki in western Wielkopolska (W. Poland). Species composition, dominance structure and abundance of malacocoenoses of selected plots were analysed. Twenty species of ten families were recorded, represented by 7,619 specimens. The main components of the malacocoenoses were Nesovitrea hammonis (Strom), Columella edentula (Drap.), Vallonia costata (O. F. Mull.), Punctum pygmaeum (Drap.), Cochlicopa lubricella (Porro) and Vertigo pusilla O. F. Mull. The species composition was stable but the abundance varied considerably.

Monacha cartusiana (O. F. Muller, 1774) was found at new sites in Poland: in Janikowo near Inowroc3aw and in Poznan. At the site in Poznan, adult individuals were relatively large, as their shell width ranged from 14.5 to 19.7 mm. It must be emphasised that adult individuals of M. cartusiana with shell width of nearly 20 mm have not been reported so far. The new sites of this snail indicate that it has become a permanent component of the Polish fauna.

Terrestrial malacocoenoses of two neighboring forest complexes in W. Wielkopolska were sampled in 2003–2004; their structure and abundance were analysed in selected plots of oak-hornbeam forest of different protection status and degree of anthropogenic transformations. Twenty species (10 families) represented by 1,588 specimens were collected. The main components of the malacocoenoses were: Nesovitrea hammonis, Aegopinella nitidula, Ae. pura, Punctum pygmaeum, Cochlicopa lubricella, Perforatella incarnata and Carychium minimum. Both species composition and abundance varied greatly between the plots. The gastropods showed no preference for old oak-hornbeam stands or plots located in nature reserves. This may suggest that even under strong anthropopressure and in much transformed environments the snails are capable of maintaining a high diversity.

Malacocoenoses containing Ruthenica filograna (Rossm.), a forest-dwelling clausiliid, were studied in two localities in Poland: the nature reserve Debno nad Wart1 (Wielkopolska region) and the Valley of Pieninski Potok (Pieniny Mts). The malacocoenoses were composed of 20 and 42 species, respectively. In Debno nad Wart1 R. filograna was the most frequent and abundant species; in the Valley of Pieninski Potok the most frequent and abundant species was Vitrea diaphana. The structure of the two communities differed also in other respects. The gastropod density in the two localities was similar (168 and 185 individuals · m, respectively), but their species composition was not, with only six species in common (Nei index 0.22).

The terrestrial gastropod fauna of the Drawa National Park includes 39 species, of which 29 (74.4%) were present in quantitative samples and subject to quantitative analysis. Dominant species were Aegopinella nitidula (Drap.), Discus rotundatus (O. F. Mull.) and Vitrea crystallina (O. F. Mull.). Snail communities of acid beech forest and a ruderal site with park tree stands in a mixed forest with beech had the highest species diversity index H’, while those of a eutrophic lowland beech forest and acid oak forest showed the highest TDI index of species diversity, and equitability index (J). The mean snail density was 19.3 indiv. m. The presence of Macrogastra ventricosa (Drap.), Laciniaria plicata (Drap.), Bulgarica cana (Held) and Helicigona lapicida (L.) is noteworthy.

Three types of columella structure have been distinguished in shells of selected snail species (Laciniaria plicata (Drap.), Chondrula tridens (O. F. Mull.), Cepaea vindobonensis (Fer.), Helix lutescens (Rossm.), Viviparus contectus (Millet), V. viviparus (L.), Helicella obvia (Menke) and Planorbarius corneus (L.)); they are correlated with the way of coiling, growth and convexity of whorls.

Cepaea vindobonensis (Ferussac, 1821) is recorded from two new localities in the Wielkopolska region, W Poland. The sites are located outside the continuous distribution range of the species, probably as remains of its more numerous insular localities in the Warta river valley.

Monacha cartusiana (O. F. Muller, 1774) was found in a new site in western Poland. The site is located in the city of Poznan, outside the continuous distribution range of the species. It is probably a result of accidental introduction by man.