Communities and diversity of land molluscs in Scandinavian woodlands. I. High diversity communities in taluses and boulder slopes in SW Sweden

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In a largely non-calcareous area in SW Sweden the most diversified land molluse communities occur in wooded talus and boulder slope habitats. There is a very strong correlation between pH and the number of snail species, whereas differences in slope aspect have a very modest influence only. Up to 34 snail species have been found co-existing in the southern slope sector, up to 29 species in the northern sector. On average there is an increase of 5 species per pH unit. Depending on the size of the sample (in the present survey standardized to 201), occasional collecting circumstances, and population dynamics of the individual species, all species of a site are in most cases not obtained in the same sample. An attempt to estimate the real diversity at different pH values has been made. Up to 37 snail species probably co-exist in the richest sites. The number of slug species is more difficult to assess. However, up to 7 species have been found together in talus and boulder slopes, and 6 species may be regularly present in the sites already at pH 5. Causes of the high diversity in wooded talus and boulder slopes are discussed. There is a rich microhabitat differentiation and good access to shelter, allowing survival during adverse periods. Furthermore, the study shows that when conditions improve species with a similar ecology tend to accumulate rather than replace each other. This indicates that competition is of subordinate importance. Compared with many other regions, and in particular with regard to the fairly high latitude (c. 58°), figures for co-existing species in SW Sweden must be regarded as high. This may depend partly on the particular geographical situation, partly on the evolutionary background, favouring species pre-adapted to a broad ecological co-existence.