SPECIES-AREA RELATIONSHIPS, WATER CHEMISTRY AND SPECIES TURNOVER OF FRESHWATER SNAILS ON THE ALAND ISLANDS, SOUTHWESTERN FINLAND

The freshwater snail fauna of 51 lakes on the Åland Islandswasinvestigated. Compared with 100 year-old data the number ofspecieshas remained relatively constant. A species turnover ratio of0.16% year^-1 was calculated for the whole area. The species-arearegressionequation gave a z-value of 0.321. The high z-value, the lowturnoverratio and a variance to mean ratio of 2.29 are interpreted asaneffect of the relative remoteness of the islands. It is suggestedthatsnails colonized the islands via brackish waters, which aregraduallycut off by land upheaval. The highest number of speciesoccurin young lakes recently isolated from the brackish Baltic Sea,whileat higher elevations the number of species declines. (Received 12 November 1999; accepted 2 June 2000)