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 Islands wasinvestigated. Compared with 100 year-old data the number of specieshas remained relatively constant. A species turnover ratio of 0.16% year^-1 was calculated for the whole area. The species-area regressionequation gave a z-value of 0.321. The high z-value, the low turnoverratio and a variance to mean ratio of 2.29 are interpreted as aneffect of the relative remoteness of the islands. It is suggested thatsnails colonized the islands via brackish waters, which are graduallycut off by land upheaval. The highest number of species occurin 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)