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 Aland Islands was investigate d. Compared with 100 year-old data the number of species has remained relat ively constant. A species turnover ratio of 0.16 % year(-1) was calculated for the whole area. The species-area regression equation gave a z-value of 0.321. The high z-value, the low turnover ratio and a variance to mean rati o of 2.29 are interpreted as an effect of the relative remoteness of the is lands. II is suggested that snails colonized the islands via brackish water s, which are gradually cut off by land upheaval. The highest number of spec ies occur in young lakes recently isolated from the brackish Baltic Sea, wh ile at higher elevations the number of species declines.