Species richness, extinction and immigration rates of vascular plants on islands in the Stockholm archipelago, Sweden, during a century of ceasing management

Survey of eighteen islands in the Stockholm archipelago from 1884–1908 was re-examined during 1996–1998 to investigate how island size, degree of isolation and human impact affect the species number, extinction and immigration rates of terrestrial vascular plants. Cattle grazing and haymaking was widely and commonly in practice on the islands, but became less intensive since the 1920s and ceased during the 1960s. In the first survey species number was positively correlated with area and negatively correlated with distance to mainland. This pattern was similar in the later survey. However, the number of surrounding islands was positively correlated with island specific species number. During the period from the first to the second survey twenty species disappeared and ninety-three appeared as new. The increased abundance of trees, bushes and shade tolerant herbs suggest that this increase in species number is the effect of an ongoing succession in the landscape. The absolute extinction rate was negatively correlated with island size and distance to mainland, while the immigration rate was positively correlated to island size. The extinction rate was negatively correlated with the distance to mainland probably due to an earlier more intense cultural exploitation closer to the mainland. Our measure of human impact, the presence of houses on islands, was not related to terrestrial plant species number or extinction rate. However, there was a trend for the immigration rate to be higher on islands with houses in 1942 than those without, indicating the importance of human activities for dispersal and/or recruitment.