Species Richness and Dispersal of Vascular Plants to Islands in Lake Mockeln, Southern Sweden

Plant species richness and other variables were analyzed on 41 islands and 2 mainland plots. The species—area coefficient was .30, but was higher (.73) for strictly terrestrial and lower (0.13) for shore species. In a stepwise multiple regression analysis, 85% of the variation in species richness was accounted for by log e surface area, percent of the ground covered by stones, and foliage, respectively. The average height of the trees correlated positively with the number of strictly terrestrial species but negatively with shore species. The difference between the number of strictly terrestrial species, predicted from the multiple regression equation, and that observed was proportionally larger on smaller islands. There was evidence that plant species carried by animals dispersed less efficiently to the islands. The flora contained proportionally fewer of these species with decreasing island area. The converse was true for water—dispersed species and some wind—dispersed (those with pappi). These observations are consistent with the hypothesis of higher extinction rates on small islands. A dynamic equilibrium in species number does not necessitate an equilibrium in the proportions of species in different dispersal categories.