Variation in dispersability among mainland and island populations of three wind dispersed plant species

Reduced dispersability of species living on islands relative to mainland has been documented in both plants and animals. One evolutionary scenario explains this trend by strong selection against dispersal, once the species has reached the island, to reduce dispersal out to sea. In this study, we compare the dispersal ability of three wind dispersed plant species (Cirsium arvense, Epilobium angustifolium, and E. hirsutum) from populations on mainland and three islands. Dispersal ability was estimated directly as drop time of diaspores, and indirectly using a morphological measure relating the weight of the diaspore to the size of the pappus (Cirsium) or seed hairs (Epilobium). Positive correlation between the morphological measure of dispersal ability and drop time of diaspores were found for all study species. Dispersal ability varied significantly among mainland and islands, and among species. C arvense showed a significant reduction in dispersal ability on islands compared to mainland, whereas the reverse was found for the two Epilobium species. Overall Epilobium diaspores had a 2-4 times higher dispersability than C. arvense, indicating that degree of isolation of islands vary among study species. Significant differences in dispersability among plants within populations were detected in all species suggesting that this trait may have a genetic component.