Colonisation and extinction of vascular plants on islands off the Swedish east coast

Abstract: Persistence of species is a function of colonisation and extinction among patches at a regional scale. A decrease in area and an increased isolation of suitable patches may result in a decreased colonisation rate and an increased extinction rate among species. A large number of species connected to semi-natural grasslands are today on the verge of extinction, due to ceased management of our semi-natural grasslands. This thesis describes patterns in colonisation, extinction and persistence among vascular plants during a century in the Blidö archipelago that have largely been transformed into forests due to the ceased management. I also examine whether species-specific traits may explain colonisation and extinction patterns both generally and more closely for Achillea millefolium and Silene viscosa.Extinction rate in the Blidö archipelago decreased with distance from mainland, which coincide with decreasing management intensity on the islands. Surprisingly we found that population size did not affect persistence among semi-natural grassland species. Locally abundant species were relatively more subjected to extinction than less abundant species, suggesting that former competitive species were more sensitive to the ongoing closing of the landscape. Effects of specific life-history traits on the processes of extinction and colonisation were examined, but could not in general explain why certain species connected to semi-natural grasslands had decreased or disappeared. Result from an analysis of physiological tolerance suggested that the observed closing of the open habitats have disfavoured species with a high light requirement.Empirical studies have shown that increased habitat fragmentation may affect plant-pollinator interactions with a reduced seed set as a result. I examined how fecundity were affected by isolation in the self-incompatibel A. millefolium, pollinated by a varity of pollinators, by doing a transplantation experiment to non-isolated and isolated islands in the Stockholm archipelago. No effects were found on pollen disposition and fecundity suggesting that species having a generalist fauna of pollinators are to a lesser extent affected by isolation than those dependent on a single pollinator or a more specialised guild of pollinators.Populations of S. viscosa were very persistent during a 40-year period and old records indicate a long-term persistence for some populations. Population size changes on islands were independent of each other even though closely situated. All this suggested that the distribution pattern approximates an Island-Mainland model, where a number of large populations are persistent and a number of smaller populations are more prone to extinctions due to catastrophes and environmental stochasticity. The patch occupancy was positively affected by isolation. This effect had disappeared 40 years later, which coincided with the appearance of the greylag goose (Anser anser), here shown to disperse seeds by occasionally eating seed capsules. This highlights the effect of dispersal on patch occupancy patterns.