Selection of islands for conservation in the urban archipelago of Helsinki, Finland

The occurrence of vascular plants was surveyed on 207 islands (size range 0 .01-390.2 ha, number of plant species 1-449) offshore from the city of Hels inki in the Baltic Sea to examine the conservation value of these islands. We calculated a rarity score for each species (1/number of islands occupied by the species) and a biodiversity score for each island (sum of the rarit y scores of each species present on the island). Positive correlations betw een species number and biodiversity score (r(s) = 0.97, p < 0.001) and betw een biodiversity score and island area (r(s) = 0.87, p < 0.001) indicated t hat these parameters are heavily dependent on island size. With the goal of including at least one occurrence (island) of all plant species, an iterat ive selection algorithm chose a set of 41 islands whose average size (29.3 ha) was four times the average size of all existing islands (7.0 ha). Stron g nestedness (N < 54) explains the concentration of plant species diversity on large islands. An operational strategy for selection of sites for prote ction is to complement the set produced by a selection algorithm with targe t species not yet included (e.g., endangered species with several occurrenc es). Comprehensive mapping and analysis of a taxonomic group will help inte grate conservation biology into land-use planning and increase the quality of the networks of protected areas.