Population size, viability and genetic diversity in the orchid Gymnadenia conopsea

Sammanfattning: In this thesis, I combined controlled crosses with genetic and demographic data to examine how a main conservation value indicator, population size, was associated with estimates of population viability. I focused on the still common, but decreasing, perennial orchid Gymnadenia conopsea at two spatial scales—locally on the island Öland, in SE Sweden, and regionally across Scandinavia. I aimed to determine whether: (1) population size or density could predict the strength of heterosis and inbreeding depression, (2) local density variation within populations affected self-pollen transfer and relatedness of individuals, (3) genetic diversity increased with population size or density, and genetic differentiation and diversity predicted the strength of heterosis, (4) population growth rate increased with population size and genetic diversity, and (5) large-scale genetic structure indicated several independent colonization events in Scandinavia, with clear genetic groups and genetic diversity hotspots.I found heterosis to decrease and inbreeding depression to increase with local density in Öland populations. The proportion of deposited self-pollen decreased with local density, but established individuals located in sparse patches were not more related than those in dense ones, possibly due to high inbreeding depression at early life stages. Genetic diversity increased with population size, but neither population genetic differentiation or within population genetic diversity was related to the strength of heterosis or inbreeding depression. I found the majority of Öland populations to be declining, and population growth rate to increase with population size. This relationship was driven by higher survival in large populations, and most likely reflects that population size was positively associated with local habitat quality and stability. Population growth rate was not related to the amount of genetic diversity within the population, suggesting declines are not driven by genetic erosion. At the Scandinavian scale, I identified three genetic groups, consistent with two independent post-glacial colonizations followed by admixture. High genetic diversity was found in southern and central populations, while low diversity was seen in marginal populations along the Atlantic coast.In conclusion, population size was associated with both genetic diversity and population viability at the Öland scale. Genetic diversity was not related to the strength of heterosis or population viability, suggesting that demographic metrics can be more informative than genetic metrics, regarding conservation priority.