Genetic diversity and fitness in Scabiosa columbaria in the Swiss Jura in relation to population size

Due to changes in land use, remnants of unfertilised, nutrient-poor calcareous grassland in the Swiss Jura are small in area and are highly fragmented. We selected 89 seed families from eleven populations of various sizes of Scabiosa columbaria for a study of molecular diversity, and used the same material in a greenhouse experiment to measure variation in fitness-related traits and the ability of populations to cope with competition. Using RAPD-PCR we detected 71 RAPD-phenotypes among 87 genotypes. Molecular diversity within populations was variable and relatively high, with an expected heterozygosity H-e ranging from 0.09 to 0.24. H-e, the Shannon index (SI) and the percentage of polymorphic bands were not correlated with population size, but the smallest populations had the lowest molecular diversity (H-e, SI). Population differentiation was moderate with 12 of the molecular diversity among populations. Measures of fitness in the greenhouse differed among seed families (P > 0.001), but not among populations. Mean above-ground biomass was largely reduced when plants had to compete with Bromus. Mean fitness of populations decreased when molecular diversity (H-e) was low, but only when plants had to compete with Bromus (P = 0.02). Accordingly, the relative competition ability of Scabiosa plants decreased when molecular diversity (H-e) was low (P = 0.01). Our results suggest an increased risk of local extinction of Scabiosa columbaria in the Swiss Jura caused by a decreased viability and reduced phenotypic plasticity due to genetic erosion in small populations.