Commonness and long-term survival in fragmented habitats: Primula elatior as study case.

In Flanders (northern Belgium), populations of Primula elatior (Primulaceae), an obligately outcrossing long-lived perennial herb, is widely distributed in fragmented ancient deciduous forests. We investigated population genetic variation and structure using 15 allozyme loci and quantified reproductive success in the form of fruit and seed production. Based on a sample of nine populations from Flemish forests, we related these variables to population size, density (interplant spacing within populations) and isolation. With only four polymorphic loci, the levels of genetic variation within populations were low compared to other species with similar life-history traits. Small populations had significantly fewer alleles per locus than large populations, but still maintained high levels of heterozygosity. No effect of plant density was found on genetic variation within populations. Genetic differentiation among populations was low, indicating moderate levels of gene flow. However, isolation by distance was observed, and small populations appeared to be more differentiated than large populations. These findings suggest genetic drift as a cause of loss of variation and divergence. Population size and plant density were correlated but differently affected fruit and seed production. A reduced seed production was found in small populations. Fruit abortion increased at high plant densities, as a result of biparental inbreeding and neighbourhood genetic structure. Our results indicate that being common is not a guarantee for long-term survival in fragmented habitats. Conservation efforts have also to focus on widely distributed species so as to prevent populations collapses and consequent species decline. Forest management has to take the preservation of the herbaceous layer into consideration, as avoiding logging practices which are damaging for the populations of forest herbs.