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

In Flanders (northern Belgium), populations of Primula elatior (Primulaceae), an obligately outcrossing long-lived perennial herb, are widely distributed in fragmented ancient deciduous forests. We used 15 allozyme loci to investigate population genetic variation and structure of P. elatior 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 with those of other species with similar life-history traits. Small populations had significantly fewer alleles per locus than large populations but 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. Isolation by distance was observed, however, 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 affected fruit and seed production differently. A reduced seed production was found in small populations. Fruit abortion increased at high plant densities as a result of biparental inbreeding and neighborhood genetic structure. Our results indicate that being common is not a guarantee of long-term survival in fragmented habitats. Conservation efforts must include widely distributed species so as to prevent population collapses and consequent species decline. Forest management has to take into consideration the preservation of the herbaceous layer and avoid logging practices that damage populations of forest herbs.